RELATIONSHIP BETWEEN PAIR DISTRIBUTION AND CUMULATION EXPANSION FOR ANHARMONIC VIBRATIONAL SYSTEMS

A new formalism for extended X-ray absorption fine structure (EXAFS) s tudies of disordered systems has been applied to analyze the vibration al disorder of a copper foil. The XAFS spectra were taken under a redu cing atmosphere in the temperature range between 300 and 873 K. The pa rameters C-2, C-3, and C-4 for the first Cu-Cu shell were found by app lying a cumulant expansion analysis to be proportional to T, T-2, and T-3, respectively. A new effective potential and pair distribution fun ction, respectively, was introduced to describe this vibrational behav ior over the entire temperature range under study and their relationsh ips to the cumulant expansion analysis was demonstrated. The EXAFS spe ctra for these disordered systems, considering anharmonic vibrations, was described completely in terms of the parameters of the proposed pa ir distribution function (PDF). As a new variable of the PDF, the skew ness parameter, was introduced and directly related to the thermal exp ansion of copper.