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

A sequence of reversible phase transitions gamma <-> beta' (at 511 degrees C) <-> alpha (at 587 degrees C) 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, alpha-Ca2B2O5, was refined by the Rietveld method at 600 degrees C. It is isostructural to the alpha-Sr2B2O5 and shows the orientational disorder of isolated B2O5 pymborate groups. Thermal expansion is strongly anisotmpic (alpha(11) = 25, alpha(22) = 7, alpha(33) = 2 x 10(-6) degrees C-1 at 400 degrees C) and dictated by preferential orientations of BO3 triangles in the structure.

Automated summary

A study on Ca2B2O5 revealed a reversible sequence of phase transitions at high temperatures, with the high-temperature modification α-Ca2B2O5 showing isostructural properties to α-Sr2B2O5 and displaying orientational disorder of isolated B2O5 pymborate groups. The thermal expansion of the material is strongly anisotropic, influenced by the preferential orientations of BO3 triangles in the structure.