(152420)Tunnel structure of tetragonal tungsten bronzes BaTa_2O_6, Ba_(0.8)Ta_2O_(5.8), and Ba_(0.5)Ta_2O_(5.5) studied using synchrotron X-ray and neutron diffraction

摘要

The complex oxide system of Ba-Ta-0 presents interesting cases of tetragonal tungsten bronze (TTB) phases being formed over a wide compositional range of Ba_(1-x)Ta_2O_(6-x). 0 ≤ x ≤ 0.65 [X. Kuang et al., Inorg, Chem., 2013, 52, 13244-13252]. Compared to the common formalism of the TTB phase AB_2O_6, Ba_(1-x)Ta_2O_(6-x) is highly off stoichiometric and is presumed to involve substantial structural modifications. In this study, Ba_(1-x)Ta_2O_(6-x) samples were prepared with the chemical compositions of BaTa_2O_6 (x = 0), Ba_(0.8)Ta_2O_(5.8) (x = 0.2), and Ba_(0.5)Ta_2O_(5.5) (x = 0.5). Rietveld refinement and Fourier electron density calculation, using high-resolution synchrotron X-ray diffraction data, revealed that the above three compo-sitiorfS 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, Ba_(0.8)Ta_2O_(5.8) and Ba_(0.5)Ta_2O_(5.5) contained segments of (TaO)~(3+) chains in the pentagonal tunnel and Ta~(5+) in the triangular tunnel (A3 site). Hence, the unit cell structure of BaTa_2O_6, Ba_(0.8)Ta_2O_(5.8), and Ba_(0.5)Ta_2O_(5.5) can be represented as Ba_5[Ta_(10)O_(30)], Ba_(4.26)(TaO)_(0.04)Ta_(0.22)[-Ta_(10)O_(30)], and Ba_(2.84)(TaO)_(1.19)Ta_(0.18)[Ta_(10)O_(30)], respectively. Dielectric constants (k) of Ba_(1-x)Ta_2O_(6-x) were measured in the temperature range of 30-450 K and at frequencies of 100 kHz-1 MHz. Compared to BaTa_2O_6, which displayed high k (≈100 at 300 K) and broad relaxation peaks below 50 K, Ba_(0.8)Ta_2O_(5.8) andBa_(0.5)Ta_2O_(5.5) exhibited markedly reduced lattice polarizations.