Crossover from First-Order Valence Transition to Kondo Behavior in C15b-Type YbCu _5-xIn_x System

YbCu5 and YbCu4In forms a solid-solution (YbCu5-xInx) with cubic AuBe5-type (C15b) structure in the In-concentration range 0.1 less than or equal to x less than or equal to 1; and below x = 0.1, it coexists with a hexagonal CaCu5-type YbCu5 phase under ambient pressure. The magnetic susceptibility and high-field magnetization measurements reveal that the first-order valence transition in YbCu4In shifts to higher temperature and becomes broadened significantly with decreasing In-content, and simultaneously the delocalized mixed-valent state of Yb ion (x = 1) shows a continuous change toward the localized trivalent state (x = 0). The electrical resistivities of YbCu5-xInx display various temperature-dependencies: ranging from a pronounced drop at x = 1, via a minimum behavior for 0.4 less than or equal to x less than or equal to 0.7, to a negative logarithmic increasing behavior (-lnT) for x less than or equal to 0.3. Above results suggest that YbCu5-xInx goes through a crossover from a valence fluctuation system to a dence Kondo one with decreasing x. At x = 0, the formation of Kondo lattice is predicted.