Cation partitioning versus pressure in Mg0.94Al2.04O4 synthetic spinel, by in situ powder neutron diffraction

Powder neutron diffraction [ISIS Facility (U.K.), POLARIS diffractometer] was used to investigate the effect of elevated pressure on cation partitioning in synthetic Mg0.94Al2.04O4 spinel. The distributions of Mg, AI, and vacancies were studied as a function of pressure, by refinement of the Tand M-site scattering lengths, and determination of the cation partitioning through numerical minimization methods. The partially disordered Mg/Al distribution, which results from the synthesis process, show an increase in ordering between 6 and 18 kbar, where Mg and AI order to the T- and Msites, respectively. Pressure effectively tends to stabilize MgAl-spinels with a "normal structure," and this behavior is supported by numerical simulations based on classical electrostatic calculations.