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NATURE 



[July 22, 1920 



Crystal Structure.^ 



By Prof. W. L. Bragg. 



THE arrangement of the atoms in many of the 

 simpler crystalline forms has now been 

 determined by X-ray analysis. In 1912 Laue 

 published his classical research on the diffraction 

 of X-rays by crystals, and the investigations thus 

 initiated have immensely increased our knowledge 

 of the nature of X-rays, of crystal structure, and 

 of the structure of the atom. Several methods 

 of analysing crystal structure have been used. 

 Laue passed a composite beam of X-rays, con- 

 sisting of radiations of all wave-lengths over a 

 continuous range, through a thin plate of crystal, 

 and he recorded the diffracted beams by allowing 

 them to fall on a photographic plate. The results 

 he obtained were too complex to admit of ready 

 interpretation, and a simpler method was realised 

 in the X-ray spectrometer devised by W. H. 



ture. In potassium chloride each potassium atom 

 is symmetrically surrounded by six chlorine atoms, 

 each chlorine atom by six potassium atoms. The 

 atoms cannot be displaced from their positions 

 without destroying the symmetry of the crystal 

 structure ; they are therefore fixed by symmetry 

 alone. Such a crystal is analysed very simply. 

 We have only to choose between various alterna- 

 tive arrangements, each quite determinate, in 

 seeking an explanation of the observed diffraction 

 effects. 



When the symmetry does not fix the exact posi- 

 tions of the atoms, the analysis is more difficult. 

 In such cases atoms may occupy any position 

 along some axis or in some plane of the crystal 

 structure, and yet be in accord with the symmetry 

 provided the other atoms of the same kind are 



Potassium chloride, KCl. 



Calcium carbonate, calcite, CaCOs. Zinc sulphide, zincblende, ZnS. 



Fig. I. 



Aluminium oxide, ruby, AI2O8. 



Bragg, in which monochromatic X-rays are re- 

 flected from individual crystal faces. In the 

 course of a series of experiments in which the 

 author took part, the structures of a number of 

 crystals such as rock-salt, the diamond, fluor, 

 zincblende, pyrites, and calcite were determined. 

 New fields were opened up by the method of 

 analysis initiated bv Debye and Scherrer, in which 

 a beam of monochromatic X-rays is passed 

 through a mass of finely powdered crystalline 

 material, and the resulting " haloes " recorded 

 photographically. Hull has extended this work 

 to a number of substances unobtainable as large 

 single crystals such as must be used in the X-ray 

 spectrometer. By these methods a wide range of 

 crystal forms has been surveyed. 



Some crystalline structures possess symmetry of 

 a high order,, examples being potassium chloride 

 and zincblende, models of which are shown in 

 Fig. I. In such cases as these every atom occu- 

 pies a symmetrical position in the crystal struc- 



1 Discourse delivered at the Royal Institution on Friday, May 28. 

 NO. 2647, VOL. 105] 



given corresponding positions. In the structure 

 of the ruby, AI2O3 (Fig. i), the unit of which the 

 structure is composed consists of a pair of 

 aluminium atoms surrounded symmetrically by 

 three oxygen atoms. The distance apart of the 

 aluminium atoms along the axis joining their 

 centres, and the distance of the oxygen atom from 

 this axis, are both indeterminate in so far as the 

 crystalline symmetry is concerned, and their exact 

 values must be deduced by the X-ray analysis. It 

 is these indeterminate parameters which make a 

 crystalline structure complex. 



The problem is simplified by regarding the 

 atoms in a crystal as a set of spheres packed 

 tightly together. This manner of regarding the 

 structure was proposed in 1907 by Barlow and 

 Pope, who assigned to the sphere representing an 

 atom a volume proportional to its valency, and 

 by packing these spheres together as closely as 

 possible they obtained structures which accounted 

 for crystal forms. We now know the structure 

 of the crystals dealt with by Barlow and Pope, 



