Mg-Al synthetic spinels: cation and vacancy distribution as a function of temperature, from neutron powder diffraction

Neutron powder diffraction data collection has been carried out at BENSC (Berlin, D), on synthetic Mg0.94Al2.04O4 spinel, in the temperature range 300-1700 K, to investigate the intracrystalline cation and vacancy order-disorder (OD) reaction, which begins to be detectable at 944 K. The Rietveld method has been used in combination with advanced minimization techniques to determine the occupancy coefficients of magnesium, aluminum, and vacancies on the tetrahedral and octahedral sites. The temperature dependence of the cell parameter, of the T-O and M-O bond lengths, and of the oxygen u-coordinate are discussed in the light of the OD-reactions. The cation site partitioning has been interpreted as a function of temperature using the thermodynamic model of O'Neill and Navrotsky, whose parameters [α = 27.4(1.4) and β = 4.3(2.7) kJ/mol] are compared with earlier determinations from literature.