Interpretation and modeling of IR-reflectance spectra of glasses considering medium range order

Abstract

Conventional qualitative and quantitative IR-reflectance spectrum interpretation of glasses is discussed in light of new findings on the optics of polycrystalline materials and the close relation of the structure of crystallites and regions of medium range order in related glasses. According to these findings, a glass spectrum must not be compared in general with the spectrum of a related polycrystalline material, if the crystallite size exceeds the resolution limit of light. As a consequence of the similarities between the spectra of glassy and related polycrystalline compounds (optically small crystallites) and based on medium range order, the macroscopic optical properties of glasses result not only from disorder and fluctuations, but also from an orientational average of the optical properties of the medium range regions similar to the macroscopic optical properties of polycrystalline materials. Consequently, the assumption of cubic symmetry, which underlies all conventional types of dispersion analysis used for glasses, is inadequate. Based on ARIT (average refractive index theory), [Appl. Spectrosc. 56 (2002) 1194], which permits modeling the optical properties of polycrystalline materials with optically small crystallites, a method is proposed to generate artificial glass spectra from single crystal data. This method is particularly useful if polycrystalline bulk samples with optically small crystallites are not available, since it enables us to determine glass structure on a semi-quantitative level by comparing measured and artificial spectra. The value of the approach is demonstrated for fresnoite glass (Ba 2TiSi 2O 8), Sr-fresnoite glass (Sr 2TiSi 2O 8) and Ge-fresnoite glass (Ba 2TiGe 2O 8). An important consequence of the assumption of an orientational average of the microscopic optical properties of medium range order regions is the prediction of the occurrence of mixed TO–LO modes in glasses. This is confirmed by the resemblance between peak shapes of model oscillators with large oscillator strengths and small damping constants and the prominent high wave number feature of vitreous silica.