Electronic structure of Yb, Ag, and Cu in the heavy-fermion system YbCu_5−x Ag_x

Abstract

The method of shifts of x-ray diffraction lines is used to study the electronic structure (the populations of 4f states in Yb, 5s states in Ag, and 4s states in Cu) in the heavy-fermion system YbCu_5−x Ag_x (0≤x≤1, T=300 K for Cu and Ag and 77, 300, and 1000 K for Yb). In the cubic phase (AuBe₅-type structure), Yb is shown to exist in the state with a noniteger valence whose magnitude is independent of the composition and is equal to $\bar m_{cub} = 2.91 \pm 0.01$. At x<0.125, in the two-phase region (a mixture of cubic AuBe₅-type and hexagonal CaCu₅-type phases), the magnitude of m decreases with decreasing x. Based on the experimental values of m in the cubic and hexagonal phases in the two-phase region, the valence of Yb in the hexagonal phase was found to be $\bar m_{hex} = 2.71 \pm 0.04$. With increasing temperature in the range of 77–1000 K, a linear decrease in m for the samples from the cubic-phase field and a linear increase in m for the samples from the two-phase field is observed. At T=1000 K, the valence of Yb in the cubic and hexagonal phases is virtually the same: m _cub=2.83±0.02 and m _mix=2.78±0.02. The cubic phase exhibits a composition-independent increase in the population of 5s states of Ag (in comparison with the metal) $\overline {\Delta n} _{5s} (Ag) = 0.69 \pm 0.07$ electron/atom and a simultaneous linear increase in Δn _4s (Cu) from about 0.1 electron/atom for x=1 to about 0.3 electron/atom for x=0.2. The difference in the behavior of the effects of Δn _s for Cu and Ag is explained by the specific features of the crystal structure of YbCu_5−x Ag_x. It follows from the analysis of microscopic and macroscopic properties that YbCu_5−x Ag_x is a system with an intermediate valence and, correspondingly, the increase in the effective mass of electrons in it is related to the shift of the 4f-electron level to the Fermi level (delocalization). The effect of the increase in the population of s states in the partners of Yb is explained by the fact that, upon the transition into the state with an intermediate valence, the 4f electron of Yb is hybridized with s electrons of neighboring atoms of Ag and Cu rather than with the electrons of Yb itself.