Sequence of phase transitions in (NH4)3SiF7.
Single crystals of silicon double salt (NH4)3SiF7= (NH4)2SiF6·NH4F = (NH4)3[SiF6]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G1) ↔ Pbam (Z = 4) (G2) ↔ P21/c (Z = 4) (G3) ↔ P1[combining macron] (Z = 4) (G4) ↔ P21/c (Z = 8) (G5). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N-HF hydrogen bonds can be a driving force of the observed phase transitions.
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036, Russia. msmolokeev@mail.ru.
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Mel'nikova
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036, Russia. msmolokeev@mail.ru and Department of Physics, Far Eastern State Transport University, Khabarovsk, 680021, Russia.
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http://orcid.org/0000-0002-8297-0945
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Molokeev
Institute of Chemistry, Far Eastern Branch of RAS, Vladivostok, 690022, Russia.
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Laptash
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036, Russia. msmolokeev@mail.ru and Siberian Federal University, Krasnoyarsk, 660074, Russia.
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Pogoreltsev
Siberian Federal University, Krasnoyarsk, 660074, Russia.
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Misyul
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036, Russia. msmolokeev@mail.ru and Siberian Federal University, Krasnoyarsk, 660074, Russia.
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Flerov