Stoichiometry, Defects, and Ordering in Lead Tantalum Oxides with Pyrochlore-Related Structures

The phase diagram in the PbO–Ta2O5system has been reinvestigated in the Pb1.5Ta2O6.5–Pb2.44Ta2O7.44composition range. The existence of the defect-pyrochlore cubic phase Pb1.5Ta2O6.5is confirmed but the apparent solid solution previously reported in the Pb2.31Ta2O7.31–Pb2.44Ta2O7.44composition range is shown to be due to the coherent intergrowth of defined rhombohedral phases with slightly different compositions.