Anomalous thermal expansion of nonstoichiometric TmSe.

Crystal lattice parameters of a nonstoichiometric and a stoichiometric sample of the intermediatevalent compound TmSe have been measured as a function of temperature by x-ray and neutron diffraction techniques. The thermal expansion behavior of the stoichiometric sample is consistent with this material showing temporal valence Auctuations. The behavior of the nonstoichiometric sample differs from the first, particularly at low temperatures where we have observed an unexpected Schottky anomaly in the thermal expansion. This anomaly may be explained in terms of the random Tm-site vacancies in the lattice which can cause a preferential freezing of Tm'+ ions at some sites and Tm + ions at other sites hindering the process of on-site valence fiuctuations.