High-Pressure Synthesis and Characterization of Perovskites with Simultaneous Ordering of Both the A- and B-Site Cations, CaCu3Ga2M2O12 (M = Sb, Ta)

Three new perovskites, CaCu3Ga2Sb2O12, CaCu3Ga2Ta2O12, and CaCu3Ga2Nb2O12, have been synthesized at 12∼12.5 GPa and 1100 °C using the uniaxial split sphere anvil type press (USSA-2000) and recovered to ambient conditions, where they have been structurally characterized using X-ray and neutron powder diffraction. The large differences in the size and bonding preferences of Ca2+ and Cu2+ stabilize a large a+a+a+ octahedral tilting distortion in all three compounds. This distortion creates a 12-coordinate icosahedral site for Ca2+ and a 4-coordinate square-planar site for Cu2+. In addition to this A-site cation ordering, CaCu3Ga2Sb2O12 and CaCu3Ga2Ta2O12 also exhibit a rock salt ordering of the octahedral B-site cations. The B-site cation order decreases from 98% in CaCu3Ga2Sb2O12 to 70% in CaCu3Ga2Ta2O12 to effectively no long range order in CaCu3Ga2Nb2O12. These materials represent the first examples in which both A-site cation order and rock salt B-site cation order have been observed simultaneously. The ...