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A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)
-
Erika J. Palin
,
Andrew M. Walker
Veröffentlicht/Copyright:
1. April 2015
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 hightemperature 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.
Keywords : Qandilite; Mg2TiO4; spinel; thermodynamics; Monte Carlo simulation; densityfunctional theory calculations
Received: 2007-12-12
Accepted: 2008-2-26
Published Online: 2015-4-1
Published in Print: 2008-8-1
© 2015 by Walter de Gruyter Berlin/Boston
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Schlagwörter für diesen Artikel
Qandilite;
Mg2TiO4;
spinel;
thermodynamics;
Monte Carlo simulation;
densityfunctional theory calculations
Artikel in diesem Heft
- Hydrothermal synthesis of hematite spherules and jarosite: Implications for diagenesis and hematite spherule formation in sulfate outcrops at Meridiani Planum, Mars
- Concentric slow cooling of a low-P–high-T terrane: Evidence from 1600–1300 Ma mica dates in the 1780–1700 Ma Black Hills Orogen, South Dakota, U.S.A.
- Monazite ages from carbonatites and high-grade assemblages along the Kambam Fault (Southern Granulite Terrane, South India)
- Forward modeling of the effects of mixed volatile reaction, volume diffusion, and formation of submicroscopic exsolution lamellae on calcite-dolomite thermometry
- A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3
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- Protonation in germanium equivalents of ringwoodite, anhydrous phase B, and superhydrous phase B
- Andreyivanovite: A second new phosphide from the Kaidun meteorite
- The crystal chemistry of whitlockite and merrillite and the dehydrogenation of whitlockite to merrillite
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- The kinetics and mechanisms of schwertmannite transformation to goethite and hematite under alkaline conditions
- Crystal chemistry of the mixed-layer sequence talc–talc-smectite–smectite from submarine hydrothermal vents
- Crystal chemical and structural characterization of fibrous tremolite from Susa Valley, Italy, with comments on potential harmful effects on human health
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- Constraints on structural models of ferrihydrite as a nanocrystalline material
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