Line broadening and enthalpy: Some empirical calibrations of solid solution behaviour from IR spectra

Following recent developments in the application of autocorrelation analysis of line broadening in powder absorption IR spectra, the possibility that elastic strain heterogeneities are responsible both for such broadening and for enthalpy variations in solid solutions has been explored systematically. Recent data for five silicate solid solutions, augite-jadeite, almandine-grossular, albite-anorthite, anthophyllite-grunerite and clinoenstatite-clinoferrosilite, are reviewed. Variations in the autocorrelation parameter for line broadening, Deltacorr, scale linearly with calorimetric data for enthalpy variations, DeltaH, associated with cation substitutions and ordering in these systems. It is proposed that, while there may not be a universal calibration of DeltaH in terms of Deltacorr for all structures, it should be possible to produce empirical calibrations for groups of solid solutions with the same basic structure. Variations in Deltacorr at different phonon frequency ranges provide additional insights into the structural mechanisms by which ordering, mixing, displacive and reconstructive processes occur in silicate solid solutions.