184- Wyckoff — Crystal Structure of Silver Oxide. 



Art. XIV. — The Crystal Structure of Silver Oxide 

 (Ag % 0) ; by Ralph W. Gr. Wyckoff. 



Introduction. — Little is known of the crystallography 

 of silver oxide (Ag 2 0) other than that it crystallizes as a 

 powder in microscopic isotropic crystals and can be grown 

 as small octahedrons. 1 It is thus impossible to assign 

 silver oxide crystals to any particular one of the classes 

 of cubic symmetry. 



Diffraction Data and the Number of Molecules within 

 the Unit Cell. — Because of the difficulty of producing 

 other than microscopic crystals the available diffraction 

 data have been limited to those to be had from powder 

 reflections. Powder photographs obtained in the custom- 

 ary manner showed lines of marked intensity. Measure- 

 ments (upon two separate films) of the first four of them 

 — all that are required to establish the structure of this 

 crystal — are given in table 1. 



The density of silver oxide 2 has been determined as 

 7.52. Because of the impossibility of immediately identify- 

 ing the different lines as reflections from particular planes 

 of atoms, the indices of the planes causing these reflections 

 and the number of chemical molecules to be associated 

 with the unit cell (or more precisely, the value of m/n 3 ) 

 can be obtained only by assuming different values for m, 

 calculating therefrom the spacings of simple planes in the 

 corresponding unit and seeing whether the observed 

 reflections can be identified in positions with some of these 

 possible planes. Since as m is varied the resultant spac- 

 ings change as the cube root, while the relative spacings of 

 different planes stand in the ratio of the square roots, it 

 should theoretically always be possible to make such an 

 identification uniquely. 3 A determination of m for silver 

 oxide by this procedure showed that if m = 2 the reflec- 

 tions observed in the photographs correspond with simple 

 planes (see table 1). Since, furthermore this agreement 

 would be equally well obtained from any unit having the 



1 Vogel, Poggendorffs Ann. 118, 145, 1863. 



2 Biedermann, Chem. Kalender, 1, 53, 1914. 



3 The actual spectrum lines are so wide, however, that such a manner of 

 determining the number of chemical molecules in the unit need be entirely 

 satisfactory only in case the correct indices of the planes actually producing 

 the reflections are simple. 



