Temperature induced valence transitions and the anomalous thermal expansion of chemically collapsed SmS

Abstract

Using a simple ionic model and thermodynamic arguments, the thermal expansion of SmS at high pressures is analysed. In this manner it is shown that the experimentally observed temperature-induced phase transition, from a metallic state with intermediate valence, to a semiconducting state, both on heating and cooling, can be accounted for. A formula is derived which enables the intermediate valence of the Sm ions, corresponding to extrema on the thermal expansion curves, to be deduced. The nature of the transitions and the critical temperature are also discussed.