Lattice strains under megabar pressures

The diamond anvil cell (DAC) can be used to compress samples to a few megabars, and the use of synchrotron radiation as a source of intense X-ray beam allows diffraction patterns to be recorded from the samples at such high pressures . Such experiments give information on the phase transitions and the lattice strains under pressure . Under truly hydrostatic pressure, the lattice strains conform to the symmetry of the unit cell . The pressure in the megabar region tend to deviate appreciably from the hydrostatic condition . The modeling of the non-hydrostatic stress in a DAC and its effect on the lattice strains are subjects of intense research activity . The analysis using appropriate theory of the lattice strain measured on a polycrystalline sample under non-hydrostatic condition can be used to estimate the shear strength and single crystal elastic moduli of the sample material as function of pressure . The pressure-volume relation of the sample material corresponding to hydrostatic compression naturally emerges from the analysis . This article reviews the experimental techniques and the present theoretical understanding of the non-hydrostatic effects