Magnetic properties of hexagonal YbCu5-xAlx - Crossover from intermediate valence to trivalence of Yb ion

YbCu6.5 with hexagonal CaCu5-type structure was found to exhibit enhanced P auli paramagnetism (similar to 10(-3) emu/mole) with a broad maximum in its susceptibility-temperature curve at similar to 1000 K for the first time. The Cu/Al substitution causes the maximum to shift systematically towards l ower temperature and to disappear at x=1. Meanwhile the magnitude of suscep tibility increases continuously. For x greater than or equal to1.5, Curie-W eiss behavior is established with an effective magnetic moment P-eff close to 4.54 mu (B) of free Yb3+ ion. Al substitution therefore drives the Yb io n from a nearly divalent state for x=0, via a strongly mixed-valence states for 0 <x <1.5, to a stable localized trivalent state for 1.5 less than or equal tox less than or equal to2. As a substantial evidence for the Yb inte rmediate valent state, an anomalous lattice contraction is observed as the Al content increases from x=1 to x=0.5. (C) 2001 Elsevier Science B.V. All rights reserved.