X-ray study of phase transitions in ferroelectric PbNb₂O₆ and related materials

Abstract

The phase transitions in PbNb₂O₆ and in compositions of the type Pb_1+x (B_xNb_1-x)O₆, where B = Ti⁴⁺, Zr⁴⁺, or Sn⁴⁺, have been investigated between 25° and 650° C. using X-ray and dilatometric techniques. The modified PbNb₂O₆ compositions possess orthorhombic PbNb₂O₆-type structure, with the additional Pb²⁺ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300° C. depending on composition. The intermediate phase of Pb_l+x(Ti₂Nb_1-z)₂O₆ appears to possess orthorhombic symmetry.