XAFS AND THERMAL AVERAGING

XAFS measures rho(r), the distribution of the magnitude r of the relat ive vector displacement (r) over right arrow between a center probe at om and its neighbors. The related rho((r) over right arrow) of the dis tribution of the vector distance between the probe atom and its neighb ors defines a pair potential V((r) over right arrow) which is thermall y averaged over the position of all of the other atoms in the material being investigated. It is shown that there are subtle differences bet ween rho(r) and rho((r) over right arrow) which lead to a new correcti on term to XAFS measurements necessary to obtain the correct thermal e xpansion of interatomic distances. Besides measuring rho((r) over righ t arrow), XAFS allows the determination of both rho((3) over right arr ow) and V((r) over right arrow) which include sigma(perpendicular to)( 2), in addition to the usual sigma(z)(2), the mean squared vibrational disorder perpendicular and along the line between the average positio n of the pair of atoms, respectively.