High-pressure synthesis and characterization of perovskites with simultaneous ordering of both the A- and B-site cations, CaCu₃Ga ₂M₂O₁₂ (M = Sb, Ta)

Three new perovskites, CaCu₃Ga₂Sb₂O ₁₂, 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 CaCu₃Ga₂Sb₂O ₁₂ to 70% in CaCu₃Ga₂Ta₂O ₁₂ to effectively no long range order in CaCu₃Ga ₂Nb₂O₁₂. 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 dielectric constants of CaCu ₃Ga₂Sb₂O₁₂ and CaCu ₃Ga₂Ta₂O₁₂ were measured to be approximately 12 and 27, respectively, at 100 kHz. Prospects for synthesizing further perovskites exhibiting ordering of both the A- and B-site ions is evaluated using the software program SPuDS.