Cation ordering and structural variations with temperature in MgAl2O4 spinel:: An X-ray single-crystal study

The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 degrees C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 degrees C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O'Neill and Navrotsky (1983) gave alpha = 23 +/- 2 KJ/mol and beta = 13 +/- 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and MO bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 Angstrom for T-O and +0.007 Angstrom for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 x 10(-6) degrees C-1 and 8.9 x 10(-6) degrees C-1 were calculated for T and M sites, respectively.