Infrared spectra from solid solutions of spessartine and yttrium aluminum garnet

Abstract

Compositional relationships were established through thin-film specffoscopy for all 17 infrared lanice vibrations in compositions along the binary join spessartine Mn3Al2Si3O12 - yttrium aluminum gamet Y3Al2Al3O12. Powder-dispersion data yielded similar results, but at lower resolution owing to prcsence of enhanced overtones, scattering components and interference fringes. Thin-film frequencies of the end members are in excellent agreement with transverse optic modes obtained from single-crystal reflectance spectra. All but a few weak modes are traceable completely across the binary, stich that most frequencies depend almost linearly on cell parameter. Dfferent behaviors are seen for the modes assigned to various atomic motions: for example, tetrahedral stretches have fwo-mode behavior (i.e., both end-member peaks are present with intensities proportional to concentration), which is consistent with the bond strength of Al-O being significantly less (0.83 times) than that of Si-O. Freguencies of the dodecahedral translations increase very slightly, indicating that the increase in average mass as Y'* replaces Mnzr is offset by the increase in bond strength zrs average valency increases. The observations suggest that the thermodl,namic properties of high-pressure garnets with minor tetrahedral substitutions (<47o) will be scarcely affected.