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, CaCu 3 Ga 2 Sb 2 O 12, CaCu 3 Ga 2 Ta 2 O 12, and CaCu 3 Ga 2 Nb 2 O 12, 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 Ca 2+ and Cu 2+ stabilize a large a + a + a + octahedral tilting distortion in all three compounds. This distortion creates a 12-coordinate icosahedral site for Ca 2+ and a 4-coordinate square-planar site for Cu 2+ . In addition to this A-site cation ordering, CaCu 3 Ga 2 Sb 2 O 12 and CaCu 3 Ga 2 Ta 2 O 12 also exhibit a rock salt ordering of the octahedral B-site cations. The B-site cation order decreases from 98% in CaCu 3 Ga 2 Sb 2 O 12 to 70% in CaCu 3 Ga 2 Ta 2 O 12 to effectively no long range order in CaCu 3 Ga 2 Nb 2 O 12 . 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 3 Ga 2 Sb 2 O 12 and CaCu 3 Ga 2 Ta 2 O 12 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.