Compressional features of orthorhombic perovskites

ABO3 perovskite oxides represent a system which offers potential developments in Materials Science (the excess physical properties, consequence of structural phase transitions, promote perovskites as high-tech ceramics) and Earth Sciences (e.g., the orthorhombic (Mg,Fe)SiO3 perovskite is the predominant phase of the Earth lower mantle). Cubic in its aristotype form (s.g. Pm-3m) ‒ a framework of corner sharing BO6 octahedra with twelve-fold coordinated A cations within the framework cavities ‒ perovskites with a lower symmetry (usually orthorhombic, s.g. Pbnm) derive from the cubic structure through a combination of octahedral tilts and distortion of the BO6 octahedra. The evolution of orthorhombic perovskites with P has been interpreted as the combination of the relative compressibility of constituent AO12 and BO6 polyhedra with the octahedral tilts. Specifically, if A has a lower formal charge than B (i.e., 2:4), the AO12 site is more compressible than the BO6 octahedron, and the volume reduction is associated to an increase of the octahedral tilting; if A and B have the same formal charge (i.e., 3:3), the AO12 site is stiffer than the BO6 octahedron, then the volume reduction is partially compensated by a decrease of the octahedral tilting. Albeit true for several perovskites, the description of above is incomplete and it was recently reviewed showing that besides to define dichotomous trends depending on the charge of A and B cations, the evolution of orthorhombic perovskites at HP is affected whether transition metal ions (TMI) are hosted at the octahedral site (Ardit, 2015). An overview on the structural evolution of perovskites upon P will be presented. Two specific cases will be also analysed. 1. The structural modifications of YAl0.25Cr0.75O3 at HP with a detailed inspection of a YM3+O3 isotype series to emphasize a possible limit case when absences of change in octahedral tilting are found, i.e., the so called "locked-tilt" YAl0.25Cr0.75O3 perovskite (Ardit et al., 2017). 2. The dualism between Cr3+ and Ga3+ hosted at the octahedral site of Nd(Ga,Cr)O3 at HP to appraise whether a specific electronic configuration allows the crystal field stabilization energy (CFSE) at octahedral sites to act as vehicle of octahedral softening (Cr3+ with partially filled 3d orbitals) or it turns octahedral into rigid units when CFSE is absent (Ga3+ with full 3d orbitals). Ardit, M. (2015): Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI). J. Phys. Chem. Solids, 87, 203-212. Ardit, M., Dondi, M., Cruciani, G. (2017): Locked octahedral tilting in orthorhombic perovskites: At the boundary of the general rule predicting phase transitions. Phys. Rev. B, 95, 024110.