186 Wychoff — Crystal Structure of Silver Oxide. 



Arrangement (c) : This grouping, deducible from the 

 space group 8 , is simply a twice scale enlargement of 

 arrangement (a). 



Arrangement (d) : In this arrangement, derived from 

 Oh 10 , the oxygen atoms lie in unique positions, the silver 

 atoms upon trigonal axes. 



The Choice of the Correct Structure. — An approximate 

 calculation of the intensity of reflection from any plane 

 for any of these possible structures can be made by the 

 method already outlined. 6 For this purpose use was 

 made of the expression 



Ioc(A 2 + B 2 ). (Vn) 2 - 35 -p, 



where I is the intensity of a line in the powder photograph, 

 p is the number of planes of the reflecting form, d/n is the 

 ratio of the spacing of the reflecting planes to the order of 

 the reflection and (A 2 + B 2 ) has the meaning previously 

 given to it. 



Arrangement (a) : For this grouping 

 If n is odd : 



When the indices of the reflecting plane are 2 odd 

 and 1 even, 



A = 20, B = 0; 

 Indices: 2 even, 1 odd, 



A = 0, B = 0; 

 Indices: all odd, 



A = 0, B = + _4 Ag. 

 If n is even : 



Indices: 2 odd, 1 even, 



A = 20 + 4 Ag, B = 0; 

 Indices: 2 even, 1 odd or all odd, 



A = 20-f 4 Ag, — when n = 2, + when 

 n =4, B = 0. 



In these computations and Ag, the scattering powers 

 of oxygen and silver, respectively, are taken equal to the 

 atomic numbers. A comparison between the estimated 

 intensities of the four principal lines of the photograph 

 and the intensities of these lines calculated for this 

 arrangement is given in table 2 ; the other planes appear- 



6 Balph W. G. Wyckoff, this Journal, 2, 239, 1921 ; Jour. Wash. Acad. Sci. 

 11, 429, 1921. 



