The β <-> α' phase transition of Sr2SiO4. II. X-ray and optical study, and ferroelasticity of the β form

Abstract

The transition between the monoclinic fl and orthorhombic a' phases of strontium orthosilicate was investigated by single-crystal diffractometry and polarizing microscopy at variable temperatures, on the basis of the disappearance or appearance of twin domains of the fl form. The phase change occurred sharply with thermal hysteresis in both experiments, at 364 and 355 K (X-rays) and at 369 and 359 K (light, best values out of a number of samples) for fl --, a' and a' --, fl, respectively; the optical study showed that transition temperatures are influenced by the size and habit of crystals. The orientations of fl and a' lattices are related by a simple rotation of the x axis. A first-order character is suggested by thermal results for the phase change, though it relates ordered and disordered structures. Crystals of fl-Sr2SiO 4 prove to be ferroelastic (Aizu's species mmmF2/m) with a spontaneous strain of 0.033. The mechanisms of the phase transition and of the transformation between mirrorrelated orientation states are discussed in relation to structural disorder of the paraelastic a' form.