M�ssbauer studies of valence fluctuations

Abstract

In the last ten years the Mossbauer technique has made a considerable contribution to the research of the phenomena of mixed valencies, valence instabilities, valence fluctuations and intermediate valencies of transition elements. The sensitivity of the hyperfine interaction parameters and in particular the isomer shift, to the valency of the Mossbauer ion, enabled the research of dynamics of valence fluctuations and the temperature, pressure and composition dependence of the ionic intermediate valence state. Studies of149Sm,152Sm and153Eu in Sm1−xRxS contributed to the understanding of the outstanding insulator-metal phase transition that occurs in these systems. Studies of57Fe and151Eu in mixed valent systems yield the charge fluctuation rates of the “hopping” mechanism, contributing to the conductivity in these systems. Studies of151Eu in EuRh2, EuCu2Si2, EuPd2Si2, EuFe4Al8, and EuPd6B4 as a function of temperature and pressure reveal many aspects of the thermodynamics of intermediate valencies. Studies of systems like Eu1−xLaxRh2, EuA2−xBx, EuA5−xBx reveal strong local environment dependence of the intermediate valency. Mossbauer spectra of169Tm in TmSe,170Yb in YbAl3 and237Np in NpOs2 also display phenomena associated with fluctuating valencies.