Hydrostatic Experiments up to Ultrahigh Pressures

Abstract

The development of the diamond-anvil-cell technology has enabled us to conduct in situ experiments at ultrahigh pressures. The current pressure limit reaches well beyond 200 GPa. The stress environment in such experiments is in general highly nonhydrostatic, and one has to be careful in evaluating the obtained data. Nonhydrostaticity affects the phase stability, transition pressures, equation of state, and various lattice properties. For detailed discussion of the change in physical properties with pressure, we have to realize hydrostatic or quasihydrostatic conditions. I describe our present understanding of the stress state of a sample compressed in a diamond-anvil cell, together with recent efforts for extending the pressure limit of hydrostatic experiments.

Hydrostatic limits of 11 pressure transmitting media

“…This is not the case for the other fluids where R1-R2 systematically indicates a considerably higher solidification pressure (figures 4, 6 and 7). In the case of helium, no minimum of R1-R2 could be detected at all up to 40 GPa, though it would appear at higher pressures [2,3,24]. The general conclusion is that, if there is only one single ruby available, the R1-width appears to be a more reliable indicator for non-hydrostaticity.…”

“…Beyond this point, the pressure across the experimental volume is generally inhomogeneous and differential (mostly uni-axial) stress and shear stresses appear. Depending on the type of measurement, this leads to a more or less dramatic decrease in the quality and accuracy of the data and often to the appearance of 'anomalies' which might be wrongly ascribed to new physical phenomena, see Takemura et al [1,2,3] for a few illustrative examples. As high pressure techniques become more sophisticated the need for high quality data and hence for hydrostatic pressure transmitting fluids becomes more urgent.…”

Equation of state of CaMnO3: a combined experimental and computational study

“…For example, the experimental value for K 0 is reported to be 528 kbar (as determined using ultrasonic measurements) [9], 1544(33) kbar (obtained by XRD at pseudohydrostatic conditions) [10], 1710 kbar (conditions close to hydrostatic in the whole pressure range studied) [11] and 2240(250) kbar (with hydrostatic conditions applied in a part of the fitting range) [12]. The importance of using hydrostatic compression and the suitability of pressure transmitting medium (PTM) formed from an alcohol-mixture for this purpose results from various recent experimental studies [13][14][15] (the hydrostaticity limit for this PTM is about 100 kbar). It is worth noting that for a related material, SrMnO 3 , three experiments differing by measurement conditions led to large discrepancies in bulkmodulus value (see [16]; the authors briefly discuss some possible reasons and remedies).…”

Stability and equation of state of face-centered cubic and hexagonal close packed phases of argon under pressure

“…In the literature, cases where non-hydrostatic stress affect the measured EoS have been discussed, such as systematic bias 16 , 19 , 20 , unphysical lattice distortions 17 , 18 . This stress should thus be evaluated here in order to discuss argon EoS.…”