Crystallochemistry and Kondo-like behaviour of the thorium and uranium arsenoselenides

We have grown single crystals of uranium and thorium arsenoselenide and ars enosulphide by the chemical vapour transport method. The crystals show a Ko ndo-like effect and its contribution to the resistivity was estimated by th e determination of the reciprocal residual resistivity ratio (RRRR) rho (4, 2)/rho (300). The ratio varied from 0.70 (for UAsS) to 2.10 (for ThAsSe). A high resolution transmission electron microscopy study of ThAsSe crystals reveals a perfect ordering of cations. The X-ray diffraction examination sh owed that the unit cells of the examined crystals belong to a tetragonal sy stem with the PbFCl-type structure (space group P4/nmm (no. 129)). Anomalou sly large anisotropic displacement factors have been observed for all the a toms composing the compounds examined. An interdependence between the displ acement factor and RRRR has allowed to ascribe both phenomena to the same s ource. It indicates that the two-level system Kondo model is most suitable for the theoretical description of the systems. (C) 2001 Elsevier Science B .V. All rights reserved.