472 Wyckoff — Crystal Structure of Ammonium Chloride. 



the side of the unit cube which would contain a single 

 chemical molecule of NH 4 C1 was found to be 3.859A.U. 

 This information is of course all that is required to cal- 

 culate the value of n\ where n is the order of the reflec- 

 tion and A is the wave length of the reflected X-rays, 

 for each spot upon the Laue photographs. If this is 

 done and if the true unit cube were a larger one contain- 

 ing not one but eight chemical molecules within it, then 



Table I. Typical Laue Photographic Data 

 (for application to Figure 2). 



idices 



Distance from 



Estimated 



Wave Length 





Central Spot 



Intensity 



(nA) 



231 



2.35 cm 



7 



0.450A.U. 



231 



2.32 



7 



.445 



251 



2.65 



0.5 



.340 



431 



2.62 



1 



.363 



131 



2.72 



4 



.573 



031 



2.87 



10 



.630 



231 



375 



6 



.650 



some reflections should be found with a value of nX less 

 than 0.24, which is the smallest wave length in the X-ray 

 beam employed in these experiments. On none of the 

 photographs, one of which was given an exposure from 

 four to five times as long as usual in order to bring out 

 faint reflections, were any spots found having such a 

 low value of ?zA; the Laue photographic data are thus 

 in agreement with the powder data in assigning one 

 chemical molecule to the correct unit cell for this form of 

 ammonium chloride crystals. The approximate inten- 

 sity calculations, carried out in the usual manner, 7 are 

 readily shown to be in accord with the only structure 

 containing one molecule within the unit which the theory 

 of space-groups indicates as possible. There can conse- 

 quently be no legitimate doubt of the correctness of the 

 usual structure assigned to ammonium chloride. In 

 this structure the atoms have the following positions : 



Nitrogen atoms : \\\. 

 Chlorine atoms : 000. 



Hydrogen atoms : uuu ; uuu ; uuu ; uuu, where the 

 value of u is not determinable. The arrangement is 

 shown in Figure 3. 



7 Kalph W. G. Wyckoff, this Journal, 50, 317, 1920. 



