The stable polymeric nitrogen and polynitrogen compounds have potential applications in high-energy-density materials. For beryllium nitrides, there is one known crystalline form, Be3N2, at ambient conditions. In the present study, the structural evolutionary behaviors of beryllium polynitrdes have been studied up to 100 GPa using first-principles calculations and unbiased structure searching method combined with density functional calculations. One stable structural stoichiometry of beyrllium polynitride has been theoretically predicted at high pressures. It may be experimentally synthesizable at high pressures less than 40 GPa. It is therefore possible to synthesize BeN4 by compressing solid Be3N2 and N2 gas under high pressure and BeN4 may be quenching recoverable to ambient conditions. The predicted high-pressure P21/c-BeN4 compound contains a novel variety of polynitrogen, extended polymeric 3D puckered N10 rings network. To the best of our knowledge, this is the first time that stable N10 rings network are predicted in alkaline-earth metal polynitrides. The decomposition of P21/c-BeN4 is expected to be highly exothermic, releasing an energy of approximately 6.35 kJ·g–1. The present results open a new avenue to synthesize polynitrogen compound and provide a key perspective toward the understanding of novel chemical bonding in nitrogen-rich compounds. Results of the present study suggest that it is possible to obtain energetic polynitrogens in main-group nitrides under high pressure.

Unique structures and properties will be introduced upon compression. To figure out the high pressure crystal structure of XeF2, in situ synchrotron X-ray diffraction, Raman spectra, UV–vis absorption spectra, and theoretical calculations have been performed up to 86 GPa. The structural dispute between reported experimental and theoretical results is settled by this study. The experimental and theoretical Raman spectra results indicated that the ambient structure of XeF2 (I4/mmm) transformed into a Immm structure at 28 GPa. Then this Immm structure transformed into Pnma structure at 59 GPa. The Rietveld refinement of the XRD results was in accordance with our Raman study. The optical absorption spectra revealed a reduction in the band gap as pressure increases. The reduction in the band gap decreases to 1.83 eV at 82 GPa while the color of the sample is getting dark. Our results provide a new insight into the high pressure behavior of noble gas compounds with the example of XeF2.

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0–300 GPa by utilizing evolutionary structure searches. TaH and TaH2 were found to be thermodynamically stable over this entire pressure range, whereas TaH3, TaH4, and TaH6 become thermodynamically stable at pressures greater than 50 GPa. The dense Pnma (TaH2), R3̅m (TaH4), and Fdd2 (TaH6) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of Fdd2 (TaH6), a calculation of electron–phonon coupling reveals the potential high-Tc superconductivity with an estimated value of 124.2–135.8 K.

Invigorated by the high temperature superconductivity in some binary hydrogen-dominated compounds, we systematically explored high-pressure phase diagrams and superconductivity of a ternary Mg–Ge–H system using ab initio methods. Stoichiometric MgGeH6 with high hydrogen content exhibiting Pm symmetry was predicted from a series of high-pressure synthesis paths. We performed an in-depth study on three distinct formation routes to MgGeH6, i.e., Mg + Ge + 3H2 → MgGeH6, MgGe + 3H2 → MgGeH6 and MgH2 + GeH4 → MgGeH6 at high pressures. By directly squeezing three elemental solids Mg + Ge + 3H2, we obtained ternary MgGeH6 at 200 GPa. By adding a little bit of the MgGe alloy into hydrogen, we found that MgGeH6 can form and stabilize at about 200 GPa. More intriguingly, upon compressing MgH2 and GeH4 to 250 GPa, we also predicted the same MgGeH6. Electron structure calculations reveal that the cubic MgGeH6 is a good metal and takes on ionic character. Electron–phonon coupling calculation reveals a large λ = 1.16 for MgGeH6 at 200 GPa. In particular, we found that ternary MgGeH6 could be a potential high temperature superconductor with a superconducting transition temperature Tc of ∼67 K at 200 GPa.

Alkali metal compounds exhibit novel characteristics under pressure, such as antimetallization in CLi4, high superconductivity at 80 K in highly compressed Li3S, and the existence of unexpected stable stoichiometries of sodium chlorides, etc., which have greatly prompted us to explore KxS compounds at pressure. We found several stable structures with a variety of stoichiometries and proposed a phase diagram on the K-rich side first. Chemical rules established at ambient pressure are frequently violated when high pressure is applied, Na3Cl and NaCl3 as unusual stoichiometries of sodium chloride have been reported in high-pressure conditions. However, KS, with its counterintuitive chemical formula, has been discovered theoretically even at ambient pressure, and possesses the same stability as K2S. The mechanism of superconductivity in Pmm K3S is deeply investigated, comparing with the reported Pmm Li3S. The weak electron–phonon coupling mainly contributes to the weak superconductivity in K3S, which is fully in contrast to the mechanism of interstitial charge localization which dominates the low Tc in the reported Li3S.

A systematic computational study on the structural, electronic, and bonding properties of binary sulfur nitrides has been performed using the projector augmented wave method based on density functional theory. The pressure–composition phase diagram of the S–N system has been established. The simulated pressure–temperature phase diagram and X-ray diffraction pattern of (SN)x explain the experimentally observed two-phase coexistence. The crystal structure of experimentally observed orthorhombic (SN)x is predicted. The high-pressure phase transition of (SN)x has been studied. Sulfur–sulfur interactions induced by localized sulfur 3pz electrons are found in the high-pressure phase of (SN)x. With increasing nitrogen composition, the coordination number of sulfur atoms increases from two to six in the S–N system. Furthermore, two nitrogen-rich sulfur nitrides SN2 and SN4 have been found at high pressure. SN4 exhibits a high energy density (2.66 kJ·g–1), which makes it potentially interesting for industrial applications as a high energy density material.

In this work, the structures, phase sequence, and metallic properties of K2S have been systematically explored. We confirm that the P63/mmc phase is the best possible candidate for the stable structure of K2S at low pressure range. Although the phases of P63/mmc and Cmcm K2S are semiconductors, two new structures of P6/mmm and Pm1 emerge with metallic characters at high pressures. The analyses of electronic localization functions reveal that the conductivity mainly comes from the electrons surrounding S atom chains, which supplies a potential way to improve the conductivity of sulfur to enhance the electrode recharge ability and rate capability in alkali sulfide battery under pressure.

The high-pressure structural evolutionary behaviors of magnesium polynitrides were studied up to 100 GPa using first-principles calculations. Using the unbiased structure searching method, five stable chemical stoichiometries of magnesium polynitrides (MgN, Mg2N3, MgN2, MgN3, and MgN4) were theoretically predicted at high pressures. The predicted MgNx compounds contain a rich variety of polynitrogen forms ranging from charged molecules (one-dimensional bent molecules N3, planar triangle N4 to benzene-like rings N6) to extended polymeric chains (N∞). To the best of our knowledge, this is the first time that stable bent molecules N3, planar triangle N4, and polymeric chains (N∞) were predicted in alkaline-earth metal polynitrides. The decomposition of P-MgN3 and P-MgN4 are expected to be highly exothermic, releasing an energy of approximately 2.83 kJ g−1 and 2.01 kJ g−1, respectively. Furthermore, P-MgN4 can be synthesized at several GPa. The results of the present study suggest that it is possible to obtain energetic polynitrogen in main-group nitrides under high pressure.

ReB2-type MnB2 has always been considered to be the ground-state structure of MnB2. However, subsequent theoretical study has revealed that this structure is easy to decompose into elemental Mn and B under ambient conditions, which motivated us to look for a stable MnB2 structure at high pressures. Using structure prediction algorithm USPEX and density functional theory calculations, we found a stable multi-layered MnB2 structure with space group Immm at high pressure. The calculated hardness of Immm-MnB2 is 22.5 GPa, which makes it a potential hard multifunctional material along with its conductive and magnetic properties. The hexagonal graphene-like boron networks of Immm-MnB2 contribute to its hardness and stability.

Aiming at finding new superconducting materials, we performed systematical simulations on phase diagrams, crystal structures, and electronic properties of vanadium hydrides under high pressures. The VH, VH2, VH3, and VH5 species were found to be stable under high pressures; among these, VH2 had previously been investigated. Moreover, all three novel stoichiometries showed a strong ionic character as a result of the charge transfer from V to H. The electron–phonon coupling calculations revealed the potentially superconductive nature of these vanadium hydrides, with estimated superconducting critical temperature (Tc) values of 6.5–10.7 K for Rm (VH), 8.0–1.6 K for Fmm (VH3), and 30.6–22.2 K for P6/mmm (VH5) within the pressure range from 150 GPa to 250 GPa.

We report a robust honeycomb boron layers sandwiching manganese layers compound, MnB2, synthesized by high pressure and high temperature. First-principle calculation combined with X-ray photoelectron spectrum unravel that the honeycomb boron structure was stabilized by filling the empty π-band via grabbing electrons from manganese layers. Honeycomb boron layers sandwiching manganese layers is an extraordinary prototype of this type of sandwiched structure exhibiting electronic conductivity and ferromagnetism. Hydrostatic compression of the crystal structure, thermal expansion, and the hardness testing reveal that the crystal structure is of strong anisotropy. The strong anisotropy and first-principle calculation suggests that B–B bonds in the honeycomb boron structure are a strong directional covalent feature, while the Mn–B bonds are soft ionic nature. Sandwiching honeycomb boron layers with manganese layers that combine p-block elements with magnetic transition metal elements could endow its novel physical and chemical properties.

We report on a first-principles study of the phase diagram, structures and properties of the Ru–H system in the H-rich regime over a wide range of pressures. The results show that RuH is thermodynamically stable and can coexist with RuH3 and RuH6 under pressure. RuH and RuH3 stoichiometries exhibit metallic character as a result of notable band structures, while RuH6 is a semiconductor. Strikingly, some hydrogen atoms pairwise couple into H2 units in the RuH6 compound. An estimation of superconducting transition temperature Tc is carried out by applying the Allen-Dynes modified McMillan equation for Fmm (RuH), Pmm (RuH3), and Pmn (RuH3) structures and the resulting Tc reaches 0.41, 3.57 and 1.25 K at different pressures, respectively.

Tantalum–boron compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Using first-principle methods, ground-state TaB3 with the monoclinic C2/m space group and high-pressure TaB4 with the orthorhombic Amm2 space group have been found. They are more stable than the previously proposed structures. High-pressure boron-rich Amm2-TaB4 can be quenched to ambient pressure. The ground-state C2/m-TaB3 and high-pressure Amm2-TaB4 are two potential ultra-incompressible and hard materials with a calculated hardness of 17.02 GPa and 30.02 GPa at ambient pressure, respectively. Detailed electronic structure and chemical bonding analysis proved that the high hardness value of Amm2-TaB4 mainly stems from the strong covalent boron–boron bonds in graphene-like B layers as well as B–B bonds between layers.

The lattice structure, electronic properties, and superconductivity of SnH4 under high pressure were determined by first-principle calculations. A new layered structure C2/m with charge transfer from the Sn to H atoms was predicted to be the most stable high-pressure form. In the layered structure, each Sn atom combines with four H atoms and two H2 units, forming six Sn–H bonds of very similar lengths. The rich and multiple Fermi surface distribution in the Brillouin zone shows metallic features with the conductivity deriving from the electrons around the hydrogen atoms. The estimated high Tc of ca. 64–74 K at 500 GPa is attributed to the strong electron–phonon coupling from the vibration phonon modes of the H atoms.

We have performed in situ synchrotron X-ray diffraction and first-principles calculations to explore the compression behavior of barium hexaboride (BaB6) under high pressure. No phase transitions in our experiment are observed up to 49.3 GPa at ambient temperature. It is found that the ambient cage structure (Pmm) is still stable with a basic covalent network during the experimental pressure run. The results of our theoretical calculations show that the ambient structure might transform into three dynamically stable structures (Cmmm, Cmcm and I4/mmm) at 78 GPa, 97 GPa and 105 GPa respectively. The energy band calculations indicate that the sample is still a semiconductor with a narrow gap at 50 GPa.

At room environment, all materials can be classified as insulators or metals or in-between semiconductors, by judging whether they are capable of conducting the flow of electrons. One can expect an insulator to convert into a metal and to remain in this state upon further compression, i.e., pressure-induced metallization. Some exceptions were reported recently in elementary metals such as all of the alkali metals and heavy alkaline earth metals (Ca, Sr, and Ba). Here we show that a compound of CLi₄ becomes progressively less conductive and eventually insulating upon compression based on ab initio density-functional theory calculations. An unusual path with pressure is found for the phase transition from metal to semimetal, to semiconductor, and eventually to insulator. The Fermi surface filling parameter is used to describe such an antimetallization process.

A novel cubic porous carbon allotrope C96 carbon with intriguing physical properties was predicted. It has 96 atoms in the conventional cell, possessing a Pmm space group. The basic building block of C96 carbon is a planar six-membered carbon ring. The structural stability, mechanical properties, and dynamical properties of C96 carbon were extensively studied. It is a semiconductor (1.85 eV) with a lower density (2.7 g cm−3) and a larger bulk modulus (279 GPa) and is stable under ambient conditions. The hardness of C96 carbon (25 GPa) is larger than that of T carbon (5.6 GPa). Due to the structural porous feature and lower density, C96 carbon can also be expected to be a good hydrogen storage material.

The structures, electron properties, and potential superconductivity of indium hydrides are systematically studied under high pressure by first-principles density functional calculations. Upon compression, two stable stoichiometries (InH3 and InH5) are predicted to be thermodynamically stable. Particularly, in the two compounds, all hydrogen atoms exist in the form of H2 or H3 units. The stable phases present metallic features with the overlap between the conduction and the valence bands. The Bader analysis indicates that charges transfer from In atoms to H atoms. Electron–phonon calculations show that the estimated transition temperatures (Tc) of InH3 and InH5 are 34.1–40.5 and 22.4–27.1 K at 200 and 150 GPa, respectively.

Tungsten–nitrogen (W–N) compounds are studied via a combination of first-principles calculations and variable-composition evolutionary structure searches. New candidate ground states and high-pressure phases at 3:2, 1:1, and 5:6 compositions are uncovered and established for possible synthesis. We found that the structures in 4/5-fold N coordination (i.e., NbO–WN and W5N6) are more favoured for the W–N system at low-pressures compared with the conventional 6-fold phases (rs-WN and δ-WN). We attribute the low N coordination feature of W–N ground states to the enhanced W 5d–N 2p orbital hybridization and strong covalent W–N bonding, which involves the full-filling of W–N bonding and antibonding states and can remarkably improve the mechanical strength and hardness. These findings not only clarify the phase diagram of the W–N system, but also shed light on the correlations of hardness with microscopic crystal and electronic structures.

The high-pressure structures and superconductivity of iodine-doped hydrogen have been studied by ab initio calculations. Above 100 GPa, we discover a stable phase with Pnma symmetry in the H2I stoichiometry that consists of a monatomic iodine tube trapping hydrogen molecular units. Interestingly, H2 molecular units dissociate and form a novel atomic phase with Rm symmetry at 246 GPa. Further electron–phonon coupling calculations predict the critical temperature of superconductivity Tc to be 3.8 K for the Pnma phase and 33 K for the Rm phase at 240 GPa. Significantly, the Tc of the Rm phase is enhanced approximately 8 times that of the Pnma phase, which is mainly attributed to the reason that H2 molecules are broken exhibiting an atomic character in the Rm phase.

Niobium–nitrogen compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Based on ab initio evolutionary structural searches, we predict three ground states (oP6-Nb2N, CW-NbN, and hP22-Nb5N6) and six stable high pressure phases (ε-NbN, AsNi-NbN, U2S3-Nb2N3, oC24-NbN2, mP8-NbN3, and mP20-NbN4) for Nb–N compounds at pressures up to 100 GPa. Among them, the oP6-Nb2N, oC24-NbN2, mP8-NbN3, and mP20-NbN4 have never been reported, and N-rich oC24-NbN2, mP8-NbN3, and mP20-NbN4 high pressure phases are recoverable to ambient pressure. We find that the structure of N-rich Nb–N compounds consists of NbNx polyhedral stacking configurations and connected with Nn (n = 2, 3, 4, and n) polymerizations, which can remarkably improve the elastic modulus. It is found that CW-NbN and mP20-NbN4 are two potential ultra-incompressible and hard materials with the hardness calculated to be 24.56 and 19.86 GPa, respectively, while other N-rich phases such as U2S3-Nb2N3, oC24-NbN2, and mP8-NbN3 are soft materials. Detailed electronic structure and chemical bonding analysis proved that the high hardness of CW-NbN and mP20-NbN4 stems from the strong covalent bonding and the fullfilled Nb–N bonding and antibonding states.

The evolutionary structure-searching method discovers that the energetically preferred compounds of germane can be synthesized at a pressure of 190 GPa. New structures with the space groups Ama2 and C2/c proposed here contain semimolecular H2 and V-type H3 units, respectively. Electronic structure analysis shows the metallic character and charge transfer from Ge to H. The conductivity of the two structures originates from the electrons around the hydrogen atoms. Further electron–phonon coupling calculations predict that the two phases are superconductors with a high Tc of 47–57 K for Ama2 at 250 GPa and 70–84 K for C2/c at 500 GPa from quasi-harmonic approximation calculations, which may be higher than under actual conditions.

The Mg4Nb2O9 crystals grown by floating zone technology were used as prototypes to investigate optoelectronic parameters by measuring the absorption and transmittance spectra along the c-axis from 200 to 800 nm at room-temperature. The imaginary and real parts of the complex dielectric constants, extinction coefficient and refractive index for Mg4Nb2O9 were obtained. The Cauchy–Sellmeier equation for Mg4Nb2O9 was validated in the Urbach tail region (4.0–5.0 eV). The dispersion energy Ed and oscillator energy E0 were obtained via the single oscillator approximation. More interestingly, the self-trapped exciton and disorder of Mg2+ and Nb5+ between MgO6 and NbO6 octahedrons participating in the transition in the Urbach tail region were confirmed by the excitation spectrum.

A first-principles calculation is applied to perform a comprehensive study of the Sn–H system. Besides the common tetravalent hydride, a novel SnH8 crystal with the space group Im2 is reported with the most dominant enthalpy from structure searching techniques. All the H atoms of SnH8 are in the form of H2 or H3 units with electrons localized around them, showing covalent bond character. The rich and multiple Fermi surface distribution displays a metallic feature. Further electron–phonon coupling calculations reveal the high Tc of 63–72 K at 250 GPa.

First-principles calculations were performed to investigate the structural, electronic, and elastic properties of N2O4 and N2O5. Two new phases, namely, P42212 N2O4 and P21/m N2O5, are determined under high pressure through an ab initio evolutionary algorithm. For N2O4, the Im3 phase transforms to the P42212 structure at 35 GPa. The pressure–volume curves of N2O4 and N2O5 show that this transition has a first-order nature. The calculated phonon dispersion and elastic constants of Im3 N2O4, P42212 N2O4, and P21/m N2O5 demonstrate that the dynamic and mechanical stable pressure ranges are 2–35, 35–80, and 29–120 GPa, respectively. The electronic properties and projected density of states imply that these three structures are insulators. Furthermore, the N–N and N–O bond length of nitrogen oxides under high pressure are discussed.

Stable compounds, crystal structures and properties of polonium hydrides have been systematically investigated through the first-principles calculations based on the density functional theory. With the increasing pressure, several stoichiometries (PoH, PoH2, PoH4 and PoH6) are predicted to be stabilized in an excess hydrogen environment. Except for PoH, other stable stoichiometries exhibit intriguing structural character with the appearance of H2 units. Moreover, the electronic band structure and projected density of states (PDOS) demonstrate that these energetically stable phases are metallic. The application of the Allen–Dynes modified McMillan equation with the calculated electron-phonon coupling parameter reveals that PoH4 is a superconductor with a critical temperature Tc of 41.2–47.2 K at 200 GPa.

We demonstrate that corundum Mg4Ta2O9 crystals, free of low-angle grain boundaries and bubbles, were prepared for the first time by the infrared heating optical floating zone method. The X-ray diffraction results showed that the crystals had corundum structure, cleaved parallel to the c plane and grew along the a-axis. The temperature-dependent optical phonon behavior of Mg4Ta2O9 were also investigated in the range from 84 to 834 K.

The high-pressure behavior of ammonium iodide (NH4I) has been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 40 GPa. The first-order phase transition from phase IV to V is confirmed by XRD measurements for the first time. Fitting the measured volumetric compression data to the third order Birch–Murnaghan equation of state reveals bulk moduli of B0 = 14.3 ± 1.3 and 28.5 ± 2.6 GPa (with fixed B′0 = 4) for phases IV and V, respectively. Still, by analyzing the red shift of the N–H symmetric and asymmetric modes and the intramolecular distances, it is concluded that hydrogen bonding is the dominant effect upon compression over the whole pressure range.

Iron mononitride has attracted much interest because of its interesting magnetoelectric properties. However, whether the ground state of FeN has a rock-salt (rs) or a zinc-blende (zb) structure is still controversial. Clarification of this issue has been impeded by the complex magnetic ordering and strong electron correlation effects. Here, we study the relative stability of rs and zb FeN toward different spin orderings (ferromagnetic, antiferromagnetic, and paramagnetic) at pressures of 0–100 GPa, with the GGA-PBE, LDA+U, and HSE hybrid exchange–correlation functionals. We find that the competition between direct and indirect exchange interactions can drive magnetic structure phase transitions for rs-FeN at high pressures, whereas zb-FeN is still nonmagnetic. Strikingly, the energy difference between rs and zb FeN decreases and finally vanishes as the occupied minority-spin t2g orbitals of rs-FeN are depleted when 3d electron correlations are considered. These results demonstrate that an appropriate treatment of electron correlations is important for determining the stability and properties of 3d transition metal nitrides.

We perform first-principles density functional theory calculations to examine the stability of nitrogen-doped wurtzite ZnO under pressure. Our calculations indicate that both the stability of the nitrogen-doped ZnO and the defect concentration increase with pressure. As the pressure increases from 0 to 9 GPa, the density of states at the Fermi level decreases, and the states have a tendency to move to lower energy levels. Electron-localization function and Bader charge analysis have been used to understand the pressure effect on the defect. Under the basic growth conditions (using ε-N2 for nitrogen atoms), the calculated formation enthalpies decrease with pressure, which suggests a rise in the defect concentration. Applying pressure has great impact on the nitrogen-doped defects, and can be used as an efficient approach to form p-type ZnO.

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials and phase transitions, germanium-hydride compounds under high pressure were studied by a genetic algorithm. Enthalpy calculations suggest that the Ge and H will form Ge3H, Ge2H, GeH3, and GeH4 at about 32, 120, 280, and 280 GPa, respectively. These four germanium-hydride compounds are all stable up to at least 300 GPa. For Ge3H, the most stable structure is P-Ge3H at 32–220 GPa and P63/m-Ge3H at 220–300 GPa. All the germanium-hydride compounds are metallic phases as demonstrated by the band structure and density of states.

Platinum (Pt) has been widely studied for pressure calibration in high pressure–temperature ranges. We have for the first time performed in situ synchrotron X-ray diffraction (XRD) with laser-heated diamond anvil cells to study the P–V–T equation of state (EOS) for Pt up to 95 GPa and 3150 K. MgO was used for pressure calibration. A detailed analysis of the room-temperature compression curve was fitted with the third-order Birch–Murnaghan (BM) EOS, which yields ambient volume V0 = 60.3 Å3, isothermal bulk modulus K0 = 308 GPa, and its pressure derivative K′0 = 4.1. A least-squares fit of the P–V–T data to a high-temperature (BM) EOS yielded K′0 = 5.5 ± 2, K0 = 274 ± 36 GPa, αKT(V0, T) = 0.003 ± 0.0003 GPa K−1 and (∂KT/∂T)V = 0.03 ± 0.01 GPa K−1 with V0 = 60.3 Å3. Within a reasonable range, it is found that the EOS of this study is consistent with the known EOS of Pt. The present technique and results cover the P–T range between the resistive heating and the shock compression experimental data in the literature.

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials, the high-pressure structures of AlH3(H2) in the pressure range of 25–300 GPa were extensively explored by using a genetic algorithm. We found an insulating P1 phase, a semiconducting P phase and an intriguing sandwich-like metallic phase with a space group of P21/m-Z (containing Z shape net layers of Al atoms). We found that the H2 molecules in the environment of AlH3 became metallic and showed a molecular semi-molecular phenomenon. The application of the Allen–Dynes to modify the McMillan equation yields remarkably high superconducting temperatures of 132–146 K at 250 GPa, which is among the higher values reported so far for phonon-mediated superconductors. In this paper, we reveal a unique superconducting mechanism, which shows that the direct interactions between H2 and AlH3 at high pressure play a major role in the high superconductivity, while the contribution from the H2 vibration is minor.

•Four calculation strategies (ultrasoft pseudopotential and norm conserving pseudopotential with/without Van der Waals force) are performed to determine the reasonable phase transition sequence of crystal hydrogen over high pressures.•We got four reasonable phase transition sequence of crystal hydrogen (Cmca-12, Cmca, Pca21, and P63/m). The transition pressures of four cases are different.•We also calculated the vibrational Raman/IR spectra of the P63/m, Pca21, Cmca, and Cmca-12, and it can provide guides to experiment.The crystal structure of solid hydrogen under high pressure has been extensively investigated using first-principles density functional calculations. We discussed the impact of two pseudopotentials (ultrasoft pseudopotential and norm conserving pseudopotential) with/without Van der Waals force on hydrogen system, and conducted a series of geometry optimization on 11 structures (C2, P63/m, Pca21, C2/c, Cmca, Cmca-12, P63/mmc, P21/c, C2/c-1, Pbcn, and Pa3), which was obtained by referring to literature, finally we got four reasonable phase transition sequence of crystal hydrogen(Cmca-12, Cmca, Pca21, and P63/m). The transition pressures of four cases are different. We also calculated the vibrational Raman/IR spectra of the four sequences of which the enthalpies are nearly equal and it can provide guides to experiment.

•The formation enthalpy of oxygen vacancy increases with pressure, which makes the defect formatted harder under pressure.•The formation enthalpy of several native point defects is related to a fine interplay between the charges on the defects and applied external pressures.•The defect electronic transition levels strongly depend on the pressure.We investigate the formation enthalpies and transition energy levels for several native point defects in B1 phase of ZnO under applied hydrostatic pressure using density functional theory. The formation volume decreases gradually with increasing pressure, and increases linearly with the number of electrons adding to the system. In negatively charged state, the calculated formation enthalpy decreases with pressure, suggesting an increase in the equilibrium defect concentration. The behavior of the positively charged state is on the contrary, consistent with the results of the formation volume. In particular, the formation enthalpy of oxygen vacancy increases with pressure, which makes the defect formation harder under pressure. Under Zn-rich conditions, the “negative-U” phenomenon of oxygen vacancy, which appears under ambient conditions, vanishes with further increase in pressure when the Fermi enthalpy is close to the conduction band minimum.

The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n > 1) are metastable up to 300 GPa, whereas for HnCl (n > 1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H3+ species are unexpectedly found in stoichiometries H2F with [H3]+[HF2]−, H3F with [H3]+[F]−, H5F with [H3]+[HF2]−[H2]3, and H5Cl with [H3]+[Cl]−[H2] above 100 GPa. Importantly, formation processes of H3+ species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H3+ species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.

The pressure-induced new structures and properties of osmium hydrides were systematically explored in a wide pressure range 0–300 GPa using ab initio methods. Three stable stoichiometries, that is, OsH, OsH3, and OsH6, are predicted above 50 GPa. The above hydrides exhibit metallic character with the notable band structures exception of OsH6. It is interesting to note that the phase P21/c of hydrogen-rich OsH6 adopts intriguing structures with H2 units. The electron–phonon coupling calculations indicate that the superconducting critical temperature (Tc) values of Fm-3m-OsH is 2.1 K at 100 GPa. Comparing to pure Os, the addition of hydrogen is in favor of improving the superconducting temperature.

The high-pressure behavior of SnI2 has been investigated in a combined experimental and theoretical study by angle-dispersive X-ray diffraction, Raman scattering measurements, and ab initio calculations. Both the Raman and XRD results confirm that the SnI2 crystal undergoes a gradual crystal to amorphous transition and subsequently recrystallizes to a new crystal structure upon compression. The intensity of the Sn–I symmetric stretching mode greatly decreases and manifests a red shift at 2.15 GPa in Raman spectra. The XRD patterns show a bonding break phenomenon before the pressure-induced amorphization. We propose that the bond breaking between neighboring layers leads to the formation of the amorphous phase under high pressure. The sample recrystallizes into a new high-pressure crystal phase at 33.18 GPa. The experimental and theoretical results provide a good candidate structure for the recrystallized phase with C2/m space group.

By ab initio molecular dynamics simulations, ammonium azide (AA, NH4N3) is predicted to be an effective precursor to form polynitrogen. Our simulations at 60 and 90 GPa show that the critical temperatures for nitrogen polymerization are about 2200 and 1600 K, respectively. Compared with molecular nitrogen (110 GPa and 2000 K), the synthesis pressure of polymeric nitrogen in AA significantly lowers. In the obtained polymeric nitrogen compounds, there are kinds of nitrogen backbones: one-dimensional chains, branched chains, and five-membered rings. By annealing simulations at 90 GPa, a one-dimensional pure nitrogen periodic chain is formed. Our finding might open a way for the practical application of polymeric nitrogen compounds as further depressurization simulations at 300 K confirm that both hydrogen-passivated polymeric networks and five-membered rings can be preserved at ambient conditions.

Based on ab initio evolutionary algorithms, a high-pressure close-packed phase of boron with hexagonal P63/mcm symmetry is predicted, named as B10, which is stable over α-Ga phase above 375 GPa to at least 500 GPa. High pressure makes the typical B12 icosahedron collapse to form an incompressible linear atomic chain arrangement together with an isosceles triangle arrangement. The electron localization function calculations confirm that the B10 has strong covalency in this special atomic arranged structure. The vibration of the three atom's isosceles triangle in the framework of linear atomic chains induces an unusual superconductivity in B10. Electron–phonon calculations indicate that electron–phonon coupling parameter λ is 0.82 and the superconducting critical temperature is 44 K at 400 GPa.

The structural, electronic, and dynamical properties and intermolecular interactions of CH4H2 are investigated based on first-principles calculations. Enthalpy calculations indicate that the P structure is the most stable phase below 15.6 GPa. On compression, the P212121 phase possesses the lowest enthalpy, then the P21/C becomes energetically favorable above 98.2 GPa. The pressure-induced hardening behavior of H2-bonds in the CH4H2 system reproduces the experimental data well. In addition, the CH4 molecules remain as tetrahedra due to the weak interactions of molecular H2 and CH4. At higher pressure, the orientation of H2 molecules in CH4H2 is also disordered and this resembles the findings for pure H2. It shows that comparing to pure H2, the addition of CH4 does not have much limitations on the rotation of the H2 molecules.

The ternary transition metal carbide Mo0.5W0.5C was synthesized under high pressure and high temperature and the crystalline structure was confirmed by Rietveld refinements as being hexagonal (Pm2). The mechanical properties, bulk modulus and Vickers hardness were also investigated by in situ high-pressure X-ray diffraction and Vickers microhardness testing, respectively. The fitting bulk modulus of the ternary transition metal carbide is 399.9 ± 9.3 GPa, which is as compressible as diamond, and its asymptotic Vickers hardness is 15.3 GPa, nearly 60% harder than molybdenum carbide. The high bulk modulus is attributed to the high valence electron density and the greater hardness compared with γ-MoC is due to the strong bond between tungsten and carbon atoms.

First-principle calculations were performed to investigate the structural, elastic, and electronic properties of iridium diboride (IrB2). It was demonstrated that the new phase of IrB2 belongs to the monoclinic C2/m space group, and we have named it m-IrB2. Its structure is energetically much superior to the recently proposed Pmmn-type IrB2. Further calculations of phonon and elastic constants confirm that m-IrB2 is dynamically and mechanically stable. The calculated high shear modulus reveals that it is a potentially a material of low compressibility. An analysis of the density of its states and chemical bonding show that the strongly directional covalent B–B and B–Ir bonds in m-IrB2 make a considerable contribution to its stability.

The important transition metal dihydride ZrH2 has been characterized using in situ synchrotron X-ray diffraction combined with diamond anvil cell techniques and ab initio calculations. The effect of a pressure-transmitting medium on the structural stability and compressive behavior was investigated under both hydrostatic pressure and nonhydrostatic pressure conditions. The ambient I4/mmm structure is stable under both nonhydrostatic and hydrostatic compressions. The supplementary theoretical calculations have proposed that the I4/mmm structure transformed into the P4/nmm structure above 100 GPa confirming the stability of the I4/mmm structure during the experimental runs. The difference in the volume reduction between the two compressions becomes larger with increasing pressure. Up to about 50 GPa, the volume collapse of nonhydrostatic compression is 6% relative to the hydrostatic compression. The present study offers a new approach with nonhydrostatic compression or shear stress for finding higher volumetric density hydrogen structures in metal hydrides.

Using the ab initio evolutionary algorithm for crystal structure prediction, we successfully obtained the high pressure crystal structure of BI3 with P21/c space group, which was characterized as the B2I6 dimer analogous to the diborane molecule. This structure has been supported by excellent agreement between the theoretical and experimental equations of state, X-ray diffraction data, and the positive pressure dependence of Tc. Moreover, the P21/c structure is both dynamically and mechanically stable through phonon and elastic constants calculations. Further analysis reveals a superconducting mechanism in which the vibrations of iodine atoms dominate the superconductivity of BI3. In addition, the positive pressure dependence Tc is mainly attributed to the increase of the EPC parameter λ, the logarithmic average phonon frequency ωlog, and the electronic density of states at the Fermi level N(εf).

The structural stability, mechanical properties, and dynamical properties of T carbon-like structures were extensively studied by first-principles calculations using density functional theory. A novel modulated T carbon-like carbon allotrope (T-II carbon) is predicted by means of first principles calculations. This structure has 8 atoms in the unit cell, possesses the Pnm space group, and can be derived by stacking up two T carbons together. T-II carbon is a semiconductor with band gap 0.88 eV and has a higher hardness (27 GPa) than that of T carbon (5.6 GPa). The calculations of ideal strength and the electron localization function indicate that T-II carbon has better ability to resist shear strain than T carbon.

The phase stability, mechanical properties and metallic properties of tantalum nitrides are extensively studied by means of first principles calculations. The relationship between nitrogen concentration and physical properties of tantalum nitrides has been systematically investigated. With the nitrogen concentration increasing, it is found that the feature of covalent bonding enhances and the directionality of the covalent bonding and hardness of tantalum nitrides reduce. While these make the ductility of tantalum nitrides improve with the nitrogen concentration increasing. The intensity of metallic properties of tantalum nitrides can be effectively adjusted by controlling the nitrogen concentration and pressure. When the tantalum: nitrogen ratio reaches Ta:N = 1:3, remarkable nitrogen–nitrogen bonds are found in TaN3. The hardness of TaN3 abnormally increases with reference to that of the preceding composition Ta3N5-II. The potential synthesis routes of tantalum nitrides are suggested.

The high-pressure behavior of hydrazine has been investigated by in situ Raman spectroscopy and synchrotron X-ray diffraction experiments under pressure up to 46.5 and 33.0 GPa, respectively. It is found that the liquid hydrazine solidifies into phase I at about 1.2 GPa. The symmetry of phase I is confirmed to be space group P21 by the peak assignment, group theory analysis, and Rietveld refinement of XRD patterns. A solid–solid transition from phase I to phase II is observed in both Raman spectroscopy and XRD experiments at about 2.4 GPa, which is ascribed to the formation of new hydrogen bonds between hydrazine molecules. At 18.4 GPa, an isostructural transition from phase II to the final phase III is observed. The pressure-induced adjustment of bifurcated hydrogen bond is first researched and regarded as the origin of the isostructural transition. Above 20.6 GPa, a clear softening behavior occurs in the NH2 symmetric stretching mode. The coupling of optical vibrations derived from enhancement of the hydrogen bond is proposed as a crucial role in this softening process.

Structures of ammonium bromide under high pressure were investigated through ab initio evolutionary algorithm and total-energy calculations based on density functional theory. Static enthalpy calculations indicate that the low-pressure phase V (space group P4/nmm) transforms into a monoclinic P21/m structure at 71 GPa and then an orthorhombic structure Cmma at 130 GPa, which is found to be energetically stable up to 264 GPa. Mechanism of phonon softening at the P4/nmm → P21/m transformation is discussed. Ab initio calculations show that the band overlap in the molecular Cmma phase, which causes the pressure-induced insulator-to-metal transition, occurs at about 240 GPa. Enthalpy calculations show that Cmma NH4Br becomes unstable and dissociates into NH3 and HBr above 264 GPa.

The behavior of pyrrole under high pressure has been investigated by in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman scattering up to 34 GPa. A solid-to-solid transition at about 6.2 GPa with a large collapse of volume (∼40%) from Pnma to P21/c has been found after a liquid-to-solid transition at 0.6 GPa, which is caused by the molecular rotational repacking of π-stacking. This new phase P21/c plays a central role in the following pressure-induced polymerization due to the formation of a closed dimer, a very important precursor. The threshold of C···C distance with steric hindrance in dimer is about 1.62 Å at ∼10.2 GPa. After this steric hindrance is overcome, a crystal–amorphous transformation starts at ∼14.3 GPa. When completely released from 34 GPa, the recovered solid product with single bond is identified by in situ Raman measurement.

Technetium nitrides with various ideal stoichiometries have been investigated with the first-principle method at the pressures between 0–60 GPa. It have been found that there could be many stable technetium nitrides including Tc3N, Tc2N, TcN, Tc2N3, TcN2, TcN3, and TcN4, among which Tc3N and Tc2N subnitrides are synthesizable at zero pressure and could be applied to nuclear waste management, such as separate radioactive 99Tc from nuclear fuel cycle. Moreover, N-rich TcN3 and TcN4 exhibit remarkable bulk properties and can be potential ultrastiff and hard materials.

The high-pressure behavior of SnBr4 has been explored by angle-dispersive X-ray diffraction, Raman spectroscopy, optical absorption/transmission measurements, and ab initio calculations. The joint of experimental and theoretical results shows that there is a crystal–crystal transition induced by molecular dimerization, which begins at ∼10.8 GPa and completes at 21.5 GPa. Above 21.5 GPa, the gradual crystal–amorphous transition induced by the molecular dissociation appears, and the sample remains in partial amorphous state up to 43.8 GPa. At ∼40.3 GPa, the possible insulator–metal transformation is discovered by in situ optical transmission measurements. It is proposed that the observed amorphous phase is a nonmolecular phase by theoretical calculations.

In 2009, a super-hard MnB2 with ReB2-type structure was predicted as being in the ground state. However, it has not been synthesized successfully in about two years either by high temperature and high pressure (HTHP) method or by the arc-melting method. To obtain the accurate synthesis conditions, the P–T phase boundary between AlB2-type and ReB2-type MnB2 has been studied by first-principles lattice dynamics calculations within quasi-harmonic approximation (QHA). Our results show that the ReB2-type MnB2 can be synthesized only below 1020 K at ambient pressure. Pressure effect makes their transition temperature decrease. If the pressure is higher than 38 GPa, only AlB2-type MnB2 can be obtained. The synthesis temperatures of previous experiments (either HTHP or arc-melting method) are all above 1020 K, so that only AlB2-type MnB2 can be synthesized. Therefore, it is essential to control the temperature accurately to synthesize the ReB2-type MnB2. On the other hand, the pressure should be controlled to be as low as possible. Further analyses show that the thermodynamic stability of MnB2 at high temperature mostly depends on the vibration frequency of Mn atoms. The stronger interactions between Mn and B in the ReB2-type MnB2 induce the vibration frequencies of Mn atoms shift to higher and increase the Gibbs free energy, causing the thermodynamics instability of ReB2-type MnB2 at high temperature. Therefore, there is no ReB2-type MnB2 synthesized at the temperature higher than 1020 K.

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

The structural studies of lithium amide (LiNH2) have been performed by synchrotron X-ray diffraction measurements and ab initio density functional theoretical calculations up to 28.0 GPa. It is revealed that LiNH2 undergoes a reversible pressure-induced phase transitions from tetragonal phase (I-4) into the monoclinic phase (P21), which starts from about 10.3 GPa and completes at about 15.0 GPa. This transition is accompanied by about 11% large volume collapse, and this volume collapse is much larger than other complex ternary hydrides. The experimental pressure–volume data for the two phases of LiNH2 are fitted by third-order Birch–Murnaghan equation of state, yielding B0 of 37.2 (1.7) GPa for the tetragonal phase and 7.6 (4.9) GPa for the monoclinic phase with the pressure derivatives at 3.5. We also have calculated the total and partial density of states of the two phases in order to explore the mechanism of the volume reduction.

The high-pressure phases of bromoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density functional theory. A new high-pressure polar phase, ε, with space group CC has been found after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from β phase to γ phase occurs at 1 GPa, then the γ phase transforms to the ε phase at 90 GPa. In addition, the Br···Br and C−H···Br interactions are the key factors for the polar aggregation in the ε phase. Further calculations show that the insulate−metal transition in ε phase due to band overlap happens at ∼130 GPa.

There is a great interest in the behavior of diatomic molecular solids under extremely high-pressure conditions that lead to pressure-induced metallization, molecular dissociation, and formation of atomic phase. The consensus has been that the phase-transition sequence that happened in both solid bromine and iodine is from a molecular phase (phase I), to an incommensurate phase (phase V), and then to an atomic phase (phase II), with increasing pressure. However, a puzzle remains unresolved for both solids: pressure-induced X and Y bands were observed in the Raman spectra in the molecular phase at low pressures, even before the onset of phase V. Here, we suggest a phase for solid iodine in such a low-pressure range (designated as phase I′) in which two different covalent intramolecular bonds coexist, based on first-principles calculations and later corroborated by x-ray diffraction experiments. The pressure dependence of the X and Y bands and other vibrational frequencies measured experimentally can be explained nicely by combining the vibrational modes of phase I and phase I′. These results help improve our understanding on the pressure-induced molecular dissociation and metallization in diatomic solids and may shed some light on the investigation of similar phenomena in solid H₂.

We investigate, through first-principles calculations, the pressure dependence of formation volumes and formation enthalpies of boron and nitrogen vacancies (VB, VN) in cubic boron nitride (cc-BN) using a supercell approach. We find that VB3− and VN3+ have the lowest formation enthalpy and −3 and +3 can be considered as the dominant charges occurring in VB and VN at ambient pressure, respectively. And the charge states which have the lowest formation enthalpies do not change with pressure in the pressure range from 0 to 20 GPa. The formation enthalpy decreases with pressure for VB, while for VN it exhibits positive dependence of the formation enthalpy on pressure. Energy levels of defects under different pressures are also discussed. Our results suggest that for VN, pressure can strengthen its conductivity, while for VB, pressure effect is not obvious in our investigated pressure range.

•Four calculation strategies (ultrasoft pseudopotential and norm conserving pseudopotential with/without Van der Waals force) are performed to determine the reasonable phase transition sequence of crystal hydrogen over high pressures.•We got four reasonable phase transition sequence of crystal hydrogen (Cmca-12, Cmca, Pca21, and P63/m). The transition pressures of four cases are different.•We also calculated the vibrational Raman/IR spectra of the P63/m, Pca21, Cmca, and Cmca-12, and it can provide guides to experiment.The crystal structure of solid hydrogen under high pressure has been extensively investigated using first-principles density functional calculations. We discussed the impact of two pseudopotentials (ultrasoft pseudopotential and norm conserving pseudopotential) with/without Van der Waals force on hydrogen system, and conducted a series of geometry optimization on 11 structures (C2, P63/m, Pca21, C2/c, Cmca, Cmca-12, P63/mmc, P21/c, C2/c-1, Pbcn, and Pa3), which was obtained by referring to literature, finally we got four reasonable phase transition sequence of crystal hydrogen(Cmca-12, Cmca, Pca21, and P63/m). The transition pressures of four cases are different. We also calculated the vibrational Raman/IR spectra of the four sequences of which the enthalpies are nearly equal and it can provide guides to experiment.

The near-edge X-ray absorption fine structure (NEXAFS) spectra and crystal structures of the ε-O8 phase and the metallic ζ-O2 phase have been investigated using full-potential linearized augmented plane-wave (FLAPW) and pseudopotential plane-wave methods. The change in NEXAFS spectra for the k-edge of the oxygen atom suggests a means for identifying the phase transition from ε-O8 to ζ-O2 experimentally. The abrupt change of the bond length L (O1–O1) induced by pressure is believed to be responsible for the phase transition. There is no pressure-induced softening behavior in our calculated phonon dispersion curves near the phase transition pressure.

In 2009, a super-hard MnB2 with ReB2-type structure was predicted as being in the ground state. However, it has not been synthesized successfully in about two years either by high temperature and high pressure (HTHP) method or by the arc-melting method. To obtain the accurate synthesis conditions, the P–T phase boundary between AlB2-type and ReB2-type MnB2 has been studied by first-principles lattice dynamics calculations within quasi-harmonic approximation (QHA). Our results show that the ReB2-type MnB2 can be synthesized only below 1020 K at ambient pressure. Pressure effect makes their transition temperature decrease. If the pressure is higher than 38 GPa, only AlB2-type MnB2 can be obtained. The synthesis temperatures of previous experiments (either HTHP or arc-melting method) are all above 1020 K, so that only AlB2-type MnB2 can be synthesized. Therefore, it is essential to control the temperature accurately to synthesize the ReB2-type MnB2. On the other hand, the pressure should be controlled to be as low as possible. Further analyses show that the thermodynamic stability of MnB2 at high temperature mostly depends on the vibration frequency of Mn atoms. The stronger interactions between Mn and B in the ReB2-type MnB2 induce the vibration frequencies of Mn atoms shift to higher and increase the Gibbs free energy, causing the thermodynamics instability of ReB2-type MnB2 at high temperature. Therefore, there is no ReB2-type MnB2 synthesized at the temperature higher than 1020 K.

The high-pressure structural evolutionary behaviors of magnesium polynitrides were studied up to 100 GPa using first-principles calculations. Using the unbiased structure searching method, five stable chemical stoichiometries of magnesium polynitrides (MgN, Mg2N3, MgN2, MgN3, and MgN4) were theoretically predicted at high pressures. The predicted MgNx compounds contain a rich variety of polynitrogen forms ranging from charged molecules (one-dimensional bent molecules N3, planar triangle N4 to benzene-like rings N6) to extended polymeric chains (N∞). To the best of our knowledge, this is the first time that stable bent molecules N3, planar triangle N4, and polymeric chains (N∞) were predicted in alkaline-earth metal polynitrides. The decomposition of P-MgN3 and P-MgN4 are expected to be highly exothermic, releasing an energy of approximately 2.83 kJ g−1 and 2.01 kJ g−1, respectively. Furthermore, P-MgN4 can be synthesized at several GPa. The results of the present study suggest that it is possible to obtain energetic polynitrogen in main-group nitrides under high pressure.

A novel cubic porous carbon allotrope C96 carbon with intriguing physical properties was predicted. It has 96 atoms in the conventional cell, possessing a Pmm space group. The basic building block of C96 carbon is a planar six-membered carbon ring. The structural stability, mechanical properties, and dynamical properties of C96 carbon were extensively studied. It is a semiconductor (1.85 eV) with a lower density (2.7 g cm−3) and a larger bulk modulus (279 GPa) and is stable under ambient conditions. The hardness of C96 carbon (25 GPa) is larger than that of T carbon (5.6 GPa). Due to the structural porous feature and lower density, C96 carbon can also be expected to be a good hydrogen storage material.

We report on a first-principles study of the phase diagram, structures and properties of the Ru–H system in the H-rich regime over a wide range of pressures. The results show that RuH is thermodynamically stable and can coexist with RuH3 and RuH6 under pressure. RuH and RuH3 stoichiometries exhibit metallic character as a result of notable band structures, while RuH6 is a semiconductor. Strikingly, some hydrogen atoms pairwise couple into H2 units in the RuH6 compound. An estimation of superconducting transition temperature Tc is carried out by applying the Allen-Dynes modified McMillan equation for Fmm (RuH), Pmm (RuH3), and Pmn (RuH3) structures and the resulting Tc reaches 0.41, 3.57 and 1.25 K at different pressures, respectively.

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

The evolutionary structure-searching method discovers that the energetically preferred compounds of germane can be synthesized at a pressure of 190 GPa. New structures with the space groups Ama2 and C2/c proposed here contain semimolecular H2 and V-type H3 units, respectively. Electronic structure analysis shows the metallic character and charge transfer from Ge to H. The conductivity of the two structures originates from the electrons around the hydrogen atoms. Further electron–phonon coupling calculations predict that the two phases are superconductors with a high Tc of 47–57 K for Ama2 at 250 GPa and 70–84 K for C2/c at 500 GPa from quasi-harmonic approximation calculations, which may be higher than under actual conditions.

Niobium–nitrogen compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Based on ab initio evolutionary structural searches, we predict three ground states (oP6-Nb2N, CW-NbN, and hP22-Nb5N6) and six stable high pressure phases (ε-NbN, AsNi-NbN, U2S3-Nb2N3, oC24-NbN2, mP8-NbN3, and mP20-NbN4) for Nb–N compounds at pressures up to 100 GPa. Among them, the oP6-Nb2N, oC24-NbN2, mP8-NbN3, and mP20-NbN4 have never been reported, and N-rich oC24-NbN2, mP8-NbN3, and mP20-NbN4 high pressure phases are recoverable to ambient pressure. We find that the structure of N-rich Nb–N compounds consists of NbNx polyhedral stacking configurations and connected with Nn (n = 2, 3, 4, and n) polymerizations, which can remarkably improve the elastic modulus. It is found that CW-NbN and mP20-NbN4 are two potential ultra-incompressible and hard materials with the hardness calculated to be 24.56 and 19.86 GPa, respectively, while other N-rich phases such as U2S3-Nb2N3, oC24-NbN2, and mP8-NbN3 are soft materials. Detailed electronic structure and chemical bonding analysis proved that the high hardness of CW-NbN and mP20-NbN4 stems from the strong covalent bonding and the fullfilled Nb–N bonding and antibonding states.

Tantalum–boron compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Using first-principle methods, ground-state TaB3 with the monoclinic C2/m space group and high-pressure TaB4 with the orthorhombic Amm2 space group have been found. They are more stable than the previously proposed structures. High-pressure boron-rich Amm2-TaB4 can be quenched to ambient pressure. The ground-state C2/m-TaB3 and high-pressure Amm2-TaB4 are two potential ultra-incompressible and hard materials with a calculated hardness of 17.02 GPa and 30.02 GPa at ambient pressure, respectively. Detailed electronic structure and chemical bonding analysis proved that the high hardness value of Amm2-TaB4 mainly stems from the strong covalent boron–boron bonds in graphene-like B layers as well as B–B bonds between layers.

Tungsten–nitrogen (W–N) compounds are studied via a combination of first-principles calculations and variable-composition evolutionary structure searches. New candidate ground states and high-pressure phases at 3:2, 1:1, and 5:6 compositions are uncovered and established for possible synthesis. We found that the structures in 4/5-fold N coordination (i.e., NbO–WN and W5N6) are more favoured for the W–N system at low-pressures compared with the conventional 6-fold phases (rs-WN and δ-WN). We attribute the low N coordination feature of W–N ground states to the enhanced W 5d–N 2p orbital hybridization and strong covalent W–N bonding, which involves the full-filling of W–N bonding and antibonding states and can remarkably improve the mechanical strength and hardness. These findings not only clarify the phase diagram of the W–N system, but also shed light on the correlations of hardness with microscopic crystal and electronic structures.

The high-pressure structures and superconductivity of iodine-doped hydrogen have been studied by ab initio calculations. Above 100 GPa, we discover a stable phase with Pnma symmetry in the H2I stoichiometry that consists of a monatomic iodine tube trapping hydrogen molecular units. Interestingly, H2 molecular units dissociate and form a novel atomic phase with Rm symmetry at 246 GPa. Further electron–phonon coupling calculations predict the critical temperature of superconductivity Tc to be 3.8 K for the Pnma phase and 33 K for the Rm phase at 240 GPa. Significantly, the Tc of the Rm phase is enhanced approximately 8 times that of the Pnma phase, which is mainly attributed to the reason that H2 molecules are broken exhibiting an atomic character in the Rm phase.