We report a systematic study of the face-centered-cubic compounds YbXCu 4 (XϭAg, Au, Cd, Mg, Tl, and Zn͒, as well as their corresponding nonmagnetic analogues LuXCu 4 . X-ray diffraction, heat capacity, magnetic susceptibility, high-field magnetization, electrical resistivity, Hall effect, and L III -edge absorption measurements have been performed. The compounds have Kondo temperatures that range from about 10 K to nearly 1000 K. Although the single-impurity Kondo model qualitatively describes the physical properties of these materials, the quantitative details are not well described and the quality of the fits varies strongly from compound to compound. Compound-to-compound variations in crystal-electric fields, effective valence, and the strength of f-ligand hybridization effects, as well as the influence of intrinsic disorder, may help to explain these discrepancies. ͓S0163-1829͑99͒07209-4͔

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An unrealistically large value of the Gruneisen parameter is required to explain the valence transition which occurs at 42 K in YbInCu4 as due to a Kondo Volume Collapse. A hint as to the origin of the transition lies in the large change in carrier density which occurs at the transition, from trivalent semimet~lic behavior at high tem~rature to mixed valent metallic behavior at low temperature. In this paper we report measurements of the Hall coefficient RH for temperatures in the range 15-300 K for a series of RXCu4 compounds (R = Yb, Lu and X = Au, Zn, Cd, Mg, Tl) that form in the cubic C15b structure. For all X the Hall coefficients are small (-lo-" m3/C!) so that the transport appears to be metallic. The observation that low carrier density is unique to RInCu4 leads us to hypothesize that the valence transition (which is also unique to YbInCus) is connected with the existence of a quasigap, which is a common feature of the band structure of RXCu4. The quasigap allows for two competing hybridization states of the 4f electrons: a small TK semimetallic state and a large TK metallic state.

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