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Orientational order‐disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr 2 B 2 O 5 pyroborate and crystal structures of β and α phases
Author(s) -
Volkov Sergey,
Dušek Michal,
Bubnova Rimma,
Krzhizhanovskaya Maria,
Ugolkov Valery,
Obozova Ekaterina,
Filatov Stanislav
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520617012689
Subject(s) - triclinic crystal system , monoclinic crystal system , crystallography , superstructure , crystal structure , phase transition , phase (matter) , chemistry , diffraction , differential scanning calorimetry , x ray crystallography , physics , condensed matter physics , organic chemistry , optics , thermodynamics
Crystal structures of γ‐ , β‐ and α‐Sr 2 B 2 O 5 polymorphs resulting from the γ ↔ (at 565 K) β ↔ (at 637 K) α′ ↔ (at 651 K) α sequence of reversible first‐order phase transitions are studied by high‐temperature single‐crystal X‐ray diffraction, high‐temperature X‐ray powder diffraction, differential scanning calorimetry and impedance spectroscopy. Out of these phases, the structure of γ‐Sr 2 B 2 O 5 was already known whereas the structures of β‐ and α‐Sr 2 B 2 O 5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β‐Sr 2 B 2 O 5 phase and monoclinic low‐temperature γ‐Sr 2 B 2 O 5 as well as high‐temperature α‐Sr 2 B 2 O 5 phase. Taking the α‐Sr 2 B 2 O 5 phase with space group P 2 1 / c as a parent structure, the γ‐Sr 2 B 2 O 5 phase was refined as a twofold superstructure with symmetry P 2 1 / c , whereas the β‐Sr 2 B 2 O 5 phase was a sixfold superstructure with symmetry P . To construct a unified structure model for all Sr 2 B 2 O 5 modifications, phases of γ‐ and β‐Sr 2 B 2 O 5 were also refined as commensurately modulated structures using the basic unit cell of the parent α‐Sr 2 B 2 O 5 . The phase transitions are related to the orientational order–disorder arrangement of B 2 O 5 pyroborate groups, where the degree of disorder grows towards the high‐temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO 3 triangles in the structure. The intermediate phase α′‐Sr 2 B 2 O 5 , stable over a narrow temperature range (637–651 K), features the largest anisotropy of expansion for the known borates: α 11 = 205, α 22 = 57, α 33 = −81 × 10 −6 K −1 .