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We present neutron diffraction results for YbInCu 4 , and Rietveld refinements of the structure. The ground state has the same structure ͑C15B͒ as the high temperature state, so that the first-order phase transition at T s ϭ40 K is indeed isomorphic, i.e., the lattice constant changes without a change of crystal symmetry. The site disorder and the diffraction linewidths decrease systematically on going from polycrystal samples with two transitions ͑at 40 and 70 K͒ to polycrystals with a single ͑40 K͒ transition to flux-grown single crystals with a sharp transition at 40 K. We argue that in site-disordered samples, the effect of doping Yb onto the In site is to increase T s and cause the transition to become continuous. ͓S0163-1829͑96͒05734-7͔

Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial offtce and in production AR. apid Communication in Physical Review 8 should be no longer than four printed pages and must be accompanied by an abstract Pag. e proofs are sent to authors.We report measurements to 6 kbar of the room-temperature compressibilities of C60 for both the fcc and sc phases using He, Ne, and Ar as pressure media. The effects of rare-gas intercalation on the fcc-to-sc transition and on the compressibilities are discussed and we compare our results with the widely disparate literature values reported for the fcc compressibility.

to be published in Phys. Rev. Lett., LA-UR-98-3125) X-ray-absorption fine-structure measurements of the local structure in UCu4Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with 24 ± 3% of the Pd atoms occupying nominally Cu sites. A "Kondo disorder" model, based on the effect on the local Kondo temperature TK of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments.

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