Phase Transitions of AlFeO3and GaFeO3from 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 Al0.5Ga0.5FeO3 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.