TiNiSi Structure Research Articles

Low-temperature properties of the Yb-based heavy-fermion antiferromagnets YbPtIn, YbRhSn, and YbNiGa

The crystallographic and low-temperature properties of the equiatomic Yb-based compounds YbPtIn, YbRhSn (hexagonal ZrNiAl structure), and YbNiGa (orthorhombic $\ensuremath{\epsilon}$-TiNiSi structure) are studied by means of x-ray diffraction, magnetic susceptibility, magnetization, electrical resistivity $(\ensuremath{\rho}),$ magnetoresistance, thermoelectric power, and specific-heat ${(C}_{P})$ measurements. The magnetic susceptibility follows a Curie-Weiss law with effective magnetic moments close to the value of ${\mathrm{Yb}}^{3+}$ and shows strong magnetocrystalline anisotropy. At low temperature the three compounds exhibit the typical properties of antiferromagnetic (AFM) Kondo lattices, namely a logarithmic increase of $\ensuremath{\rho}(T),$ enhanced electronic ${C}_{P}$ coefficients ${\ensuremath{\gamma}}_{o},$ and a reduced entropy gain below the ordering temperature. The values of ${\ensuremath{\gamma}}_{o}g350 {\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ are among the largest of known Yb-based magnetic Kondo lattices. Two AFM transitions in each compound evidence a competition between single-ion anisotropy and frustration of the magnetic interactions due to the topology of the underlying crystal structure. The ground-state properties are discussed and compared with those of other f-electron compounds showing similar characteristics concerning magnetic frustration.

CaRhIn with TiNiSi Type Structure and CaTIn2 (T = Rh, Ir) with a New Filled Version of the Zintl Phase CaIn2

CaRhIn, CaRhIn2, and CaIrIn2 were synthesized by reacting the elements in glassy carbon crucibles under an argon atmosphere in a high-frequency furnace. CaRhIn adopts the TiNiSi structure: Pnma, a = 730.0(4) pm, b = 433.1(2) pm, c = 828.8(4) pm, wR2 = 0.0707, 630 F2 values, 20 variables. The CaRhIn structure consists of strongly puckered Rh3In3 hexagons with Rh–In distances ranging from 273 to 276 pm. Due to the strong puckering each rhodium atom has a distorted tetrahedral indium environment. The calcium atoms fill the channels within the three-dimensional [RhIn] polyanion. CaRhIn2 and CaIrIn2 crystallize with a new structure type: Pnma, a = 1586.2(3) pm, b = 781.4(2) pm, c = 570.9(1) pm, wR2 = 0.0385, 1699 F2 values, 44 variables for CaRhIn2, and Pnma, a = 1588.7(3) pm, b = 780.8(1) pm, c = 574.0(1) pm, wR2 = 0.0475, 1661 F2 values, 44 variables for CaIrIn2. The structures of CaRhIn2 and CaIrIn2 can be described as an orthorhombically distorted rhodium respectively iridium filled CaIn2. The motif of transition metal filling is similar to that found in MgCuAl2 type compounds CaTIn2 (T = Pd, Pt, Au) and SrTIn2 (T = Rh, Pd, Ir, Pt), but constitute a different tiling. Semi-empirical band structure calculations for CaRhIn and CaRhIn2 reveal strong bonding In–In and Rh–In but weaker Ca–Rh and Ca–In interactions. Magnetic susceptibility and resistivity measurements of compact polycrystalline samples of CaRhIn2 indicate weak Pauli paramagnetism and metallic conductivity with a room temperature value for the specific resistivity of 230 ± 50 μΩcm.

Differences and Similarities between the Isotypic AntimonidesMFe1−xSb, ScCo1−xSb, andMNiSb (M=Zr, Hf)

The new antimonides MFe1−xSb can be synthesized by arc-melting of M, Fe, andMSb2(M=Zr, Hf). All title compounds crystallize in the TiNiSi structure type (space groupPnma,Z=4). The lattice parameters of the new phases MFe1−xSb, as obtained from the bulk samples of the nominal compositionsMFeSb, are (a=681.4(1) pm,b=417.87(7) pm,c=740.3(1) pm for ZrFe1−xSb anda=674.0(1) pm,b=412.0(2) pm,c=729.7(2) pm for HfFe1−xSb. Under the reaction conditions used, the occupancy factors of the iron position content of ZrFe1−xSb does not exceed 68(1)% (i.e.x=0.32(1)). Extended Hückel calculations, performed on the hypothetical model structures “ZrFeSb” and “ZrFe0.75Sb”, point to the phase ZrFe1−xSb being metallic, independent of thexvalue. The band structure of “ZrFeSb”, obtained withab initioLMTO calculations, reveals a three-dimensional metallic conductivity and a nonmagnetic ground state.

The TiNiSi Family of Compounds: Structure and Bonding

We report the synthesis and structure and discuss in substantial detail the bonding of a remarkable family of compounds spanning most of the transition series. All members of the MTSi (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni; T = Co, Ni) series have now been prepared, and their crystal structures have been determined accurately using X-ray diffraction and, for MnNiSi and FeNiSi, neutron diffraction. Each of these compounds, four of which were previously unknown, crystallizes in the TiNiSi structure type. In contrast, for T = Cu, one has only representatives with M = Sc and Ti. A simple Zintl picture, which works so well for the three-dimensional four-connected indium net of the related BaIn2 structure, is not applicable to these intermetallics; there is substantial M−T and M−Si bonding, and the “extra” electrons in the MTSi structure enter orbitals essentially nonbonding within the TSi network. A structural change observed for M = Fe and higher d-electron counts, distorting the six-rings in the structure, is tr...

NMR of 51V in VRuP: Possible Onset of Charge-Density-Waves

Abstract NMR experiments are reported for VRuP which has an orthorhombic TiNiSi structure and shows anomalies in the temperature dependence of the electric conductivity and static susceptibility. A field-swept spectrum of 51V perturbed by a quadrupole interaction has been observed, which shows no anomalous temperature dependence. However, the metallic spin-lattice relaxation rate decreases in a stepwise fashion with decreasing temperature at around 200 K, in accordance with the resistivity and the susceptibility data, suggesting a possible CDW formation in this material.

Specific heat of UNiGe in high magnetic fields

Specific-heat measurements were performed on a single crystal of UNiGe in magnetic fields up to 17.5 T. This compound crystallizes in the orthorhombic TiNiSi structure and undergoes two magnetic phase transitions at 41.5 and 50 K. Besides these transitions, the specific heat also shows a Schottky contribution due to the crystal-field splitting. The effects of an applied magnetic field are compared with results from previous neutron diffraction, magnetoresistance, and magnetization measurements.

Magnetism in URhSi

Our neutron-powder-diffraction experiment revealed that URhSi crystallizes in the orthorhombic TiNiSi (space group Pnma) structure and orders ferromagnetically at low temperatures with the U magnetic moments of 0.11 μB aligned along the c axis. Anomalies in the temperature dependence of the magnetic susceptibility, specific heat and electrical resistivity indicate that URhSi orders below 9.5 K. The enhanced Cp/T value (extrapolated to 0 K) of 186 mJ/mol K2 can be partially reduced in magnetic fields, which indicates a considerable magnetic contribution even at very low temperatures. The ferromagnetic ground state is documented also by magnetization measurements at low temperatures. The high-field magnetization data obtained on oriented powder reveal a strong magnetocrystalline anisotropy. All the results obtained on polycrystalline samples classify URhSi as an itinerant 5f ferromagnet with very reduced U magnetic moments.

The Four-Connected Net in the CeCu2 Structure and Its Ternary Derivatives. Its Electronic and Structural Properties

The crystallochemistry of and the bonding in the orthorhombic four-connected nets of BaIn(2) (CeCu(2) structure) and of CaPtSn (TiNiSi structure, a derivative of the CeCu(2) structure) are analyzed with approximate molecular orbital calculations. Following the Zintl concept, in BaIn(2) the In(-) ions are isoelectronic with group IV tin and should adopt a four-connected structure. In contrast to alpha-tin, which has a cubic diamond structure, the indium ions in BaIn(2) build up an orthorhombic three-dimensional four-connected net containing distorted tetrahedra and ladder polymers of four-membered rings. In the CeCu(2) structure (space group Imma) two bond angles in these distorted tetrahedra are fixed at 90 degrees. The four-connected net in the CeCu(2) structure is topologically related to the layers in black phosphorus (space group Cmca). In CaPtSn (TiNiSi structure) the orthorhombic four-connected net is formed by (PtSn)(2)(-) ions in an ordered arrangement. Calculations on BaIn(2) and CaPtSn show that the four-connected nets are increasingly stabilized as the valence electron count is increased from 16 to 30 valence electrons per 4 formula units. For more than 30e, the nets are destabilized due to filling of M-E antibonding states. Structural data obtained by precise single crystal investigations for the TiNiSi series CaPdIn (20e), CaPdSn (24e), CaPdSb (28e), and CaAgSb (32e), confirm the results of the extended Hückel calculations. We find an interesting and understandable angular asymmetry of the tetrahedral sites in these ternary compounds.

Metamagnetism in EuPdIn and EuAuIn

EuPdIn and EuAuIn were prepared by reaction of the elemental components in tantalum tubes. They crystallize with the TiNiSi structure in space group Pnma and with Z= 4 formula units per unit cell. Both structures were refined from single-crystal X-ray data: a= 748.30(13) pm, b= 447.20(8) pm, c= 853.50(14) pm, V= 0.28562(5) nm3, wR2 = 0.0408, 449 F2 values, 20 variables for EuPdIn; and a= 755.2(2) pm, b= 472.2(1) pm, c= 842.3(2) pm, V= 0.3004(2) nm3, wR2 = 0.0470, 524 F2 values, 20 variables for EuAuIn. Magnetic susceptibility measurements show Curie–Weiss behaviour above 40 K for both compounds with experimental magnetic moments of 7.6(1)µB/Eu (EuPdIn) and 7.5(1)µB/Eu (EuAuIn), which are close to that of the free Eu2+ ion, namely µeff= 7.94 µB/Eu. At an external field strength of 0.01 T, EuPdIn and EuAuIn order antiferromagnetically at 13.0(5) and 21.0(5) K, respectively. Magnetization measurements at 5 K indicate metamagnetism with a magnetic moment of 5.9(1)µB/Eu at 5.5 T for both compounds. The critical fields amount to 3.1(1) T for EuPdIn and 0.25(2) T for EuAuIn. Both compounds are good metallic conductors with specific resistivities of 23 (EuPdIn) and 96 µΩ cm (EuAuIn) at room temperature.

Low-temperature magnetic structure of UNiGe.

We report on the magnetic structure of single-crystalline UNiGe in its low-temperature commensurate antiferromagnetic phase. UNiGe crystallizes in the orthorhombic TiNiSi structure type with space group {ital Pnma} and undergoes two magnetic transitions at 51 and 42 K. Our data, taken at 20 K, rule out both of the previously reported magnetic structures for the low-temperature phase, though we find an ordering vector {ital q}=(0,1/2,1/2) in agreement with the previous powder study of Murasik {ital et} {ital al}. [J. Phys. Condens. Matter {bold 3}, 1841 (1991)]. We observe both domains of the single-{ital q} structure, and show that this is preferred over a 2-{ital q} structure. We extract a moment of 0.91{mu}{sub {ital B}} per U atom, and it is likely that the moment lies within the {ital b}-{ital c} plane in a collinear moment-density-wave configuration. However, out-of-plane moment components are allowed by symmetry and our best refinement gives noncollinear canting out of the plane by approximately 20{degree}. This structure belongs to one of the two possible irreducible representations, and we discuss its relationship to a more conventional Shubnikov-group analysis. {copyright} {ital 1996 The American Physical Society.}

On the rare-earth palladium aluminides LnPdAl

A hexagonal ZrNiAl- or Fe2P-type high-temperature modification was identified for the compounds LnPdAl with LnSm, Gd ... Tm, Lu, Y. YbPdAl and its Pt analogue were obtained only in the orthorhombic TiNiSi structure, LaPdAl only in the orthorhombic LaNiAl structure. CePdAl transformed to an unknown low-temperature modification, while PrPdAl and NdPdAl retained the ZrNiAl structure on annealing at 700 °C. ScPdAl crystallizes in the hexagonal MgZn2 structure. Antiferromagnetic ordering was observed in hexagonal TbPdAl, whereas DyPdAl and HoPdAl appear to undergo a transition to ferromagnetic order.

Magnetic and crystallographic structures in UTX intermetallic compounds

Uranium, together with other actinides and lanthanides, forms a large group of ternary intermetallic compounds of stoichiometry UTX (Ttransition metal, Xp electron metal). These compounds are formed in several structure types and the occurrence and stability of particular structures with respect to the transition metal content suggests reasonable systematics. We have also investigated the magnetic structures of selected UTX compounds and it is revealing to relate the crystallographic and magnetic structures, because of the relationship between the magnetic symmetry and that of the U atom environment produced by the 5f—ligand hybridization, and the consequent anisotropic exchange. Those of ZrNiAl structure type are collinear, with moments along the hexagonal c axis. In the orthorhombic TiNiSi structure type, the moments are confined to the b− c plane (perpendicular to the uranium chains) and the structures are often incommensurate. In the hexagonal CaIn 2 (or GaGeLi) structure type, the magnetic structures form in an orthorhombic cell and, at least in the disordered centric group, again the moments lie perpendicular to the nearest-neighbour uranium spacing.

Distribution des atomes metalliques dans les structures apparentees des trois composes ternaires equiatomiques : RhMnSi, isotype de Co 2P, RhMnGe, isotype de TiFeSi et PdMnGe, isotype de Fe 2P

The distribution of metallic elements has been studied in three ternary equiatomic transition metal silicides and germanides : RhMnSi with TiNiSi (ordered anti-PbCl 2) structure, PdMnGe with Fe 2P structure and RhMnGe with the closely related TiFeSi structure. The large influence of the relative electronegativity of transition metals upon the crystallographic order was shown.

A note on the crystal structure of two ScCuSi phases

Two phases in the system Sc-Cu-Si were prepared and investigated. Their compositions were established to be very close to 1:1:1. The crystal structure was investigated by means of single-crystal and powder methods. The compounds belong to the TiNiSi and Fe 2P structure types. The peculiarities of the behaviour of scandium in intermetallics are discussed.

Ternary equiatomic transition metal silicides and germanides

The phases TiRhSi, TiPdSi, MnRhSi, ZrFeGe, ZrRhGe, ZrPdGe, and NbRhGe crystallize with the TiNiSi (ordered anti-PbCl 2) structure. ZrRuSi possesses the Fe 2P structure, while TiRuSi has the closely related TiFeSi structure. Cell parameters for these phases and a refinement of the structure of ZrRuSi from X-ray powder data are reported. Relationships among the observed structural types as well as the “filled” NiAs (Ni 2In) type are pointed out. The crystal chemistry of ternary equiatomic phases of two transition metals and silicon or germanium is discussed, emphasizing differences in metal-metal bonding.