Infrared spectroscopy and lattice dynamical calculations of Gd3Al5O12, Tb3Al5O12 and Lu3Al5O12 single crystals

The infrared (IR) reflectance spectra of Gd3Al5O12, Tb3Al5O12 and Lu3Al5O12 single crystals are studied for the first time. At room temperature, 15 infrared active modes for Lu3Al5O12 and 14 for Gd3Al5O12, Tb3Al5O12, out of the 17 theoretically predicted, have been experimentally recorded. The complex dielectric function the refractive index, the absorption coefficient as well as the longitudinal (omega(LO)) and transverse (omega(TO)) frequencies of the long-wavelength T-lu modes are determined by the Kramers-Kronig transformation of the reflectance spectra. The experimental data are compared and discussed with the theoretical results obtained by the rigid ion model. Our theoretical analysis reveals that the bonds in the tetrahedra exhibit a covalent character while those in the dodecahedra almost ionic character, which is in accordance with the results for other materials of this crystal family. (C) 2003 Elsevier Science Ltd. All rights reserved.