Phase stability and properties of Bi6Te2O13+δ (δ = 0 or 2)

Highlights: • The orthorhombic Bi6Te2O15 is the stable phase at room temperatures. • The cubic phase Bi6Te2O13 is obtained at temperatures higher than 856 °C. • The cubic phase can be maintained at room temperatures for more than two year. • Maintaining the cubic phase suggest its technological use at ambient conditions. The crystalline Bi₆Te₂O₁₅ and Bi₆Te₂O₁₃ phases with orthorhombic and cubic symmetries, respectively, were synthesized by solid-state reaction synthesis. A study of the thermal stability of both phases was carried out, being verified that the high temperature cubic phase can be kept at room temperature in a metastable condition during long time. The Raman scattering spectrum, optical reflectivity and photoluminescence emission of these phases were measured for the first time. The optical band gaps E_g = 3.29eV, for the orthorhombic phase, and E_g = 3.08eV, for the cubic phase, were calculated from the reflectivity spectra. Using the brick-layer model the electrical conductivity of a dense ceramic of Bi₆Te₂O₁₅ was measured, taking in account the grain and grain-boundary effects.