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Symmetry and lattice-dynamic aspects of structural phase transitions in (CH3NH3)2MnCL4and related compounds

Authors
Journal
Journal of Physics and Chemistry of Solids
0022-3697
Publisher
Elsevier
Publication Date
Volume
36
Issue
9
Identifiers
DOI: 10.1016/0022-3697(75)90181-x
Disciplines
  • Physics

Abstract

Abstract A group theoretical analysis of the second-order structural phase transition in (CH 3NH 3) 2MnCL 4 at 394°K and of similar transitions in ethyl and propyl compounds ( D 17 4 h → D 18 2 h ) was performed. The soft mode transforms according to the τ x 5-irreducible representation at the X-point of the Brillouin-zone boundary and its eigenvector is discussed. The transition is of the order-disorder type and is caused by a slowing down of the hindered rotation of NH 3-groups. Knowing the symmetry of the order parameter, a thermodynamic potential expansion was constructed and expected anomalies in material constants around the transition temperature are briefly discussed. The high temperature phase transitions in analogous copper compounds are explained as a sequence D 17 4 h → D 18 2 h → D 15 2 h . The second of these phase transitions is driven by a soft mode transforming as the τ Y 7-representation at the D 18 2 h Brillouin zone boundary.

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