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Phase Transitions in [M(NH 3 ) 6 ] (XY 4 ) 2 Compounds
Author(s) -
Parlinski K.
Publication year - 1980
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2220980210
Subject(s) - brillouin zone , monoclinic crystal system , crystallography , space group , crystal structure , phase transition , ion , lattice (music) , irreducible representation , symmetry group , phase (matter) , group (periodic table) , point group , diffraction , chemistry , x ray crystallography , physics , condensed matter physics , geometry , optics , mathematics , quantum mechanics , acoustics , organic chemistry
The group‐theoretical analysis of the phase transitions in the ionic crystals [Ni(NH 3 ) 6 ] (ClO 4 ) 2 , [Mg(NH 3 ) 6 ](ClO 4 ) 2 , and [Ni(NH 3 ) 8 ](BF 4 ) 2 , accompanied by the lowering of the space groups from Fm3m to P2 1 /b is carried out. It is shown that the symmetry reduction is associated with the two‐dimensional irreducible representation τ 10 of the star { k 10 }, which corresponds to the point X of the f.c.c. Brillouin zone. The analysis of the lattice normal modes in Fm3m phase allowed one to specify the structural changes created at the phase transition point. The monoclinic crystal structure with the space group P2 1 /b may arise as a result of translational displacements of all ions in the plane parallel to the unique symmetry plane. The relative orientations of anions become also fixed.