The concept of "reaction field" introduced by Onsager is applied to the case of a Heisenberg system in the paramagnetic phase. The procedure to obtain the static q-dependent susceptibility and pair correlation functions is reviewed and extended to calculate the magnetic specific heat. The result of this calculation is compared with experiment.Das von Onsager eingefuhrte Konzept eines ,,Reaktionsfeldes" wird auf den Fall des Heisenberg Systems in der paramagnetischen Phase angewendet. Die Prozedur, urn die statische q-abhangige Suszeptibilitat und Paarkorrelation zu erhalten, wird zusammenfassend beschrieben und auf die Berechnung der magnetischen spezifischen Wiirme erweitert. Das Resultat dieser Rechnung wird mit Experimenten verglichen.

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We present a theoretical model to explain the valence transitions of Sm in Sml-xMxS alloys, where M is a transition metal (e.g., Y or La). The f-level of Sm is described as a zero-width band with a finite Coulomb repulsion U, hybridized with the d band of the alloy, which is treated in the coherent potential approximation (CPA). The results show a change of the number of 4f electrons as a function of the concentration, which explains the valence transition observed in these alloys. 433

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