Dethroning Bi as an alternative replacement for Pb in tetragonal tungsten bronzes

Pb₅Nb₁₀O₃₀ (PN) with the so-called “in-plane polarisation”, is known to form tetragonal tungsten bronze (TTB) solid solutions with morphotropic phase boundaries (MPB). A corresponding lead-free MPB system is envisaged by replacing PN with a similar TTB containing Bi. However, a recent study has demonstrated that the Bi-containing compounds K₄Bi₂Nb₁₀O₃₀ and Rb₄Bi₂Nb₁₀O₃₀ are not ferroelectric [1]. In this study, we have systematically synthesised and characterised Bi-based TTB compositions with the aim to increase the Bi content beyond two Bi³⁺ per unit cell. Higher Bi-content was enabled by substitution of Nb⁵⁺ with lower valence cations Ti⁴⁺, Fe³⁺ or Zr⁴⁺ or by increasing the cation vacancy concentration to maintain the overall charge balance. All explored compositions resulted in the formation of a second phase as soon as two Bi³⁺ per unit cell were exceeded. The apparent instability of TTBs with a higher Bi-content was analysed in terms of chemical and geometrical considerations including the tolerance factor. These findings demonstrate that the in-plane polarisation known in PN is most likely not possible to realise in Bi-based TTBs and Bi³⁺ is not a proper replacement for Pb²⁺ in TTBs.

[1] Nylund, I.-E. & Zeiger, C. R.; Peng, D.; Vullum, P. E.; Walker, J.; Einarsrud, M.-A. and Grande, T. Exploring the Role of Bi 6s Lone Pair in Tetragonal Tungsten Bronzes A₄Bi₂Nb₁₀O₃₀ (A=Na, K, Rb). Submitted.