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

¹ Dipartimento di Geoscienze, Unversità di Padova, Corso Garibaldi 37, I-35137 Italy
² Bayerisches Geoinstitut, University of Bayreuth, Universitaetstrasse 37, D-95447 Germany
³ Geological Institute, University of Copenhagen, Oester Volgade 10, DK-1350 Denmark
⁴ Dipartimento di Scienze della Terra, Università di Trieste, Via E. Weiss 8, I-34127 Italy

Correspondence: ^* E-mail: fabrizio.nestola{at}unipd.it

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, K_T0, 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) ų, K_T0 = 193(1) GPa, K' = 5.6(3) for the ordered sample and V₀ = 528.39(2) ų, K_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 MgAl₂O₄ spinel.

Key Words: X-ray single-crystal diffraction • spinel • cation ordering • high pressure