Tunnel structure of tetragonal tungsten bronzes BaTa2O6, Ba0.8Ta2O5.8, and Ba0.5Ta2O5.5 studied using synchrotron X-ray and neutron diffraction

Abstract The complex oxide system of Ba-Ta-O presents interesting cases of tetragonal tungsten bronze (TTB) phases being formed over a wide compositional range of Ba1−xTa2O6−x, 0 ≤ x ≤ 0.65 [X. Kuang et al., Inorg. Chem., 2013, 52, 13244−13252]. Compared to the common formalism of the TTB phase AB2O6, Ba1−xTa2O6−x is highly off stoichiometric and is presumed to involve substantial structural modifications. In this study, Ba1−xTa2O6−x samples were prepared with the chemical compositions of BaTa2O6 (x = 0), Ba0.8Ta2O5.8 (x = 0.2), and Ba0.5Ta2O5.5 (x = 0.5). Rietveld refinement and Fourier electron density calculation, using high-resolution synchrotron X-ray diffraction data, revealed that the above three compositions have distinct local structural features, and the irregularity escalates with the increase of x. In all the three compositions, the Ba position inside the pentagonal tunnel (A2 site) was lightly split, reflecting the oversize polyhedral cavity. Additionally, Ba0.8Ta2O5.8 and Ba0.5Ta2O5.5 contained segments of (TaO)3+ chains in the pentagonal tunnel and Ta5+ in the triangular tunnel (A3 site). Hence, the unit cell structure of BaTa2O6, Ba0.8Ta2O5.8, and Ba0.5Ta2O5.5 can be represented as Ba5[Ta10O30], Ba4.26(TaO)0.04Ta0.22[Ta10O30], and Ba2.84(TaO)1.19Ta0.18[Ta10O30], respectively. Dielectric constants (κ) of Ba1−xTa2O6−x were measured in the temperature range of 30–450 K and at frequencies of 100 kHz−1 MHz. Compared to BaTa2O6, which displayed high κ (≈100 at 300 K) and broad relaxation peaks below 50 K, Ba0.8Ta2O5.8 and Ba0.5Ta2O5.5 exhibited markedly reduced lattice polarizations.