A crystal-chemical framework for relaxor versus normal ferroelectric behavior in tetragonal tungsten bronzes

Abstract

Tetragonal Tungsten Bronzes (TTBs) - an important class of oxides known to exhibit ferroelectricity - undergo complex distortions, including rotations of oxygen octahedra, which give rise to either incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behaviour rather than sharper frequency-independent normal ferroelectric anomalies but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behaviour in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition-structure-properties relations for a broad range of TTB systems.