Symmetry change of majorite solid-solution in the system Mg3Al2Si3O12-MgSiO3
Symmetry change of majorite solid-solution in the system Mg3Al2Si3O12-MgSiO3
复制标题
Mg3Al2Si3O12-MgSiO3系中多数石固溶体的对称性变化
DOI：
10.2138/am-1999-7-816
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发表时间：
1999
影响因子：
3.1
通讯作者：
O. Ohtaka
中科院分区：
文献类型：
--
作者：
A. Nakatsuka;A. Yoshiasa;T. Yamanaka;O. Ohtaka
用“6-8”型单轴劈裂球装置在20 Gpa、2000℃下合成了六种不同x=0.05、0.13、0.24、0.38、0.52和0.64(马来石固溶体)的镁铝硅单晶。单晶X射线衍射研究表明，摩尔体积、等效各向同性温度因子(BEQ)和平均键长在x=0.24到0.38之间的组成依赖关系不连续。成分范围为0≤x≤0.2 4的单晶无双折射，x=0.6 4的单晶有轻微的光学各向异性。此外，用同步加速器X射线辐射获得的x=0.64的晶胞对称性是四方的，与立方体略有偏离。根据立方精修得到的BEQ的组成依赖关系所期望的位置分裂，在0.38≤x≤0.64范围内最可能的空间群是I41/Acd(四方)，这是空间群Ia3̅d(立方体)的最大子群。考虑到以前报道的0.8≤x≤1.0的晶体具有四方空间群I41/a，该系统中的马来石固溶体经历了一系列的对称性变化，Ia3̅d→I41/Acd→I41/a，随着MgSiO_3组分的增加。从Ia3̅d到I41/Acd的对称性变化不能用八面体上的阳离子有序性来解释。从原子热运动和电子密度分布观察到十二面体(Mg2+)和四面体(Si4+)阳离子之间有很强的静电相互作用。由于I41/Acd结构中两个不等价的四面体位置的其中一个位置对称性随Ia3̅d向I41/Acd的对称性降低而失去对称性，因此对称性降低可能是由于相邻阳离子-阳离子相互作用引起的阳离子的电子极化引起的。
Abstract Six single crystals of Mg3(MgxSixAl2-2x)Si3O12 with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 °C with a “6-8” type uniaxial splitsphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (Beq), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 ≤ x ≤ 0.24 show no birefringence, whereas those of x = 0.64 have a slight optical anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using synchrotron X-ray radiation is tetragonal with a slight deviation from cubic. On the basis of site splitting expected from compositional dependence of Beq obtained by cubic refinement, the most probable space group in the range 0.38 ≤ x ≤ 0.64 is I41/acd (tetragonal), which is the maximal subgroup of the space group Ia3̅d (cubic). Given that the previous reports that crystals with 0.8 ≤ x ≤ 1.0 have the tetragonal space group I41/a, the majorite solid-solution in this system undergoes the series of symmetry changes, Ia3̅d →I41/acd→I41/a, with increasing MgSiO3 component. The symmetry changes from Ia3̅d to I41/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg2+) and tetrahedral (Si4+) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I41/acd structure loses the center of symmetry with the symmetry reduction from Ia3̅d to I41/acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.