188 R. W. G. Wyckoff — Symmetry and Crystal 



Aet. XVIII. — The Symmetry and Crystal Structure of 

 Zinc Bromate Hexahydrate, Zn(Br0 3 ) 2 .6H 2 0; by 

 Ralph W. Gr. Wyckoff. 1 



[Contribution from the Gates Chemical Laboratory of the California Insti- 

 tute of Technology, No. 16.] 



Introduction. 



This paper has the two-fold purpose of adding confir- 

 mation to the previously assigned structure of nickel 

 nitrate hexammonate 2 by the study of an analogous 

 compound and, more especially, it is intended to be an 

 illustration of the application of those criteria for distin- 

 guishing between the cubic spa'ce groups which are 

 described in the preceding article^ 



Excellent crystals of Zn(Br0 3 j 2 .6H 2 0, mostly octahe- 

 dral in habit, are formed from solutions both by slow 

 cooling and by slow evaporation. The crystals that grow 

 from a cooling solution usually exhibit a slight anoma- 

 lous double refraction with sectoring. 4 The Laue photo- 

 graphs to which these crystals give rise do not, however, 

 show any anomalous effects. Completely isotropic speci- 

 mens are obtained by gradual evaporation. 



Zn(Br0 3 ) 2 .6H 2 is one of a group of isomorphous 

 crystals to which belong the chlorates of nickel, cobalt 

 and probably copper, and the bromates of nickel, cobalt 

 and magnesium. 5 



The Structure of Zinc Bromate Hexahydrate. 



A reflection photograph from the octahedral face 

 combined with an estimation of the density of the salt 

 indicates that four chemical molecules are to be asso- 

 ciated with the unit cube. 6 The length of the side of this 

 unit was found to be 10.31 A.U. (10.31 X 10- 8 cm.). 



Laue photographs were prepared through both octahe- 



1 Member of the Staff of the Geophysical Laboratory of the Carnegie 

 Institution of Washington. 



2 Ealph W. G. Wyckoff, Jour. Am. Chem. Soc, June, 1922. 



3 See page 175 of this Journal. 



4 Marbach, Poggendorffs Ann. d. Phys., 99, 465, 1856. 

 5 P. Groth, Chemische Krystallographie, II, p. 112, Leipzig, 1908. . 

 fl Ealph W. G. Wyckoff, Jour. Am. Chem. Soc, 42, 1100, 1920 ; Ealph W. G. 

 Wyckoff and Eugen Posnjak, ibid. 43, 2292, 1921. 



