In situ powder neutron diffraction of cation partitioning vs. pressure in Mg0.94Al2.04O4 synthetic spinel

Powder neutron diffraction [ISIS Facility (U.K.), POLARIS diffractometer] w as used to investigate the effect of elevated pressure on cation partitioni ng in synthetic Mg0.94Al2.04O4 spinel. The distributions of Mg, Al, and vac ancies were studied as a function of pressure, by refinement of the T- and M-site scattering lengths, and determination of the cation partitioning thr ough numerical minimization methods. The partially disordered Mg/Al distrib ution, which results from the synthesis process, show an increase in orderi ng between 6 and 18 kbar, where Mg and Al order to the T- and M-sites, resp ectively. Pressure effectively tends to stabilize MgAl-spinels with a "norm al structure," and this behavior is supported by numerical simulations base d on classical electrostatic calculations.