THE CRYSTAL STRUCTURE
THE CRYSTAL STRUCTURE
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发表时间：
2010
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通讯作者：
C. Prewitt
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作者：
C. Prewitt
部分满足理学硕士学位的要求。钙铝石是少数几种被归为4类晶体的晶体之一。旋进研究表明其衍射球为4/m I-/-，空间群为I4，I4 E和14/.由于已知的4形态，它必须归属于空间群I4。晶胞尺寸为a = 7.11A，o = 6.201，含有两个分子量的Ca BAsO（OH）L。用单晶衍射仪测定了晶体的结构。Patterson的合成首先是对沿c、a和110方向的投影进行沿着的合成。原子的原子序数的比例为As：Ca：0：B = 33：20：8：5，因此Patterson峰由含有砷作为该对的一个成员的原子对支配。由于体心晶胞中只有两个砷原子，因此可以任意将一个As指定为原点。然后Patterson合成的主峰位于结构中原子的位置。以这种方式近似确定的结构通过二维差异图进行细化，然后使用CuKec辐射可记录的所有hkl反射通过最小二乘进行细化。该结构由I的四面体包围的氧在距离为1.67和B四面体包围的氧在距离为1.47。虽然四面体不共享氧，但它们通过氢键连接在一起。每个Ca原子被8个氧原子环绕，平均间距为2.46。由于对使用等倾角单晶衍射仪系统地收集三维衍射强度感兴趣，在附录中导出了可由数字计算机计算的T和#的一般解析表达式。另一附录给出了晶体结构分析中的一些实验技术的分析。导言1
partial fulfillment of the requirements for the degree of Master of Science. Cahnite is one of the few crystals which had been assigned to crystal class 4. A precession study showed that its diffraction sphol is 4/m I-/-, which contains space groups I4, I4E, and14/. Because of the known 4 morphology, it must be assigned to space group I4. The unit cell, whose dimensions are a = 7.11A, o = 6.201, contains two formula weights of Ca BAsO (OH)L. The structure was studied with the aid of in ensity medsurements made with a single-crystal diffractometer. Patterson s ntheses were first made for projections along the c, a, and 110 directions. The atomic numbers of the atoms are in the ratio As:Ca.0:B = 33:20:8:5, so that the Patterson peaks are dominated by the atom pairs containing arsenic as one member of the pair. Since there are only two arsenic atoms in a body-centered cell, one As can be arbitrarily assigned to the origin. Then the major peaks of the Patterson syntheses are at locations of atoms in the structure. The structure, determined approximately in this manner, was refined by two-dimensional difference maps, and later by least squares using all hkl reflections recordable with CuKecradiation. The structure consist I of As tetrahedrally surrounded by oxygen at a distance of 1.67 and B tetrahedrally surrounded by oxygen at a distance of 1.47. Although the tetrahedra do not share oxygens, they are linked together by hydrogen bonds. The Ca atoms are each swrrounded by 8 oxygen atoms at an average distance of 2.46A. Because of interest in the use of the equi-inclination single-crystal diffractometer for the systematic collection of three-dimensional diffraction intensities, general analytical expressions for T and #which can be evaluated by a digital computer have been derived in an appendix. Another appendix gives an analysis of some experimental techniques in crystal structure analysis. Abstract Introduction 1