Hydrothermal Synthesis of Pyrochlore-Type Pentavalent Bismuthates Ca₂Bi₂O₇ and Sr₂Bi₂O₇

The pyrochlore-type Ca₂Bi₂O₇ and Sr₂Bi₂O₇ have been synthesized from a low-temperature hydrothermal route using NaBiO₃·nH₂O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca₂Bi₂O₇ and Sr₂Bi₂O₇, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.