Effect of local A-site strain on dipole stability in A₆GaNb₉O₃₀ (A = Ba, Sr, Ca) tetragonal tungsten bronze relaxor dielectrics

A series of isovalently A-site substituted relaxor dielectric tetragonal tungsten bronzes of general formula Ba_6-x-ySr_xCa_yGaNb₉O₃₀ were investigated. The long-range (average) crystal structure as determined by conventional diffraction techniques varies monotonically according to Vegard’s law. The dielectric properties, however, do not display a similar, simple “average size” dependence and instead show a dependence on the statistical size variance, i.e. size mismatch, of the A-cation. The difficulties in Vogel-Fulcher analysis of relative permittivity and the complementary approach of using dielectric loss data fitted to Jonscher’s empirical Universal Dielectric Relaxation model is discussed.