JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 11 November 4, 1921 No. 18 



INORGANIC CHEMIvSTRY.— r/^t- crystal structures of the alkali 

 halides} I. Ralph W. G. Wyckoff, Geophysical Laboratory, 

 Carnegie Institution of Washington. (Communicated by Arthur 

 L. Day.) 



Introduction. — A knowledge of the cr^^stal structures of a large 

 number of chemical compounds will furnish information concerning 

 the nature of the atoms themselves and of their manner of combination 

 one with another. Though the structures of most complicated com- 

 pounds, especially those having a symmetry that is other than cubic 

 or rhombohedral, cannot yet be obtained with certainty and exactness 

 because of the absence of precise knowledge concerning the laws of 

 scattering, there are many simple substances in which the arrangement 

 of their constituent atoms can be determined with all of the assurance 

 of correctness with which the structure of any crystal can now be 

 obtained. Of these substances the simplest, aside from the elements 

 themselves, are compounds of the type RX, where R is an electro- 

 positive atom and X is a more electronegative (electron-seeking) atom 

 of equal valence. This paper furnishes an account of the study of 

 the crystal structures of one class of the compounds RX, the alkali 

 halides. 



The possibility, which seems to arise from a consideration of the 

 structures of simple crystals, of assigning definite sizes to the various 

 atoms and building up crystals by the close packing of these atoms 

 has already been given especial attention.- Most of the data upon 

 which this work was based were obtained by assuming a particular 

 arrangement of the atoms in the crystals and by calculating the di- 

 mensions of the unit cells with the aid of the published values of the 

 densities of the crystals. In view of the unreliable character of these 

 density determinations and because of the inabiUty to choose between 

 the different structures that are possible for these simple crystals, 



' Received October 10, 192L 

 2 W. L. Bragg. PhiL Mag. (6) 40: 169. 1920. 



429 



