Dielectric Materials for Wireless Communication

CHAPTER NINE

CATION-DEFICIENT PEROVSKITES

9.1 INTRODUCTION

The ABO₃ perovskite structure is flexible and can incorporate a considerable amount of vacancies. A large number of anion- or cation-deficient perovskites exist and both A-site and B-site vacant perovskites have been reported in the literature [1–5]. The vacancies are often ordered forming superstructures with interesting dielectric properties. The B-site vacancies are more often found and these structures, with the general formula A_nB_n−1O_3n, have blocks of (n−1) corner sharing octahedra separated by vacant octahedral layers. These oxides which form a series of trigonal structures are observed [6] in various systems with A = Ba²⁺, Sr²⁺, Ca²⁺, La³⁺ and B = Mg²⁺, Al³⁺, Ti³⁺, Ti⁴⁺, Nb ...