THERMOELECTRIC POWER FOR LIQUID Tl-S ALLOYS CORRELATED TO THE PHASE DIAGRAM

The thermoelectric power was measured in three liquid Tl-S alloys (54; 50 and 45at.% Tl) in a temperature range including the pre-freezing region. For all alloys a dual behavior was identified in the Seebeck coefficient dependence on temperature. In the high temperature range the behavior was similar to the one obtained in a previous research for liquid alloys richer in thallium (57.1 and 64.7at.% Tl), namely decrease of the Seebeck coefficient as the temperature increases (a typical behavior for a one band semiconductor). In the low temperature range down to the liquidus point, a different behavior was manifest, namely increase of the Seebeck coefficient as the temperature increases. The overheating range above the liquidus point in which this second type of behaviour was noticed became broader as the Tl content decreased. The two types of behavior have been associated with the phase transformations that take place during heating in view of melting, more specifically with the nature of the last crystals to dissolve in the melt. The nature of these crystals was different in the alloys exhibiting a unique semiconductor behavior (Tl2S crystals) and in alloys exhibiting a dual behavior (Tl4S3 or TlS crystals).