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 BiFe1-x Alx O3 (x=0.2, 0.3, 0.4) perovskites with R3c structure. The ordered compound undergoes antiferromagnetic ordering at TN ≈280 K. The butterfly nature of piezoelectric displacement loop further confirms the polar nature of the cation-ordered Bi2 FeAlO6 .