Hydrostaticity in high pressure experiments: some general observations and guidelines for high pressure experimenters

ABSTRACT

The characteristics of hydrostatic stress conditions are discussed and compared with real experimental observations made under high pressure with a diamond-anvil cell. While fluid pressure-transmitting media give a well-defined single stress condition, solid pressure-transmitting media give a variety of stress conditions within the limit of their shear strength under high pressure. Owing to its low shear strength, solid helium would be the best choice for a pressure-transmitting medium to at least 100 GPa. However, helium is so compressible that care should be taken on minimizing irregular deformation of a gasket hole, which often causes complicated stress states. A review is given on the quasi-hydrostatic limits of solidified pressure-transmitting media, and some ways to reduce nonhydrostaticity are discussed including the case of low-temperature experiments.

KEYWORDS:

• Hydrostaticity
• pressure-transmitting medium
• solid helium
• diamond-anvil cell

Acknowledgments

The author would like to thank K. Syassen, S. Klotz, K. Aoki, T. Koide, and anonymous reviewers for valuable comments and discussions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Although MEW with a mixing ratio of 16:3:1 by volume was reported to be fluid up to about 14.4 GPa at room temperature [18,19], recent investigations revealed that it undergoes glass transition above ∼10.5 GPa [20,21]. Furthermore, slight deviation of the mixing ratio lowers the glass-transition pressure [18], which could be the case for this experimental run.

2 One can change the coordinates so that shear stresses vanish and only normal stresses (principal stresses) remain [31]. In the present case, however, the axes are fixed to the laboratory system (e.g., the load axis of a DAC) and accordingly shear stress components are present in general.

3 The term quasi-hydrostatic is still ambiguous, since there is no clear distinction between nonhydrostatic and quasi-hydrostatic stress conditions.

4 Sodium is reported to melt at room temperature near 100 GPa [37] and may serve as a unique fluid PTM in this pressure range.

5 Negative uniaxial stress means σ₃ < σ₁, namely, the lateral stress becomes larger than the axial one. It is unusual but was observed in several experiments with a He-PTM [17,45,47].