Crystal structures and stabilities of cristobalite-helium phases at high pressures
Crystal structures and stabilities of cristobalite-helium phases at high pressures
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DOI：
10.2138/am.2014.4637
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发表时间：
2014-01-01
影响因子：
3.1
通讯作者：
Funamori, Nobumasa
中科院分区：
文献类型：
--
作者：
Matsui, Masanori;Sato, Tomoko;Funamori, Nobumasa
采用第一性原理计算方法研究了方石英-He I和II在高压下的结构和能量性质，这两种性质是最近在室温下用氦压力介质进行的α-方石英高压粉末X射线衍射实验中发现的。这些计算表明，方石英-He I和II都含有一个氦原子/SiO2，分子式为SiO2 He。它也已被揭示，方石英-He I是积极的优势高于6.4 GPa，方石英-He II是稳定的相之间的压力在1.7和6.4 GPa，和方石英II和结晶He的混合物是更稳定的比任何方石英-He I或II低于1.7 GPa，在一般协议与观察。方英石-He Ⅰ和Ⅱ分别为单斜晶系，空间群为P2（1）/c，菱方晶系，空间群为R（3）。方石英-He I和II的晶胞参数重新确定从先前测量的高压X射线衍射数据的基础上，这些预测的细胞。有一个很好的协议之间的观察（重新确定）和计算的压力依赖性的细胞参数的两个阶段。计算的方石英-He I和II的X射线衍射图也与观测数据一致。据预测，由于氦原子渗透到结构的大空隙中，方石英-He I和II在10 GPa下的摩尔体积比方石英II大21%，在4 GPa下的摩尔体积比方石英II大23%。
First-principles calculations were used to study the structural and energetic properties of cristobalite-He I and II at high pressures, both of which were recently found in high-pressure powder X-ray diffraction experiments of alpha-cristobalite with helium pressure-medium at room temperature. These calculations have revealed that both cristobalite-He I and II contain one helium atom per SiO2 with the formula SiO2He. It has also been revealed that cristobalite-He I is energetically favored above 6.4 GPa, cristobalite-He II is the stable phase at pressures between 1.7 and 6.4 GPa, and the mixture of cristobalite II and crystalline He is more stable than either cristobalite-He I or II below 1.7 GPa, in general agreement with the observation. Cristobalite-He I and II have been predicted to be monoclinic with space group P2(1)/c, and rhombohedral with space group R (3) over barc, respectively. The unit-cell parameters of both cristobalite-He I and II were re-determined from the previously measured high-pressure X-ray diffraction data on the basis of these predicted cells. There is an excellent agreement between the observed (re-determined) and calculated pressure dependence of the cell parameters for the both phases. The calculated X-ray diffraction patterns for both cristobalite-He I and II are also consistent with the observed data. Cristobalite-He I and II have been predicted to have molar volumes 21% larger at 10 GPa and 23% larger at 4 GPa than cristobalite II due to the penetration of helium atoms into large voids of the structure.