File:X rays and crystal structure (1915) (14778089875).jpg

Identifier: xrayscrystalstru00braguoft (find matches)
Title: X rays and crystal structure
Year: 1915 (1910s)
Authors: Bragg, William Henry, Sir, 1862-1942 Bragg, William Lawrence, Sir, 1890-1971
Subjects: Crystallography X-rays
Publisher: London : G. Bell
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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ted, and all spots are intermediate in intensitybetween their neighbours. Compare this with the diagram Fig. 74 represent-ing the Laue photograph of rock salt. It is evident that the perfect regularity has dis-appeared. Instead of there being a spot at everyintersection of the circles, there are gaps. Forinstance, there are spots corresponding to reflectionin the planes (511), (531) ^^^ none corresponding toreflection in the planes (501), (521), (541). There are (331), (35O ^^^ ^° (340 ^^^ ^° ^^ There is,in fact, a preference for the odd as against the evenindices ; the series of planes (Jikl) is divided intotwo sets, one of which contains all planes withwholly odd indices and the other the rest of theplanes. 216 ANALYSIS OF LAUE PHOTOGRAPHS The two-dimensional examples gives the key tothe explanation of this division. The photographapplies to a crystal structure such as that shown inFig. 72, p. 212. We are dealing with points of twokinds, arranged alternately in a three-dimensional
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Fig. 74. structure analogous to that of Fig. 28. Potassiumbromide and iodide repeat the peculiarities of rocksalt in a more marked manner, as would be ex-pected, for in these crystals the difference betweenthe two kinds of diffracting points is accentuated. The division between odd and even planes asreflectors is the characteristic of the face-centred ANALYSIS OF LAUE PHOTOGRAPHS 217 cubic lattice. This is only another way of stating-the conclusion already arrived at. The face-centredlattice may be looked on as an extreme case of therock salt structure, in which one atom has becomenegligible in comparison with the other. The broadfeatures of the Laue photograph are a guide to theunderlying space lattice of the crystal structureexactly as are the positions of the first spectraobtained by the spectrometer. The crystal which Laue employed for exactinvestigation of his new discovery was zincblende.This crystal, as we have seen, is composed of twointerpenetrating face-centred cubic lattice

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