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New Method of Modeling Infrared Spectra of Non-Cubic Single-Phase Polycrystalline Materials with Random Orientation

A new approach has been used to model optical properties of noncubic single-phase polycrystalline materials with random orientation and crystallites small compared to the resolution limit of light. This method has been applied to fresnoite (Ba2TiSi2O8) and proved to provide widely adequate results to effective medium approximation (EMA) and an excellent correspondence between simulated and measured spectra. In contrast to EMA, the new theory predicts that the deviation between damping constants of polycrystalline and single crystal fresnoite can be adjusted by one common factor for all damping constants. Based on EMA and the new model, much smaller damping constants have been determined for polycrystalline fresnoite compared to conventional dispersion analysis; the latter is able to provide a relevant dielectric function, but seems to be of little reliability concerning the determination of meaningful oscillator parameters due to the implicit assumption of an arithmetic average of the principal dielectric functions. This approximation, which can be derived in a similar form from EMA and is of widespread use, is shown to be inferior to an approximation based on the arithmetic average of the principal refractive indices.