HYDROSTATIC LIMITS OF PRESSURE-TRANSMITTING MEDIA BY SINGLE-CRYSTAL DIFFRACTION

In 1980 Bell and Mao (Carnegie Institution Yearbook, 80:404) reported the first attempts to measure the practical limits of hydrostaticity in helium, neon and argon. They associated the onset of non-hydrostatic conditions with the appearance of significant differences in apparent pressures obtained from ruby chips distributed across the sample chamber of a diamond-anvil cell. Subsequently, changes in the widths and the R1-R2 splitting of ruby fluorescence spectra have been widely used as indicators of non-hydrostatic conditions, but single-crystal X-ray diffraction measurements of soft crystals offer a complimentary method of detecting the onset of significant non-hydrostaticity in pressure media held within a diamond-anvil cell. We have now examined a number of commonly-used pressure media including nitrogen, argon, water, isopropanol and various grades of silicone oil by examining the diffraction maxima from single quartz crystals loaded with each of these pressure media in turn. In all cases, the onset of non-hydrostatic stresses is first detectable as the broadening of the rocking curves of single crystals loaded with this pressure medium. With increasing non-hydrostatic conditions, this is followed by a distortion of the unit-cell parameters of the quartz with the c/a ratio diverging from the trend measured in hydrostatic conditions. Finally, at higher non-hydrostatic stresses, the molar volume of the quartz single crystal also deviates from the hydrostatic equation of state. We detect the onset of broadening of the rocking curves of quartz at ~3.5 GPa in nitrogen at room temperature, 2 GPa in argon, 4.2 GPa in isopropanol, and 2.5 GPa in water. These pressures, which in some cases are significantly lower than hydrostatic limits quoted in the literature, should be considered as maximum limits to the hydrostatic behavior of these materials.