B-cation effects in relaxor and ferroelectric tetragonal tungsten bronzes

The properties of a family of relaxor materials with nominal formula Ba6M3+Nb9O30 (M3+, Ga3+, Sc3+ and In3+) with the tetragonal tungsten bronze (TTB) structure are reported. The dielectric curves are dramatically shifted (>200 K) to higher temperature with increasing M3+ cation size, in contrast to the opposite effect observed with increasing M4+ ionic radius in Ba6M4+2Nb8O30 TTBs. Previously, variation in dielectric properties with ionic radii in these materials has been discussed in terms of geometry (tolerance factor) arguments as used in perovskite ferroelectrics, however we demonstrate the seeming contradictory results for tri- and tetravalent Bsite substitutions can be unified by describing the degree of structural distortion such that polar stability is governed by ‘tetragonality’. Using this approach the observed peaks in dielectric constant with temperature, Tm, are displaced to higher temperatures with increasing tetragonality irrespective of substituting cation valence or absolute unit cell parameters.