Pyrochlore-related phases in the lead oxide-tantalum oxide system

Pyrochlore-related phases in the PbOTa₂O₅ system were reinvestigated. Three such phases are described: cubic Pb_1.5Ta₂O_6.5 with a narrow range of composition, rhombohedral Pb₂₂Ta₁₈O₆₇, and orthorhombic Pb₁₄Ta₁₀O₃₉. A new pyrochlore-related structure is proposed for the orthorhombic phase. There is also a range of rhombohedral solid solution, which may be metastable only. The existence of the previously reported Pb₂Ta₂O₇ is not confirmed.