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

Palin, EJ; Walker, AM; Harrison, RJ; (2008) A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite). AM MINERAL , 93 (8-9) 1363 - 1372. 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 ail inverse-spinel ground state. We therefore used the quantum-mechanical simulations to derive pair interaction parameters (for nearest-neighbor tetrahedral-tetrahedral, octahedra I-octahedral, and tetrahedra I-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 P4(1)22 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.

Type:Article
Title:A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)
DOI:10.2138/am.2008.2896
Keywords:qandilite, Mg2TiO4, spinel, thermodynamics, Monte Carlo simulation, density-functional theory calculations, MONTE-CARLO, CATION DISTRIBUTIONS, SYNTHETIC QANDILITE, 1ST PRINCIPLES, THERMODYNAMICS, MINERALS, TITANIUM, SIMULATION, PSEUDOPOTENTIALS, DISTANCES
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Earth Sciences

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