Structure of strontium tellurite glass, anti-glass and crystalline phases by high-energy X-ray diffraction, reverse Monte Carlo and Rietveld analysis

The structures of xSrO-(100 - x)TeO₂ (x = 5, 7.5, 8.5 and 10 mol.%) glass, anti-glass and crystalline samples were studied by high-energy X-ray diffraction (HEXRD), reverse Monte Carlo (RMC) simulations, atomic pair distribution function analysis and Fullprof Rietveld refinement. The atomic pair distributions show the first peak at 1.90 Å due to the Te-O equatorial bonds and the Te-O peak is asymmetrical due to the range of Te-O bond lengths in glass, anti-glass and crystalline samples. The short-range structural properties of glasses such as Te-O bond lengths, Te-O speciation, Te-Te distances and O-Te-O bond angle distributions were determined by RMC simulations. The average Te-O coordination number (N_Te-O) for 5SrO-95TeO₂ glass is 3.93 which decreases to 3.59 on increasing the SrO concentration to 10 mol.%. The changes in N_Te-O revealed that the glass network predominantly contains TeO₄ units with a small amount of TeO₃ units and there is a structural transformation TeO₄ → TeO₃ with an increase in SrO concentration. The O-Te-O bond angle distributions have a peak at 79° and reveal that the O_equatorial-Te-O_equatorial bonds are the most abundant linkages in the tellurite network. Two glass samples containing 7.5 and 8.5 mol.% of SrO were annealed at 350°C for 1 h to produce anti-glass phases; they were further annealed at 450°C for 4 h to transform them into crystalline phases. The anti-glass samples are disordered cubic SrTe₅O₁₁ and the disordered monoclinic SrTeO₃ phases, whereas the crystalline samples contain monoclinic SrTeO₃ and the orthorhombic TeO₂ phases. The unit-cell parameters of the anti-glass and crystalline structures were determined by Fullprof Rietveld refinement. Thermal studies found that the glass transition temperature increases with an increase in SrO mol.% and the results on the short-range structure of glasses from Raman spectroscopy are in agreement with the RMC findings.