Crystal Structure, Cation Occupation, and Phase Transitions in Ba(LiNa)NbO 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 BaNaNbO and tetragonal BaLiNbO TTBs. Ceramics of the composition Ba(LiNa)NbO were achieved through solid-state synthesis. The crystal structures were investigated by powder X-ray diffraction, which revealed a transition from orthorhombic (2) to tetragonal (4) symmetry for Li content, , between 0.3 and 0.35. For materials of compositions close to this transition, a 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 > 0.60, a second phase, with a TTB-related structure, appears. This work has increased the understanding of the composition-structure relationship in TTBs.