Arrangement of Atoms in Crystals. 



171 



in crystalline structure, and not by variations in the space 

 occupied by the atoms. 



This additive law is only intended to be regarded as a 

 working approximation, an aid to the analysis of complex 

 structures. In analysing such a structure, various arrange- 

 ments of the atoms have to be tried to explain the intensities 

 of the reflected spectra. It will be shown that, when mar- 

 shalling the atoms together, each atom must be given a 

 certain space in the structure, so that two atoms may not be 

 placed closer together than a distance equal to the sum of the 

 radii of the spheres representing them. This greatly facili- 

 tates the determination of the parameters, which are con- 

 fined to a much more limited range. 



The diameter of the sphere representing an atom is, for 

 the sake of brevity, called the diameter of the atom, and is 

 expressed in Angstrom units (A=10~ 8 cm.). 



269 A" > 



Unit of Iron Pyrites Structure. 



3. In the Iron Pyrites structure*, the iron atoms are 

 situated on a face-centred cubic lattice. If the unit cube of 

 this lattice is divided into eight smaller cubes, each of these 

 latter will have an iron atom situated at four of its eight 

 corners. Figure la represents such a unit of the structure 

 * W. L. Brag-g-, Proc. Roy. Soc. lxxxix. (Nov. 19] 3). 

 N 2 



