Several compositions in the PbnNb2O5+n system have been ascribed nearly identical distorted pyrochlore symmetries, with the degree of distortion increasing with PbO content. Rietveld refinements of neutron diffraction data for the n=1.5 composition have confirmed its cubic structure. Defect layers form for n > 1.5, and their density increases with n. The temperature coefficient of resonant frequency (TCF) of Pb2Nb2O7 is 814 MK−1. The substitution of Ta for Nb decreases TCF significantly, down to 223 MK−1 for Pb2(Nb0.5Ta1.5)O7, and increases the defect density slightly. Further substitution of Ca for Pb significantly alters the structure of these defect layers, although the product remains single-phase. The structure and properties of Ca2Nb2O7 and Ca2Ta2O7 have also been studied. Ca2Nb2O7 is monoclinic and ferroelectric at room temperature, containing (100) domains but no defect layers. Monoclinic Ca2Ta2O7, on the other hand, is not ferroelectric but does contain defect layers similar in appearance to those found in Pb-niobate-tantalates.