The electronic properties and structure of liquid Tl-Se and Ga-Se alloys.

Liquid alloys of the group (III) elements (Ga, In and Tl) and the group (VI) elements (S, Se and Te) show a diverse range of behaviour. It is presumed that these properties arise due to the two possible valence states of the group (III) element and the tendency for the chalcogenide element to form pairs in the liquid state. In this paper results are presented showing that liquid GaxSe1-x remains semiconducting with sigma < similar to 20 Ohm(-1) cm for all compositions between 0.4 < x < 0.5 but becomes progressively more metallic for x > 0.5. In contrast liquid TlxSe1-x remains a good semiconductor for all compositions up to x = 0.67 and then becomes more metallic. Using recent neutron diffraction data, it is demonstrated that the low conductivity of Tl-Se alloys is fully explained in terms of an ionic type semiconductor where the conductivity remains low due to the formation of charged Se-2 pairs in the liquid for x < 0.67, On the other hand, the semiconducting alloys of liquid Ga-Se are shown to be characterised as a low density tetrahedrally coordinated covalent type structure reminiscent of the solid state structures in which Ca-Ga bonding is known to occur. These results give strong clues in understanding the particularly unusual properties of liquid In-Se alloys.