Changes of infrared and Raman spectra induced by structural phase transitions. I. General considerations

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Abstract
The frequency and infrared and Raman strength changes of lattice modes due to structural phase transitions of Landau type have been investigated. The lowest-order invariants in the thermodynamic potential describing the effect of interest have been determined by a group-theoretical method. It is shown that in the first approximation all changes are proportional to some positive integer power of the spontaneous value of the order parameter eta . General formulae are derived which enable the authors by simple group-theoretical means to determine phenomenologically all strength changes and frequency shifts due to a particular phase transition. The frequency splitting is discussed for cases when splitting into two components occurs. The limits of validity of the theory are briefly discussed.