Infrared and Raman Spectroscopy of Single Crystal (Nb,In) co-doped Rutile

Co-doped rutile single crystals of (Nb,In)_0.02Ti_0.98O₂ (2-NITO) were grown using float zone melting. These crystals exhibit dielectric permittivity (f< MHz) significantly larger than pure rutile at low temperature, a feature which has not yet been conclusively explained. Polarized infrared and Raman spectroscopy measurements (backscattering geometry) were made on oriented (100) and (001) crystal faces at several temperatures between 4 and 300 K. The A_2u and the lowest frequency E_u infrared modes both soften significantly with decreasing temperature, similar to pure rutile. Turning to the Raman spectra, co-doping produces no significant changes in the frequencies of the Raman modes.