Carbon nitride compounds with high N : C ratios and graphitic to polymeric structures are being investigated as potential next-generation materials for incorporation in devices for energy conversion and storage as well as for optoelectronic and catalysis applications. The materials are built from C- and N-containing heterocycles with heptazine or triazine rings linked via sp2-bonded N atoms (N(C)3 units) or –NH– groups. The electronic, chemical and optical functionalities are determined by the nature of the local to extended structures as well as the chemical composition of the materials. Because of their typically amorphous to nanocrystalline nature and variable composition, significant challenges remain to fully assess and calibrate the structure–functionality relationships among carbon nitride materials. It is also important to devise a useful and consistent approach to naming the different classes of carbon nitride compounds that accurately describes their chemical and structural characteristics related to their functional performance. Here we evaluate the current state of understanding to highlight key issues in these areas and point out new directions in their development as advanced technological materials.

•Significant survival of microbes is observed following shock compression into the GPa range.•Bacteria exhibit enhanced survival following shock compression compared with static compression.•Our results indicate the potential survival of viable lifeforms following bolide impact events.The possibility that life can exist within previously unconsidered habitats is causing us to expand our understanding of potential planetary biospheres. Significant populations of living organisms have been identified at depths extending up to several km below the Earth's surface; whereas laboratory experiments have shown that microbial species can survive following exposure to GigaPascal (GPa) pressures. Understanding the degree to which simple organisms such as microbes survive such extreme pressurization under static compression conditions is being actively investigated. The survival of bacteria under dynamic shock compression is also of interest. Such studies are being partly driven to test the hypothesis of potential transport of biological organisms between planetary systems. Shock compression is also of interest for the potential modification and sterilization of foodstuffs and agricultural products. Here we report the survival of Shewanella oneidensis bacteria exposed to dynamic (shock) compression. The samples examined included: (a) a “wild type” (WT) strain and (b) a “pressure adapted” (PA) population obtained by culturing survivors from static compression experiments to 750 MPa. Following exposure to peak shock pressures of 1.5 and 2.5 GPa the proportion of survivors was established as the number of colony forming units (CFU) present after recovery to ambient conditions. The data were compared with previous results in which the same bacterial samples were exposed to static pressurization to the same pressures, for 15 minutes each. The results indicate that shock compression leads to survival of a significantly greater proportion of both WT and PA organisms. The significantly shorter duration of the pressure pulse during the shock experiments (2–3 µs) likely contributes to the increased survival of the microbial species. One reason for this can involve the crossover from deformable to rigid solid-like mechanical relaxational behavior that occurs for bacterial cell walls on the order of seconds in the time-dependent strain rate.

•The annealing of TeO2-based glasses show growth of anti-glass spherulites.•The textures are similar but different to those in polyamorphic Y2O3-Al2O3 liquids.•Diffraction data for the tellurite glasses shows a discorded network formed from TeO3 and TeO4 polyhedra.•The growth of spherulites is the nucleation of disordered anti-glass nano-crystals possibly accompanying the growth of a lower density glassy form.Bi2O3-Nb2O5-TeO2 glasses show unusual annealing behavior with appearance of spherulites within the matrix glass structure for the Bi0.5Nb0.5Te3O8 composition. The textures resemble those found previously among polyamorphic Al2O3-Y2O3 glasses containing metastably co-existing high- and low-density phases produced during quenching. However the spherulites produced within the Bi2O3-Nb2O5-TeO2 glass are crystalline and can be identified as an “anti-glass” phase related to β-Bi2Te4O11. We used high energy synchrotron X-ray diffraction data to study structures of binary and ternary glasses quenched from liquids within the Bi2O3-Nb2O5-TeO2 system. These reveal a glassy network based on interconnected TeO4 and TeO3 units that is related to TeO2 crystalline materials but with larger Te…Te separations due to the presence of TeO3 groups and non-bridging oxygens linked to modifier (Bi3 +, Nb5 +) cations. Analysis of the viscosity-temperature relations indicates that the glass-forming liquids are “fragile” and there is no evidence for a LLPT occurring in the supercooled liquid. The glasses obtained by quenching likely correspond to a high-density amorphous (HDA) state. Subsequent annealing above Tg shows mainly evidence for direct crystallization of the “anti-glass” tellurite phase. However, some evidence may exist for simultaneous formation of nanoscale amorphous spherulites that could correspond to the LDA polyamorph. The quenching and annealing behavior of Bi2O3-Nb2O5-TeO2 supercooled liquids and glasses is compared with similar materials in the Al2O3-Y2O3 system.

High pressures extending up to several thousands of atmospheres provide extreme conditions for biological organisms to survive. Recent studies are investigating the survival mechanisms and biological function of microorganisms under natural and laboratory conditions extending into the GigaPascal range, with applications to understanding the Earth's deep biosphere and food technology. High pressure has also emerged as a useful tool and physical parameter for probing changes in the structure and functional properties of biologically important macromolecules and polymers encountered in soft matter science. Here we highlight some areas of current interest in high pressure biophysics and physical chemistry that are emerging at the frontier of this cross-disciplinary field.

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch–Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K′o = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta–(O,N) and Ta–Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko ∼ 370 GPa, close to the theoretically predicted value.

Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low-temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition-metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies has been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of prestructuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The orthorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

There is interest in identifying novel materials for use in radioactive waste applications and studying their behavior under high pressure conditions. The mineral zirconolite (CaZrTi2O7) exists naturally in trace amounts in diamond-bearing deep-seated metamorphic/igneous environments, and it is also identified as a potential ceramic phase for radionuclide sequestration. However, it has been shown to undergo radiation-induced metamictization resulting in amorphous forms. In this study we probed the high pressure structural properties of this pyrochlore-like structure to study its phase transformations and possible amorphization behavior. Combined synchrotron X-ray diffraction and Raman spectroscopy studies reveal a series of high pressure phase transformations. Starting from the ambient pressure monoclinic structure, an intermediate phase with P21/m symmetry is produced above 15.6 GPa via a first order transformation resulting in a wide coexistence range. Upon compression to above 56 GPa a disordered metastable phase III with a cotunnite-related structure appears that is recoverable to ambient conditions. We examine the similarity between the zirconolite behavior and the structural evolution of analogous pyrochlore systems under pressure.

Graphitic carbon nitride compounds were prepared by thermal treatment of C–N–H precursor mixtures (melamine C3N6H9, dicyandiamide C2N4H4). This led to solids based on polymerized heptazine or triazine ring units linked by −N═ or −NH– groups. The H content decreased, and the C/N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 °C due to increased layer condensation. The UV–vis spectra exhibited a strong π–π* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600–650 °C showed an additional absorption near 500 nm that is assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H2 evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h–1 H2 evolution with UV–vis irradiation, but this dropped to ∼0.23 μmol h–1 when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H2 production rates (31.5 μmol h–1) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O2 evolution was determined for RuO2-coated samples using the 300 W lamp with aqueous AgNO3 solution as the sacrificial agent. The materials all showed production rates ∼9 μmol h–1. A highly crystalline compound containing polytriazine structural units ((C3N3)2(NH)3·LiCl) prepared in this study did not show measurable photocatalytic activity.

Structural changes occurring within non-conventional Dawson-type [α/β-Mo18O54(SO3)2]4− polyanions in the form of tetrapentylammonium salts were studied by a combination of IR, Raman and visible spectroscopy at high temperature and high pressure. Evidence of the formation of bronze-type materials above 400 K and also upon pressurization to 8 GPa is presented. This conclusion is suggested to be a general result for polyoxometalate compounds subjected to extreme conditions and it opens opportunities for the design of new materials with interesting optical and electronic properties.Graphical abstractStructural changes occurring within non-conventional Dawson-type [α/β-Mo18O54(SO3)2]4− polyanions in the form of tetrapentylammonium salts were studied by a combination of IR, Raman and visible spectroscopy at high temperature and high pressure. Evidence of the formation of bronze-type materials above 400 K and also upon pressurization to 8 GPa is presented. This conclusion is suggested to be a general result for polyoxometalate compounds subjected to extreme conditions and it opens opportunities for the design of new materials with interesting optical and electronic properties.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Spectroscopy studies of non-conventional Wells–Dawson polyoxometalates (POMs) at high temperature and high pressure. ► Discussion on the stability of two POM isomers. ► Local formation of bronze-like materials: possibilities for a new synthetic method at high pressure from POM precursors.

Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na8Si46 and Na2Ba6Si46. Although their structure and properties are well described, there is little understanding of the formation mechanism. Na8Si46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si44– polyanions. In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25–500 °C at 8 °C/min) under vacuum conditions (10–4 bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state 23Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na8Si46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.Keywords: in situ studies; Na8Si46; NaSi; NMR spectroscopy; silicon clathrate; solid-state synthesis; synchrotron radiation; X-ray diffraction; Zintl phase;

We present results of an XAS and EXAFS study of the synthesis of Ge nanoparticles formed by a metathesis reaction between Mg2Ge and GeCl4 in diglyme (diethylene glycol dimethyl ether). The progress of the formation reaction and the products formed at various stages in the processing was characterised by TEM and optical spectroscopy as well as in situ XAS/EXAFS studies using specially designed reaction cells.Graphical abstractNano-Ge particles 2–10 nm in diameter were prepared by reaction between Mg2Ge Zintl phase and GeCl4 in diglyme followed by capping with BuLi and extraction into hexane. We used synchrotron X-ray absorption spectroscopy (XAS) at the Ge K edge with analysis of the EXAFS region combined with room temperature photoluminescence and TEM to characterise the nature of the nanoparticles and model compounds and to follow the course of the reaction. A TEM image of the germanium nanoparticles is shown.Highlights► In situ characteristaion of germanium nanoparticles. ► X-ray spectroscopic technique development. ► Improving quality of nanoparticles grown by metathesis route.

Sodium orthonitrate (Na3NO4) is an unusual phase containing the first example of isolated tetrahedrally bonded NO43− groups. This compound was obtained originally by heating together mixtures of Na2O and NaNO3 for periods extending up to >14 days in evacuated chambers. Considering the negative volume change between reactants and products, it was inferred that a high-pressure synthesis route might favor the formation of the Na3NO4 compound. We found that the recovered sample is likely to be a high-pressure polymorph, containing NO43− groups as evidenced by Raman spectroscopy. The high-pressure behavior of Na3NO4 was studied using Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell above 60 GPa. We found no evidence for major structural transformations, even following laser heating experiments carried out at high pressure, although broadening of the Raman peaks could indicate the onset of disordering at higher pressure.Graphical abstractWe studied Na3NO4 at high pressure using Raman spectroscopy and X-ray diffraction and determined V(P) to 64 GPa. We investigated synthesis and phase behavior of the orthonitrate at high-P,T conditions.Research highlights► Investigation of Na3NO4 at extreme compression (up to 64 GPa) using Raman spectroscopy and X-ray diffraction. ► Synthesis of Na3NO4 via high-pressure conditions. ► Estimation of the bulk moduli of Na3NO4 and Na2O and comparison with related compounds. ► Exploratory laser heating run in search for high-pressure Na3NO4 phases.

Cellulose is an important biopolymer with applications ranging from its use as an additive in pharmaceutical products to the development of novel smart materials. This wide applicability arises in part from its interesting mechanical properties. Here we report on the use of high pressure X-ray diffraction and Raman spectroscopy in a diamond anvil cell to determine the bulk and local elastic moduli of native cellulose. The modulus values obtained are 20 GPa for the bulk modulus and 200–355 and 15 GPa for the crystalline parts and the overall elastic (Young’s) modulus, respectively. These values are consistent with those calculated from tensile measurements. Above 8 GPa, the packing of the cellulose chains within the fibers undergoes significant structural distortion, whereas the chains themselves remain largely unaffected by compression.

We have synthesized a range of Mo–Nb nitrides at 2.5 GPa and T = 1600 °C or 2200 °C using high pressure–high temperature techniques. Following syntheses at 1600 °C Mo-rich compositions are hexagonal and Nb-rich phases are cubic, with a narrow two-phase region indicating the presence of a solvus. The maximum Mo content in the cubic phase derived from δ-NbN is 40–44 mol%, and the maximum Nb content in the hexagonal δ-MoN phase is 46–47 mol%. There was little variation in the superconducting transition temperature Tc for hexagonal δ-(Mo,Nb)N samples produced in the study. Previous studies showed maximum Tc values of 12–15 K for pure δ-MoN as a function of N-site ordering in high-P,T experiments. Here we recorded Tc = 14 K for the limiting Mo-rich composition prepared in the study. We observed Tc = 11.5–16 K for cubic δ-NbN depending on high-P,T synthesis and annealing conditions. This value falls to 5 K for the cubic Mo0.37Nb0.63N0.98 solid solution phase. Some high-P,T synthesis or annealing experiments were carried out at T = 2200 °C. At this temperature δ-MoN decomposes to produce γ-MoN0.54. A minor phase in this sample achieves Tc = 9.5 K. A new superconducting hexagonal oxynitride MoN0.74O0.38 with Tc = 16 K was also produced during this study.

We have used Raman and IR spectroscopy to study the oxidation reaction of sulfite groups in the unique polyoxometalate compound K7Na[WVI18O56(SO3)2(H2O)2]·20H2O with a structure related to the Wells–Dawson type in situ at high temperature and high pressure. The results give new insights into the unusual redox process that occurs in this compound with electrons and O2− ions transferred between the polyoxometalate cluster and SO32−/SO42− groups held within the cage in a process that has been termed the “Trojan Horse” effect.

We report a crystallographic study of Ti2.85O4N, a new titanium oxynitride phase discovered using chemical vapor deposition and combinatorial chemistry techniques, under high-pressure conditions. Synchrotron X-ray diffraction was used to monitor structural changes in the material during compression up to 68 GPa. The data indicate that the orthorhombic (Cmcm) ambient-pressure phase (K0 = 154 ± 22 GPa with K0′ = 5.2 ± 0.5) undergoes a first-order transition at 18 GPa to a new orthorhombic (Pmc21) structure. This new high-pressure polymorph remains stable up to 42 GPa, after which the emergence of a second high-pressure monoclinic (P21/c) phase is observed.

The graphitic layered compound C6N9H3·HCl was prepared by reaction between melamine and cyanuric chloride under high pressure–high temperature conditions in a piston cylinder apparatus and characterised using SEM, powder X-ray diffraction, UV Raman and near-IR Fourier transform Raman spectroscopy with near-IR excitation. Theoretical calculations using density functional methods permitted evaluation of the mode of attachment of H atoms to nitrogen sites in the structure and a better understanding of the X-ray diffraction pattern. Broadening in the UV and near-IR FT Raman spectra indicate possible disordering of the void sites within the graphitic layers or it could be due to electron–phonon coupling effects.The graphitic layered compound C6N9H3·HCl was prepared by reaction between melamine and cyanuric chloride under high pressure–high temperature conditions in a piston cylinder apparatus and characterised using SEM, powder X-ray diffraction, UV Raman and near-IR Fourier transform Raman spectroscopy using near-IR excitation. Theoretical calculations using density functional methods permitted evaluation of the mode of attachment of H atoms to nitrogen sites around the C12N12 voids within the layered structure and also led to better understanding of the X-ray diffraction pattern. Sharp peaks in the UV Raman spectra are due to C3N3 triazine ring units in the structure, that may be enhanced by resonance Raman effects. Broadening in the UV and near-IR FT Raman spectra indicate possible disordering within the graphitic layers or electron–phonon coupling effects.

The high-pressure behaviour of NaSi has been studied using Raman spectroscopy and angle-dispersive synchrotron X-ray diffraction to observe the onset of structural phase transformations and potential oligomerisation into anionic Si nanoclusters with extended dimensionality. Our studies reveal a first structural transformation occurring at 8–10 GPa, followed by irreversible amorphisation above 15 GPa, suggesting the formation of Si–Si bonds with oxidation of the Si− species and reduction of Na+ to metallic sodium. We have combined our experimental studies with DFT calculations to assist in the analysis of the structural behaviour of NaSi at high pressure.The high-pressure behaviour of NaSi has been studied using Raman spectroscopy and angle-dispersive synchrotron X-ray diffraction. Our studies reveal a first structural transformation occurring at 8–10 GPa, followed by irreversible amorphisation, suggesting the formation of Si–Si bonds with oxidation of the Si− species and reduction of Na+ to metallic sodium. We have combined our experimental studies with DFT calculations to assist in the analysis of the structural behaviour of NaSi at high pressure.

Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the Tg and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H2O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si4+ and Al3+ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the Tg or ηvs. P relations. This may be the case for H2O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.

We examined the solid-state water-soluble amorphous precursors that are formed by partial thermal decomposition of Al(NO3)3·9H2O (aluminum nitrate nonahydrate: ANN) using Raman and FTIR and solid-state magic-angle spinning NMR spectroscopy. We also studied the species formed in the aqueous alumosols formed by dissolution of the pre-ceramic precursors using 27Al NMR spectroscopy. Species identified in the alumosols included the Al3+(H2O)6 monomer, the [AlO4Al12(OH)24(H2O)12]7+(Al13) Keggin ion, and the Al30 polycation, [Al30O8(OH)56(H2O)24]18+, as well as various other oligomers or nanoparticles containing IV-, V- and VI-coordinated Al3+ ions.

Evidence has been presented for a density-driven phase transition occurring between supercooled liquids in the system Y2O3–Al2O3. The high- and low-density liquids were quenched to metastably coexisting glasses. Chemical analysis showed the compositions of the two glasses to be identical, and it was inferred that they differed in their densities and entropies. The entropy difference has been verified by calorimetry. Here, we confirm that the chemical compositions of the glassy materials derived from the high- and low-temperature liquids are identical. We present a direct density determination of the two glasses using sink-float techniques. The measured densities are 3.72(3) g/cm3 for the glass derived from the high-temperature liquid (i.e., the high-density amorphous or HDA polyamorph), and 3.58(1) g/cm3 for the low-temperature (low-density, LDA) polyamorph.

Besides temperature at one atmosphere, the applied pressure is another important parameter for influencing and controlling reaction pathways and final reaction products. This is relevant not only for the genesis of natural minerals, but also for synthetic chemical products and technological materials. The present critical review (316 references) highlights recent developments that utilise high pressures and high-temperatures for the synthesis of new materials with unique properties, such as high hardness, or interesting magnetic or optoelectronic features. Novel metal nitrides, oxonitrides as well as the new class of nitride-diazenide compounds, all formed under high-pressure conditions, are highlighted. Pure oxides and carbides are not considered here. Moreover, syntheses under high-pressure conditions require special equipment and preparation techniques, completely different from those used for conventional synthetic approaches at ambient pressure. Therefore, we also summarize the high-pressure techniques used for the synthesis of new materials on a laboratory scale. In particular, our attention is focused on reactive gas pressure devices with pressures between 1.2 and 600 MPa, multi-anvil apparatus at P < 25 GPa and the diamond anvil cell, which allows work at pressures of 100 GPa and higher. For example, some of these techniques have been successfully upgraded to an industrial scale for the synthesis of diamond and cubic boron nitride.

Solution phase reactions between tetrakisdimethylamidotitanium (Ti(NMe2)4) and ammonia yield precipitates with composition TiC0.5N1.1H2.3. Thermogravimetric analysis (TGA) indicates that decomposition of these precursor materials proceeds in two steps to yield rocksalt-structured TiN or Ti(C,N), depending upon the gas atmosphere. Heating to above 700 °C in NH3 yields nearly stoichiometric TiN. However, heating in N2 atmosphere leads to isostructural carbonitrides, approximately TiC0.2N0.8 in composition. The particle sizes of these materials range between 4–12 nm. Heating to a temperature that corresponds to the intermediate plateau in the TGA curve (450 °C) results in a black powder that is X-ray amorphous and is electrically conducting. The bulk chemical composition of this material is found to be TiC0.22N1.01H0.07, or Ti3(C0.17N0.78H0.05)3.96, close to Ti3(C,N)4. Previous workers have suggested that the intermediate compound was an amorphous form of Ti3N4. TEM investigation of the material indicates the presence of nanocrystalline regions <5 nm in dimension embedded in an amorphous matrix. Raman and IR reflectance data indicate some structural similarity with the rocksalt-structured TiN and Ti(C,N) phases, but with disorder and substantial vacancies or other defects. XAS indicates that the local structure of the amorphous solid is based on the rocksalt structure, but with a large proportion of vacancies on both the cation (Ti) and anion (C,N) sites. The first shell Ti coordination is approximately 4.5 and the second-shell coordination ∼5.5 compared with expected values of 6 and 12, respectively, for the ideal rocksalt structure. The material is thus approximately 50% less dense than known Tix(C,N)y crystalline phases.Amorphous and nanocrystalline titanium nitrides and carbonitrides with a very defect-rich rocksalt structure.

We prepared samples of cubic γ-MoNx (x  ∼0.5) by high-pressure–high-temperature synthesis. N atom site occupancies within the defect rock salt structure were determined from time-of-flight neutron diffraction and powder X-ray diffraction data by Rietveld analysis. The results show that N atoms occupy only octahedral sites within the structure. The semi-metallic compound is a superconductor, with Tc=5.2K determined by SQUID magnetometry. The compressibility of the material was determined by synchrotron X-ray diffraction measurements at high pressure in the diamond anvil cell. The vibrational density of states was studied by Raman scattering spectroscopy.The neutron observed (+), calculated (−) and difference profiles (bottom trace) for γ-MoN at 290 K.

A new solid-state metathesis synthesis route was applied to obtain bulk samples of amorphous or microcrystalline Si and Ge. The method involves reaction of Zintl phases such as NaSi or NaGe, with ammonium or metal (e.g., CuCl, CoBr2) halides. The driving force for the solid-state reaction is provided by the formation of alkali halides and the transition metals or metal silicides, or gaseous ammonia and hydrogen. The semiconductors were purified by washing to remove other solid products. The amorphous semiconductors were obtained in bulk form from reactions carried out at 200–300 °C. Syntheses at higher temperatures gave rise to microcrystalline semiconductors, or to micro-/nanocrystalline particles contained within the amorphous material. Similar crystalline/amorphous composites were obtained after heat treatment of bulk amorphous materials.TEM image of a region within a luminescent sample of a-Si prepared by solid-state metathesis synthesis, showing nanocrystalline particles.

We have determined the crystal structure of ordered hexagonal δ-MoN by use of powder X-ray diffraction and time-of-flight neutron diffraction. A disordered variety of the compound was first prepared by high-temperature ammonolysis of MoCl5. This material has hexagonal symmetry with the space group P63mc with a=2.87(2) and c=2.81(1) Å. Upon high pressure annealing, the N-atoms become ordered and the unit cell doubles in size: a=5.73659(10) and c=5.61884(17) Å. The superconducting transition temperature increases from 4 K in the disordered compound to 12.1 K in the ordered phase.

Metastable high-pressure transformations in germanium nitride (α- and β-Ge3N4 polymorphs) have been studied by energy- and angle-dispersive synchrotron X-ray diffraction at high pressures in a diamond anvil cell. Between P=22 and 25 GPa, the phenacite-structured β-Ge3N4 phase (P63/m) undergoes a 7% reduction in unit-cell volume. The densification is primarily concerned with the a-axis parameter, in a plane normal to the hexagonal c-axis. Based on results of previous LDA calculations and Raman spectroscopic studies, we propose that the structural collapse is due to transformation into a new metastable polymorph (δ-Ge3N4) that has a unit-cell symmetry based upon P3, that is related to the low-pressure β-Ge3N4 phase by concerted displacements of N atoms away from special symmetry sites in the plane normal to the c-axis. No such transformation occurs for α-Ge3N4, due to the different stacking of linked GeN4 layers. All three polymorphs (α-, β- and δ-Ge3N4) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured γ-Ge3N4 phase, that contains octahedrally coordinated Ge4+. Experimentally determined bulk modulus values for α-Ge3N4 (K0=165(10) GPa, K0′=3.7(4)) and β-Ge3N4 (K0=185(7) GPa, K0′=4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new δ-Ge3N4 polymorph is only determined above the β–δ transition pressure (P=24 GPa); K=161(20) GPa, assuming K′=4. Above 45 GPa, both α- and δ-Ge3N4 polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering.

Lithium monosilicide (LiSi) was formed at high pressures and high temperatures (1.0–2.5 GPa and 500–700°C) in a piston-cylinder apparatus. This compound was previously shown to have an unusual structure based on 3-fold coordinated silicon atoms arranged into interpenetrating sheets. In the present investigation, lowered synthesis pressures permitted recovery of large (150–200 mg) quantities of sample for structural studies via NMR spectroscopy (29Si and 7Li), Raman spectroscopy and electrical conductivity measurements. The 29Si chemical shift occurs at −106.5 ppm, intermediate between SiH4 and Si(Si(CH3)3)4, but lies off the trend established by the other alkali monosilicides (NaSi, KSi, RbSi, CsSi), that contain isolated Si44− anions. Raman spectra show a strong peak at 508 cm−1 due to symmetric Si–Si stretching vibrations, at lower frequency than for tetrahedrally coordinated Si frameworks, due to the longer Si–Si bonds in the 3-coordinated silicide. Higher frequency vibrations occur due to asymmetric stretching. Electrical conductivity measurements indicate LiSi is a narrow-gap semiconductor (Eb∼0.057 eV). There is a rapid increase in conductivity above T=450 K, that might be due to the onset of Li+ mobility.

In this study, we used laser-heated diamond anvil cell techniques coupled with synchrotron X-ray diffraction to investigate the synthesis and stability of nitride spinels in the Si3N4–Ge3N4 system, at pressures close to 20 GPa and at temperatures up to >2000°C. The newly discovered nitride spinels were found to be stable over the entire pressure and temperature range studied. There is little incorporation of Si3N4 component in γ-Ge3N4, but we observed formation of a new ternary nitride spinel (SixGe1−x)3N4, with x∼0.6. The analysis of the X-ray patterns indicates that Si4+, normally considered to be the smaller ion, is strongly partitioned into the octahedral sites in the spinel phase. Excess Ge4+ ions may also occupy these octahedral sites in the experimental synthesis at high pressure and temperature.

Amyloid fibrils, originally associated with neurodegenerative diseases, are now recognized to have interesting mechanical properties. By using synchrotron x-ray diffraction at high pressure in a diamond anvil cell we determined the bulk modulus of TTR105-115 amyloid fibrils in water and in silicone oil to be 2.6 and 8.1 GPa, respectively. The compression characteristics of the fibrils are quite different in the two media, revealing the presence of cavities along the axis of the fibrils, but not between the β-sheets, which are separated by a dry interface as in a steric zipper motif. Our results emphasize the importance of peptide packing in determining the structural and mechanical properties of amyloid fibrils.

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch–Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K′o = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta–(O,N) and Ta–Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko ∼ 370 GPa, close to the theoretically predicted value.

High pressures extending up to several thousands of atmospheres provide extreme conditions for biological organisms to survive. Recent studies are investigating the survival mechanisms and biological function of microorganisms under natural and laboratory conditions extending into the GigaPascal range, with applications to understanding the Earth's deep biosphere and food technology. High pressure has also emerged as a useful tool and physical parameter for probing changes in the structure and functional properties of biologically important macromolecules and polymers encountered in soft matter science. Here we highlight some areas of current interest in high pressure biophysics and physical chemistry that are emerging at the frontier of this cross-disciplinary field.

We have synthesized a range of Mo–Nb nitrides at 2.5 GPa and T = 1600 °C or 2200 °C using high pressure–high temperature techniques. Following syntheses at 1600 °C Mo-rich compositions are hexagonal and Nb-rich phases are cubic, with a narrow two-phase region indicating the presence of a solvus. The maximum Mo content in the cubic phase derived from δ-NbN is 40–44 mol%, and the maximum Nb content in the hexagonal δ-MoN phase is 46–47 mol%. There was little variation in the superconducting transition temperature Tc for hexagonal δ-(Mo,Nb)N samples produced in the study. Previous studies showed maximum Tc values of 12–15 K for pure δ-MoN as a function of N-site ordering in high-P,T experiments. Here we recorded Tc = 14 K for the limiting Mo-rich composition prepared in the study. We observed Tc = 11.5–16 K for cubic δ-NbN depending on high-P,T synthesis and annealing conditions. This value falls to 5 K for the cubic Mo0.37Nb0.63N0.98 solid solution phase. Some high-P,T synthesis or annealing experiments were carried out at T = 2200 °C. At this temperature δ-MoN decomposes to produce γ-MoN0.54. A minor phase in this sample achieves Tc = 9.5 K. A new superconducting hexagonal oxynitride MoN0.74O0.38 with Tc = 16 K was also produced during this study.

Carbon nitride compounds with high N:C ratios and graphitic to polymeric structures are being investigated as potential next-generation materials for incorporation in devices for energy conversion and storage as well as for optoelectronic and catalysis applications. The materials are built from C- and N-containing heterocycles with heptazine or triazine rings linked via sp2-bonded N atoms (N(C)3 units) or –NH– groups. The electronic, chemical and optical functionalities are determined by the nature of the local to extended structures as well as the chemical composition of the materials. Because of their typically amorphous to nanocrystalline nature and variable composition, significant challenges remain to fully assess and calibrate the structure–functionality relationships among carbon nitride materials. It is also important to devise a useful and consistent approach to naming the different classes of carbon nitride compounds that accurately describes their chemical and structural characteristics related to their functional performance. Here we evaluate the current state of understanding to highlight key issues in these areas and point out new directions in their development as advanced technological materials.