Crystalline Structure of Zinc Oxide. 649 



zinc atoms are situated, and derived from these latter by a 

 movement of translation parallel to the c axis, which brings 

 every oxygen atom into the centre of four zinc atoms 

 arranged at the corners of what is very nearly a regular 

 tetrahedron. 



The dimensions of the structure are given in fig. 2. They 

 are calculated from the axial ratio c/a = l'608 and the 

 density of zinc oxide 5'78. That they agree with the dimen- 

 sions measured by the angles of reflexion is shown by the 

 following table : — 



Plane. Calculated Spacing. Observed Spacing. 



(0001) ^=260 A.TJ. 2-58 A.TJ. 



(1010) ?_*? =2-79 A.TJ. 2-80 A.TJ. 



(1120) " r = l-61 A.TJ. 1*61 A.TJ. 



(10ll)' 246 A.TJ. 2-47 A.TJ. 



In the case of the basal plane (0001) the successive planes 

 are arranged as shown in fig. 1, oxygen and zinc atoms 

 being arranged on alternate planes. The oxygen planes 

 divide the distance between the zinc planes approximately 

 in the ratio 1 : 3, thus explaining the small second-order 

 reflexion and the large third-order reflexion. 



The first-order prism face shows an abnormally large 

 third-order reflexion, corresponding to the spacing of the 

 planes parallel to that face. The planes represent the 

 centres of equal numbers of zinc and oxygen atoms, and 

 are so spaced that the distances between successive planes 

 are alternately 0*93 A.U. and 1-87 A. U., thus reinforcing the 

 third-order reflexion. 



The symmetry of the crystal structure is dihexagonal 

 polar. The polar nature of the hexagonal axis is shown by 

 the arrangement of the planes (0001). 



Cadmium sulphide, CdS, and wurtzite, ZnS, are iso- 

 morphous. Like zincite, their symmetry is of the dihexa- 

 gonal polar type, and the axial ratio is very nearly the same 

 for the three crystals. 



ZnO a:c = 1: 1*608 



ZnS a:c = 1:1-635 



CdS a:c = 1:1-622 



Close packed structure ... a : c == I : 1*632 



Phil. Mag. S. 6. Vol. 39. No. 234. June 1920. 2 U 



