234 



Messrs. R. W. James and N. Tunstall on the 



Now, for a face-centred rhombohedral lattice, as calculated 

 from the geometry of the rhombohedron, the corresponding 

 ratios should be 



1 : 1-377 : 1*450 : 0*820 : 0*880, 



while for a simple rhombohedral lattice they would be 



1 : 1-377 : 1-450 : 1*651 : 1*761. 



Fig. 1. 



Ooo) 



(100) 





1 



1 





(no) 



5-4° 



10-9° 



16-6° 



o 



(mo) 



7-4° 



15 



0° 

 1 



{J 



On) I 



7-8° 





1 



15-8° 



24-3° 



4-4.° 

 ON) n 



89° 



1 



135° 





22-7° 



*_ (J 













< 



3-09 — f 



<--2-86- -y 





(110) 



(mo) 



* 



-2 2<i->, 



<-l-9l-> 







(III) 



(III) 



* 



-3-76---> 



<- 2 30 -* 







< 



*-l-89-> 

 3-51 --> 







Glancing angles and intensities for 

 wave-length 0-584 A.U. 



Spacings of planes. 



o 



Distances in Angstrom Units. 



The small glancing-angle for the first-order spectrum from 

 the (111) face indicates that the underlying structure is the 

 face-centred lattice. Assuming this to be the case, we find 

 from the glancing angle for the (100) face, that the edge of 

 each unit rhomb of the face-centred lattice has a length 

 of 6*20 A.U. 



Taking the density of antimony as 6*70, and calculating 

 the number of atoms in such a unit rhomb, we find the 

 number to be eight. (The number actually found was 7*96.) 

 Now, if one atom is associated with each corner and face 

 centre of the unit lattice, the number would be four only. 

 Thus it appears that the structure, as in the case of diamond *, 

 consists of two interpenetrating face-centred lattices. The 



* W. H. Bragg and W. L. Bragg, Proc. B,ov. Soc. A, vol. lxxxix. 

 p. 277. 



