Evolution of a magnetic state inYbCu5−xGax

Substitutions of Cu by Ga in ${\mathrm{YbCu}}_5$ stabilize the hexagonal ${\mathrm{CaCu}}_5$ structure and cause a crossover from a divalent behavior of the Yb-ion in ${\mathrm{YbCu}}_5$ to an almost 3+ state of Yb in ${\mathrm{YbCu}}_3$ ${\mathrm{Ga}}_2$. This gain of magnetism is accompanied by a noticeable enhancement of the electronic contribution to the specific heat. External pressure transforms those compounds, exhibiting intermediate valence, to a state which is best described in terms of the Kondo lattice model.