Effects of static non-hydrostatic stress on the R lines of ruby single crystals

The R‐line fluorescence spectroscopy of ruby (Cr:Al2O3) single crystals has been investigated under controlled deviatoric stresses at high confining pressures in a diamond anvil cell. Full stress tensors of the ruby samples were determined based on free‐slip boundary condition and cylindrical symmetry. The shift of the R2 line is insensitive to deviatoric stress and represents local mean stress (pressure). When ruby is loaded along the c‐axis, the R1‐R2 splitting changes linearly with deviatoric stress at a rate of −0.241±0.012 Å/GPa and does not depend on the confining pressure. The splitting increases non‐linearly for ruby loaded along the a‐axis. The dependencies of the splittings on deviatoric stress are in excellent agreement with theoretical calculations constrained by shock‐wave data. The widths of the fluorescence lines increase with growing inhomogeneity in pressure, but are not affected by the deviatoric stresses.