Arrangement of Atoms in Crystals. 173 



4. In Zinc sulphide (Zinc-blende) the zinc atoms lie on a 

 face-centred cubic lattice. The sulphur atoms lie on a similar 

 lattice in such a way that every sulphur atom is surrounded 

 by four zinc atoms arranged at the corners of a regular 



Fur. lb. 



Section through A C G E. The broken circle is the projection of the 

 iron atoms centered at F and H. 



tetrahedron, every zinc atom by four sulphur atoms similarly 

 arranged. The distance ^between the centres of the zinc and 

 sulphur atoms is o 2*35 A. Taking the diameter of the sul- 

 phur atom as 2 "05 A., this gives the diameter of the zinc atom 

 as 2*65 A. 



In zinc oxide * the zinc atoms have an arrangement which 

 is that of hexagonal close-packing, The oxj'gen atoms have 

 a similar arrangement, each oxygen atom being surrounded 

 by four zinc atoms at the corners of a tetrahedron and each 

 zinc atom by four oxygen atoms. The distance between 

 * W. L. Bragg, Phil. Mag. vol. xxxix. p. 647, June 1920. 



