TOF-NEUTRON DIFFRACTION STUDY OF LIQUID TL-AS(2)X(3) SYSTEMS (X-SE, TE)

Time-of-flight neutron diffraction has been measured on both liquid Tl -As2Te3 and Tl-As2Se3 systems, which exhibit the metal-non-metal trans ition at the Tl-rich composition. Liquid As2Te3 contains direct As-As bonds in the first coordination shell as well as As-Te and perhaps Te- Te bonds, while liquid As2Se3 contains only As-Se bonds in the first c oordination shell. The coordination number of component atoms in both liquids was determined to be three for As, and two for Te and Se, thro ugh a least-squares fit to the observed intensity function. The first sharp diffraction peak in S(Q) in liquid As(2)X(3) (X: Te or Se), rela ting to -As-X-As- linkages, rapidly decreases upon the addition of Tl and disappears completely in the transition composition range in both liquid Tl-As(2)X(3) systems. In the liquid Ti-As2Te3 system, As-Te bon ds are gradually replaced by partially ionic Tl-Te bonds with increasi ng Tl content, leading to the sharp metal-non-metal transition at Tl6A s2Te3. On the other hand, As-Se bonds in the liquid selenide system re main even in the vicinity of the transition composition, strongly infl uencing the electronic properties over a wide composition range.