Nonstoichiometry and ferroelectric properties of PbNb₂O₆-type compounds

Abstract

The crystal structure of PbNb₂O₆ consists of a framework of NbO₆ octahedra linked through corners. The Pb²⁺ ions occupy 5 out of the 6 available interstitial sites in this framework. Single-phase compositions have been prepared by solid-state reactions to yield nonstoichiometric compounds of the form A_1+xB₂O₆ where A and B may consist of two ions of different valency and x has values between -0.08 and 0.15. In these materials, the number of unfilled interstitial sites inherently present in PbNb₂O₆ lattice has been varied. Lattice parameter and x-ray intensity data confirm that the additional ions introduced occupy the same sites as Pb²⁺ ions. The ferroelectric Curie temperature has been studied as a function of the composition of these dielectric solids. For compositions with x>0, the Curie temperature is not very sensitive to composition. As the number of unfilled interstitial sites increases (x<0), the Curie temperature decreases sharply.