HIGH-TEMPERATURE RAMAN-SPECTROSCOPY AND QUASI-HARMONIC LATTICE DYNAMIC SIMULATION OF DIOPSIDE

We investigated the lattice vibrational properties and lattice dynamic al behaviour of diopside by combining laser micro-Raman spectroscopic measurements with quasi-harmonic lattice dynamic simulation using a tr ansferable interatomic potential. We obtained polarized Raman spectra from a Fe-poor natural diopside and the temperature dependencies of th e Raman modes to 1125 K from high-temperature Raman spectra of a Fe-po or and a Fe-rich natural diopside. The various modes display different temperature dependencies: from -0.021 cm(-1)/K to -0.004 cm(-1)/K. Th e temperature shift of low frequency modes is generally higher. A comp arison of experimentally determined frequencies and symmetries of vibr ational modes of the optical type (Raman and infrared) obtained in thi s and earlier studies with those calculated by us suggests that a cons istent characterization of the vibrational properties was achieved. Th e good agreement between the experimental and simulated data on the te mperature-dpendencies of the Raman modes (within 5%), crystal structur e (2%), bulk modulus (5%), volume thermal expansivity (6%), and consta nt volume heat capacity (0.2%) testifies to the applicability of the t ransferable interatomic potential and the lattice dynamic model to pre dicting the vibrational, physical, and thermodynamic properties. The s imulated properties from the lattice dynamic calculations are very sim ilar to those obtained by molecular dynamic calculations with the same potential model.