X rays and crystal structure (1915) (14755053146)

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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in both directions and iscommon to a whole string of cubes. We may think of the sulphur atom as initially ina symmetrical position at the cube centre. It lieson one of these chosen diao^onals, the diagonal havino-an iron atom at one end and an empty corner at theother. To arrive at the actual crystal structure thesulphur atoms are now displaced by equal amountsalong the diagonals, and it remains to be found byhow much. So long as the typical sulphur atomlies on the chosen diagonal symmetry alone givesno reason why it should be at one place more 132 ANALYSIS OF CRYSTAL STRUCTURE than another. Its exact position must be found byquaittitative measurements of the strengths of thespectra. Placing the sulphur atom at different pointsalong the diagonal, it is possible to calculate theoreti-cally the relative intensities of the spectra, in theway employed above in the case of rock salt andzincblende. These theoretical values are then com-pared with the actual spectra obtained, and it appears
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Fig. 41. that there is only one position for the sulphur atomswhich explains the facts. A first approximation to the position of the sulphuratoms can be got from the (100) spectra. The firstspectrum is large, the second and third are too small todetect, and the fourth and the fifth easily measurable.The iron atoms lie on (100) planes whose distanceapart is that of two opposite sides of the cube inFig. 40 (b\ and the sulphur atoms lie betweenthem. If the distance between sulphur planes andiron planes were (^(,oo)/4j the sulphur planes would ANALYSIS OF CRYSTAL STRUCTURE 133 tend to destroy the second spectrum (cf. diamond).If this distance were </(,^)/6, it would tend to destroythe third spectrum. Since it is actually found thatboth have become too small to observe, the distancemust be about d^^^^/^. In order that it may have this value, the sulphuratom must be displaced along the diagonal until itdivides it in the ratio 1:4. As far as these planesgo, the displacement may take place