Crystal structure of new polymorphic modification β-Ca3B2SiO8, β-α phase transition and thermal expansion of α- and β-modifications

Single crystals of the β-Ca3B2SiO8 new monoclinic modification have been obtained by cooling the melt of a stoichiometric composition. The crystal structure has been determined from the single crystal X-ray diffraction data and refined with R = 0.059 (wR = 0.069) in the monoclinic space group P21/m. The thermal behavior of the synthetic borosilicate has been studied. At 472 ± 5°С, a reversible phase transition of the first order occurs, leading to the formation of the orthorhombic α-Ca3B2SiO8 modification. The thermal expansion of α- and β-modifications of Ca3B2SiO8 is anisotropic: (α11 = 15, α22 = 16, α33 =–1, α V = 30 × 10–6°С–1) and α11 = 9, α22 = 28, α33 = 1, α V = 38 × 10–6°C–1, respectively.