Accession Number : AD0652196

Title :   ATOMIC GEOMETRY OF THE ALKALI HALIDES.

Descriptive Note : Rept. for Jun-Oct 66,

Corporate Author : AIR FORCE MATERIALS LAB WRIGHT-PATTERSON AFB OHIO

Personal Author(s) : Bradstreet,Samuel W.

Report Date : FEB 1967

Pagination or Media Count : 71

Abstract : Current data for the alkali halides (interatomic distances in crystal and vapor, specific gravities, melting temperatures, and elastic moduli) are brought together to show that several mutually consistent ionic and covalent radius values can be developed which, interpolated on the basis of ionicity, will additively describe observed interatomic distances within the required accuracy. Some of the properties of the alkali halides are compared to the properties which would be displayed by a rock-salt structure of ideal octahedral packing. In this way radius ratios can be derived; together with measured interatomic distances these serve to generate unique values for metal and halogen radii for each salt, and these in turn can be extrapolated to obtain unique ionic radii for the halogens and ionic and covalent radii for the alkali metals (for which the latter differ significantly from half the observed interatomic distance in the metals). It is concluded that the principle of additive radii is incapable of universal application since the overlap repulsion term will be influenced by the spatial requirements of packing efficiency. It is suggested that, for the alkali halides at least, correction for this can be provided by an added 'spacing' factor which depends only on the size of the host atom and on the radius ratio.

Descriptors :   (*ALKALI METAL COMPOUNDS, *CRYSTAL STRUCTURE), CRYSTAL LATTICES, MOLECULAR ASSOCIATION, IONS, ATOMIC PROPERTIES, HALIDES, CHEMICAL BONDS, MELTING

Subject Categories : Crystallography
      Atomic and Molecular Physics and Spectroscopy

Distribution Statement : APPROVED FOR PUBLIC RELEASE