Cation partitioning versus temperature in spinel: optimization of site occupants

Pavese ct al. (1999) examined cation partitioning vs. temperature in a synt hetic spinel of composition (Mg0.70Fe0.233+) Al-1.97 O-4 using structure da ta obtained from in situ neutron powder diffraction. After imposing assumpt ions on the site assignment of vacancies and Fe3+, they assigned the remain ing cations by applying least-squares minimization to chemical constraints on site-occupancy sums, site-scattering, chemical composition, and thermal expansion of the octahedral site. Their proposed site assignments exhibit a sharp discontinuity in occupancy fractions versus temperature, a necessary consequence of their assumptions on vacancy assignments. In this paper we reexamine the cation partitioning of the same spinel using the constrained least-squares formulation of OccQP (Wright et al. 2000), which optimizes si te occupancies without ad hoc assumptions. We obtain strikingly different r esults, supporting the general view that spinel undergoes transition at sim ilar to 1000 K. For all observed the residuals obtained with the OccQP assi gnments are lower than those obtained with the Pavese ct al. assignments, i n some cases by more than order of magnitude.