Wyckoff — Crystal Structure of Ammonium Chloride. 4:69 



Art. XL. — On the Crystal Structure of Ammonium 

 Chloride; by Kalph W. G. Wyckoff. 



Introduction. — Crystals of ammonium, chloride are of 

 interest because their symmetry as previously deter- 

 mined by the conventional methods of face development 

 and etch figure formation are completely at variance with 

 the symmetry of the arrangement of their atoms as found 

 from studies of crystal structure using X-rays. 



Studies of face development and of etch figures that 

 seem fairly definite both agree in assigning crystals of 

 ammonium chloride to the enantiomorphic hemihedral 

 (plagihedral) class of cubic symmetry. 1 X-ray powder 

 measurements 2 (and a single spectrometric observation), 3 

 on the other hand, have been interpreted as yielding a 

 structure which is possessed of hemimorphic hemihedral 

 (tetrahedral) cubic symmetry. On the basis of space- 

 group reasoning it has also been shown that the powder 

 data are such as to seem to preclude the possibility of 

 any atomic arrangement possessing enantiomorphic 

 symmetry being in agreement with them. 4 In view of 

 the importance of the conclusion which must be drawn 

 from these conflicting observations, both for crystal 

 structure study and for crystallography itself, it has been 

 considered desirable to study Laue photographs of 

 ammonium chloride to see if the greater number of data 

 which they can furnish will be in agreement with the 

 powder measurements. 



Laue Photographic Data for Ammonium Chloride. 5 — 

 Clear optically isotropic crystals in the form of rectangu- 

 lar prisms several millimeters on one side and usually 

 from one to two millimeters thick were obtained by desic- 

 cating a solution of ammonium chloride containing urea. 



A measurement of the refractive index of one of these 

 crystals for the purpose of insuring their purity was 



x See P. Groth, Chemische Krystallographie, Vol. I, p. 182 (1906) for 

 references. 



2 G. Bartlett and I. Langmuir, J. Am. Chem. Soc. 43, 84, 1921. 



3 W. H. and W. L. Bragg, X-rays and Crystal Structure, p. 110 (London, 

 1918). 



4 Ealph W. G. Wyckoff, this Journal, 3, 177, 1922. 



5 Some of these experimental data Tvere collected in the Gates Chemical 

 Laboratory of the California Institute of Technology. 



