Dependence of R fluorescence lines of rubies on Cr³⁺ concentration at various temperatures, with implications for pressure calibrations in experimental apparatus

The R fluorescence lines of rubies that contain 0.022, 0.068, 0.211, 0.279, 0.556, 1.221, and 1.676 wt% of Cr₂O₃ were measured at temperatures of 100–600 K and at atmospheric pressure. The R₁ line wavenumbers of all of the ruby samples shifted linearly as the temperature increased from 298 to 600 K at atmospheric pressure, and the temperature dependence increased from −0.157 ± 0.001 cm⁻¹/K to −0.149 ± 0.001 cm⁻¹/K as the Cr₂O₃ content in the rubies increased from 0.022 to 1.676 wt%, which suggests a significant dependence on Cr³⁺ concentration. At room temperature and atmospheric pressure, the full-width at half maximum (FWHM) of the peak height of the R lines also appears to be linearly related to the Cr³⁺ concentration. The relative intensity ratios of the R₂ to R₁ lines (I₂/I₁) of ruby samples with different Cr³⁺ concentrations show several non-linear variations with temperature from 100 to 600 K, and the maximum values, (I₂/I₁)_max, occur near room temperature. The effect of Cr³⁺ doping on the temperature dependence of the R line wavenumbers should be considered when rubies are used to calibrate the pressure or temperature in high-pressure and high-temperature experiments.