Vibrational spectroscopy and intrinsic dielectric properties of Sr₂RE8(SiO₄₎₆O₂ (RE = rare earth) ceramics

This work reports the vibrational and intrinsic dielectric properties of apatite-type Sr2RE8(SiO4)6O2 (RE = rare earth) ceramics. Raman scattering and infrared-reflectivity spectroscopy were employed under optimized conditions to determine the phonon modes in this series. The results showed good agreement with the theoretical predictions, in which all 33 Raman-active bands and 18 of the 20 predicted polar phonons were visualized and their positions evaluated as a function of the lanthanide ionic radii under investigation. Dispersion analyses were conducted in all Sr2RE8(SiO4)6O2 ceramics using the Lorentz model and Kramers-Kro center dot nig relations to obtain the infrared optical functions and polar phonon characteristics of the materials, from which their intrinsic dielectric properties were calculated. The relationships between the intrinsic properties obtained by infrared spectroscopy and the extrinsic properties determined by direct microwave measurements (postresonator method and resonant cavity procedures), as well as with the values predicted by the Clausius-Mossotti equation were discussed.