Testing interaction models by using x-rayabsorption spectroscopy: solid Pb

Abstract

Structural models obtained using classical molecular dynamics (MD)simulations and realistic interatomic potentials for solid metalsare tested using experimental results obtained by x-ray absorptionspectroscopy (XAS). Accurate L-edgeextended x-ray absorption fine-structure (EXAFS) measurements of Pbgrains dispersed in BN and graphite matrices have been collected fortemperatures up to the melting point. The thermal expansion of thegrains was measured by energy-dispersive x-ray diffractiontechniques and found to be coincident with that of pure Pb up to thelimit of the present measurements. L3-edge EXAFSmeasurements of solid Pb at various temperatures have been analysedusing advanced data-analysis techniques (GNXAS)based on exact spherical-wave multiple-scattering simulation of theabsorption cross-section. Realistic structural models for solid Pbwere obtained from MD simulations using an empirical pair potential(Dzugutov, Larsson and Ebbsjo (DLE)), a tight-binding (TB) square-root functional, and anembedded-atom (EA) model potential parametrized by us. Theshort-range pair distribution function g(r) reconstructed by meansof EXAFS is compared with those obtained by MD simulations. Theempirical DLE potential, originally designed for the liquid state,is too soft, showing too-large values for the average distance R,variance σ2, and skewness β. The TB and EA potentialsare both compatible with XAS data as regards the average distanceand skewness of the first neighbours. The distance variance,associated with the thermal vibration amplitudes, is underestimatedfor the TB potential, while the EA model is found to be in agreementwith XAS data. The present results are also compared with thosefrom a previous EXAFS study on solid lead, where the cumulantexpansion and a simple one-dimensional anharmonic oscillator modelwere used. The need for realistic interaction models andappropriate simulation schemes for reliable XAS data analysis isemphasized, while differences from and improvements with respect toprevious approaches are only briefly discussed.