Recent neutron-diffraction work on the perovskite ${\mathrm{LaMnO}}_3$ has revealed that the Néel temperature increases under high pressure and reaches a maximum at P∼12 GPa. Given the uncertainty in structural determinations from experiment, the origin of this phase transition remains elusive. We simulated the evolution of crystal structure as a function of pressure based on density-functional theory with the exchange-correlation functional at the levels of both standard Perdew-Burke-Ernzerhof and hybrid functional approximations. We confirm a previously postulated phase transition from the orthorhombic phase to the rhombohedral phase at P∼12 GPa by calculating the enthalpy of both structures. Our simulations also demonstrate that the orbital ordering and the orbital mixing as extracted from the local structural distortion are progressively suppressed under high pressure in the orthorhombic phase, which matches the results of neutron diffraction. In addition, our calculations have been extended to simulate the general pressure effect on the structure in orthorhombic $A^{3+}{B}^{3+}{\mathrm{O}}_3$ perovskites. Orthorhombic distortion in the perovskites is described by the difference in lattice parameters b-a. Our simulations reproduce well the contrasting pressure effect on the orthorhombic distortion in different perovskites, i.e., reduced b-a under pressure for perovskites having initially smaller b-a (like ${\mathrm{LaMnO}}_3$, ${\mathrm{LaGaO}}_3$) and increased b-a under pressure for perovskites having larger b-a (like ${\mathrm{LuMnO}}_3$, ${\mathrm{LuGaO}}_3$). Local structural changes revealed from the simulation, which yet remain to be confirmed by experiment, provide an important clue to the origin of these opposite pressure effects exerted on the same type of crystal structure.

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• Electronic structure
• Jahn-Teller effect
• Magnetic phase transitions
• Pressure effects
• Transition metal oxides
• Density functional calculations