Incommensurate modulation and thermal expansion of Sr3B2 + xSi1 − xO8 − x/2solid solutions

Abstract

Crystal structures of Sr3B2 + xSi1 − xO8 − x/2solid solutions with nominal compositionsx= 0.28, 0.53, 0.78 in the Sr3B2SiO8–Sr2B2O5section of the SrO–B2O3–SiO2system are refined using single-crystal X-ray diffraction data. Incommensurate structure modulations are mainly associated with various orientations of corner-sharing (B,Si)-polyhedra. Preference is given to the (3 + 2)-dimensional symmetry groupPnma(0βγ)000(0{\rm{\bar \beta }}γ)000 for a single crystal compared with an alternate model of a twin formed by monoclinic components, each of them corresponding to the (3 + 1)-dimensional symmetry groupP21/n(0βγ). Single-phase polycrystalline samples of solid solutions are investigated by high-temperature X-ray powder diffraction in air. Orientation preferences of the BO3units lead to a strong anisotropy of thermal expansion. Negative expansion is observed along theaaxis over the temperature range 303–753 K. Anisotropy decreases both on heating and decreasing of the boron content.