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Sequence of phase transitions in (NH 4 ) 3 SiF 7 .

Authors :
Mel'nikova SV
Molokeev MS
Laptash NM
Pogoreltsev EI
Misyul SV
Flerov IN
Source :
Dalton transactions (Cambridge, England : 2003) [Dalton Trans] 2017 Feb 21; Vol. 46 (8), pp. 2609-2617.
Publication Year :
2017

Abstract

Single crystals of silicon double salt (NH <subscript>4</subscript> ) <subscript>3</subscript> SiF <subscript>7</subscript> = (NH <subscript>4</subscript> ) <subscript>2</subscript> SiF <subscript>6</subscript> ·NH <subscript>4</subscript> F = (NH <subscript>4</subscript> ) <subscript>3</subscript> [SiF <subscript>6</subscript> ]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) (G <subscript>1</subscript> ) ↔ Pbam (Z = 4) (G <subscript>2</subscript> ) ↔ P2 <subscript>1</subscript> /c (Z = 4) (G <subscript>3</subscript> ) ↔ P1[combining macron] (Z = 4) (G <subscript>4</subscript> ) ↔ P2 <subscript>1</subscript> /c (Z = 8) (G <subscript>5</subscript> ). 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.

Details

Language :
English
ISSN :
1477-9234
Volume :
46
Issue :
8
Database :
MEDLINE
Journal :
Dalton transactions (Cambridge, England : 2003)
Publication Type :
Academic Journal
Accession number :
28157243
Full Text :
https://doi.org/10.1039/c6dt04874a