Study of the total scattering and pair distribution function of materials under high pressure in the diamond cell offers new possibilities, revives old issues, and answers new questions

By C. David Martin

New possibilities:

1) Studies of the local structure of materials under pressures greater than 10 GPa are possible.

2) We can resolve structure characteristics from amorphous and crystalline materials with both strong and weak scattering elements. (Such information is not available from Rietveld analysis of Bragg diffraction alone, considering the relative transparency of oxygen to x-rays and the limited angular range available for scattering from samples in the diamond anvil cell.)

How:

Using the diamond anvil cell, static pressures in excess of 50 GPa are possible and are routinely performed.

Structure features of materials are illuminated by the combined scattering power of the strong- and weak-scattering atom-pair.

Old issue:

Are applied pressures hydrostatic? This is important because analysis of Bragg diffraction from materials subject to non-hydrostatic pressures yield non-reproducible pressure-volume relationships.

New Questions:

Is the pair distribution function immune to the undesirable effects of non-hydrostatic pressure?
If not, what will be the effect of non-hydrostatic pressures on the pair-distribution function?

Answers:

The pair distribution function, including the short-range order (< 5 Å), is subject to the stress condition in the diamond cell

As expected from decades-old experiments examining Bragg diffraction, the total-scattering from samples under non-hydrostatic pressures in the diamond cell appears to resist the pressure applied to the sample. This effect is due to the parallel direction of the x-ray beam and principle axis of force during the experiment and leads to an underestimation of the compressibility and an incorrect estimation of phase transition pressures. The phase transition from alpha to epsilon iron is one example

.

Loading a pressure-transmitting medium, such as helium gas or an alcohol mixture, with the sample is ideal for high pressure studies.