Heavy-Fermion behavior in YbCu5-xAgx

The phase relation and physical properties of the YbCu5-xAgx system have been investigated as functions of temperature and composition x. Powder x-ray-diffraction experiments showed that YbCu5-xAgx crystallizes in a single phase of the cubic AuBe5-type structure in the range of 0.125 less than or equal to x less than or equal to 1.0. Magnetic susceptibility chi, electrical resistivity rho, 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, chi(0), the T-2 coefficient of resistivity, A, and the electronic specific heat coefficient, gamma, are enhanced with the decrease of x (0.125 less than or equal to x less than or equal to 1.0). The gamma value increases monotonically from 210 mJ/mol K-2 for x = 1.0 to 460 mJ/mol K-2 for x = 0.125 with decreasing x. This substantial increase of gamma 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 less than or equal to x less than or equal to 0.1, the system consists of Two phases with the cubic AuBe5-type and the hexagonal CaCu5-type structures. These results mean that the YbCu5 exists with the cubic AuBe5-type structure as well as with the hexagonal CaCu5-type one, although only the hexagonal CaCu5-type YbCu5 has been reported so far. The value of gamma for the hypothetical cubic YbCu5 of single phase is expected to reach about 500 mJ/mol K-2 from an extrapolation of the gamma values of YbCu5-xAgx (0.125 less than or equal to x less than or equal to 1.0) to x = 0.