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 Bi2FeAlO6 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+ andAl3+ ions are ordered in a rock salt manner. However, these ions remain disordered in BiFe1-xAlxO3 (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 Bi2FeAlO6 .
Figure: Crystal structure obtained from the refinement of the neutron diffraction data. a) View along  with c-axis up showing the rocksalt arrangement of Fe and Al ions. b) Coordination of oxygen ions around Fe and Al ions with different bond length.
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