Line-broadening and anharmonic effects in the attenuated total reflectance infrared spectra of calcite

The attenuated total reflectance (ATR) spectra of two calcite samples have been measured using Ge and diamond internal reflection elements. One of the samples was obtained by precipitation from solution and displays sub-micrometer sized particles. The second sample was obtained by grinding a single-crystal of calcite ("Iceland spar" variety). Theoretical spectra were obtained by computing the reflection coefficient of the interface between the ATR internal reflection element and a homogeneous effective medium representing the powder sample. The dielectric properties of the effective medium have been determined from those of bulk calcite using the Bruggeman approach which is adequate for standard powders with intermediate fractions of solid and empty porosity. Although this modeling approach provides a quantitative interpretation of the broadening of strong absorption bands in the calcite ATR spectra, it fails to reproduce the apparent splitting of the strong nu (sub 3) CO (sub 3) band. This problem was solved by combining the electrostatic modeling approach with a semi-quantum model of the dielectric tensor of bulk calcite, which allows for a frequency-dependent damping of the nu (sub 3) CO (sub 3) excitation. The frequency-dependence is ascribed to a multiphonon-related resonance in the nu (sub 3) CO (sub 3) phonon self-energy at a frequency close to its transverse optical frequency. The combination of approaches exposed in the present study makes it possible to discriminate among physically different processes affecting the powder infrared spectra of calcite, some being related to the long-range nature of electrostatic interactions in polar materials and others being related to atomic-scale anharmonic interactions between vibrational modes.