Magnetic structure of DyCo3

Oxysulfides in sheets formed by rare earths and second metals* zyxwvutsrqponmlkjihg J. FLAHAUT, P. LARUELLE. M. GUITTARD and D. CARRE zyxwvutsrqponmlkjihgfedcbaZYXWVUT Laboratoire de Chimie Minerale Structurale, associe au CNRS, UA 200 Fact&C des Sciences Pharmaceutiques, 4 Avenue de l’observatoire, 75270 Paris Cedex 06(France) Rare earth oxysulfides (RO),S combine with different metal sulfides to form quaternary compounds displaying original structures. Some of them have true three-dimensional networks, in which the two non-metals are bonded to the two metals, without selectivity in these bonds. They are for example RCrSZO, R,V,S,O,, La,,,, Ga,S,,O. Most of them have layered structures or structures closely related to the layered structures in which layers are divided into ribbons by shear mechanisms. These two series of structures have the same structural features: all the oxygen atoms are in the centre of tetrahedra formed by rare earth RE atoms. These tetrahedra share edges with other tetrahedra, and constitute (RO) sheets of tetragonal or pseudotetragonal symmetry in the layered structures, or ribbons of various stoichiometry in the shear structures. Thus, oxygen atoms are only bound to RE atoms. All the second-metal atoms M are only bound to the sulfur atoms, usually in tetrahedral coordination (Ag, Cu, Ga, In, Ge, Sn) sometimes in octahedral coordination (In, Bi). The R-0 distances are short and indicate a relatively large covalent character. In contrast, the R-X distances are relatively long, and the sheets or the ribbons are well-defined in the networks. The M-S distances have the usual values, as in other sulfides. In the layered structures, while the (RO) sheets exhibit a constant arrangement, the (MS,) sheets display widely varying structures. The (MS,) sheets can be divided into three groups according to the number of their S layers: 2 S layers: (LaO)CuS. (NdO),Ga$, crystal types; 3 S layers: (CeO),Ga,S,, (LaO),Ga, TzS, 5H, (LaO),Sn,S, crystal types; 4 Slayers: (CeO)BiS,, (LaO)GaS, crystal types. The oxysulfide UOS does not combine with such a large series of metal sulfides as the RE oxysulfides. Only compounds (UO),RS, were obtained by reaction of UOS with RS monosulfides (R I Gd to Lu and Y). In this tetragonal structure, space group Z4/mmm, uranium has the apparent valency 3.5. The RS, sheet is formed by 3 sulfur layers, in which R’+ atoms have octahedral environments of sulfur atoms. As in the preceding layered compounds, oxygen is only bound to uranium atoms inside OU, tetrahedra, and the RE atoms are only bound to sulfur atoms. Magnetic structure of DyCo,* L. JIN”, W. J. JAMES”, R. LEMAIREb and J. RHYNE’ a Department of Chemistry and Graduate Centerfor Materials Research, University of Missouri-Rolla, Rolla, MO 65401 (U.S.A.) ’ Laboratoire Louis Neel, C.N.R.S., 166X, 38042Grenoble-Cedex (France) ’ Reactor Radiation Division, National Bureau of Standards, Washington, DC 20234(U.S.A.) Powder neutron diffraction measurements have been carried out on the intermetallic compound DyCo, at 4 and 295 K. A modified Rietveld profile refinement program was used to fit the neutron data. It was found that a non-colinear magnetic structure model, coplaner in the a-c plane, gives a satisfactory fit to the data obtained at 4 and 295 K. At 4 K, the cobalt moments on the three crystallographically non-equivalent sites lie at 211.0 to the c-axis; the dysprosium moments on the two non-equivalent sites are parallel at 36.9 to the c-axis and in the opposite direction to that of the *Abstract of a paper presented at the 17th Rare Earth Research University, Hamilton, Ontario. Canada, June 9912,1986. 0022.5088/87/$3.50 Conference, ‘(;I Elsevier Sequoia/Printed McMaster in The Netherlands cobalt moments. The magnitudes of the dysprosium moments are close to the free ion value, but that of cobalt, 1.2 p++is smaller than the free ion value. At 295 K, the structure is still non-colinear, but all moments lie much closer to the c-axis. The dysprosium moments on nonequivalent sites are no longer parallel, their magnitudes are reduced to 6.4 and 6.6 pLB, respectively. The cobalt moment is reduced to 0.6 pfl and makes an angle of 192.6” with the c-axis. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Local structure and thermal gadolinium-rich alloys* T. TYLISZCZAKP, E. KOLAWAf, Institute of Physics, Jagiellonian annealing J. A. SAWICKQ processes in iron-implanted and B. D. SAWICKAS zyxwvutsrqponmlkjihgfedcbaZYX University, 30-059 Cracow (Poland) The residence sites and the features of alloying processes in iron-implanted gadolinium have been studied with Mijssbauer 57Fe spectroscopy. Samples were obtained by implantation of 50 keV 57Co or 70 keV 57Fe ions at doses from 2 x 1O’j at.cm-’ to 4 x 1016 at.cm-’ into polycrystalline Gd targets kept at room temperature. The doses correspond to iron concentrations from 0.01 to 12at.%, respectively. The spectra of as-implanted samples reveal clearly the tendency of matrix amorphisation but the nature of this process seems to be different for concentrations lower and higher than about 3at.%. Two main iron atom sites are observed: an iron atom completely surrounded by gadolinium atoms and Fe-Fe dimers. The variation in the population and local structure of each of these two environments was studied as a function of the implanted dose and annealing temperature. The investigated alloys are characterized by low stability against thermal treatment and begin to undergo structural relaxation effects at about 400K. At temperatures higher than 500 K one observes a transition, which is thought to be associated either with interstitial ordering processes of iron atoms within the gadolinium matrix or with formation of the intermetallic GdFe phase. Arguments for the former interpretation, based on the Miedema model of alloying are presented. The unusual features of the alloys under study are governed by the dramatic atomic volume mismatch and strong electron charge transfer between iron and gadolinium constituents. Effect of group II elements on the retention and superconductivity high temperature b.c.c. X-La phase* J. W. HERCHENROEDER and K. A. GSCHNEIDNER, of the JR. Ames Laboratory and Department of Materials Science and Engineering Iowa State University, Ames, IA 50011 (U.S.A.) A series of La-M alloys, where M is a Group II element, were rapidly quenched from the liquid state to metastably retain the high temperature b.c.c. y-La structure. X-ray analysis showed that the y phase could be retained with magnesium, calcium, cadmium and mercury additions but not with zinc as the alloying element. The superconducting transition temperature of the metastable y phase was found to decrease linearly with increasing solute with departures from linearity occurring at *Abstract of a paper presented at the 17th Rare Earth Research Conference, University, Hamilton, Ontario, Canada, June g-12,1986. t Present address: Institute for Materials Research, McMaster University, Canada L6S 4Ml. $ Present address: California Institute of Technology, Pasadena, California 91125, $Present address: Atomic Energy of Canada Limited, CRNL, Chalk River, Canada 0022-5088/87/$3.50 0 Elsevier Sequoia/Printed McMaster Hamilton, U.S.A. KOJ 1JO. in The Netherlands