Crystal Structure, Cation Occupation, and Phase Transitions in Ba₄(LiNa_1-)₂Nb₁₀O₃₀ Tetragonal Tungsten Bronzes

The chemical flexibility of the tetragonal tungsten bronze (TTB) structure offers a large potential for compositional engineering. Cation size and vacancy concentration are known to affect its structure, cation disorder, and functional properties. However, the compositional complexity also makes the TTB structure challenging to understand. Here, we investigate the solid solution between orthorhombic Ba₄Na₂Nb₁₀O₃₀ and tetragonal Ba₄Li₂Nb₁₀O₃₀ TTBs. Ceramics of the composition Ba₄(LiNa_1-)₂Nb₅O₃₀ were achieved through solid-state synthesis. The crystal structures were investigated by powder X-ray diffraction, which revealed a transition from orthorhombic (Cmm2) to tetragonal (P4bm) symmetry for Li content, x, between 0.3 and 0.35. For materials of compositions close to this transition, a T_C of 557-572 °C was observed. Site occupancies of Li-, Na-, and Ba-ions were investigated through Rietveld refinement, complemented by DFT calculations, and were discussed with respect to the stability of the TTB structure. While the Na configuration on the A1- and A2-sites depends on the Na concentration, the Ba configuration appears constant over the compositional range. The Li-ions solely occupy C-sites, and with increasing Li contents, higher vacancy concentrations are created on the A-sites. For compositions of x > 0.60, a second phase, with a TTB-related structure, appears. This work has increased the understanding of the composition-structure relationship in TTBs.