A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)

Palin, E. J. and Walker, A. M. and Harrison, R. J. (2008) A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite). American Mineralogist, 93 (8-9). pp. 1363-1372. DOI https://doi.org/10.2138/am.2008.2896

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Abstract

We have used a combination of classical and quantum-mechanical atomistic calculations, together with Monte Carlo simulations, to study order-disorder phenomena in the spinel mineral qandilite, Mg2TiO4. Using an interatomic potential model akin to those previously used for 2-3 spinels yielded a general increase in energy E as a function of inversion parameter x, and thus incorrectly predicted a normal-spinel ground state, whereas the E(x) behavior as modeled by density-functional theory exhibited a maximum at an intermediate degree of inversion and correctly predicted an inverse-spinel ground state. We therefore used the quantum-mechanical simulations to derive pair interaction parameters (for nearest-neighbor tetrahedral-tetrahedral, octahedral-octahedral, and tetrahedral-octahedral interactions) and chemical potential to use in Monte Carlo simulations of order-disorder in qandilite. The simulated cation distributions compared favorably with those obtained experimentally, although the long-range ordering transition to the tetragonal P4122 phase was not observed when using only nearest-neighbor interactions. However, this transition could be observed following the addition of two extra parameters to the model. The simulations were used to calculate the effect of short- and long-range cation order on the configurational entropy of qandilite as a function of temperature. The calculated entropy of the high-temperature cubic phase was in very good agreement with the experimental value recently determined, supporting the suggestion that there is considerable short-range order in qandilite.

Item Type:	Article
Uncontrolled Keywords:	08AREP; IA57;
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences
Journal or Publication Title:	American Mineralogist
Volume:	93
Page Range:	pp. 1363-1372
Identification Number:	https://doi.org/10.2138/am.2008.2896
Depositing User:	Sarah Humbert
Date Deposited:	20 May 2009 15:01
Last Modified:	23 Jul 2013 09:54
URI:	http://eprints.esc.cam.ac.uk/id/eprint/991

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