Thermal evolution of the Ca2B2O5 crystal structure through γ ↔ β′ ↔ α phase transitions and the crystal structure of the incommensurately modulated β′-phase†

Crystal structures of γ-, β′- and α-Ca₂B₂O₅ polymorphs formed during a sequence of reversible phase transitions are studied by high-temperature single-crystal X-ray diffraction. Here, we present a unified description for the three polymorphic modifications using the superspace approach. The structure of the intermediate β′ phase is incommensurately modulated with the P2₁/c(αβγ)00(α–βγ)00 superspace group and modulation vectors q₁ = −0.141a* + 0.381b* + 0.112c* and q₂ = −0.141a* − 0.381b* + 0.112c*. Phase transitions are associated with changes in the degree of ordering of the B₂O₅ pyroborate groups. While the low-temperature lock-in γ-phase contains only groups with left and right orientations, the intermediate β′ phase contains left-oriented, right-oriented and orientationally disordered groups, resulting in a complex ordering of three types of groups. Left-oriented groups replace right-oriented groups, forming linear zones ∼6 Å wide, with orientationally disordered groups between these zones. In the structure of the high-temperature α-phase, all pyroborate groups are orientationally disordered. The unified description allows changes in the calcium coordination to be analysed. Upon heating, the coordination polyhedron tends to take the form of a regular octahedron, while the seventh oxygen atom becomes more distant from calcium.