General behavior of chalcogenides of rare-earth metals in transition to the intermediate valence state under high pressures

High-precision measurements of the electric resistance, thermopower, and volume of TmS, TmSe, and TmTe under hydrostatic pressures up to 8.5 GPa were conducted. Comparison of the behavior of the electron-transport characteristics and volume of TmTe and SmTe in the electron transition region demonstrates a complete analogy up to the quantitative coincidence. We found that the thermopower of all samarium and thulium chalcogenides in the lattice-collapse region and during the subsequent reconstruction of the electronic spectrum obeys the universal dependence, which corresponds to the intersection of the Fermi level with the peak of the electron density of states. The results obtained testify in favor of the exciton nature of the intermediate valence state in chalcogenides of the rare-earth metals.