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Orientational order-disorder ? ? ? ? ?? ? ? phase transitions in Sr2B2O5 pyroborate and crystal structures of ? and ? phases

ABSTRACT: Hitherto unknown crystal structures of ?- and ?-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the ? ? ? ? ?? ? ? thermal phase transitions are revisited and extended. Crystal structures of ?-, ?- and ?-Sr2B2O5 polymorphs resulting from the ? ? (at 565?K) ? ? (at 637?K) ?? ? (at 651?K) ? 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 ?-Sr2B2O5 was already known whereas the structures of ?- and ?-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate ?-Sr2B2O5 phase and monoclinic low-temperature ?-Sr2B2O5 as well as high-temperature ?-Sr2B2O5 phase. Taking the ?-Sr2B2O5 phase with space group P21/c as a parent structure, the ?-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the ?-Sr2B2O5 phase was a sixfold superstructure with symmetry P

SUBMITTER: Volkov S

PROVIDER: S-EPMC5744407 | biostudies-literature | 2017 Oct

REPOSITORIES: biostudies-literature

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