The dependence of structural and magnetic properties of rare-earth orthoferrites (in their Pbnm ground state) on the rare-earth ionic radius is systematically investigated from first principles. The effects of this 'chemical pressure' on lattice constants, Fe-O bond lengths, Fe-O-Fe bond angles and Fe-O bond length splittings are all well reproduced by these ab initio calculations. The simulations also offer novel predictions (on tiltings of FeO6 octahedra, cation antipolar displacements and weak magnetization) to be experimentally checked. In particular, the weak ferromagnetic moment of rare-earth orthoferrites is predicted to be a linear function of the rare-earth ionic radius. Finally, the effects of applying hydrostatic pressure on structural and magnetic behavior of SmFeO3 is also studied. It is found that, unlike previously assumed, hydrostatic pressure typically generates changes in physical properties that are quantitatively and even qualitatively different from those associated with the chemical pressure.
First-principles calculations are performed to compare the energetics of several phases, including hexagonal polar P6(3)cm and perovskite non-polar Pbnm-like states, of epitaxial RFeO3 films (with R = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Lu) grown on different cubic (1 1 1)- and hexagonal (0 0 0 1)-oriented substrates. The P63cm phase is found to be the ground state for large enough in-plane lattice parameters in all investigated RFeO3 films, and its polarization is tunable by the amount of epitaxial strain. Series of available substrates allowing the growth of hexagonal polar RFeO3 films, as well as other phenomena of fundamental and technological importance (e.g. different ground states and coexistence between several phases) are also predicted.
A first-principles-based effective Hamiltonian is developed and employed to investigate finite-temperature structural properties of a prototype of perovskite halides, that is CsPbI 3 . Such simulations, when using first-principles-extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic Pm3m upon heating, unlike measurements that reported the occurrence of an intermediate long-range-tilted tetragonal P4/mbm phase in-between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first-principle methods can be trusted to mimic hybrid perovskites, can be resolved by "only" changing one short-range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic Pm3m phase is highly locally distorted and develops strong transverse antiphase correlation between firstnearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling.
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