Structural and electrical properties of tetragonal tungsten bronze K4Bi2Nb10O30-Ba4Na2Nb10O30 solid solutions across two proximate phase transitions

Tetragonal tungsten bronzes allow for broader compositional engineering compared to perovskites. Here we investigate solid solutions between Ba4Na2Nb10O30 (BNN) and K4Bi2Nb10O30 (KBiN). A transition from orthorhombic to tetragonal symmetry was observed at 30-35% KBiN. An expansion of the in-plane and a contraction of the out-of-plane parameter were observed with increasing KBiN content. Ferroelectricity, shown by a maximum in permittivity, was observed for 20-35% KBiN, while diffuse dielectric behaviour was exhibited from 40% KBiN. Non-linear P-E loops are evident up to 35% KBiN, and vaguely present for 40% KBiN. The most prominent ferroelectric character, assigned by large remanent polarisation and opening of the strain hysteresis response was exhibited at 35% KBiN. A ferroelastic-paraelastic phase transition between 30 and 35% KBiN and a ferroelectric-paraelectric phase transition between around ~40% KBiN were inferred. Thus, the two distinct phase transitions mimic the temperature dependent structural evolution observed for BNN.