Locked-tilt perovskites

A recent synchrotron structural investigation at high-pressure (HP) sets YAl0.25Cr0.75O3 orthorhombic perovskite as the possible prototype of the "locked-tilt" perovskites [1]. Placed at the boundary of the previously described structural evolution models for GdFeO3-type perovskites at HP [e.g., 2, 3], YAl0.25Cr0.75O3 represents the first finding of a perovskite characterized by the absence of changes in the octahedral tilting and a volume reduction with P exclusively controlled by an isotropic polyhedral compression. Although a theoretical modeling of a new locked-tilt perovskite can be done with a high degree of accuracy, the only way to confirm the possible occurrence of a perovskite belonging to this family is through a HP experiment. The use of geochemical constraints, and the assessment of the "normalized cell distortion factor with pressure, dnorm(P)" for several perovskite solid solutions [1 ,3, 4], has allowed the identification of three possible locked-tilt perovskite formulations, i.e., La(Mn0.69Ga0.31)O3, Ca(Ti0.95Ge0.05)O3, and (Sc0.86Y0.14)AlO3, respectively. The aim of our proposal is to investigate at HP the above identified perovskite samples which were previously characterized at ambient conditions through structural refinements from XRPD. Besides to extend the locked-tilt perovskite family, this investigation will provide a deeper comprehension on the role of these compounds in view of their application as functional materials (e.g., multiferroics, layered perovskites). Furthermore, the structural modification at HP of a hypothesized locked-tilt perovskite where the cubic site has a lower formal charge than the octahedral site (i.e., the 2:4 Ca(Ti0.95Ge0.05)O3), will be extremely useful from a geophysical viewpoint to outline a more accurate Earth' mantle compressional model.