DEBYE-WALLER FACTORS OF ALKALI HALIDES

Using very-high-intensity ({approximately}70 Ci) {sup 183}Ta M{umlt o}ssbauer sources, we have measured the Debye-Waller factors (DWF{close_quote}s) of sodium chloride, potassium chloride, and potassium bromide single crystals for several of the (h00) and (nnn) Bragg reflections. We have used an approach which properly accounts for thermal expansion over the temperature range of our experiment, from 90 K to 900 K, about 100 K below the melting point of our crystals. We have found that a procedure used to analyze data by earlier workers leads to incorrect parameters in the Debye-Waller factor exponent, and our procedure does not require empirical parameters to account for the effects of thermal expansion. Additionally, we find three items of significance. Contrary to earlier results, we observe that the cations and the anions have identical DWF{close_quote}s in NaCl and also in KBr. We observe terms in the expansion of the DWF exponential which are quartic in the scattering wave vector {rvec Q} in NaCl and KCl, with some evidence for a Q{sup 4} term in KBr. The size of the Q{sup 4} contribution is reported and varies with the direction of momentum transfer. We also observe that the Debye temperature and the coefficient of the anharmonic Q{supmore » 2} term also vary with the direction of momentum transfer. We believe our data are the definitive evidence for a nonspherical thermal cloud in a cubic crystal; the ions have a larger amplitude of oscillation in the [h00] direction than in the [nnn] direction, contrary to the commonly held view of crystallographers that the most general form of the mean-square thermal motion is of an ellipsoid shape. {copyright} {ital 1998} {ital The American Physical Society}« less