The Anisotropy of Photoluminescence of Gemstones and Materials: Cr-Bearing Corundum and Chrysoberyl

Abstract

Raman and photoluminescence spectroscopy measurements were conducted on corundum and chrysoberyl in various orientations. This research primarily focused on elucidating the anisotropy of photoluminescence in gemstones and related materials. For this purpose, two classic gem materials—corundum (a uniaxial crystal) and chrysoberyl (a biaxial crystal)—were utilized. The investigation of photoluminescence anisotropy in these materials provided deeper insights into the structural characteristics of gemstones, including local symmetry environment and multisite structure. The analysis of the photoluminescence spectra revealed that both corundum and chrysoberyl exhibit clear anisotropy. Additionally, these spectra were dependent on the observed polarization. Specifically, the band shape and relative intensity of R-lines varied under different crystallographic orientations. Furthermore, their intensities attained local maximum or minimum values when the crystallographic orientation aligned with the polarization direction of the scattered light. When combined with polarized Raman spectroscopy, it was confirmed that the intensities achieved local maximum or minimum values under parallel polarization configurations when the sample's polarization direction was parallel to the crystallographic orientation. Therefore, photoluminescence spectroscopy emerges as a rapid, precise, and nondestructive method for identifying the crystallographic orientation of gemstones and materials. Upon comparing the photoluminescence spectra and their corresponding polarization degrees of chrysoberyl and ruby, it became apparent that chrysoberyl, due to its lower symmetry and multisite structure encompassing Cr-bearing octahedra, displayed more pronounced anisotropy in its spectrum along a specific polarization direction (E⊥c) compared with corundum. Moreover, chrysoberyl exhibited anisotropy in a greater number of directions than corundum.