Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases

Sergey Volkov, Michal Dušek, Rimma Bubnova, Maria Krzhizhanovskaya, Valery Ugolkov, Ekaterina Obozova, Stanislav Filatov

Research output

3 Citations (Scopus)

Abstract

Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565K) β ↔ (at 637K) α′ ↔ (at 651K) 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-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order-disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637-651K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = -81 × 10-6 K-1.Hitherto unknown crystal structures of β- and α-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the γ ↔ β ↔ α′ ↔ α thermal phase transitions are revisited and extended.

Original languageEnglish
Pages (from-to)1056-1067
Number of pages12
JournalActa Crystallographica Section B: Structural Science, Crystal Engineering and Materials
Volume73
Issue number6
DOIs
Publication statusPublished - 2017

Fingerprint

Order disorder transitions
Phase Transition
Crystal structure
Phase transitions
disorders
crystal structure
Temperature
symmetry
X-Ray Diffraction
Hot Temperature
borates
diffraction
triangles
Dielectric Spectroscopy
Powder Diffraction
thermal expansion
Borates
x rays
heat measurement
Anisotropy

Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Atomic and Molecular Physics, and Optics
  • Metals and Alloys
  • Materials Chemistry

Cite this

@article{a352d57aff4646558935b87efb625f0d,
title = "Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases",
abstract = "Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565K) β ↔ (at 637K) α′ ↔ (at 651K) 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-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order-disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637-651K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = -81 × 10-6 K-1.Hitherto unknown crystal structures of β- and α-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the γ ↔ β ↔ α′ ↔ α thermal phase transitions are revisited and extended.",
keywords = "commensurate modulated structure, high-temperature X-ray diffraction, order-disorder, phase transitions, thermal expansion",
author = "Sergey Volkov and Michal Dušek and Rimma Bubnova and Maria Krzhizhanovskaya and Valery Ugolkov and Ekaterina Obozova and Stanislav Filatov",
year = "2017",
doi = "10.1107/S2052520617012689",
language = "English",
volume = "73",
pages = "1056--1067",
journal = "Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials",
issn = "2052-5192",
publisher = "International Union of Crystallography",
number = "6",

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TY - JOUR

T1 - Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases

AU - Volkov, Sergey

AU - Dušek, Michal

AU - Bubnova, Rimma

AU - Krzhizhanovskaya, Maria

AU - Ugolkov, Valery

AU - Obozova, Ekaterina

AU - Filatov, Stanislav

PY - 2017

Y1 - 2017

N2 - Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565K) β ↔ (at 637K) α′ ↔ (at 651K) 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-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order-disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637-651K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = -81 × 10-6 K-1.Hitherto unknown crystal structures of β- and α-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the γ ↔ β ↔ α′ ↔ α thermal phase transitions are revisited and extended.

AB - Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565K) β ↔ (at 637K) α′ ↔ (at 651K) 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-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order-disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637-651K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = -81 × 10-6 K-1.Hitherto unknown crystal structures of β- and α-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the γ ↔ β ↔ α′ ↔ α thermal phase transitions are revisited and extended.

KW - commensurate modulated structure

KW - high-temperature X-ray diffraction

KW - order-disorder

KW - phase transitions

KW - thermal expansion

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U2 - 10.1107/S2052520617012689

DO - 10.1107/S2052520617012689

M3 - Article

AN - SCOPUS:85037674347

VL - 73

SP - 1056

EP - 1067

JO - Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials

JF - Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials

SN - 2052-5192

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ER -