Effects of oxygen-deficiency on crystal structure, dielectric and ferroelectric properties in Sr5SmTi3+2xNb7−2xO30−x with tungsten bronze structure
Oxygen-deficient tungsten bronze ceramics with general formula Sr5SmTi3+2xNb7−2xO30−x (x = 0, 0.1, 0.25) are prepared. The effects of oxygen deficiency on the crystal structure, dielectric and ferroelectric properties are investigated. With increasing oxygen deficiency, the crystal structure becomes less stable and the lattice shrinks, while the ferroelectricity is weakened, and the low temperature relaxation enhanced. Commensurate superlattices observed in Sr5SmTi3Nb7O30, corresponding to the onset of the ferroelectric transition as reported previously. In compositions with x = 0.1 and 0.25, two factors that A-site cross occupancies and disturbance of oxygen vacancies on the ferroelectric order are noticed and related to the change from ferroelectric behavior to diffuse one. Three orders of incommensurate superlattice reflections are observed in x = 0.1 and 0.25 compositions. The first-order incommensurate superlattice reflections appear at positions (h + 1/4 − δ, k + 1/4 − δ, l + 1/2), reflecting the common feature in tungsten bronzes with relaxor or diffuse ferroelectric nature. The second-order incommensurate superlattice reflections are observed on the (00l) (l = 0, 1, 2, 3, …) planes around the (h + 1/2, k + 1/2, l) positions, while the third-order incommensurate superlattice reflections are observed on the (00l) (l = 1/2, 3/2, …) planes, by the side of the first-order ones. The appearance of higher-order is believed to reduce the high structural energy induced by the oxygen deficiency.