The deformations of the ions in NaCl and AgCl crystals and the temperature parameters of ions in some alkali halides

Results of X-ray powder intensity measurements and spherical harmonic expansions have been used for investigating deformations of the ions in NaCl and AgCl crystals. It is found that within the limits of experimental accuracy the ions in crystalline NaCl are spherically symmetric. Compared with free ions, the charge around Ag in AgCl has spread farther and a part of it is located inside the Cl sphere. The electron clouds inside the Ag and Cl spheres are, however, almost spherically symmetric. It has been shown that the results of recent X-ray measurements concerning the Debye-Waller coefficients of the ions in alkali halide crystals have one prominent feature: the square of the relative vibration amplitude of an individual ion, which is proportional to the ratio B/a² (B is the Debye-Waller coefficient and a the length of the edge of the unit cell), increases when the number of electrons of its company increases. It is proposed that this dependence is mainly due to the repulsive forces between the next-nearest-neighbours.