Heavy-fermion behavior in ${\mathrm{YbCu}}_{5\mathrm{-}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$s

The phase relation and physical properties of the ${\mathrm{YbCu}}_{5\mathrm{-}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ system have been investigated as functions of temperature and composition x. Powder x-ray-diffraction experiments showed that ${\mathrm{YbCu}}_{5\mathrm{-}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ crystallizes in a single phase of the cubic ${\mathrm{AuBe}}_5$ -type structure in the range of 0.125⩽x⩽1.0. Magnetic susceptibility χ, electrical resistivity ρ, and specific heat C measurements revealed that this system belongs to a series of dense Kondo compounds, and the characteristic temperature of Kondo effect shifts to lower temperatures with decreasing x. It was also observed that the susceptibility at zero temperature, χ(0), the ${\mathrm{T}}^2$ coefficient of resistivity, A, and the electronic specific heat coefficient, γ, are enhanced with the decrease of x (0.125⩽x⩽1.0). The γ value increases monotonically from 210 mJ/mol ${\mathrm{K}}^2$ for x=1.0 to 460 mJ/mol ${\mathrm{K}}^2$ for x=0.125 with decreasing x. This substantial increase of γ with decreasing x can be explained by the 'chemical pressure' effect, which is estimated from the composition dependence of the lattice parameters. Furthermore, the coefficient A depending on 'chemical pressure' is well parallel to that for x=1.0 under hydrostatic pressures. In the composition range of 0.0⩽x⩽0.1, the system consists of two phases with the cubic ${\mathrm{AuBe}}_5$ -type and the hexagonal ${\mathrm{CaCu}}_5$ -type structures. These results mean that the ${\mathrm{YbCu}}_5$ exists with the cubic ${\mathrm{AuBe}}_5$ -type structure as well as with the hexagonal ${\mathrm{CaCu}}_5$ -type one, although only the hexagonal ${\mathrm{CaCu}}_5$ -type ${\mathrm{YbCu}}_5$ has been reported so far. The value of γ for the hypothetical cubic ${\mathrm{YbCu}}_5$ of single phase is expected to reach about 500 mJ/mol ${\mathrm{K}}^2$ from an extrapolation of the γ values of ${\mathrm{YbCu}}_{5\mathrm{-}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ (0.125⩽x⩽1.0) to x=0.