Extended x-ray—absorption fine-structure study of anharmonicity in CuBr

The extended x-ray—absorption fine structure (EXAFS) above the Cu and Br $K$ edges in CuBr has been measured at 72, 210, and 295 K. The first-shell contributions were isolated by Fourier filtering the EXAFS data, and the resulting single-shell data were analyzed using the ratio method. It is found that the third and fourth cumulants of the nearest-neighbor distance distribution are quite significant even at 210 K and cannot be ignored when making least-squares fits to amplitude ratios and phase differences. At 295 K we obtain values of (0.37±0.05) × ${10}^{-3\mathrm{}}$ ${\mathrm{\AA }}^3$ and (0.72±0.12) × ${10}^{-4\mathrm{}}$ ${\mathrm{\AA }}^4$ for the third and fourth cumulants, respectively, as well as an anharmonic contribution of (0.16±0.12) × ${10}^{-2\mathrm{}}$ ${\mathrm{\AA }}^2$ to the mean-square relative displacement. The data cannot be explained by a disorder model in which a Cu ion is allowed to occupy four equivalent off-center sites as well as its ideal position; the non-Gaussian contribution must be due to intrinsic anharmonicity. Interpretation of the results in terms of an anharmonic effective single-particle potential for the Cu ions indicates that the fourth-order potential term is just as important as the third and cannot be neglected. This model gives a reasonable description of the temperature dependence of the second, third, and fourth cumulants.