Comparative compressibility and structural behavior of spinel Mg[Al.sub.2][O.sub.4] at high pressures: the independency on the degree of cation order

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural Mg[Al.sub.2][O.sub.4] spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, [V.sub.0], the bulk modulus, [K.sub.T0], and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: [V.sub.0] = 529.32(2) [[Angstrom].sup.3], [K.sub.T0] = 193(1) GPa, K' = 5.6(3) for the ordered sample and [V.sub.0] = 528.39(2) [[Angstrom].sup.3], [K.sub.T0] = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the Mg[Al.sub.2][O.sub.4] spinel. Keywords: X-ray single-crystal diffraction, spinel, cation ordering, high pressure