Comments on A new theory for X-ray diffraction.
Comments on A new theory for X-ray diffraction.
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DOI：
10.1107/s2053273318003959
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发表时间：
2018-09-01
期刊：
影响因子：
--
通讯作者：
Wark JS
中科院分区：
文献类型：
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作者：
Fraser JT;Wark JS
Fewster [（2014），Acta Cryst.A70，257-282]声称需要X射线衍射的新理论，并且小的微晶将以正好两倍于布拉格角的角度引起散射，无论它们的取向如何。本文论证了这一理论是错误的。在一篇题为X射线衍射的新理论[Fewster（2014）. Acta Cryst.A70，257-282]，下文称为NTXRD，声称当X射线从小微晶散射时，无论其尺寸和形状如何，衍射图案都将包含在精确的2θB角处的增强散射，无论晶体的取向如何。有人声称，以这种方式从粉末散射，随机取向的晶体，引起布拉格散射，即使布拉格条件是从来没有满足由一个单独的微晶。对NTXRD中提出的理论进行了检验，发现它们是错误的。虽然对于晶体形状的某个限制集，当微晶远离布拉格条件取向时，可以获得接近（但不完全在）布拉格角的一些衍射，但通常不是这种情况。此外，与NTXRD中所作的声明相反，对所描述的效应类型的起源的认识并不新鲜，并且自X射线衍射的最早时期以来就已为人所知。
Fewster [(2014), Acta Cryst. A70, 257–282] claimed that a new theory of X-ray diffraction is required, and that small crystallites will give rise to scattering at angles of exactly twice the Bragg angle, whatever their orientation. This article demonstrates that this theory is in error. In an article entitled A new theory for X-ray diffraction [Fewster (2014). Acta Cryst. A70, 257–282], hereafter referred to as NTXRD, it is claimed that when X-rays are scattered from a small crystallite, whatever its size and shape, the diffraction pattern will contain enhanced scattering at angles of exactly 2θB, whatever the orientation of the crystal. It is claimed that in this way scattering from a powder, with randomly oriented crystals, gives rise to Bragg scattering even if the Bragg condition is never satisfied by an individual crystallite. The claims of the theory put forward in NTXRD are examined and they are found to be in error. Whilst for a certain restricted set of shapes of crystals it is possible to obtain some diffraction close to (but not exactly at) the Bragg angle as the crystallite is oriented away from the Bragg condition, this is generally not the case. Furthermore, contrary to the claims made within NTXRD, the recognition of the origin of the type of effects described is not new, and has been known since the earliest days of X-ray diffraction.