CRYSTALLOGRAPHY AND MAGNETISM OF THE HEAVY-FERMION COMPOUND YBBIPT

The super-heavy-fermion compound YbBiPt has the largest known linear s pecific-heat coefficient gamma = 8 J mol-1 K-2, and the source of this enormous ''electronic'' specific heat is of great current interest. H ere we describe neutron-diffraction studies that indicate its previous ly reported crystallographic structure to be incorrect. We find that t he Pt atom is on the unique site and can be thought of as an interstit ial in a fictitious rock-salt structure YbBi, which can in turn be tho ught of as an ordered form of elemental bismuth. We find no evidence o f disorder between sites, occupancy on the nominally vacant site, nor for any tetragonal or rhombohedral distortions or displacements. Furth ermore, any ordered magnetic moment at low temperature must be less th an 0.25mu(B). The sample contains 8.1 wt.% elemental Bi, and if this i s typical of other samples, the previously published values for molar susceptibilities and specific heats should be scaled up by this amount to obtain the intrinsic properties of YbBiPt alone.