A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)
A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)
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DOI：
10.2138/am.2008.2896
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发表时间：
2008-08
影响因子：
3.1
通讯作者：
E. Palin;A. Walker;R. Harrison
中科院分区：
文献类型：
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作者：
E. Palin;A. Walker;R. Harrison
摘要我们采用经典和量子力学原子计算相结合的方法，结合蒙特卡罗模拟，研究尖晶石矿物镁钛铁矿中的有序-无序现象。使用类似于先前用于2-3尖晶石的那些的原子间势模型产生能量E作为反转参数x的函数的一般增加，并且因此不正确地预测正常尖晶石基态，而由密度泛函理论建模的E（x）行为在中间程度的反转处表现出最大值，并且正确地预测了反尖晶石基态。因此，我们使用量子力学模拟来推导对相互作用参数（最近邻四面体-四面体，八面体-八面体和四面体-八面体相互作用）和化学势，以用于蒙特卡罗模拟qandilite中的有序-无序。模拟的阳离子分布与实验获得的结果相比，虽然没有观察到的长程有序过渡到tetraorthane P4122相时，只使用最近邻相互作用。然而，在向模型添加两个额外参数后可以观察到这种转变。模拟被用来计算的短，长距离阳离子顺序上的qandilite作为温度的函数的构型熵的效果。高温立方相的熵的计算值与最近确定的实验值非常吻合，支持的建议，有相当大的短程有序qandilite。
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.