On the structure of crystalline and molten cryolite: Insights from the ab initio molecular dynamics in NpT ensemble,The Journal of Chemical Physics

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On the structure of crystalline and molten cryolite: Insights from the ab initio molecular dynamics in NpT ensemble
The Journal of Chemical Physics ( IF 3.1 ) Pub Date : 2016-02-09 15:12:03 , DOI: 10.1063/1.4941333
Tomáš Bučko _{1,

2} , František Šimko ₂

Affiliation

Ab initiomolecular dynamics simulations in isobaric-isothermal ensemble have been performed to study the low- and the high-temperature crystalline and liquid phases of cryolite. The temperature induced transitions from the low-temperature solid (α) to the high-temperature solid phase (β) and from the phase β to the liquid phase have been simulated using a series of MD runs performed at gradually increasing temperature. The structure of crystalline and liquid phases is analysed in detail and our computational approach is shown to reliably reproduce the available experimental data for a wide range of temperatures. Relatively frequent reorientations of the AlF₆ octahedra observed in our simulation of the phase β explain the thermal disorder in positions of the F⁻ ions observed in X-ray diffraction experiments. The isolated AlF₆³⁻, AlF₅²⁻, AlF₄⁻, as well as the bridged Al2Fm6−m ionic entities have been identified as the main constituents of cryolite melt. In accord with the previous high-temperature NMR and Raman spectroscopic experiments, the compound AlF52− has been shown to be the most abundant Al-containing species formed in the melt. The characteristic vibrational frequencies for the AlFn3−n species in realistic environment have been determined and the computed values have been found to be in a good agreement with experiment.

中文翻译：

关于结晶和熔融冰晶石的结构：从NpT集合的从头算分子动力学中获得的见解

在等压-等温集成中进行了从头算分子动力学模拟，以研究冰晶石的低温和高温结晶相和液相。使用在逐渐升高的温度下进行的一系列MD运行，模拟了温度诱导的从低温固相（α）到高温固相（β）的转变以及从相β到液相的转变。详细分析了结晶相和液相的结构，并显示了我们的计算方法能够可靠地重现广泛温度范围内的可用实验数据。在我们的相模拟中观察到AlF ₆八面体的相对频繁的重新定向β解释的F的位置处的热紊乱^-在X射线衍射实验中观察到的离子。的分离的AlF ₆^3-，的AlF ₅^2-，的AlF ₄^-，以及桥接Al2Fm6-m的离子实体已经被鉴定为冰晶石熔体的主要成分。与先前的高温NMR和拉曼光谱实验一致，化合物AlF52-被证明是在熔体中形成的最丰富的含Al物质。确定了现实环境中AlFn3-n物种的特征振动频率，发现计算值与实验结果吻合良好。

更新日期：2016-02-10

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