ANHARMONICITY EFFECTS ON THE EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE- THE CASE OF CADMIUM SELENIDE

Extended x-ray absorption fine structure (EXAFS) has been measured at the K edge of Se in bulk CdSe in the temperature range from 18 to 300 K. The first four cumulants have been obtained as a function of temper ature from a data analysis based on the phase difference and amplitude ratio method, taking the 18-K spectrum as reference. The effective di stributions of distances P(r,lambda) and the corresponding mean force potentials V(r) have been reconstructed from the cumulants. The potent ials exhibit a shift of the minimum position as a function of temperat ure. The temperature dependence of the first cumulant depends on both the shape and position of the anharmonic mean force potential. The the rmal expansion measured by EXAFS is larger than the one measured by di ffraction; the anharmonicity monitored by EXAFS does not reflect only the crystal anharmonicity, but is enhanced by thermal atomic vibration s normal to the bond direction. The correlation of vibrational motion is different in radial and transverse directions.