First principles insights into lead-free ba-based tetragonal tungsten bronzes

Tetragonal tungsten bronzes (TTBs), with the general formula A2₄A1₂C₄B1₂B2_8­O­­₃₀, are a family of ferroelectric materials, which, due to their broad compositional space and structural flexibility make them a suitable framework for tuneable lead-free oxide ferroelectrics.

The Ba-containing TTBs such as Ba₄Na₂Nb₁₀O₃₀ (BNN) with a T_c of ~560^oC[1] is of particular interest in high temperature applications where there is a lack of suitable materials. Previous experimental work on substituting the A-site Na cation with larger alkali metals such as K and Rb, shows a systematic decrease in T_c in addition to uncovering the integral role that cation disorder plays on the structural parameters in these systems.[1]

However, there is little in terms of mechanistic understandings of these compositions using first principles characterisation techniques. In this work, density functional theory (DFT) calculations using both standard and hybrid functionals are performed on the compositions: Ba₄A₂M₁₀O₃₀ (A = Na, K, Rb ; M = Nb, Ta). In particular, we probe the origin and trends of the high T_c associated with the compositional engineering, as well as adress the thermodynamics of disorder and its effect on structural and electronic properties. The conclusions of this study are discussed in line with experimental work done in parallel as well as implications for the future direction of these materials.

[1] I.E. Nylund, N.S. Løndal, J. Walker, P.E. Vullum, M.-A. Einarsrud, T. Grande, Cation disorder in ferroelectric Ba₄M₂Nb₁₀O₃₀ (M=Na, K, Rb) tetragonal tungsten bronzes, Inorg. Chem. 2022 (61) 15540-15546.