We report a first-principles study of the recently predicted Pmc21 phase of the multiferroic BiFeO3 material, revealing a novel magnetoelectric effect that makes it possible to control magnetism with an electric field. The effect can be viewed as a two-step process: Switching the polarization first results in the change of the sense of the rotation of the oxygen octahedra, which in turn induces the switching of the secondary magnetic order parameter. The first step is governed by an original trilinear-coupling energy between polarization, octahedral tilting, and an antiferroelectric distortion. The second step is controlled by another trilinear coupling, this one involving the predominant and secondary magnetic orders as well as the oxygen octahedral tilting. In contrast with other trilinear-coupling effects in the literature, the present ones occur in a simple ABO3 perovskite and involve a large polarization.

High-melting-point oxides of chemical formula ABO3, with A = Ca, Sr, Ba and B = Zr, Hf, are investigated as a function of hydrostatic pressure up to 200 GPa, by combining first-principles calculations with a particle swarm optimization method. Ca-and Sr-based systems (1) first undergo a reconstructive phase transition from a perovskite state to a novel structure that belongs to the post-post-perovskite family; and (2) then experience an isostructural transition to a second, also new post-post-perovskite state at higher pressure, via the sudden formation of a specific out-of-plane B-O bond. In contrast, the studied Ba-compounds evolve from a perovskite phase to a third novel post-post-perovskite structure, via another reconstructive phase transition. Original characteristics of these three different post-post-perovskite states are emphasized. Unusual electronic properties, including significant piezochromic effects and an insulator-metal transition, are also reported and explained.

The quest for materials possessing both a magnetic ordering temperature above room temperature and a large electrical polarization is an important research direction in order to design novel spintronic and memory devices. Up to now, BiFeO3 and related systems are the only known compounds simultaneously possessing such characteristics. Here, first-principles calculations predict that another family of materials, namely epitaxial films made of rare-earth orthoferrites (RFeO3), can also exhibit such desired features. As a matter of fact, applying a large enough strain to these compounds, which are nominally paraelectric and have a high magnetic transition temperature, is predicted to render them ferroelectric, and thus multiferroic. At high compressive strain, the resulting ferroelectric phase of RFeO3 systems having large rare-earth ions is even a tetragonal state characterized by a giant polarization and axial ratio. For large tensile strain, two striking inhomogenous ferroelectric phases--including one never observed before in any perovskite--are further predicted as having significant polarization. A multiphase boundary also occurs, which may lead to optimization of properties or unusual features. Finally, many quantities, including electrical polarization and magnetic ordering temperature, are tunable by varying the epitaxial strain and/or chemical pressure.

Using first-principles calculations, the elastic constants, thermodynamic properties and structural phase transition of NbN under high pressure are investigated by means of the pseudopotential plane-waves method, in addition to the effect of metallic bonding on its hardness. Three candidate structures are chosen to investigate NbN, namely, rocksalt (NaCl), NiAs and WC types. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt (Pt = 200.64 GPa) between the WC phase and the NaCl phase of NbN is predicted for the first time. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Poisson's ratio of NbN are derived. The calculated results are found to be in good agreement with the available experimental data and theoretical values. According to the quasi-harmonic Debye model, the Debye temperature under high pressure is derived from the average sound velocity. Moreover, the effect of metallic bonding on the hardness of NbN is investigated and the hardness shows a gradual decrease rather than increase under compression. This is a quantitative investigation on the structural and thermodynamic properties of NbN, and it still awaits experimental confirmation.

Using first-principles calculations, the elastic constant, structural phase transition and effect of metallic bonding on the hardness of OsN2 under high pressure are investigated by means of the pseudopotential plane-waves method. Five candidate structures are chosen to investigate for OsN2, namely, the pyrite, CoSb2-type, marcasite, simple hexagonal and tetragonal structures. A comparison among the formation energies of OsN2 explains the synthesis of OsN2 marcasite under high pressure. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt (Pt = 223 GPa) between the marcasite and simple tetragonal phase is determinated. Elastic constants, shear modulus, Young's modulus, Poisson's ratio and Debye temperature are derived. The calculated values are, generally speaking, in good agreement with experiments and other theoretical calculations. Our calculation indicates that the N-N bond length is one determinative factor for the ultrahigh bulk moduli of the heavy-transition-metal dinitrides. Moreover, based on Mulliken overlap population analysis in first-principles technique, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bonding is presented. The effect of metallic bonding on the hardness of OsN2 is investigated and the hardness shows a gradual decrease rather than increase under compression, which is different from diamond. This is a quantitative investigation on the structural properties of OsN2, and it still awaits experimental confirmation.