Lattice Dynamical Properties of the Rare Earth Aluminum Garnets (RE3Al5O12)

Abstract

PACS: 63.20.Dj; 78.30.Am; 87.64.Je This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al-O and the dodecahedral RE-O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.

Pressure-induced structural evolution and amorphization in Eu3Ga5O12

“…These bands correspond to the translational motion of the cations in the tetrahedral, octahedral, and dodecahedral sites. 47,48 This indicates that the cationic sublattice retains its symmetry at least up to 68 GPa, which is in agreement with the results of the x-ray diffraction.…”

“…Papagelis et al indicate that the force strength of the R-O bond in the REE garnets is much weaker than that of the tetrahedra and octahedra. 47 Therefore, the increase of the local oxygen environment around Eu 3þ with increasing pressure can be viewed from the deformation of the tetrahedra and octahedra. On the other hand, the pressure forces the tetrahedra, octahedra, and dodecahedra to get closer to each other linearly with the shrinkage of the lattice constant (a) (inset of Fig.…”

“…Previous studies indicate that the Raman bands in the intermediate-and highfrequency regions (>170 cm À1 ), to first approximate, can be assigned to the bending vibration and internal stretching of the tetrahedra GaO 4 . 47,48 The change in the relative intensity of these peaks can be explained as a result of the lattice tilting and distortion of the GaO 4 tetrahedra. More especially, the high-frequency band at $738 cm À1 splits into two peaks at 10.3 GPa (Fig.…”

Phase transition, structure and luminescence of Eu:YAG nanophosphors by co-precipitation method

“…However, it was absent in the spectra of YAG single crystal. According to the theoretical rigid ion model calculation of the rare earth aluminum garnets (RE 3 Al 5 O 12 ) by Papagelis et al [11], the band at 478 cm −1 is associated with T 2g symmetry, and it was also absent in their experimental spectra. Additionally, the relatively intensity of the band at 748 cm −1 associated with E g symmetry in Eu:YAG nanocrystalline powder was much stronger than that in YAG crystal.…”

Synthesis, morphology and spectroscopy of Nd:GSAG nano-powders

“…As raised to 800 • C, the spectrum changed greatly because the structure of the precursor was destroyed. The new bands at 479 cm −1 and 744 cm −1 should be due to certain molecular modes of AlO 4 units[9], indicating the structure of the sample sintered at 800 • C is not completely amorphous. For the sample sintered at 1000 • C, new bands at 413 cm −1 , 475 cm −1 , 651 cm −1 , 679 cm −1 , 745 cm −1 appeared in the low frequency range of 800-400 cm −1 , which correspond to characteristic metal-oxygen (M-O) vibrations and can be attributed FT-IR spectra of the Nd:GSAG precursor and the samples sintered at 800 • C and 1000 • C for 1 h.…”