Moessbauer studies of valence fluctuations

In the last ten years the Moessbauer technique has made a considerable contribution to the research of the phenomena of mixed valencies, valance instabilities, valance 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 Moessbauer ion, enabled the research of dynamics of valence fluctuations and the temperature, pressure and composition dependence of the ionic intermediate valence state. Studies of 149Sm, 152Sm and 153Eu in Smsub(1-x)Rsub(x)S contributed to the understanding of the outstanding insulator-metal phase transition that occurs in these systems. Studies of 57Fe and 151Eu in mixed valent systems yield the charge fluctuation rates of the 'hopping' mechanism, contributing to the conductivity in these systems. Studies of 151Eu 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 Eusub(1-x)Lasub(x)Rh2, EuAsub(2-x)Bsub(x), EuAsub(5-x)Bsub(x) reveal strong local environment dependence of the intermediate valency. Moessbauer spectra of 169Tm in TmSe, 170Yb in YbAl3 and 237Np in NpOs2 also display phenomena associated with fluctuating valencies. (Auth.)