Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite
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Attenuated total reflectance (ATR) infrared spectra were measured on a synthetic and a natural fluorapatite sample. A modeling approach based on the computation of the Fresnel reflection coefficient between the ATR crystal and the powder sample was used to analyze the line shape of the spectra. The dielectric properties of the samples were related to those of pure fluorapatite using an effective medium approach, based on Maxwell–Garnett and Bruggeman models. The Bruggeman effective medium model leads to a very good agreement with the experimental data recorded on the synthetic fluorapatite sample. The poorer agreement observed on the natural sample suggests a more significant heterogeneity of the sample at a characteristic length scale larger than the mid-infrared characteristic wavelength, i.e., about 10 micrometers. The results demonstrate the prominent role of macroscopic electrostatic effects over fine details of the microscopic structure in determining the line shape of strong ATR bands.
KeywordsApatite ATR-FTIR Infrared spectroscopy Effective medium Bruggeman model Attenuated total reflectance
We thank G. Morin for providing us with the fluorapatite samples. Support by M. Guillaumet, I. Estève and the IMPMC spectroscopy and SEM–FIB facilities is acknowledged. We thank Prof E.K.H. Salje and an anonymous reviewer for thoughtful and constructive reviews of this manuscript. This work was supported by French state funds managed by the ANR within the Investissements d’Avenir program under reference ANR-11-IDEX-0004-02 and, more specifically, within the framework of the Cluster of Excellence MATISSE led by Sorbonne Universités. Support from the Convergence Program “Environnements & Société” of Sorbonne Universités is acknowledged.
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