The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System

Perovskite-type materials containing Bi3+ cations at A sites are interesting from the viewpoints of applications and fundamental science as the lone pair of Bi3+ cations often stabilizes polar, ferroelectric structures. This can be illustrated by a lot of discoveries of different new functionalities in bulk and thin films of BiFeO3 and its derivatives. In this work, we investigated solid solutions of BiCr1−xFexO3 with 0.1 ≤ x ≤ 0.4 prepared by a high-pressure (HP) method and post-synthesis annealing at ambient pressure (AP). HP-BiCr1−xFexO3 modifications with 0.1 ≤ x ≤ 0.3 were mixtures of two phases with space groups C2/c and Pbam, and the amount of the C2/c phase decreased with increasing x. The amount of the C2/c phase was also significantly decreased in AP-BiCr1−xFexO3 modifications, and the C2/c phase almost disappeared in AP-BiCr1−xFexO3 with 0.2 ≤ x ≤ 0.3. Fundamental, strong reflections of HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 were almost unchanged; on the other hand, weak superstructure reflections were different and showed clear signs of strong anisotropic broadening and incommensurate positions. These structural features prevented us from determining their room-temperature structures. On the other hand, HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 showed high-temperature structural phase transitions to the GdFeO3-type Pnma modification at Tsrt = 450 K (x = 0.1), Tsrt = 480 K (x = 0.2), Tsrt = 510 K (x = 0.3), and Tsrt = 546 K (x = 0.4). Crystal structures of the GdFeO3-type Pnma modifications of all the samples were investigated by synchrotron powder X-ray diffraction. Magnetic properties of HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 were quite close to each other (HP vs. AP), and the x = 0.2 samples demonstrated negative magnetization phenomena without signs of the exchange bias effect.