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Crystal structure of Nd3Au4 and Nd17Au36
No full textThe crystal structures of two phases in the neodymium-gold system were determined by single crystal X-ray diffractometer methods, '/.(MoKx = 0.7107 A. Nd3Au4(hR42) is trigonal, R3 (n. 148), a = 13.739(3) A, c = 6.088(2) A, Z = 6, R = 0.083 with 405 reflections, isotypic with Pu3,Pd4. Nd,7Au36(tP 106) is tetragonal, P4/nmm (n. 129), a = 15.835(4) A, c = 9.278(2) A, Z = 2, R = 0.082 with 540 reflections. The structure of Nd17Au36, which represents a new type, can be described as packing of bicapped square antiprisms with vertices occupied by five Nd and five Au atoms and centered by part of the gold atoms. This kind of polyhedron is identical to that of aluminium in the pyramidal site of the BaAl4 structure. In the Nd —Au system the phase Nd17Au36 replaces the phase given as “NdAu2”. © 1994, R. Oldenbourg Verlag. München. All rights reserved
Examples of linear structures of intermetallic compounds described as intergrowth of segments of simple basic structures
No full textSegments of the basic structures BaAl4, AlB2, W, Po, Cu3Au, CaF2 or hybrids obtained combining two segments of these types are used with the mechanism of intergrowth to describe nine linear structures of binary and ternary intermetallic compounds formed by rare earth or alkaline earth metals. Such a representation allows to display relationship among the structures. All examined compounds with one exception contain AlB2 fragments, that means rare earth or alkaline earth face-sharing trigonal prisms centred by the small atoms. Depending on the segments between which the AlB2 fragment is comprised, the atoms centring the prisms show a different behaviour in their waist contacts
Electrical resistivity of the RMBi phases with R = Ca, Yb and M = Cu, Ag, Au
No full textThe electrical resistivity of the phases CaAgBi, CaAuBi, YbAgBi in the range 10-300 K and CaCuBi, YbCuBi, YbAuBi in the range 10-600 K was measured. All compounds show metallic behaviour. The phonon contribution with an additional cubic term are used to represent the temperature dependence of the resistivity for the gold and silver phases. Anomalous features in the thermal cycling resistivity, imputable to the occurrence of microcracks, are found for the CaCuBi and YbCuBi samples. Moreover, YbCuBi shows a phase transition at 375 K, probably due to a crystallographic transformation from the LiGaGe to the ZrBeSi structure type
Different stacking of the gold and silicon atoms in the phases RAuSi(R Sc, Y, Lu)
No full textThe structures of ScAuSi, YAuSi and LuAuSi were determined by single-crystal diffractometry. ScAuSi, hP6, hexagonal P6m2 (no. 187), a=4.212(1) A ̊, c=7.546(3) A ̊, Z=2, R=0.039 using 66 reflections, represents a new ternary ordered type. YAuSi, hP6, hexagonal P63mc (no. 186),a=4.288(2) A ̊, c=7.546(3) A ̊, Z=2, R=0.039 using 66 reflections, is isotypic with LiGaGe (ternary ordered derivative of the CaIn2 structure). LuAuSi, isotypic with ScAuSi, a=4.267(2) A ̊, c=7.151(3) A ̊, was refined to R=0.052 using 90 reflections. In both ScAuSi and YAuSi structures trigonal prisms of scandium or yttrium sharing their lateral faces are stacked along the c axis. The gold and silicon atoms forming rumpled layers centre the prisms in an ordered way. The difference between the two structures consists in the different stacking of these layers along the c axis. In YAuSi atoms of gold in a layer alternate with atoms of silicon in the subsequent layer, and vice versa; in ScAuSi the layers become equally oriented so that Au-Au and Si-Si pairs are formed
RMX compounds formed by alkaline earths, europium and ytterbium II: Ternary phases with M ≡ Zn, Cd and X ≡ Si, Ge, Sn, Pb
No full text25 new RMX phases (R ≡ Ca, Sr, Ba, Eu, Yb; M ≡ Zn, Cd; X ≡ Si, Ge, Sn, Pb) were found to crystallize in the structure types ZrBeSi, LiGaGe, TiNiSi, ZrNiAl and CeCu2. Single-crystal refinements were made for YbZnSn (LiGaGe type) with complete atomic ordering, and for SrCdSn, BaCdSn and BaCdPb with disordered CeCu2 structure. A two-dimensional structural map is proposed accounting for 83 RMX phases formed by alkaline earths, europium and ytterbium with the same valence electron concentration ( 8 3 electrons per atom)
Crystal structure and electrical properties of the new R2TSi2 compounds (R=rare earths; T=Fe,Co)
No full textThe R2FeSi2 compounds with R=Gd, Tb, Dy, Ho, Er, Tm, Y, and R2CoSi2 with R=Ho, Er, Tm, were synthesized and studied by X-ray powder diffraction. All phases crystallize in the Sc2CoSi2 monoclinic structure type, and the atomic positional and thermal parameters were refined by single crystal data for Tb2FeSi2, Er2FeSi2 and Er2CoSi2. The measurement of the electrical resistance in the 13–300 K temperature range showed for all phases, save the yttrium compounds, a transition point which indicates an order–disorder magnetic change. The scaling of these temperatures with the de Gennes factor gives a good correlation, with some slight deviations probably due to crystal electric field effects
RMX compounds formed by alkaline earths, europium and ytterbium-I. Ternary phases with M ≡ Cu, Ag, Au; X ≡ Sb, Bi
No full text19 intermetallic compounds with the general formula RMX (R ≡ Ca, Sr, Ba, Eu, Yb; M ≡ Cu, Ag, Au; X ≡ Sb, Bi) were synthesized and structurally studied by X-ray methods on powders and single crystals. The ternary phases crystallize in four ordered structural types: ZrBeSi, LiGaGe, TiNiSi and MgAgAs. Some comments are given on the parameters controlling the structure of these phases, and the volume effects are calculated
Crystal chemical features of ternary phases in the R-Li-Ge (R = rare earth element) systems
No full textThe synthesis and structural characterization of phases in the R–Li–Ge (R = rare earth) systems are reported. The R4LiGe4 compounds with R = La–Nd, Sm complete the series, where the analogues with R = Gd–Tm, Lu were confirmed, all crystallizing with the Tm4LiGe4 type. Nd4LiGe4 and Ho4LiGe4 were structurally studied by single crystal X-ray diffraction (space group Pnma, Z = 4, Pearson code oP36). Nd4LiGe4: a = 7.367 (1) Å, b = 15.116 (5) Å, c = 7.947 (3) Å, wR2 = 0.054; Ho4LiGe4: a = 7.077 (4) Å, b = 14.585 (6) Å, c = 7.670 (3) Å, wR2 = 0.067. The phase Pr7Li8Ge10 crystallizes with a novel orthorhombic structure (space group Cmmm, Z = 2, Pearson code oS50): a = 6.8945 (7) Å, b = 33.531 (6) Å, c = 4.4400 (2) Å, wR2 = 0.060. Isotypic compounds were found also with R = La, Ce, Nd. A third series R3Li4Ge4 with R = La–Nd, Sm crystallizes with the Gd3Cu4Ge4 type. The structure was refined for the Nd analogue with the final composition Nd3Li3.65(2)Ge4.35(2) (space group Immm, Z = 2, Pearson code oI22): a = 4.3935 (5) Å, b = 6.811 (1) Å, c = 14.619 (3) Å, wR2 = 0.068. The Pr7Li8Ge10 structure is closely related to both Ce2Li2Ge3 and Gd3Cu4Ge4 types. They belong to a structural series with general formula R2m+nM4mX2m+2n formed by m and n segments of the MgCuAl2 and AlB2 types, respectively. All the examined structures are characterized by the trigonal prismatic coordination around the germanium atoms, which represents a common feature of the R–Li–Ge compounds
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