High-temperature γ ↔ β′ ↔ α phase transitions in Ca2B2O5: Thermal expansion and crystal structure of α-phase

A sequence of reversible phase transitions γ ↔ β′ (at 511°С) ↔ α (at 587°С) was found in Ca2B2O5 by high-temperature X-ray powder diffraction, DSC and in the high-temperature Raman spectroscopy. The crystal structure of high-temperature modification, α-Ca2B2O5, was refined by the Rietveld method at 600°С. It is isostructural to the α-Sr2B2O5 and shows the orientational disorder of isolated B2O5 pyroborate groups. Thermal expansion is strongly anisotropic (α11 = 25, α22 = 7, α33 = 2 × 10−6 °C−1 at 400 °C) and dictated by preferential orientations of BO3 triangles in the structure.