ABX3 (A = organic cation; B = Sn, Pb; and X = halogen) organohalide perovskites have recently attracted much attention for their photovoltaic applications. Such hybrid compounds are derived from the replacement of the inorganic monovalent metal element by an organic cation, for example, methylammonium ion (MA = CH3NH3+) and formamidinium ion (FA= +HC(NH2)2). In particular, since the organic cations are polar, it is interesting to investigate their possible long-range ordering and the corresponding Rashba spin-split bands. In this work, by using density functional theory calculations, we estimate the ferroelectric polarization corresponding to a complete ordering of dipole moments for the optimized structures of 12 perovskite halides, with A = MA, FA; B = Pb, Sn; X = Cl, Br, I. The adiabatic path and functional mode analysis have been discussed for all cases. The calculated values of the polarization may be as high as a conventional inorganic ferroelectric compound, such as BaTiO3. The concomitant inversion symmetry breaking, coupled to the sizable spin–orbit coupling of Pb and Sn, results in a fairly large Rashba spin-splitting effect for both valence and conduction bands. We highlight a rather anisotropic dispersion of spin–orbit split bands which gives rise to different Rashba parameters in different directions perpendicular to the polar axis in k-space. Furthermore, we found a weak and positive correlation between the magnitude of polarization and relevant spin-split band parameters. Since the mechanism for enhanced carrier lifetime in 3D Rashba materials is connected to the reduced recombination rate due to the spin-forbidden transition, our study could aid in the understanding of the fundamental physics of organometal halide perovskites and the optimization and design of materials for better performance.

The theoretical ferroelectric polarization of the low-temperature (monoclinic, P21) phase and the high-temperature (hexagonal, P63) phase of hydroxyapatite Ca10(PO4)6(OH)2 is calculated based on the density functional theory (DFT). In the monoclinic structure, the value of ferroelectric polarization is found to be 9.87 μC cm−2 along the [001] direction. In the hexagonal structure, the ferroelectric polarization is 7.05 μC cm−2 along the [001] direction. The main contribution to the electric polarization comes from ordered hydroxyl OH− anions for both phases, although the inorganic Ca5(PO4)3 apatite framework also gives a non-negligible contribution. A detailed analysis of ferroelectric polarization and structural change of the hydroxyapatite is presented for a better understanding of this important biomaterial.

First principles calculations based on density functional theory were performed to study the electronic structure and magnetic properties of β-Ga2O3 in the presence of cation vacancies. We investigated two kinds of Ga vacancies at different symmetry sites and the consequent structural distortion and defect states. We found that both the six-fold coordinated octahedral site and the four-fold coordinated tetrahedral site vacancies can lead to a spin polarized ground state. Furthermore, the calculation identified a relationship between the spin polarization and the charge states of the vacancies, which might be explained by a molecular orbital model consisting of uncompensated O2− 2p dangling bonds. The calculations for the two vacancy systems also indicated a potential long-range ferromagnetic order which is beneficial for spintronics application.

Boron-based two-dimensional materials have extremely rich structures and excellent physical properties. Using a particle-swarm optimization (PSO) method and first-principles calculations, we performed a comprehensive search for the structure of a two-dimensional BeB2 monolayer. We found new configurations with lower energy compared with the previously reported α phase, namely the β, γ, and δ structures. Among those structures, the δ phase is found to have the lowest energy and we examined its dynamic as well as its thermodynamic stabilities. Then through strain engineering, we found a metal–semimetal transition in the α phase (under about 5% biaxial compressive strain) and in the δ phase (under about 3.2% and 7% biaxial tensile strain). As the compressive strain increases to 7%, the BeB2 sheets of the β phase and γ phase strongly twist, becoming more stable than the δ system. More interestingly, we found that Be atoms could penetrate the B atomic layer in the γ system under 2.5% tensile strain. All the predicted effects demonstrate the rich physical properties of the two-dimensional BeB2 monolayer.

•A TiO2 hard template with periodic structure was demonstrated by imprinting technique combined with a sol-gel process.•The TiO2 hard template can be used in both UV imprinting and hot embossing lithography.•The TiO2 patterns have a high mechanical strength which can be applied in high pressure imprinting process.The fabrication of imprinting stamps plays a vital role in the nanoimprinting lithography. In this work, we proposed a facile method to fabricate TiO2 hard stamp by patterning sol-gel films. A polydimethysiloxane (PDMS) soft stamp with highly ordered micropit array was used as an original stamp to imprint the TiO2 sol which was spin coated on a quartz wafer. The nano-hardness and Young’s modulus of TiO2 hard stamp are in the range of 7.7–10.0 GPa and 118.9–130.0 GPa, which is much higher than the conventional PDMS material. Afterwards the TiO2 hard stamps are demonstrated in both UV and hot embossing lithography with a promising life cycle. The proposed stamp can be extended to other metal oxides, which possesses a promising prospective for large scale patterning because of the low cost and excellent mechanical durability.TiO2 hard template with periodic microstructure was successfully developed through a sol-gel method, where PDMS template with highly ordered micropit array was used as original template to imprint the spin-coated TiO2 solution on quartz substrate. The TiO2 hard template has a high mechanical strength which can be applied in high pressure imprinting process, which has been characterized in UV imprinting and hot embossing lithographyDownload high-res image (125KB)Download full-size image

•Ruthenium carbides with various stoichiometries were studied by density functional theory calculations.•Ground states of Ru2C, RuC, Ru2C3, RuC2, RuC3 and RuC4 are found, pressure-induced phase transitions are uncovered.•Two carbon-rich structures are found to have large Vickers hardness 45.1 GPa and 41.5 GPa.Compounds formed by transition metals and light elements have attracted increasing attention owing to superior functionalities. Here, high throughput first-principles calculations are employed to investigate the crystal structures and physical properties of ruthenium carbides with various stoichiometries. It is found that the R3¯m-Ru2C, R3¯m-RuC, P3¯m1-Ru2C3, P3¯m1-RuC2, P3¯m1-RuC3 and C2/c-RuC4 are the ground states for the respective chemical compositions at ambient pressure, from a systematical investigation of both thermodynamic and mechanical stabilities, as well as phonon dispersions. Further calculations indicate that P3¯m1-RuC3 and P63/mmc-RuC4 are ultra-incompressible with high bulk and shear modulus. Subsequent empirical calculations predict that the carbon-rich P3¯m1-RuC3 and P63/mmc-RuC4 are superhard materials with a large Vickers hardness of 45.1 GPa and 41.5 GPa, respectively. In addition, a strong covalent CC bonding was observed from the electronic localization function contours of all the ground states, which is crucial for their excellent mechanical properties.Download high-res image (183KB)Download full-size image

•Ho0.5Pr0.5FeO3 single crystal was grown by optical floating zone method.•It shows an abrupt jump of magnetization along a axis at low temperature.•The jump height and temperature is sensitive to external applied magnetic field.•It is attributed to the spin reversal of the rare earth ions.We report temperature-induced spin reorientation and magnetization jump effects in the rare earth (RE) orthoferrite Ho0.5Pr0.5FeO3 single crystal. The single crystal of about 6 mm in diameter and 50 mm in length was successfully grown by optical floating zone method. Both X-ray diffraction and Laue photograph confirmed the homogeneity and high quality of the crystal. Magnetic properties of Ho0.5Pr0.5FeO3 single crystal are studied over a wide temperature range from 2 to 300 K. Spin reorientation transition from Γ2 to Γ4 phase is observed in the temperature range of 75–90 K. At lower temperature, the Ho0.5Pr0.5FeO3 shows an abrupt jump of magnetization along the a-axis, which occurs only in the field-cooling process, and is sensitive to external applied magnetic field. By analyzing the jump temperature and magnitude of the magnetization, we conclude that it is caused by the spin reversal of the rare earth ions. The isothermal magnetization versus field hysteresis loop measurements along a axis explain the spin configuration variation from 3 K to 60 K.

Multiferroic materials are currently the subject of intensive research worldwide, because of both their fundamental scientific problems and also possible technological applications. Among a number of candidates in the laboratories, compounds consisting of rare earth and transition metal perovskite oxides have very unusual structural and physical properties. In contrast to the so-called type I multiferroics, ferroelectricity may be induced by magnetic ordering or by applying external fields. In this review, the recent progress on the experimental and theoretical studies of some selected type II multiferroics is presented, with a focus on the perovskite oxides containing rare earth and transition metal elements. The rare earth orthoferrite crystals, rare earth titanate strained film, and rare earth-based superlattices are systematically reviewed to provide a broad overview on their promising electric, magnetic, and structural properties. The recent experimental advances in single-crystal growth by optical floating zone method are also presented. First-principles investigations, either supported by experimental results or awaiting for experimental verifications, are shown to offer useful guidance for the future applications of unconventional multiferroics.

Multiferroic nanodots can be harnessed to aid the development of the next generation of nonvolatile data storage and multi-functional devices. In this paper, we review the computational aspects of multiferroic nanodot materials and designs that hold promise for the future memory technology. Conception, methodology, and systematical studies are discussed, followed by some up-to-date experimental progress towards the ultimate limits. At the end of this paper, we outline some challenges remaining in multiferroic research, and how the first principles based approach can be employed as an important tool providing critical information to understand the emergent phenomena in multiferroics.

Highlights•Density functional theory computations are carried out to study inverse Heusler alloys.•Electronic and magnetic properties are studied from first principles.•Spin magnetic moments are found to obey Slater-Pauling behavior.Using augmented plane wave+local orbital basis, we have calculated the electronic structure and magnetic properties of X2YZ (X=Cr; Y=Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, Sb) inverse Heusler alloys from first principles. We employ the Hg2CuTi type L21 structure which indeed provides the low energy solution. The calculated total magnetic moments considered here follow the generalized Slater-Pauling behavior (Zt−24) except Cr2NiSb, which shows (Zt−28) instead. These materials show a systematic trend of ferrimagnetic Cr-Cr exchange interaction which increases with the number of valence electrons. The half-metallic and ferrimagnetic behaviors make them promising candidates for spintronic materials and devices.

Highlights•Density functional theory computations are carried out to study inverse Heusler alloys.•Electronic and magnetic properties are studied from first principles.•Spin magnetic moments are found to obey Slater-Pauling behavior.Using augmented plane wave+local orbital basis, we have calculated the electronic structure and magnetic properties of X2YZ (X=Cr; Y=Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, Sb) inverse Heusler alloys from first principles. We employ the Hg2CuTi type L21 structure which indeed provides the low energy solution. The calculated total magnetic moments considered here follow the generalized Slater-Pauling behavior (Zt−24) except Cr2NiSb, which shows (Zt−28) instead. These materials show a systematic trend of ferrimagnetic Cr-Cr exchange interaction which increases with the number of valence electrons. The half-metallic and ferrimagnetic behaviors make them promising candidates for spintronic materials and devices.

•Co4Nb2O9 single crystals are grown by the optical floating zone method.•Laue and X-ray studies confirmed high quality of the single crystal.•Magnetic measurements and first-principle calculations indicate that the spin flop occurs along a axis.•Compared with polycrystalline Co4Nb2O9, single crystal ones show smaller critical magnetic fields for triggering the spin flip.A single crystal of Co4Nb2O9 about 7 mm in diameter and 55 mm in length was successfully grown by an optical floating zone method. X-ray powder diffraction (XRD) indicates that it has a single phase corundum-type structure. Clear Laue spots and sharp XRD peaks confirm the good quality and crystallographic orientations. Below TN, magnetization of Co4Nb2O9 along a axis shows dramatically different behaviors between H=1 kOe and 20 kOe, suggesting a spin flop occurs along a axis. Ma(H) curve at 5 K shows a change of slope at a critical magnetic field of 7.5 kOe for triggering the spin flip.

•Sm0.7Dy0.3FeO3 single crystal was grown by optical floating zone method.•SmxDy1−xFeO3 (x = 0.3–1, step 0.1) single crystals grown by the hetero-seeded method.•SmxDy1−xFeO3 (x = 0.2, 0.1, 0) grown together by hetero-seed and hetero-feed method.•Growth direction and components of these single crystals are very accurate.•SDFO single crystals show the typical anisotropic magnetic property.We studied samarium-dysprosium rare-earth orthoferrites SmxDy1−xFeO3 (SDFO, x = 0–1, interval 0.1) with 11 different x concentration values. All of the SDFO single crystals were successfully grown by a hetero-seed and hetero-feed optical-floating-zone technique in flowing air. The XRD powder patterns illustrate that the lattice mismatch of two samples with Δx = 0.1 is about 0.1–0.2% in ac plane, which is considered appropriate for our hetero-seed and hetero-feed crystal growth. Thus, we could successfully grow the series of SDFO single crystals continuously, for example, the x = 0.8 single crystal was grown on the x = 0.7 seed rod. X-ray back-reflection Laue photographs indicate good quality of all the as-grown SDFO single crystals. Composition analysis of SDFO single crystals were conducted by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), which demonstrate the accurate cation stoichiometry for each crystal. Moreover, we show that such crystal system possesses particular anisotropic magnetic property. The hetero-seed growth method and hetero-feed single crystal relay growth based on optical-floating-zone technique will be useful in high-throughput crystal materials growth.