Crystallography and magnetism of the heavy-fermion compound YbBiPt

Abstract

The super-heavy-fermion compound YbBiPt has the largest known linear specific-heat coe5cient y =8 Jmol ' K, 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. %'e 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. %'e 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 0.25pz. The sample contains 8.1 wt. %%u oelementa 1 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.The cubic heavy-fermion compound YbBiPt was discovered in 1991 by CanfMld and co-workers' and has a very large linear specific-heat coefficient y = 8 Jmol 'K 2, an order of magnitude more than that of typical heavy-fermion compounds and three orders of magnitude larger than that of conventional metals. YbBiPt forms in the C lb or MgAgAs structure type (space group E43trt), which is a common Heusler-alloy structure, and in which some uranium-based ternaries such as the UNiSn form. There is only one formula unit per primitive unit cell. It is well known that the conventional face-centered-cubic unit cell is not the primitive cell, and typically the simpler primitive rhombohedral cell is employed. The atoms are then distributed on three of the four sites along the [111] long diagonal of the rhombohedra1 ce11 is 1isted in Table I. All four sites have tetrahedral point-group symmetry 43m, but the sequence is not arbitrary in the sense that, if three sites are occupied, one can be thought of as an ordered vacancy, two of the sites have this vacancy as a nearest neighbor, and the fourth site is unique, in that it has no vacant neighbor sites. A previous x-ray study reported that Bi was on the unique site, and to date two band-structure calculations7's have assumed this sequence.In this paper, we describe the results of neutron and xray powder-difFraction studies. For these elements the neutron-diffraction contrast is approximately twice that of x rays and opposite in sign. We find that Pt actually occupies the special site rather than Bi. In addition, we address a number of other questions. Firstly, the @SR response is reminiscent of spin-glass behavior, and it has been suggested that this might be due to nonzero occupancy on the nominally vacant site, or to disorder on the occupied sites. We Snd no evidence for either. Secondly, there is evidence from inelastic neutron-scattering studies' '" of the crystal-Geld excitations that the cubic symmetry is broken. We Snd no evidence for this at room temperature, nor at 0.4...

High-resolution x-ray diffraction study of the heavy-fermion compound YbBiPt

“…1 are consistent with the lattice being described by a face-centered-cubic space group, which is the symmetry possessed by the lattice at T = 300 K. 9 There are also no indications of the formation of a superstructure, such as the occurrence of additional Bragg peaks upon cooling. In addition to the data shown in Fig.…”

“…9 The Yb ions are located at the 4d Wyckoff position, forming a face-centered-cubic magnetic sublattice, and the Bi and Pt are located at the 4c and 4a Wyckoff positions, respectively. 9 All three sites possess 43m tetrahedral (i.e. cubic) point-group symmetry.…”

“…However, the ratio of the integrated intensity of the broad peak to that of the narrower peak is ≈ 12 : 1, and previous experiments have estimated that the static ordered moment is 0.1 to 0.25 µ B . 9,10 Many questions regarding the nature of the low-temperature magnetism and the proposed quantum-critical point remain to be answered. For example, several previous studies have suggested that a lattice distortion occurs at T ≈ 6 K which results in a lowering of the cubic symmetry.…”

Electron spin resonance of Er3+ in YBiPt

“…Interest in this structure [3,4] has grown since the discovery by Canfield et al [5,6] that the compound YbBiPt is a cubic heavy-fermion and has the largest known linear specific-heat coefficient, 7 = 8 J/molK 2. This is ten times that of typical heavy-fermions [7] and about a thousand times that of conventional metals.…”

Low-Energy Excitations and the Electronic Specific Heat of YbBiPt

“…study [4], we have shown that the Pt ion is on the unique site, in contrast to what had been reported previousl [2]. We have conducted inelastic neutron scattering experiments using the LAM-40 spectrometer [5] at the Japanese National Laboratory for High-Energy Physics (KEK), the QENS spectrometer [6] at Argonne National Laboratory, and the Cold-Neutron Fermi-Chopper Spectrometer [7] at the National Institute of Standards Fig.…”