Topaz and Kyanite Luminescence Under High Pressure

Abstract:

The luminescence spectra of Cr³⁺ in heat-treated topaz Al₂SiO₄(OH,F)₂ and natural kyanite Al₂SiO₅  were measured from 650 - 800 nm in a hydrostatic environment up to pressures of 15 GPa. The R₁ and R₂ peaks of topaz shift at average rates of 0.30 nm/GPa and 0.22 nm/GPa, respectively, implying that the deformation of the Cr³⁺ octahedra increases with pressure. Three peaks are fit under each R line of topaz at both room and high pressure, and these peaks are associated with different Al sites into which the Cr substitutes. The shift of the R lines in topaz under pressure is remarkably linear, which appears to be a general feature of many Cr³⁺-bearing oxides: the underlying cause of this linearity may lie in anharmonic coupling with lattice vibrations. In this context, we also characterize the frequency shifts of two vibronic peaks within topaz. The R₁ and R₂ peaks of kyanite shift at 0.37 nm/GPa and 0.88 nm/GPa respectively. Two peaks are fit under R₁ and three peaks are fit under R₂ of kyanite at both room and high pressure; this result is also consistent with three different Cr³⁺ sites in this material. The R lines in kyanite are notably optically anisotropic, depending strongly on crystallographic orientation: this is most strongly manifested in the R₂ peak. The Cr³⁺ luminescence in these materials provides a sensitive probe of pressure-dependent shifts in the local geometry of the Al-sites in these materials, which are analyzed in the context of previous single-crystal x-ray diffraction measurements.