Vibrational Anisotropy

The mean square relative displacement parallel to the bond direction (parallel MSRD) is directly measured by the EXAFS Debye-Waller exponent. For crystals, the comparison of the thermal expansions measured by EXAFS and by Bragg diffraction allows one to obtain also the perpendicular MSRD. The ellipsoid of relative thermal displacements is anisotropic with respect to the bond direction for all studied systems. The quantitative study of anisotropy gives original insights on the local lattice dynamics and can be exploited to investigate the vibrational mechanism of negative thermal expansion in crystals. The basic theory is here reviewed and the results of some recent experimental studies on crystals with the diamond-zincblende structure are critically compared.