Formation and Transformation of Cubic Lead Niobate Pyrochlore Solid Solutions

Formation of lead niobate pyrochlores by the reaction of PbO with Nb 2 O 5 in various molar ratios and at various temperatures has been investigated. It has been shown that for a PbO/Nb 2 O 5 molar ratio between 1.5 and 3.0, a pyrochlore solid solution having cubic symmetry is formed at low temperatures (∼500‐600°C). X‐ray diffraction data show evidence for the existence of compositional inhomogeneity for this phase which contains part of the Pb in the Nb site leading to the formula Pb 1.5+ x (Nb 2‐ y Pb y )O 7‐delta (0.0 < x < 0.5; 0.0 < y < 0.5). A phase‐pure Pb‐deficient pyrochlore (Pb 1.5 Nb 2 O 6.5 ) and Pb‐excess pyrochlores (Pb 2.5 Nb 2 ‐O 8 , Pb 3 Nb 2 O 8 ) are formed by the decomposition of this cubic solid solution and subsequent reaction at high temperatures. These results indicate that the mechanism of the structural transformation from the low‐temperature (LT) cubic solid solution to the corresponding high‐temperature (HT) phases is reconstructive.