Ionic radii and diffraction patterns of molten alkali halides

Abstract

The mean spherical approximation for a fluid of charged hard spheres of different radii is applied to the evaluation of diffraction patterns for the whole family of molten alkali halides. The ionic radii entering the model are determined from the liquid‐state compressibility. The liquid‐state radii are shown to be characteristic temperature‐dependent lengths for each ion through the family of salts, and a good one‐parameter fit of the whole set of compressibilities at each temperature is obtained through the assumption that the radius ratios are those for basic radii yielded by the Born–Mayer theory of alkali halide crystals. The analysis also provides new evidence against the older sets of crystal ionic radii. Calculations of partial structure factors and of x‐ray diffraction patterns are then reported for all the salts and compared with the results of x‐ray and neutron diffraction experiments wherever possible. The model emerges as having semiquantitative value in the prediction of x‐ray diffraction from simple ionic fluids and in its detailed interpretation.