High-pressure evolution of structural distortions in perovskites: A comparative RFeO₃ vs RMnO₃ study

The physical properties of perovskite materials, with general formula ABO₃, are known to be strongly dependent on their structural distortions, which can be changed through temperature, applied electric/magnetic fields, and epitaxial strain. The understanding of the delicate energy balance involving the different structural instabilities is therefore a key point for the design of multifunctional perovskite-based materials. Hydrostatic pressure allows modifying the interatomic distances and tune the interactions to a much larger extent than any other external parameters.

Rare-earth orthoferrites (RFeO₃), and orthomanganites (RMnO₃) (with R lanthanide atom) exhibit interesting bulk and strain-engineered magnetic and multiferroic phases, motivating a better understanding of the compressibilities of these structures. All of RFeO₃ and the RMnO₃, with R = La to Dy, crystallize in the Pnma space group, characterized by two tilts of the oxygen octahedra. For the same rare-earth, the RFeO₃ and RMnO₃ exhibit the same the tolerance factor, as Mn³⁺ and Fe³⁺ have the same ionic radius, however, the MnO₆ octahedra exhibits Jahn-Teller distortion.

To disclose the structural distortions high-pressure behavior, in this work we studied the RFeO₃ and RMnO₃ by x-ray diffraction and Raman scattering. In a general scenario, both types of compounds undergo a structural phase transition at P_C between 35 to 50 GPa. At P_C, there is a volume collapse of around 5%, accompanied by an insulator-to-metal transition, explained by a high-spin to low-spin state change of the Fe³⁺ and Mn³⁺ cations. We focus on the similarities and differences of their high-pressure behavior, explaining how the structural distortions can be either enhanced or suppressed, giving special evidence to the role played by the Jahn-Teller distortion.