Anharmonicity and the Debye-Waller factors in BCC metals

The harmonic, quasi-harmonic and the anharmonic contributions to the Debye-Waller factors of two BCC metals-sodium and alpha iron are calculated at different temperatures by using a non-central force model proposed by authors. To estimate the effect of anharmonic terms in the crystal potential, the authors have followed the Willis anharmonic approach. A comparative study of the computed values and the experimental results reveal the fact that the anharmonic consideration in the crystal potential has greatly improved the results compared with those due to the harmonic and the quasi-harmonic approximation.