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Raman scattering study of the pressure‐ and temperature‐induced phase transitions in the chain compounds (CH 3 ) 4 NMnCl 3 and (CH 3 ) 4 NCdCl 3
Author(s) -
Couzi M.,
Mlik Y.
Publication year - 1986
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250170124
Subject(s) - raman spectroscopy , monoclinic crystal system , phase diagram , chemistry , octahedron , lattice (music) , space group , crystallography , crystal structure , phase (matter) , phase transition , raman scattering , spectral line , x ray crystallography , condensed matter physics , diffraction , physics , organic chemistry , acoustics , optics , astronomy
The Raman spectra of (CH 3 ) 4 NMnCl 3 (TMMC) and (CH 3 ) 4 NCdCl 3 (TMCC) single crystals have been studied at different pressures between 0 and 6 kbar and at temperatures ranging from 80 to 300 K. The P, T phase diagrams of TMMC and TMCC have been determined, and a generalized phase diagram for these systems is proposed. This diagram includes four different structural modificiations whose structures are discussed on the basis of the present Raman results and of previously published structural data. Phase I is hexagonal ( P 6 3 / m, Z = 2 space group) and exhibits orientational disorder of the (CH 3 ) 4 N + groups; phases II, III and IV are monoclinic and apparently ordered. Phase III ( P 2 1 / m, Z = 2) exhibits a re‐entrant character. The Raman spectra of phase II are consistent with the P 2 1 / a, Z = 4 space group induced by anti‐phase translations of the octahedra chains, but do not reveal any feature that could be related to an incommensurability. It is concluded that the structure of phase IV is very complex (for instance P 2 1 / m, Z = 8 or P 2 1 / a, Z = 12). The order‐disorder processes and their possible coupling with other lattice dynamic properties are considered, to explain the I ↔ II and I ↔ III transitions; additional structural information is needed in order to choose between the different models examined.