LXV. Structure and Phase Transitions of Ferroelectric Sodium Cadmium Niobates

A study of the electrical properties and structure of ceramic specimens has shown that sodium-cadmium niobates form single phase solid solutions of the perovskite type. The cadmium niobate enters into solid solution as CdNb2O6 rather than in the more familiar form of Cd2Nb2O7; pairs of Bodium atoms in the NaNbO3 lattice are replaced by one cadmium atom and one vacancy up to the composition with Na: Cd vacancy in the ratio 6:1:1. Beyond this limit a second phase, either Cd2Nb2O7 or CdNb2O6 is always present. In the solid solution region a new tetragonal phase is introduced below the Curie temperature and ferroelectric properties are observed in this phase and in the orthorhombio phase whioh is displaced to lower temperatures. Superlattice effects indicate that throughout the solid solution range the structure cell is multiple compared with the simple perovskite cell. The ferroelectric properties ore associated with a doubled unit cell, whereas antiferroelectric sodium niobate has a quadrupled cell. By observation of the superlattice lines which distinguish these multiple structure cells it has been possible to establish that for certain compositions, of fairly low cadmium niobate concentration, and in certain temperature regions, the ferroelectric and antiferroelectric structures coexist within the perovskite lattice. In this condition the properties are weakly ferroelectric. With increasing cadmium niobate content the proportion of antiferroelectric substructure decreases until the material is fullv ferroelectric.