Phase transitions of AlFeO₃ and GaFeO₃ from the chiral orthorhombic (Pna2₁) structure to the rhombohedral (R3̅c) structure

Abstract

AlFeO₃ and GaFeO₃, which crystallize in a chiral orthorhombic (Pna2₁) structure, transform to a rhombohedral (R3̅c) structure when subjected to ball-milling. There is a distinct difference between the transformations of AlFeO₃ and GaFeO₃. AlFeO₃ first transforms to an orthorhombic P2₁2₁2₁ structure followed by its transformation to the R3̅c structure, while GaFeO₃ goes directly to the R3̅c structure. The transformations have been characterized by X-ray diffraction and Raman spectroscopy. Magnetic properties of Pna2₁ and the transformed phases show significant differences. It is noteworthy that partial substitution of aluminum by gallium in AlFeO₃ as in Al_0.5Ga_0.5FeO₃ eliminates the intermediate P2₁2₁2₁ phase, causing direct transformation of the Pna2₁ structure to the R3̅c structure. All of the transformations are thermodynamically first-order associated with significant changes in volume. We have used first-principles simulations to determine the pressure-dependent properties of AlFeO₃ and GaFeO₃ in orthorhombic and corundum structures and have estimated the critical pressures for the structural phase transition between the two structures. On the basis of this information, we also comment on the differences in the behavior of AlFeO₃ and GaFeO₃ under ball-milling.