G. Venturini

Nouveaux isotypes de CeNiSi2 dans les systemes R-M-X (R ≡ La-Lu, M ≡ metaux des groupes 7 A 11 ET X ≡ Ge, Sn). I Compositions et parametres cristallins

Resume De nombreux germaniures et stannures isotypes de CeNiSi 2 ont ete caracterises par diffraction X de poudres et analyse a la microsonde electronique. La plupart de ces composes sont non-stoechiometriques, avec de larges domaines dhomogeneite; leurs formules sont donc RM x X 2 avec 0 x min x x max 2 de type ZrSi 2 . De simples considerations geometriques ne permettent pas dexpliquer les fortes variations des indices x min et x max , observees en fonction de la nature du constituant M. Deux nouveaux germaniures binaires sont aussi decrits: LuGe 2 et TmGe 2 , isotypes de ZrSi 2 .

Nouveaux isotypes de CeNiSi2 dans les systemes R-M-X (R ≡ La-Lu, M ≡ Metaux des groupes 7 A 11 ET X ≡ Ge, Sn). II Essai d'interpretation de leur non-stoechiometrie

Resume Cette deuxieme partie du memoire apporte une interpretation des differences de composition relevees entre les nouveaux germaniures et stannures nonstoechiometriques, RMxX2, decrits en I. Les structures de type CeNiSi2 de ces composes resultent de linsertion datomes M dans des sites pyramidaux a base carree de sous-reseaux RX2 de type ZrSi2. Cette insertion depend non seulement des tailles relatives des atomes M et de leurs sites mais aussi des interactions entre ces atomes et certains de leurs premiers voisins R. Laffinite des atomes M pour les elements R des terres rares croit avec leur numero atomique dans chacune des 4e et 5e periodes. Dans les chaines X1-X1, la force des liaisons varie aussi en fonction de M et elle est maximale dans les composes du cuivre.

Phase equilibria in the Nb-Fe-Cr-Si system

Abstract Efforts to improve the CVD process used to elaborate protective coatings against oxidation for niobium alloys have needed a good knowledge of the quaternary Nb–Fe–Cr–Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb 6.6 Fe 1.4 Cr 4 Si 8 , phase has been identified and studied using single X-ray diffraction data. The crystal structure is of a new type, derived from the Cr 11 Ge 8 -type structure (space group Pnma, Z =4, a =13.649(2) A, b =4.953(1) A, c =16.386(3) A, R = R w =5.7%). Higher temperatures stabilize the ternary Nb 2 Cr 3 Si 3 silicide in the Mn 5 Si 3 -type structure (space group P6 3 /mcm, Z =2, a =7.190(2) A, c =4.860(1) A, R =3.2%, R w =2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges.

Vacancy-ordering effects in AlB2-type ErGe2 −x(0.4 <x≤ 0.5)

In the Er-Ge system, the compostion range ErGe(2) to Er(2)Ge(3) has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(alpha0gamma)0s, X representing the centring vector ((1/2), (1/2), 0, (1/2)). In this description the modulation vector q = (alphaa* + gammac*) is shown to be a direct measure of the Ge content as ErGe(2 - alpha) (alpha falls in the range 1over 3 to (1/2)). The large composition range is achieved by extended vacancy ordering in the planar 6(3) net of Ge with subsequent relaxation.

A Neutron Diffraction and Mössbauer Effect Study of the (FeO 2 ) n Ferromagnetic Sheets in Antiferromagnetic α-Fe 2 (PO 4 )O

An unusual feature of the mixed-valance compounds alpha -Fe2(PO4)O is the presence of zig-zag chains of face-sharing octahedra running parallel to (010) and containing, alternately, Fe2+ and Fe3+ ions. These chains form sheets with the composition (FeO2)n and, because of the presence of face-sharing octahedra, have a very short Fe2+-Fe3+ distance of 2.92 AA. The magnetic properties of alpha -Fe2(PO4)O have been studied by powder neutron diffraction between 4.2 and 280 K and by the Mossbauer effect between 4.2 and 295 K. Below the Neel temperature of 218(3) K, the neutron diffraction data indicate a magnetostriction predominantly along the c-axis, magnetic moments at 4.2 K of 3.82 (4) mu B for Fe2+ and 4.26(5) mu B for Fe3+ oriented parallel to (010), and the presence of ferromagnetic sheets, containing the zig-zag chains, which are coupled into an overall antiferromagnetic structure. The net antiferromagnetic coupling is the result of a dominant 134 degrees Fe3+-0(1)-Fe3+ intersheet superexchange pathway. Ferromagnetic coupling dominates within the sheets and, as a result, the entire magnetic structure avoids any spin frustration. Above 220 K, the Mossbauer effects spectra are paramagnetic and show two quadrupole doublets with hyperfine parameters corresponding to the discrete Fe2+ and Fe3+ valence states. Below 220 K, the Mossbauer effect spectra show two magnetic components with hyperfine parameters characteristic of discrete Fe2+ and Fe3+ ions and in agreement with the corresponding paramagnetic values and the magnetic structure.

Unexpected Magnetic Ordering on the Cr Substructure in UCr2Si2C and Structural Relationships in Quaternary U-Cr-Si-C Compounds

Previous experimental and theoretical studies revealed that carbon insertion into the RCr2Si2 compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCr2Si2C (R = Y, La-Sm, Gd-Er) compounds in contrast to RCr2Si2 (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCr2Si2C compound. While specific heat and magnetic measurements suggest a Pauli paramagnetic behavior, neutron powder diffraction reveals an antiferromagnetic ordering of the chromium substructure at high temperature ( TN > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCr2Si2C carbides involving a magnetic lanthanide, is discussed in relation with the singularity of its crystal structure among the series. Moreover, the crystallographic structures and the structural stability of UCr2Si2C and of two other quaternary U-Cr-Si-C compounds (i.e., UCr3Si2C and U2Cr3Si2C3), based on the full occupancy of interstitial sites by carbon atoms, are discussed and compared to those of the related ternary intermetallics. Finally, the low-temperature form of UCr2Si2, corresponding to a displacive transformation around 210 K of the ThCr2Si2-type structure, is reinvestigated by considering a higher symmetry monoclinic unit cell ( C2/ m) instead of the previously reported triclinic cell ( P1̅). The antiferromagnetic ordering at low temperature ( TN = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.

Refinement of Sr3Rh4Pb13, isostructural with Yb3Rh4Sn13

Sr 3 Rh 4 Pb 13 a une structure cubique (cP40, groupe despace, P 3 m3n) avec les parametres suivants a=10,05 A, V=10,6 A, Z=2, D x =11,00 Mgm −3 . Determination de laffinement par diffraction RX