Crystallographic and hydriding properties of the system La1−xCexY2Ni9 (xCe=0, 0.5 and 1)

The structural properties of the system La_1−xCe_xY₂Ni₉ with x_Ce=0, 0.5 and 1 have been investigated by electron probe microanalysis, powder X-ray diffraction and absorption spectroscopy. The compound LaY₂Ni₉ adopts a rhombohedral structure of PuNi₃-type (R-3m space group, Z=3). It can be described as an intergrowth between RM₅ (Haücke phase) and RM₂ (Laves phase) type structures. Among the two available crystallographic sites for R atoms, lanthanum occupies preferentially the site 3a leading to a partially ordered ternary compound. Substitution by cerium involves anisotropic variations of the cell parameter with a decrease of a and an increase of c leading to an overall cell volume reduction. Increasing cerium content does not induce any symmetry change but leads to a statistical distribution of the rare earths over the two sites 3a and 6c involving an evolution toward a pseudo-binary compound. This behavior is related to the intermediate valence state of cerium observed by X-ray absorption spectroscopy. The hydriding properties of the two compounds LaY₂Ni₉ and CeY₂Ni₉ are described in relation with their crystallographic structure.     

R12Pt7In (R=Ce, Pr, Nd, Gd, Ho)—new derivatives of the Gd3Ga2-type

A new compound Ce₁₂Pt₇In was synthesized and its crystal structure at 300 K has been determined from single crystal X-ray data. It is tetragonal, space group I4/mcm, Z=4, with the lattice parameters: a=12.102(1) Å and c=14.542(2) Å, wR2=0.1102, 842 F² values, 33 variable parameters. The structure of Ce₁₂Pt₇In is a fully ordered ternary derivative of the Gd₃Ga₂-type. Isostructural compounds has been found to form with Pr (a=11.976(1) Å, c=14.478(2) Å), Nd (a=11.901(1) Å, c=14.471(2) Å), Gd (a=11.601(3) Å, c=14.472(4) Å), and Ho (a=11.369(1) Å, c=14.462(2) Å). Magnetic properties of Ce₁₂Pt₇In, Pr₁₂Pt₇In and Nd₁₂Pt₇In were studied down to 1.7 K. All three ternaries order magnetically at low temperatures with complex spin arrangements. The electrical resistivity of Ce₁₂Pt₇In and Nd₁₂Pt₇In is characteristic of rare-earth intermetallics.     

Ternary rare-earth nickel arsenides R3Ni7As5 (R=La, Ce, Pr, Nd, Sm) with a new variant of the BaAl4-type: crystal structure and physical properties

The ternary rare-earth nickel arsenides R₃Ni₇As₅ (R=La, Ce, Pr, Nd, Sm) were prepared by arc melting the elemental components and subsequent annealing at T=1070 K. The crystal structure of Ce₃Ni₇As₅ was determined from single-crystal X-ray data: space group Pmmn, Z=2; a=1.24210(6), b=0.40797(2), c=0.96436(5) nm, R_F=0.037 (R_w=0.044); 596 independent reflections; 53 variable parameters. It is a new structure type, which belongs to the family of BaAl₄-related structures. The magnetic properties are as follows: La₃Ni₇As₅ is a Pauli-type paramagnet above 4.2 K, Pr₃Ni₇As₅ remains paramagnetic in the temperature range investigated and Nd₃Ni₇As₅ undergoes a ferromagnetic ordering at T_C=24 K. Sm₃Ni₇As₅ orders antiferromagnetically at a Néel temperature of T_N=18 K followed by a spin flip towards parallel spin-alignment below T_C=6 K. Ce₃Ni₇As₅ reveals a strong deflection of the linear temperature dependence of the inverse susceptibility due to an intermediate valence behavior. The temperature dependence of the electrical resistivities for the La, Pr, Nd, Sm containing samples corroborates with the metallic state of the non-magnetic (La) and the magnetically ordered compounds, whereas in case of Ce₃Ni₇As₅ the resistivity seems to be determined by an interplay of Kondo scattering and crystalline field effects. L_III X-ray absorption spectra confirm the demagnetization effects owing from valence fluctuations, the actual valence thereby changes from ν=3.10-3.14 at room temperature and 10 K, respectively.     

On the structural and magnetic properties of the ternary silicides Ce6M1.67Si3 (M=Co, Ni) and Ce5Ni1.85Si3

Contrary to that reported previously, the ternary silicide “Ce₆Ni₂Si₃” does not exist. The melting of this alloy, followed or not by annealing, leads to the existence of the two new ternary compounds, Ce₆Ni_1.67Si₃ and Ce₅Ni_1.85Si₃. The investigation of these ternary silicides based on nickel and Ce₆Co_1.67Si₃ by X-ray diffraction on single crystal reveals an ordered distribution between Ni (or Co) and Si atoms. The nickel or cobalt positions in the chains of face-shared octahedra of cerium are not fully occupied with a strong delocalisation of their electron density. The structural investigations of these compounds confirm that the “Ce₆Ni₂Si₃” and “Ce₅Ni₂Si₃” structural type have to be rewritten as Ce₆Ni_2−xSi₃ and Ce₅Ni_2−xSi₃. Magnetisation and specific heat measurements evidence a magnetic ordering at 3.8(2) K for Ce₆Ni_1.67Si₃ and a heavy fermion behaviour for Ce₆Co_1.67Si₃.     

Crystal structure and magnetic properties of Ce7Ni5±xGe3±xIn6 and Pr7Ni5±xGe3±xIn6

The indides Ce₇Ni_5±xGe_3±xIn₆ and Pr₇Ni_5±xGe_3±xIn₆ were synthesized from the elements by arc-melting of the components. Single crystals were grown via special annealing sequences. Both structures were solved from X-ray single crystal diffraction data: new structure type, P6/m, Z=1, a=11.385(2), c=4.212(1) Å, wR₂=0.0640, 634F² values, 25 variables for Ce₇Ni_4.73Ge_3.27In₆ and a=11.355(6), c=4.183(2) Å, wR₂=0.0539, 563F² values, 25 variables for Pr₇Ni_4.96Ge_3.04In₆. Both indides show homogeneity ranges through Ni/Ge mixing (M sites). This new structure type can be derived from the AlB₂ structure type by a substitution of the Al and B atoms by CeM₁₂ and NiIn₆Ce₃ polyhedra (tricapped trigonal prism). Magnetic susceptibility measurements on a polycrystalline sample of Ce₇Ni₅Ge₃In₆ indicated Curie-Weiss like paramagnetic behavior down to 1.71 K with the effective magnetic moment slightly reduced in relation to the value expected for trivalent cerium ions. No magnetic ordering is evident.     

Crystal structure and magnetic properties of the UFe7Al5 uranium-iron aluminide

The ternary compound UFe₇Al₅ was synthesized by arc melting, followed by annealing at 850°C. The crystal structure was determined by single-crystal X-ray diffraction and refined to a residual value of R=0.039 (S=1.030), with lattice parameters a=8.581(2) Å and c=4.946(1) Å. This compound is a new extreme composition in the family of intermetallics with general formula UFe_xAl_12−x crystallizing in the tetragonal ThMn₁₂-type structure, space group I4/mmm. In contrast to UFe_xAl_12−x within the composition range 4?x?6, in UFe₇Al₅ the additional iron atom is found in the 8i equipositions. Magnetization measurements versus temperature show two magnetic transitions at 363 and 275 K, respectively, with a ferromagnetic behavior below the highest temperature transition. ⁵⁷Fe Mössbauer data indicate that the high-temperature transition is related to the ordering of the iron atoms. The dependence of the isomer shifts and magnetic hyperfine fields on the crystallographic site and on the number of the iron nearest neighbors is similar to that observed in the other UFe_xAl_12−x and rare-earth analogues. The magnetic hyperfine field values of iron atoms on 8i sites is larger than in the other sites, in agreement with previous data obtained for other ThMn₁₂-type compounds.     

Preparation and structural study from neutron diffraction data of Pr5Mo3O16

The title compound has been prepared as polycrystalline powder by thermal treatments of mixtures of Pr₆O₁₁ and MoO₂ in air. In the literature, an oxide with a composition Pr₂MoO₆ has been formerly described to present interesting catalytic properties, but its true stoichiometry and crystal structure are reported here for the first time. It is cubic, isostructural with CdTm₄Mo₃O₁₆ (space group Pn-3n, Z=8), with a=11.0897(1) Å. The structure contains MoO₄ tetrahedral units, with Mo-O distances of 1.788(2) Å, fully long-range ordered with PrO₈ polyhedra; in fact it can be considered as a superstructure of fluorite (M₈O₁₆), containing 32 MO₂ fluorite formulae per unit cell, with a lattice parameter related to that of cubic fluorite (a_f=5.5 Å) as a≈2a_f. A bond valence study indicates that Mo exhibits a mixed oxidation state between 5+ and 6+ (perhaps accounting for the excellent catalytic properties). One kind of Pr atoms is trivalent whereas the second presents a mixed Pr³⁺-Pr⁴⁺ oxidation state. The similarity of the XRD pattern with that published for Ce₂MoO₆ suggests that this compound also belongs to the same structural type, with an actual stoichiometry Ce₅Mo₃O₁₆.     

Nb28Ni33.5Sb12.5, a New Representative of the X-phase

The first ternary compound in the Nb–Ni–Sb system, Nb₂₈Ni_33.5Sb_12.5, has been synthesized and its structure has been determined by single-crystal X-ray diffraction methods. Nb₂₈Ni_33.5(2)Sb_12.5(2) adopts the X-phase structure type (orthorhombic, space group Pnnm, Z=1, a=13.2334(5) Å, b=16.5065(7) Å, c=5.0337(2) Å), which belongs to the set of tetrahedrally close-packed (TCP) structures adopted by many intermetallic compounds. Typical of such TCP structures, the atoms reside in sites of high coordination number, with Ni and Sb in CN12 and Nb in CN14, -15, and -16 sites. The relative importance of various metal–metal bonding interactions is discussed on the basis of extended Hückel band structure calculations. Nb₂₈Ni_33.5Sb_12.5 displays metallic behavior with a room-temperature resistivity of 2.3×10⁻⁴ Ω cm.     

Synthesis, crystal structure, and electrical and magnetic properties of Ce3MoO7

Powder samples and single crystals of the ternary oxide Ce₃MoO₇ were obtained by solid state reaction. The structure was determined by single-crystal X-ray diffraction. Ce₃MoO₇ crystallizes in the orthorhombic space group P2₁2₁2₁ (no. 19) with unit-cell parameters a=7.5395(2) Å, b=7.6769(1) Å, c=10.9769(2) Å and Z=4. Full-matrix least-squares refinement on F² using 3903 independent reflections for 101 refinable parameters results in R₁=0.0281 and wR₂=0.0473. The structure consists of chains of corner-linked MoO₆ octahedra running parallel to the b-axis and separated from each other by seven- or eight-coordinate Ce-O polyhedra. The trend of the unit-cell parameters of the Ln₃MoO₇ series, plotted versus the R³⁺ ionic radius, shows a linear behavior, which strongly suggests a trivalent state for the Ce atoms. Magnetic susceptibility measurements confirm that the oxidation state of the Ce atoms is +3. Resistivity measurements on a single crystal show that the Ce₃MoO₇ compound is a semi-conductor with a band gap of about 2 eV.     

New borides Er0.917Ni4.09B and ErNi7B3 and their crystal structures

New borides have been synthesized and their crystal structures have been determined using X-ray single-crystal methods, namely: Er_0.917Ni_4.09B, own structure type, space group P6/mmm, a=14.8399(3), c=6.9194(3) Å, R_F=0.0545, and ErNi₇B₃, own structure type, space group I4₁/amd, a=7.6577(2), c=15.5798(5) Å, R_F=0.0451. The relationship between these structures and the structure types of CeCo₄B, Y_0.915Ni_4.12B and Sc₄Ni₂₉B₁₀ has been discussed.     

X-rays structural analysis and thermal stability studies of the ternary compound α-AlFeSi

From literature data presently available, the decomposition temperature and the nature of the decomposition reaction of the ternary compound α-AlFeSi (also designated as α_H or τ₅) are not clearly identified. Moreover, some uncertainties remain concerning its crystal structure. The crystallographic structure and thermochemical behaviour of the ternary compound α-AlFeSi were meticulously studied. The crystal structure of α-AlFeSi was examined at room temperature from X-ray single crystal intensity data. It presents hexagonal symmetry, space group P6₃/mmc with unit cell parameters (293 K) a=12.345(2) Å and c=26.210(3) Å (V=3459 Å³). The average chemical formula obtained from refinement is Al_7.1Fe₂Si. From isothermal reaction-diffusion experiments and Differential Thermal Analysis, the title compound decomposes peritectically upon heating into θ-Fe₄Al₁₃(Si), γ-Al₃FeSi and a ternary Al-rich liquid. Under atmospheric pressure, the temperature of this reversible transformation has been determined to be 772±12 °C.     

Synthesis, crystal structure and magnetic properties of U2Co6Al19

The new compound U₂Co₆Al₁₉ was prepared by reaction of the elemental components in an arc-melting furnace followed by a heat treatment at 1050°C for 500 h. Its chemical composition was checked by energy-dispersive X-ray analyses and its crystal structure was determined by single crystal X-ray diffraction experiments. It crystallizes with four formula units in the monoclinic space group C2/m in a unit cell of dimensions a=17.4617(3)Å, b=12.0474(2)Å, c=8.2003(1)Å, β=103.915(1)°. The crystal structure of U₂Co₆Al₁₉ can be regarded as a superstructure of NdCo_4−xGa₉ structure type. This complex structure consists of a three-dimensional Co-Al framework delimiting tunnels where the U atoms reside. The shortest U-U distances are found in the c direction with alternating values of 3.98(1) and 4.22(1) Å. Temperature-dependent magnetization shows a first peak at 12.5 K and a weak ferromagnetic character below the temperature T_C=8 K. Magnetization at 1.9 K reaches almost saturation in 5 T with the moment of 0.36 μ_B/U atom. The complex magnetic behavior of U₂Co₆Al₁₉ may be ascribed to a canted spin structure resulting from an antiparallel arrangement of the magnetic moments not fully compensated at low temperature. At higher temperature, the compound displays simple paramagnetic behavior.     

Crystal structure and hydrogenation properties of pseudo-binary Mg6Pd0.5Ni0.5 complex metallic alloy

The crystal structure of the Ni-substituted Mg_6.10(2)Pd_0.52(2)Ni_0.41(2) complex metallic alloy has been determined by X-ray and neutron powder diffraction. The reaction of this compound at 573 K towards deuterium absorption for pressures up to 23 bar has also been studied. The crystal structure of Mg_6.10(2)Pd_0.52(2)Ni_0.41(2) compound was determined in the light of Samson's [Acta Crystallogr. B 28 (1972) 936) and Makongo's (Philos. Mag. 86 (2006) 427] models for the binary Mg₆Pd compound. It crystallizes in space group with lattice parameter 20.13331(7) Å. The refined unit-cell composition is Mg₃₄₂₍₁₎Pd₂₉₍₁₎Ni₂₃₍₁₎ with Z=56. Nickel by palladium substitution is not fully random. Nickel atoms preferentially locate on Pd sites with low coordination number due to steric effects. Deuterium uptake is 9.6 D/f.u. under the given conditions of pressure and temperature. Upon absorption, the intermetallic compound disproportionates into MgD₂, Mg₅Pd₂ and Mg₂NiD₄ phases. The Mg₂NiD₄ phase is observed to crystallize in the orthorhombic LT2 modification for which an averaged crystal structure in the Pcc2 space group is proposed.     

Ba14Zn5−xAl22+x: a new polar intermetallic compound with a novel 2D network

A new ternary, intermetallic compound, Ba₁₄Zn_5−xAl_22+x, was synthesized by heating the pure elements at 900°C. This compound crystallizes in the monoclinic space group I2/m, Z=2, with a=10.474(2) Å, b=6.0834(14) Å, c=34.697(8) Å and β=90.814(4)°. The crystal structure of Ba₁₄Zn_5−xAl_22+x consists of [Zn_5−xAl_22+x] slabs that are built with a novel, two-dimensional (2D) network of Zn and Al atoms involving eight-membered rings sandwiched between two layers of trigonal bipyramids interconnected by three-center bonding. Tight-binding, linear muffin-tin orbital (TB-LMTO-ASA) calculations have been performed to understand the relationship between composition and orbital interactions in the electronegative element framework. This new structure is closely related to the high-pressure, cubic Laves-type structure of BaAl₂ as well as the ambient pressure binary compound, Ba₇Al₁₃. The degree of valence electron charge transfer from the electropositive Ba atoms is related to the Al:Ba molar ratio in the Ba-Zn-Al system.     

Structure, and magnetic and transport behavior of twinned Ce2Rh3(Pb,Bi)5

Single crystals of a new compound, Ce₂Rh₃(Pb,Bi)₅, have been grown via a flux-growth technique using molten Pb as a solvent. The compound has been characterized by single crystal X-ray diffraction and found to be of the orthorhombic Y₂Rh₃Sn₅ structure type [Cmc2₁ (No. 36), Z=4] with lattice parameters a=4.5980(2), b=27.1000(17) and c=7.4310(4) Å, with V=925.95(9) Å³. Ce₂Rh₃(Pb,Bi)₅ has a complex crystal structure containing Ce atoms encased in Rh-X (X=Pb/Bi) pentagonal and octagonal channels in [100], with polyanions similar to those found in Ce₂Au₃In₅ and Yb₂Pt₃Sn₅. Magnetization measurements find that Ce₂Rh₃(Pb,Bi)₅ is a quasi-two-dimensional system, where the Ce moments are spatially well-localized. Heat capacity measurements show a transition at the Néel temperature of 1.5 K. Evidence for Fermi surface nesting is found in electrical resistivity measurements, and we argue that Ce₂Rh₃(Pb,Bi)₅ is very near a metal-insulator transition in zero field.     

Temperature-dependent crystallographic studies and electronic structure of Ba2Cd3Bi4

The ternary compound Ba₂Cd₃Bi₄ crystallizes in the C-centered orthorhombic space group Cmce (No. 64) with its own type (Pearson's symbol oC36; a=7.019(3) Å, b=17.389(7) Å and c=9.246(3) Å determined at -23 °C). Although the structure of this intermetallic compound with transition metal in d¹⁰ configuration has already been established, details such as the rather unusual coordination of the Cd-atoms and the elongation in specific direction of their anisotropic displacement parameters had not been explained. These facts, along with the higher than 12% R-values from the original structure determination prompted the systematic structural studies by single-crystal X-ray diffraction at several different temperatures. The results from these studies confirm strong temperature dependence of the cadmiums’ anisotropic displacement parameters, concomitant rather large thermal expansion along the crystallographic b-axis. Electronic band structure calculations performed by the TB-LMTO-ASA method are also reported.     

Structural and physical properties of the new intermetallic compound Yb3Pd2Sn2

The crystal structure of the ternary intermetallic compound Yb₃Pd₂Sn₂ has been determined ab initio from powder X-ray diffraction data. The compound crystallizes as a new structure type in the orthorhombic space group Pbcm and lattice constants a=0.58262(3), b=1.68393(8), c=1.38735(7) nm. Yb₃Pd₂Sn₂ is composed of a complex _∞[Pd₂Sn₂]^δ− polyanionic network in which the Yb ions are embedded. A comparison between this structure and those of Eu₃Pd₂Sn₂ and Ca₃Pd₂Sn₂, other novel polar intermetallic compounds, was made. DC susceptibility and ¹⁷⁰Yb Mössbauer spectroscopic measurements indicate a close-to divalent Yb behavior. Moreover, a hybridization between 4f and conduction electrons is suggested by electronic structure calculations and heat capacity measurements.     

Structural study of copper-nickel aluminate (CuxNi1−xAl2O4) spinels

CuAl₂O₄, NiAl₂O₄, and three ternary spinels Cu_xNi_1?xAl₂O₃ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, NiO, and Al₂O₃ at 1223 K. X-Ray powder diffractometry, coupled with adequate computational methods, allowed determination of the unit-cell length, oxygen positional parameter, and cation distribution for each compound. Interdependence of these structural parameters is closely analyzed on the ternary oxide spinels. The one-electron difference between the Cu²⁺ and Ni²⁺ ions was found to be enough to render them distinguishable by X-ray powder diffraction.     

Synthesis and crystal structures of the new metal-rich ternary borides Ni12AlB8, Ni12GaB8 and Ni10.6Ga0.4B6—examples for the first B5 zig-zag chain fragment

Single crystals of the new borides Ni₁₂AlB₈, and Ni_10.6Ga_0.4B₆ were synthesized from the elements and characterized by XRD and EDXS measurements. The crystal structures were refined on the basis of single crystal data. Ni₁₂AlB₈ (oC252, Cmce, a=10.527(2), b=14.527(2), c=14.554(2) Å, Z=12, 1350 reflections, 127 parameters, R₁(F)=0.0284, wR₂(F²)=0.0590) represents a new structure type with isolated B atoms and B₅ fragments of a B-B zig-zag chain. Because the pseudotetragonal metric crystals are usually twinned. Ni_10.6Ga_0.4B₆ (oP68, Pnma, a=12.305(2), b=2.9488(6), c=16.914(3) Å, Z=4, 1386 reflections, 86 parameters, R₁(F)=0.0394, wR₂(F²)=0.104) is closely related to binary Ni borides. The structure contains B-B zig-zag chains and isolated B atoms. Ni₁₂GaB₈ is isotypical to the Al-compound (a=10.569(4), b=14.527(4) and c=14.557(5) Å).     

Rare earth-nickel-indides Dy5Ni2In4 and RE4Ni11In20 (RE=Gd, Tb, Dy)

The new rare earth metal (RE)-nickel-indides Dy₅Ni₂In₄ and RE₄Ni₁₁In₂₀ (RE=Gd, Tb, Dy) were synthesized from the elements by arc-melting. Well-shaped single crystals were obtained by special annealing sequences. The four indides were investigated by X-ray diffraction on powders and single crystals: Lu₅Ni₂In₄ type, Pbam, Z=2, a=1784.2(8), b=787.7(3), c=359.9(1) pm, wR₂=0.0458, 891 F² values, 36 variables for Dy₅Ni₂In₄, U₄Ni₁₁Ga₂₀ type, C2/m, a=2254.0(9), b=433.8(3), c=1658.5(8) pm, β=124.59(2)°, wR₂=0.0794, 2154 F² values, 108 variables for Gd₄Ni₁₁In₂₀, a=2249.9(8), b=432.2(1), c=1657.9(5) pm, β=124.59(2)°, wR₂=0.0417, 2147 F² values, 108 variables for Tb₄Ni₁₁In₂₀, and a=2252.2(5), b=430.6(1), c=1659.7(5) pm, β=124.58(2)°, wR₂=0.0550, 2003 F² values, 109 variables for Dy₄Ni_10.80In_20.20. The 2d site in the dysprosium compound shows mixed Ni/In occupancy. Most nickel atoms in both series of compounds exhibit trigonal prismatic coordination by indium and rare earth atoms. Additionally, in the RE₄Ni₁₁In₂₀ compounds one observes one-dimensional nickel clusters (259 pm Ni₁-Ni₆ in Dy₄Ni_10.80In_20.20) that are embedded in an indium matrix. While only one short In₁-In₂ contact at 324 pm is observed in Dy₅Ni₂In₄, the more indium-rich Dy₄Ni_10.80In_20.20 structure exhibits a broader range in In-In interactions (291-364 pm). Together the nickel and indium atoms build up polyanionic networks, a two-dimensional one in Dy₅Ni₂In₄ and a complex three-dimensional network in Dy₄Ni_10.80In_20.20. These features have a clear consequence on the dysprosium coordination, i.e. a variety of short Dy-Dy contacts (338-379 pm) in Dy₅Ni₂In₄, while the dysprosium atoms are well separated (430 pm shortest Dy-Dy distance) within the distorted hexagonal channels of the [Ni_10.80In_20.20] polyanion of Dy₄Ni_10.80In_20.20. The crystal chemistry of both structure types is comparatively discussed.