Analysis of deviatoric stress from nonhydrostatic pressure on a single crystal in a diamond anvil cell: The case of monoclinic aegirine, NaFeSi₂O₆

A synthetic crystal of aegirine was compressed in a 4-pin diamond anvil cell with 4:1 methanol:ethanol+water mixture as a pressure-transmitting medium. The pressure was monitored using the shift of ruby R1-R2 lines. X-ray diffraction patterns were recorded at 17 pressures to 13.52 GPa using unfiltered Mo radiation. The pressure dependencies of the monoclinic cell parameters showed normal behavior to 11.55 GPa. The zero-pressure axial compressibilities were χ_a = 0.0029, χ_b = 0.0031, and χ_c = 0.0027 GPa^{- 1}. Fitting the third-order Birch-Murnaghan equation to the pressure-volume data yielded K₀ = 117 (1) GPa, K₀^′ = 3.2 (2), and V₀ = 429.40 (9) ų. The average peak widths in all the runs to 11.55 GPa were ∼0.08° in ω with small spread (standard deviation). At 12.62 and 13.52 GPa, a monoclinic unit cell did not fit well, as the standard deviations in cell parameters were 5-20 times the values obtained below 11.55 GPa. The cell parameters showed marked deviations from the trends exhibited by the data up to 11.55 GPa. Axial compressibilities over the range from 12.62 to 13.52 GPa were dramatically changed; χ_a = - 0.0008, χ_b = 0.0026, and χ_c = 0.0006 GPa^{- 1}. The resulting discontinuity at 12.62 GPa in the pressure-volume plot was reminiscent of a pressure-induced phase transition. The average peak widths at these two pressures increased to ∼0.24° with large spread. The analyses of the stress and strain tensors associated with the deviations in the cell parameter versus pressure plots at 12.62 GPa indicate the onset of nonhydrostatic stresses. At 13.52 GPa, just 2 GPa above the onset of nonhydrostatic strain, the magnitude of the nonhydrostatic stress was computed to be 1.2 GPa. Such a situation is likely to arise in high-pressure studies on single crystals. It is important to recognize in order to avoid incorrect interpretation of the data. The analysis presented in this article can be used in detecting the onset of nonhydrostatic stresses due either to freezing of the pressure-transmitting medium or to the sample bridging the anvils.