On the Solubility of Bi in Tetragonal Tungsten Bronzes

Abstract

Ferroelectric Pb5Nb10O30 (PN) tetragonal tungsten bronze (TTB) forms solid solutions that exhibit morphotropic phase boundaries (MPBs). Successful replacement of PN with TTBs containing Bi could advance lead-free TTB MPBs. The most studied Bi containing TTB, K4Bi2Nb10O30, was recently demonstrated to be paraelectric, and we have systematically explored the possibility to increase the Bi-content beyond two Bi3+ ions per unit cell aiming to induce ferroelectricity. Higher Bi content was incorporated by either increasing the cation vacancy concentration on the A-site or by B-site substitution with lower valency cations. An additional pyrochlore phase was found in almost all the explored compositions exceeding two Bi3+ per unit cell. The instability of the TTB structure with higher Bi3+ concentrations was investigated in terms of first-principles calculations, combined with chemical considerations and including the tolerance factor. Consequently, an in-plane polarisation found in PN is unlikely to exist in Bi-based TTBs, meaning that Bi3+ cannot fully replace Pb2+ in ferroelectric TTBs.