Crystallography and magnetism of the heavy-fermion compound YbBiPt

The super-heavy-fermion compound YbBiPt has the largest known linear specific-heat coefficient [gamma]=8 J mol[sup [minus]1] K[sup [minus]2], and the source of this enormous electronic'' specific heat is of great current interest. Here we describe neutron-diffraction studies that indicate its previously reported crystallographic structure to be incorrect. We find that the Pt atom is on the unique site and can be thought of as an interstitial in a fictitious rock-salt structure YbBi, which can in turn be thought of as an ordered form of elemental bismuth. We find no evidence of disorder between sites, occupancy on the nominally vacant site, nor for any tetragonal or rhombohedral distortions or displacements. Furthermore, any ordered magnetic moment at low temperature must be less than 0.25[mu][sub [ital B]]. The sample contains 8.1 wt. % elemental Bi, and if this is 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.