AN X-RAY DIFFRACTION STUDY OF TETRAGONAL BARIUM TITANATE
AN X-RAY DIFFRACTION STUDY OF TETRAGONAL BARIUM TITANATE
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DOI：
10.1107/s0365110x61002989
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发表时间：
1961-01-01
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影响因子：
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通讯作者：
EVANS, HT
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文献类型：
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作者：
EVANS, HT
为了准确测定铁电体钛酸四钡的晶体结构参数，用特制的盖革计数器从一个小的、非孪晶的单晶中收集了X射线衍射强度数据。收集了99个独立的hOl数据，并通过精确积分法校正了吸收和二次消光。其中，83个用于结构细化，其余的（sin 0）2/2< 0.250）经受初级消光效应。首先通过试错法，然后通过最小二乘分析，首先基于4参数模型，最后使用12参数模型，对结构进行了改进。当试图求解12参数模型（加上比例因子）的完整最小二乘矩阵时，得到了异常的结果，很明显结构本质上是不确定的。这种情况是由每个原子的z和B33参数之间的总相互作用引起的。这些参数的异常大的标准偏差已经从最小二乘矩阵计算出来。有人建议，这种异常的错误可能会遇到晶体具有极性空间群，但偏离只有小原子位移从一个更高的对称群。铁电晶体通常是这种类型的，并且通常可以以任何合理的真实精度不确定。
In order accurately to determine the parameters of the crystal structure of ferroelectric tetragonal barium titanate, X-ray diffraction intensity data were gathered from a small, untwinned, single crystal by means of a Geiger counter apparatus specially built for the purpose. 99 independent hOl data were collected, and corrected for absorption and secondary extinction by an exact integration method. Of these, 83 were used for the structure refinement, the remainder (with (sin 0) 2/)~ 2< 0.250) being subject to primary extinction effects. The structure was refined first by trial and error methods, then by least-squares analysis; first on the basis of a 4-parameter model, and finally with a 12-parameter model. When an attempt was made to solve the full least-squares matrix of the 12-parameter model (plus scale factor), anomalous results were obtained, and it became apparent that the structure is essentially indeterminate. This situation is caused by gross interactions between the z and B33 parameters, among others, for each of the atoms. Anomalously large standard deviations for these parameters have been calculated from the least-squares matrix. It is suggested that such anomalous errors may be encountered in crystals having a polar space group, but which deviate only by small atomic displacements from a higher symmetry group. Ferroelectric crystals are generally of this type and may be generally indeterminate with any reasonable, true accuracy.