Anharmonic interatomic potentials of octahedral Pt-halogen complexes studied by extended x-ray-absorption fine structure

The temperature dependence of Pt LIII- and I K-edge extended x-ray-absorption fine-structure spectra of octahedrally coordinated Pt X₆^2-(X=Cl, Br, and I) complexes has experimentally been investigated not only for the first-nearest-neighbor (NN) shells but for higher NN ones. The second- and third-order cumulants of a radial distribution function in the octahedral system have been evaluated quantum mechanically by employing the interatomic potential including the third-order force constants. The experimentally obtained second-order cumulants agree with the values expected by the vibrational data and the third-order cumulants have successfully given the anharmonic force constants. Contribution of the bending motions to the cumulants of higher NN shells is discussed in detail. {} 1996 The American Physical Society.