TOF-neutron diffraction study of liquid Tl-As 2X 3 systems (X: Se, Te)

Abstract

Time-of-flight neutron diffraction has been measured on both liquid Tl-As 2Te 3 and Tl-As 2Se 3 systems, which exhibit the metal-non-metal transition at the Tl-rich composition. Liquid As 2Te 3 contains direct As-As bonds in the first coordination shell as well as As-Te and perhaps Te-Te bonds, while liquid As 2Se 3 contains only As-Se bonds in the first coordination shell. The coordination number of component atoms in both liquids was determined to be three for As, and two for Te and Se, through a least-squares fit to the observed intensity function. The first sharp diffraction peak in S(Q) in liquid As 2X 3 (X: Te or Se), relating 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 2X 3 systems. In the liquid Tl-As 2Te 3 system, Ass-Te bonds are gradually replaced by partially ionic Tl-Te bonds with increasing Tl content, leading to the sharp metal-non-metal transition at Tl 6As 2Te 3. On the other hand, As-Se bonds in the liquid selenide system remain even in the vicinity of the transition composition, strongly influencing the electronic properties over a wide composition range.