The modulated average structure of mullite.
The modulated average structure of mullite.
复制标题
莫来石的调制平均结构。
DOI：
10.1107/s205252061500757x
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
R. Fischer
中科院分区：
文献类型：
--
作者：
J. Birkenstock;V. Petříček;B. Pedersen;H. Schneider;R. Fischer
用X射线和中子衍射方法研究了用提拉法生长的2：1莫来石（Al（4.8）Si（1.2）O（9.6））单晶的均匀性和无夹杂物。所观察到的漫散射与卫星反射的图案一起证实了先前公布的数据，因此是莫来石结构的固有特征。通过微探针分析（Al（4.82（3））Si（1.18（1））O（9.59（5）和平均结构细化证实，接近满足理想组成。 在四面体三簇的T* 位置发现8（5）至20（13）%的有效Si。 莫来石中无序的强烈趋势可以从假设的超结构的考虑来理解，所述超结构相对于2：1莫来石的三维平均晶胞必须是n-5倍，并且在3：2莫来石的情况下必须是n-4倍。在任何一种情况下，空位和四面体单元的可能排列都不可避免地是不利的。确定了三个方向的无公度调制：q1 = [0.3137（2）0 ½]，q2 = [0.4021（5）0.1834（2）]和q3 = [0.4009（5）-0.1834（2）]。 与q1相关的一维连续调制晶体结构首次使用超空间方法进行了改进。调制占主导地位的调和职业调制的原子中的二和三簇的四面体单元莫来石。调制幅度小，谐波字符意味着调制的结构仍然代表一个平均结构的整体无序排列的空位和四面体结构单元。换句话说，当以几十个平均莫来石晶胞的尺度将局部组件投影到由调制矢量q1、q2或q3中的任一个确定的晶胞中时，弱平均调制导致四面体单元的平均占据因子略微变化。结果表明，莫来石的真实的结构是局部有序的（如前所述），但在长程上其平均结构并不完全无序，莫来石的调制结构可称为真正的“莫来石平均结构”。
Homogeneous and inclusion-free single crystals of 2:1 mullite (Al(4.8)Si(1.2)O(9.6)) grown by the Czochralski technique were examined by X-ray and neutron diffraction methods. The observed diffuse scattering together with the pattern of satellite reflections confirm previously published data and are thus inherent features of the mullite structure. The ideal composition was closely met as confirmed by microprobe analysis (Al(4.82 (3))Si(1.18 (1))O(9.59 (5))) and by average structure refinements. 8 (5) to 20 (13)% of the available Si was found in the T* position of the tetrahedra triclusters. The strong tendencey for disorder in mullite may be understood from considerations of hypothetical superstructures which would have to be n-fivefold with respect to the three-dimensional average unit cell of 2:1 mullite and n-fourfold in case of 3:2 mullite. In any of these the possible arrangements of the vacancies and of the tetrahedral units would inevitably be unfavorable. Three directions of incommensurate modulations were determined: q1 = [0.3137 (2) 0 ½], q2 = [0 0.4021 (5) 0.1834 (2)] and q3 = [0 0.4009 (5) -0.1834 (2)]. The one-dimensional incommensurately modulated crystal structure associated with q1 was refined for the first time using the superspace approach. The modulation is dominated by harmonic occupational modulations of the atoms in the di- and the triclusters of the tetrahedral units in mullite. The modulation amplitudes are small and the harmonic character implies that the modulated structure still represents an average structure in the overall disordered arrangement of the vacancies and of the tetrahedral structural units. In other words, when projecting the local assemblies at the scale of a few tens of average mullite cells into cells determined by either one of the modulation vectors q1, q2 or q3 a weak average modulation results with slightly varying average occupation factors for the tetrahedral units. As a result, the real structure of mullite is locally ordered (as previously known), but on the long-range its average is not completely disordered, the modulated structure of mullite may be denoted the true 'average structure of mullite'.