Mössbauer studies of valence fluctuations

In the last ten years the Mössbauer 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 Mössbauer 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 Sm_1?xR_xS 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 Eu_1?xLa_xRh₂, EuA_2?xB_x, EuA_5?xB_x reveal strong local environment dependence of the intermediate valency. Mössbauer spectra of¹⁶⁹Tm in TmSe,¹⁷⁰Yb in YbAl₃ and²³⁷Np in NpOs₂ also display phenomena associated with fluctuating valencies.