TY - JOUR

T1 - Thermal evolution of the Ca2B2O5 crystal structure through γ ↔β′ ↔α phase transitions and the crystal structure of the incommensurately modulated β′-phase

AU - Копылова, Юлия Олеговна

AU - Волков, Сергей

AU - Кржижановская, Мария Георгиевна

AU - Банару, Александр

AU - Юхно, В. А.

AU - Аксенов, С.М.

AU - Бубнова, Римма Сергеевна

PY - 2025/5/15

Y1 - 2025/5/15

N2 - 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.

AB - 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.

UR - https://www.mendeley.com/catalogue/1cdc49f0-bda4-3093-9e0a-06c38c2f0146/

U2 - 10.1039/d5ce00200a

DO - 10.1039/d5ce00200a

M3 - Article

VL - 27

SP - 4196

EP - 4208

JO - CrystEngComm

JF - CrystEngComm

SN - 1466-8033

IS - 24

M1 - 4196

ER -