B. Roques

Nouveaux stannures ternaires: MMn6Sn6 (M = Sc, Y, Sm, Gd-Tm, Lu) ET ScFe6Sn6

Abstract New stannides with formula MMn 6 Sn 6 (M = Sc, Y, Sm, Gd-Tm, Lu) and ScFe 6 Sn 6 have been prepared at 973 K. They are isostructural with HfFe 6 Ge 6 , except SmMn 6 Sn 6 which is isostructural with YCo 6 Ge 6 , a disordered variant of HfFe 6 Ge 6 . The manganese compounds of HfFe 6 Ge 6 -type are magnetically ordered at room temperature.

De nouveaux isotypes de U2Co3Si5 ou Lu2Co3Si5 dans les systems R-T-Ge (R = Elements Des Terres Rares; T = Ru, Co, Rh, Ir). Supraconductivite de Y2Ir3Ge5

New ternary germanides R2T3Ge5 have been prepared at 1173 K. From their X-ray powder diagrams, some of them are isostructural with U2Co3Si5: R2Ru3Ge5 (R = Y, La-Sm, Gd-Tm), Ce2Co3Ge5, R2Rh3Ge5 (R = La-Pr) and R2Ir3Ge5 (R = Y, La-Sm, Gd-Dy); others crystallize with the Lu2Co3Si5 - type structure, a monoclinic deformation variant of the U2Co3Si5 type: R2Co3Ge5 (R = Y, Pr-Sm, Gd-Er) and R2Rh3Ge5 (R = Y, Nd, Sm, Gd-Tm). The Lu2Co3Si5-type structure of Ho2Rh3Ge5 was refined from single-crystal X-ray data to a conventional R value of 0.034 for 371 independent reflections (Rw = 0.034). Superconductivity above 1.3 K was found only in Y2Ir3Ge5.

De nouvelles séries de germaniures, isotypes de Yb3Rh4Sn13 et BaNiSn3, dans les systèmes ternaires TRTGe où TR est un élément des terres rares et T ≡ Co, Rh, Ir, Ru, Os

Resume Quarante-sept nouveaux germaniures ternaires ont ete caracterises par analyse radiocristallographique de poudres: 34 isotypes de Yb 3 Rh 4 Sn 13 : TR 3 Co 4 Ge 13 (TR ≡ Y, Sm, Gd, …, Lu), TR 3 Rh 4 Ge 13 (TR ≡ Y, Nd, Sm, Gd, …, Lu) et TR 3 Ir 4 Ge 13 (TR ≡ Y, Ce, …, Sm, Gd, …, Lu), et 13 isotypes de BaNiSn 3 : TRCoGe 3 (TR ≡ La, …, Nd, Eu), TRRhGe 3 (TR ≡ La, …, Pr, Eu), TRIrGe 3 (TR ≡ La, Eu), LaRuGe 3 et LaOsGe 3 . Les evolutions structurales observees sont discutees.

LaPt2Ge2, a monoclinic variant of the tetragonal CaBe2Ge2-type structure

Abstract New ternary germanides MPt2Ge2 (M ≡ Ca, Y, LaDy) were obtained by reaction of the elemental components in powder form. X-ray diffraction examination of a single crystal was used to establish the crystal structure of LaPt2Ge2. This structure is a monoclinic deformation variant of the tetragonal CaBe2Ge2-type structure with the following characteristics: a = 4.404 A , b = 4.421 A , c = 9.851 A , β = 90.50 ° , space group P21 (R = 0.073 for 15 variable parameters and 121 structure factors). LaPt2Ge2 was previously reported to have the tetragonal CaBe2Ge2-type structure. However, this could not be corroborated. The monoclinic deformation of MPt2Ge2 compounds depends on the size and the valence (2 or 3) of the M atoms.

Une famille de siliciures ternaires isotypes de V6Si5: (T, T′)6Si5 OÙ T = V, Cr, Mn ET T′ = Ti, Nb, Ta

This study reports on nine ternary suicides, isostructural with V6Si5 and Ti6Ge5, which have the general formula (T, T′)6Si5 where T = V, Cr, Mn and T′ = Ti, Nb, Ta. These suicides are ordered, with the following occupation of the three metallic sites. The smallest site, 8 (f), is reserved for T metals, which form rectilinear chains with the same environment as in other ternary suicides with similar structures: Nb4CrSi3 (W5Si3 type); Nb2V3Si3 (Mn5Si3 type) and (T, T′)11Si8 (Cr11Ge8 type) where T = Cr, Mn and T′ = Ta, Mo, Nb. The larger of the two sites, 8 (j), is occupied preferentially by T′ metals, whereas the smaller 8 (j) second site prefers the T atoms, but accepts with equal facility their substitution by T′.

De nouveaux isotypes lacunaires de CeNiSi2: Les germaniures RCoxGe2, R = Y, La-Sm, Gd-Lu, 0 < x ≦ 1

Abstract The new germanides RCoxGe2, R = Y, La-Sm, Gd-Lu, crystallize in the orthorhombic CeNiSi2-type structure with cobalt vacancies. Their cobalt deficiency regularly increases through the series according to the lanthanide contraction: x vary from 1 to 0.34 between R = La and Lu. The structure of NdCo0.83Ge2 was refined from single-crystal data to a conventional R value of 0.034 for 210 independent reflections (Rw = 0.033). The La, Y and Lu compounds show Pauli paramagnetism and are not superconducting above 1.5 K.

De nouveaux stannures ternaires de rhodium et d'elements des terres rares : TR2Rh3Sn5 OU TR = Y, Gd, Tb, Dy, Ho. structure cristalline et proprietes electriques et magnetiques de Y2Rh3Sn5

Abstract New ternary stannides are reported : RE 2 Rh 3 Sn 5 (RE = Y, Gd, Tb, Dy, Ho). Y 2 Rh 3 Sn 5 has been studied by single - crystal X-ray diffraction analysis. Its structure is of a new type with space group Cmc2 1 and Z = 4 : a = 4,387(2), b = 26,212(4), c = 7,1550(8) A , D x = 8,71 Mgm −3 , μ (AgK α ) = 17 mm −1 , F(000) = 1851, R = 0,045 for 478 independant reflexions (R w = 0,046). This structure is characterized by a tridimensionnal lattice of RhSn covalent bonds with two very different sites for yttrium. Y 2 Rh 3 Sn 5 is diamagnetic and semimetallic according to its electrical properties.

De nouveaux stannures ternaires de rhodium et d'elements des terres rares: TR1+xRh2Sn4−x, 0 ⩽ x ⩽ 0,5, TR = La - Sm

Abstract New ternary stannides are reported: RE1+xRh2Sn4−x, 0 ⩽ x ⩽ 0,5 (RE = La − Sm). NdRh2Sn4 has been characterized by single - crystal X-ray diffraction analysis. Its structure is of a new type with space group Pnma and Z = 4: a = 18,535(3), b = 4,463(1), c = 7,229(1) A , Dx = 9,16 g.cm−1, μ(AgKα) = 13,6 mm−1, F(000) = 1300, R = 0,040 for 496 independent reflexions (Rw = 0,047). One of the tin sites can be partly occupied by rare earths atoms. The structure types of NdRh2Sn4 and Y2Rh3Sn5 are closely related. LaRh2Sn4 does not exhibit superconductivity above 1.4 K.

New rare earth — Transition metal stannides with Sc5Co4Si10 - and TiMnSi2 - type structures. Atomic size effects on their stability

Abstract The following new stannides have been identified by X-ray powder diffraction : M5Rh4Sn10 (M = CeSm and M5Ir4Sn10 (M = LaPr) with Sc5Co4Si10 type structures; MMnSn2 (M = Y, DyTm, Lu) with TiMnSi2 type structures. Their existence had been predicted using a simple geometric criterion. Because Sn atoms squeeze up together, there are less stannides than isostructural silicides and germanides.

Effects de la taille des terres rares sur les structures de type YIrGe2. Repartition atomique dans LaTmIr2Ge4

Abstract LaTmIr2Ge4 has an ordered quaternary YIrGe2-type structure with the following characteristics: a = 4.3230(7) A , b = 16.101(3) A , c = 8.825(2) A , space group Immm, La in 4(i), Tm in 4(h), (R = 0.039 for 29 variable parameters and 217 structure factors). The distribution of the lanthanum and thulium atoms agrees with their relative size. This quaternary structure allows us to define the structural adjustments occurring in the series MIrGe2 (M ≡; Y, Pr-Er) with varying M radius.