锆石结构天然硅酸盐的晶格动力学和高压相稳定性
Lattice Dynamics and High Pressure Phase Stability of Zircon Structured Natural Silicates
摘要：
我们报告了天然硅酸盐MSiO 4（M=Zr，Hf，Th，U）作为压力（P）和温度（T）的函数，这是很重要的，在他们的背景下，在核废料存储介质中使用的各个阶段的相对稳定性的晶格动力学研究。扩展了我们以前对ZrSiO 4的工作，在P和T的范围内，使用可转移的原子间作用势计算了各个相的吉布斯自由能。由于振动熵和原子堆积之间有趣的相互作用，锆石（体心四斜晶系，I41/amd）、白钨矿（体心四斜晶系，I41/a）和胡氏石（单斜晶系，P21/n）相出现在不同的P和T下。结果表明，对于高P下的ThSiO_4，在高T和低T下，分别有利于生成Huttonite和白钨矿相。然而，对于USiO 4和HfSiO 4两者，膨润土相是动态不稳定的，并且白钨矿相作为高压相是稳定的。事实上，计算表明，膨润土相的稳定性是由M-原子的离子尺寸决定的;该相对于具有较小Hf和U离子的硅酸盐是不稳定的，并且对于较大Th离子是稳定的。计算出的相图与报道的实验观察结果相当一致。计算的结构、声子谱和各种热力学性质也相当好地再现了可用的实验数据。发现导致其大振动熵的Huttonite相中的低能声子涉及硅酸盐四面体单元的振动运动。
英文摘要：
We report a lattice dynamics study of relative stability of various phases of natural silicates MSiO4 (M=Zr, Hf, Th, U) as a function of pressure (P) and temperature (T), which is important in the context of their use in nuclear waste storage media. Extending our previous work on ZrSiO4, the Gibbs free energy has been calculated using a transferable interatomic potential in various phases over a range of P and T. Due to an interesting interplay between the vibrational entropy and atomic packing, the zircon (body centered tetragonal, I41/amd), scheelite (body centered tetragonal, I41/a) and huttonite (monoclinic, P21/n) phases occur at different P and T. It is shown that for ThSiO4 at high P, the huttonite and scheelite phases are favored at high and low T respectively. However, for both USiO4 and HfSiO4 the huttonite phase is dynamically unstable and the scheelite phase is stable as the high pressure phase. In fact, the calculations reveal that the stability of the huttonite phase is determined by the ionic size of the M-atom; this phase is unstable for the silicate with the smaller Hf and U ions and stable with the larger Th ion. The calculated phase diagrams are in fair agreement with the reported experimental observations. The calculated structures, phonon spectra, and various thermodynamic properties also fairly well reproduce the available experimental data. The low-energy phonons in the huttonite phase that contribute to its large vibrational entropy are found to involve librational motion of the silicate tetrahedral units.
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