Abstract

Measurement and theoretical estimate of the anharmonic non-quadratic contribution to the Debye-Waller factor for NaCl are reported. For the experiment the Mossbauer -ray diffraction technique was used to find the elastic diffracted intensity separated from the thermal diffuse scattering. The theoretical treatment makes use of the asymptotic form of the displacement correlation functions to give simple explicit expressions for the non-quadratic term. The role of the relevant lattice dynamical parameters is discussed. The deviation from the Gaussian form of the Debye-Waller factor is shown to be large enough to be observed, and the possibility of estimating the third-order anharmonic coupling constant from such a measurement is indicated.