Temperature Dependence of the Structure and Transport Properties of Liquid Thallium

Using a theta—theta diffractometer and quartz-crystal monochromatized MoKα radiation, x-ray scattering from liquid thallium has been measured as a function of temperature ranging from 350° to 700°C. The radial distribution functions (RDF) which have been obtained by Fourier inversion of the normalized intensity do not show any unusual feature and indicate a close-packed structure for the liquid thallium. The modulation-free part of the RDF in the region of small r where r is the radial distance suggests a high degree of reliability in the calculated value of atomic scattering factors of thallium and a considerable accuracy in the measured intensity. The interatomic distance r1=3.30 Å of thallium does not change with temperature, and the coordination numbers change very little from 11.5 atoms at 350°C to 10.9 at 700°C. This decrease may be explained with excess- or free-volume theory. The resistivity and thermoelectric power have been determined using the x-ray interference functions and theoretically calculated values of the pseudopotential elements. The predicted values of the resistivities Rpred are little lower than the measured values Rexpt1 (e.g., Rpred=63 μΩ·cm, Rexpt1=79 μΩ·cm at 500°C). Further, the temperature coefficient of the predicted resistivities is also smaller than that of the measured resistivities. The observed discrepancy has been attributed to the inadequacy in pseudopotential elements.