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Abstract
Hard modes are, in the context of this review, optically active phonons which show systematic changes of their Raman and/or IR spectra when the structural properties of a material are changed (e.g. by heating, application of pressure or chemical reactions).
As the characteristic length of high-frequency phonons is very short (the Ornstein–Zernike correlation length) the structural variations are measured on an atomic scale. This feature is a great advantage for the analysis of heterogeneous materials, e.g. exsolution pattern, disordered systems.
The interpretation of frequency shifts, variations of the intensities and line width of optical spectra is largely based on symmetry arguments which show that the renormalization of phonon spectra is, in most cases, proportional to AQ 2 + BQ 4, where Q is a structural order parameter and A, B are numerical constants. Recipes for the analysis of phonon spectra including the use of reference spectra, profile analysis and the application of spectral autocorrelation functions are discussed.
In the case of powder IR spectra the effect of the embedding matrix in a powder pellet has to be analysed. A simple approach to the “effective medium theory” is reviewed.
The effect of short-range structural order on the phonon spectra is discussed using the phase transitions in lead phosphate (Pb3(PO4)2), titanite CaTiSiO5 and lead scandium tantalate PbSc0.5.Ta0.5O3.
