Thermal vibration and melting from a local perspective

X-ray-absorption fine-structure (XAFS) measurements of lead from 10 K through and above the melting transition at 600 K have been carried out. A cumulant expansion analysis provides data on the first four moments of the radial distribution of the first shell, which are used to construct a consistent temperature-dependent radial distribution function for the solid. The distribution is one given by a simple one-dimensional anharmonic oscillator. The results also demonstrate that XAFS, a local probe, can clearly distinguish between the liquid and solid state. In the liquid, the apparent coordination number is decreased by the fraction of the time the atom is diffusing. An improved method for determining the background at low k is discussed that gives a reliable determination of the background much closer to the threshold than allowed by standard methods.