INFRARED-SPECTRA OF GEO2 WITH THE RUTILE STRUCTURE AND PREDICTION OF INACTIVE MODES FOR ISOSTRUCTURAL COMPOUNDS

A thin-film infared (IR) spectrum of very fine-grained synthetic GeO2 having the rutile structure shows all four bands expected from symmetry analysis. Through comparison of these data to previous contradictory reflection data (Kahan et al., 1971; Roessler and Albers, 1972), longitudinal optic (LO) and transverse optic (TO) positions were accurately established for all bands in both the E(u) and A2u polarizations. Examination of the available vibrational data on 11 rutile structure compounds shows that Raman frequencies for each type of anion smoothly decrease as a function of both cell volume and cation-anion bond length. IR frequencies follow similar trends regardless of type of anion present, except for SnO2, which is probably caused by the much heavier mass of Sn, or a different percentage of covalent or ionic character in its bonding from that in other rutile structure compounds. The consistency of the trends, along with two measurements and one calculated value for inactive modes in TiO2, allows reasonable estimates to be made for 2B1u and A2g in all the other rutile types. Our estimates appear to be at least as accurate (+/- 50 cm-1) as recent lattice dynamic calculations (e.g., Maroni, 1988). Comparisons of vibrational frequencies made using the Gordy formulation for the force constant produced the same type of smooth, nonlinear trends as did the dependence of frequency on cell volume, which suggests that the force constants are not a simple relationship of reduced mass, bond length, and electronegativity or valance. We suggest that for solids, additional factors affecting the value of vibrational frequencies are relative bond compressibilities and bond strengths.