Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order

Abstract

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 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, V0, the bulk modulus, KT0, and its first pressure derivative, K′, were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) Å3, KT0 = 193(1) GPa, K′ = 5.6(3) for the ordered sample and V0 = 528.39(2) Å3, KT0 = 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 MgAl2O4 spinel

On the energetics of cation ordering in tungsten-bronze-type oxides

“…Largely normal spinels such as MgAl 2 O 4 tend to become more inverse with increasing pressure, and it has been observed that this compound disorders more with temperature under high-pressure conditions. 35,36 However, at ambient temperature, high pressure alone has only a negligible effect on the cation distribution, 37 which is also one of the assumptions in the thermodynamic model employed in this work.…”

The elasticity of MgAl2O4-MnAl2O4 spinels by Brillouin scattering and an empirical approach for bulk modulus prediction

“…To date, much effort has been devoted to the experimental and computational study of physical properties of spinel compounds as a function of pressure (Kiefer et al 1997;Nestola et al 2002;Levy et al 2003;Reichmann and Jacobsen 2006), temperature (Askarpour et al 1993;Andreozzi et al 2000;Suzuki et al 2000), and at high magnetic and electric fields (e.g., Hong and Yoo 2006;Ito et al 2011). Nevertheless, there are still many open issues regarding the dependencies of the physical properties of spinels as a function of composition and crystal-chemistry.…”

Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

“…There has been excellent agreement on the T effect on the Mg-Al disorder process of the MgAl2O4-Sp at ambient P: x increases as T increases [42,47,48,50,51,[53][54][55][56][57]. As to the P effect at ambient T, discrepancy presumably exists because the order-disorder reaction could not be readily activated and did not adequately approach its equilibrium during the course of a conventional high-P study [52,90,91]. Thanks to Méducin et al [45] who conducted an investigation under simultaneously high-P and high-T conditions (up to 3.2 GPa and 1318 °C), the P effect at relatively high T has been well established: x increases as P increases.…”