PHYSICAL REVIE%' VOLUME 33, NUMBER 5 Magnetic structure of the heavy-fermion compound MARCH 1986 U2Znt7 D. E. Cox, G. Shirane, and S. M. Shapiro Physics Department, A Tcf T Brookhaven National Laboratory, Bell Laboratories, Upton, New York G. Aeppli 600 Mountain Avenue, Murray Hill, New Jersey 07974 Z. Fisk and J. L. Smith Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 l. Kjems Risd National Laboratory, Roskilde, Denmark H. R. Ott Laboratorium I'ur I estkorperphysik, Eidgenossische Technische Hochschule-Honggerberg, 8093 Zurich, Switzerland (Received 26 September 1985) The phase transition of U2zni7 at 9. 7 K has been investigated by neutron powder diffraction. The transi- tion corresponds to the onset of antiferromagnetic order where the U moments are oriented antiparallel to their nearest neighbors within the basal planes and the near neighbor along the c axis of this rhombohedral compound. At 5 K, the ordered moments lie within the basal planes and are of magnitude (0. 8+0. 1) p, &, which is substantially below the paramagnetic moment of 2.25', &/U atom given by high-temperature sus- ceptibility data. The list of heavy-fermion systems, which display ex- traordinarily large ( & 400m, ) effective conduction electron masses as deduced from the low-temperature specific heat, and metals remaining now includes both superconductors Recent magnetic paramagnetic to the lowest temperatures. susceptibility and specific-heat measurements suggest that there are also heavy-fermion systems which undergo transi- tions to magnetically ordered ground states. ' In this paper, we present the first neutron-scattering determination of magnetic order in a heavy-fermion system, U2Zni7. The essential results are as follows. First, the ordering, which 10 K, is exceedingly simple, with the mag- sets in at T~ netic unit cell identical to the nuclear unit cell, and the mo- ments associated with the two U atoms in the primitive unit cell oriented in opposite directions. Second, the ordered moment is (0. 8+0. 1)p, s/U atom which is well below the moment of 2. 25@, s /U atom deduced from the high- temperature (bulk) susceptibility. U2Zni7 was made by heating the appropriate amounts of U and Zn to 1050'C in an evacuated BeO crucible. Ap- proximately 20 g of material were crushed into a coarse pounder and loaded into a cylindrical aluminum sample hold- er in an atmosphere of helium. Data were collected at the Brookhaven High Flux Beam Reactor with 2.353-A neu- trons from a pyrolytic-graphite monochromator in the (002) filter suppressed higher-order setting. A pyrolytic-graphite wavelengths. For the structure determination at 15 K ( T~), a pyrolytic-graphite analyzer in the (004) setting was used in order to optimize the resolution at higher scattering angles. The collirnations were 20'-open-40'-20' for in-pile, monochromator-sample, sample-analyzer, and analyzer-detector, respectively. The resulting diffraction pattern showed a number of weak impurity peaks in addi- tion to those characteristic of U2Zni7. The major impurity was identified as Zn, estimated to be about 5'/0 by weight. The remaining peaks were an order of magnitude weaker «1% of the strong U2Zn~7 peaks) and (with intensities could not be identified. The d spacings are listed in the cap- tion to Fig. 1, and do not correspond to those of UZni2 or e-U. At room temperature, U2Zn&7 (Ref. 3) has the Th2Zn~7- type structure, which is one of a series of ordered structures [e. g. , Pu3Zn22, UZnt2 (Ref. 5)] which can be derived from that of CaCu5. In U2Zni7, Zn occupies the Cu sites and U is ordered on two-thirds of the Ca sites, the remaining one- third being replaced by pairs of Zn atoms about 2. 6 A apart. The ordered structure has rhombohedral symmetry, space group R3m, with hexagonal lattice constants v 3a and 3c with respect to those of the parent CaCu5-type cell. Be- cause the structural parameters for U2Zni7 near its 10-K transition were unknown, we carried out a Rietveld analysis of the data sho~n in Fig. 1. Regions around the impurity peaks and Al reflections from the sample holder were ex- cluded from the refinement, and background contributions were estimated by interpolation between values obtained by averaging over regions where no Bragg peaks were present. Table I displays the lattice parameters and atomic coordi- nates given by the profile refinement. In the upper frame of Fig. 1, the solid line represents the calculated profile which best fits the data, while in the lo~er frame, the difference between the calculated and observed profiles is shown. The results are in exce11ent agreement with a room-temperature x-ray scattering determination of the structure. Belo~ 10 K some very weak additional scattering at some of the low-angle nuclear peak positions was observed. To gain more intensity tics to be obtained and thus allow adequate counting statis- in a reasonable period of time, the
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May 14, 2024
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