UC Irvine

We report measurements of the low-temperature specific-heat coefficient (Formula presented) cell volume (Formula presented) Hall coefficient (Formula presented) and valence (Formula presented) [where the Yb hole occupation (Formula presented) was determined from (Formula presented) x-ray absorption] of single crystals of (Formula presented) Alloying (Formula presented) with Ag increases the temperature (Formula presented) of the first-order isomorphic phase transition and causes it to terminate at a critical point at (Formula presented) and (Formula presented) The variation of (Formula presented) near the critical point is well described by a mean-field equation of state. The phase transition involves a large change in the Kondo temperature, and the transition temperatures (Formula presented) are of order of the Kondo temperatures (Formula presented) of the high-temperature state. The cell volume is found to vary proportionally to (Formula presented) At low temperatures, well away from the transition, the Wilson ratio of the susceptibility χ(0) and specific heat coefficient γ falls within 20% of the value predicted for a Kondo impurity, and (Formula presented) and χ(0) are roughly proportional as predicted from the Anderson model. The temperature dependence (Formula presented) for temperatures away from the phase transition also fits the predictions of the Kondo model. The small volume discontinuity (Formula presented) observed at (Formula presented) suggests that the phase transition is not due to a Kondo volume collapse. The large Hall coefficients (Formula presented) observed for (Formula presented) and (Formula presented) suggest instead that a low carrier density in the high-temperature state plays a key role in the phase transition. © 1997 The American Physical Society.