Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7
Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7
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天然黄长石五维结构细化，(Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
G. Chapuis
中科院分区：
文献类型：
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作者：
L. Bindi;P. Bonazzi;M. Dušek;V. Petříček;G. Chapuis
意大利翁布里亚圣维南佐黄长石晶体的通式为X2T1(T2)2O7，其中X=Ca0.945Sr0.005Na0.04K0.01，T1=Mg0.92Al0.08，T2=Si0.99Al0.01。结构为四方结构，超空间群PAR 421m：P4 mg，晶胞参数a=7.860 (1)，c=5.024 (1) A，调制向量q1=0.2815 (3)(a*+b*)，q2=0.2815 (3)(−a*+b*)。数据采集是在KumaCCD衍射仪上进行的。结构从7606个反射精炼到最终的R=0.0481。需要对JANA2000精化程序进行特殊修改，以考虑在积分过程中不能适当分离的卫星反射重叠m×n=±1。最终的模型包括对原子位置的调制以及对热参数的调制。后者是由实际调制位置附近的强烈差异引起的。X和T1位点的职业调节都不显著，推测这两个位点分别只被钙和镁占据。由于钙和氧的位置调制，整个结构中发生了6、7和8倍的钙配位，热椭球也相应地改变了形状。原子的位置调制引起原子间距离的变化，但对键价和影响不大，但分别导致T1和T2四面体的扁平化和旋转。
The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X2T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group Par 421m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) A, modulation vectors q1 = 0.2815 (3)(a* + b*), q2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.