Isovalent Cation Ordering in the Polar Rhombohedral Perovskite Bi₂ FeAlO₆

Ordering of cations in different structural types occurs when there is a significant difference in the oxidation states and ionic radii of the ions involved. Herein we report an unusual ordering of isovalent cations Fe³⁺ and Al³⁺ in the polar rhombohedral R3 double perovskite structure of Bi₂ FeAlO₆ synthesized at high-pressure (6 GPa) and high-temperature (1000 °C). This ordered structure is derived from the 1:1 combination of the polar oxides BiFeO₃ (R3c) and BiAlO₃ (R3c), which results in reduction of symmetry to an R3 structure where the Fe³⁺ and Al³⁺ ions are ordered in a rock salt manner. However, these ions remain disordered in BiFe_1-x Al_x O₃ (x=0.2, 0.3, 0.4) perovskites with R3c structure. The ordered compound undergoes antiferromagnetic ordering at T_N ≈280 K. The butterfly nature of piezoelectric displacement loop further confirms the polar nature of the cation-ordered Bi₂ FeAlO₆ .