NOV. 4, 1921 



WYCKOFF: STRUCTURE ALKALI HALIDES 



431 



case by averaging the values calculated from these first three or four 

 lines. 



Unless the true s}'mmetry of these crystals is either tetrahedral or 

 tetratohedral cubic, — and there is no evidence for either of these two 

 kinds of symmetry, — the number of arrangements having four mole- 

 cules within the unit cell is limited to two: the "sodium chloride" 

 arrangement or the "zinc sulfide" arrangement. 



R • X 



Fig. 1. The unit cell of the sodium chloride grouping. 



Sodium chloride 

 The spectrograph used in all of these experiments was standardized 

 using sodium chloride, the length of the unit celP of which was taken 

 to be 5.628X10-^ cm. 



Sodium bromide 

 The positions of the spectrum lines together with the previously 

 determined value of the density clearly show four molecules to be 

 associated with the unit cell. 



hkl 



Hid) 



100(2J 



110(2) 

 Calculated density' =3.20 

 Spacing: </ioo = 5.95=^0.01 A. U. 

 Structure: XaCl grouping (Fig. 1). 



»W DuANE. Bull. Xat. Research Council 1:383. 1920. 



