IR reflectance spectra of natural ilmenite: comparison with isostructural compounds and calculation of thermodynamic properties

Polarized single-crystal IR reflectance spectra measured for natural ilmenite (FeTiO₃) possessed the eight peaks mandated by symmetry plus weak bands attributable to overtones. IR and Raman peaks were correlated between these data and previous polycrystalline studies of 14 isostructural compounds by comparing intensity patterns. Different trends exist for high- and low-frequency bands indicating that the former are essentially internal motions of octahedra containing X⁴⁺, while the latter are translations of the M²⁺. Heat capacities calculated from vibrational spectra lie 5% below calorimetric measurements from 150 to 1100 K. Measured entropy is bracketed by the sum of lattice entropy calculated from a Kieffer-type model plus a constant magnetic contribution and the sum S_lat plus S_mag plus constant d-electron exchange and Fe-Ti charge transfer contributions. -from Author