Interactions In Intermetallic Compounds Research Articles

Cluster Formation in the Superconducting Complex Intermetallic Compound Be21Pt5.

Materials with the crystal structure of γ-brass type (Cu5Zn8 type) are typical representatives of intermetallic compounds. From the electronic point of view, they are often interpreted using the valence electron concentration approach of Hume-Rothery, developed previously for transition metals. The γ-brass-type phases of the main-group elements are rather rare. The intermetallic compound Be21Pt5, a new member of this family, was synthesized, and its crystal structure, chemical bonding, and physical properties were characterized. Be21Pt5 crystallizes in the cubic space group F4̅3m with lattice parameter a = 15.90417(3) Å and 416 atoms per unit cell. From the crystallographic point of view, the binary substance represents a special family of intermetallic compounds called complex metallic alloys (CMA). The crystal structure was solved by a combination of synchrotron and neutron powder diffraction data. Besides the large difference in the scattering power of the components, the structure solution was hampered by the systematic presence of very weak reflections mimicking wrong symmetry. The structural motif of Be21Pt5 is described as a 2 × 2 × 2 superstructure of the γ-brass structure (Cu5Zn8 type) or 6 × 6 × 6 superstructure of the simple bcc structural pattern with distinct distribution of defects. The main building elements of the crystal structure are four types of nested polyhedral units (clusters) with the compositions Be22Pt4 and Be20Pt6. Each cluster contains four shells (4 + 4 + 6 + 12 atoms). Clusters with different compositions reveal various occupation of the shells by platinum and beryllium. Polyhedral nested units with the same composition differ by the distance of the shell atoms to the cluster center. Analysis of chemical bonding was made applying the electron localizability approach, a quantum chemical technique operating in real space that is proven to be especially efficient for intermetallic compounds. Evaluations of the calculated electron density and electron localizability indicator (ELI-D) revealed multicenter bonding, being in accordance with the low valence electron count per atom in Be21Pt5. A new type of atomic interactions in intermetallic compounds, cluster bonds involving 8 or even 14 atoms, is found in the clusters with shorter distances between the shell atoms and the cluster centers. In the remaining clusters, four- and five-center bonds characterize the atomic interactions. Multicluster interactions within the polyhedral nested units and three-center polar intercluster bonds result in a three-dimensional framework resembling the structural pattern of NaCl. Be21Pt5 is a diamagnetic metal and one of rather rare CMA compounds revealing superconductivity (Tc = 2.06 K).

Tuning the magnetic and structural transitions in TbCo2Mnx compounds

The wide ranging magnetic behavior in intermetallic compounds continues to attract broad interest. Effective control of their magnetic properties is of great importance for fundamental research and potential applications. In this work the structural and magnetic properties of $\mathrm{TbC}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{x}$ compounds are studied by a combination of temperature dependent synchrotron x-ray diffraction, neutron powder diffraction, specific heat, and magnetic measurements. Magnetization measurements show that the addition of Mn can modify the magnetic behavior significantly: first, the magnetic transition temperatures increase from \ensuremath{\sim}227 K to 332 K with $x=0.0\phantom{\rule{0.28em}{0ex}}\mathrm{to}\phantom{\rule{0.28em}{0ex}}0.3$; secondly, the nature of the magnetic transitions change from the first order to second order, as identified by three methods (Banerjee criterion, master curves of magnetic entropy changes, and detailed crystal structure analysis through neutron diffraction). Both synchrotron x-ray diffraction and neutron diffraction confirm that a structural transition, from cubic $Fd\overline{3}m$ to rhombohedral $R\overline{3}m$ on cooling, occurred accompanying the magnetic transition. To further clarify the nature of the second order magnetic phase transitions, we have carried out a detailed critical exponent analysis. The derived critical exponents are close to the theoretical prediction from the mean-field model, indicating the magnetic interactions are long range. This work benefits our general understanding of magnetic interactions in intermetallic compounds and provides guidance to design a functional magnetic material for room temperature magnetic devices.

Solute-solute interactions in intermetallic compounds

Two types of solute-solute interactions are investigated in this work. Quadrupole interactions caused by nearby Ag-solute atoms were measured at nuclei of 111In/Cd solute probe atoms in the binary compound GdAl2 using the method of perturbed angular correlation of gamma rays (PAC). Locations of In-probes and Ag-solutes on both Gd- and Al-sublattices were identified by comparing site fractions in Gd-poor and Gd-rich GdAl2(Ag) samples. Interaction enthalpies between solute-atom pairs were determined from temperature dependences of observed site fractions. Repulsive interactions were observed for close-neighbor complexes In/Gd/+Ag/Gd/ and In/Gd/+Ag/Al/ pairs, whereas a slightly attractive interaction was observed for In/Al/+Ag/Al/. Interaction enthalpies were all in the range +/- 0.15 eV. Temperature dependences of site fractions of In-probes on locally defect-free Gd- and Al-sites yields a transfer enthalpy that was found to be 0.343 eV in a previous study of undoped GdAl2. The corresponding values in GdAl2(Ag) samples are much smaller. This is attributed to competition of In- and Ag-solutes to occupy sites of the same sublattice. While the difference in site-enthalpies of In-solutes on Gd- and Al-sites is temperature independent, it is proposed that the transfer of Ag-solutes from Gd- to Al-sites leads to a large temperature dependence of degeneracies of levels available to In-solutes, resulting in an effective transfer enthalpy that is much smaller than the difference in site-enthalpies.

Synthesis of hydrides by interaction of intermetallic compounds with ammonia

Interaction of intermetallic compounds with ammonia was studied as a processing route to synthesize hydrides and hydridonitrides of intermetallic compounds having various stoichiometries and types of crystal structures, including A2B, AB, AB2, AB5 and A2B17 (A=Mg, Ti, Zr, Sc, Nd, Sm; B=transition metals, including Fe, Co, Ni, Ti and nontransition elements, Al and B). In presence of NH4Cl used as an activator of the reaction between ammonia and intermetallic alloys, their interaction proceeds at rather mild P–T conditions, at temperatures 100–200°C and at pressures of 0.6–0.8MPa. The mechanism of interaction of the alloys with ammonia appears to be temperature-dependent and, following a rise of the interaction temperature, it leads to the formation of interstitial hydrides; interstitial hydridonitrides; disproportionation products (binary hydride; new intermetallic hydrides and binary nitrides) or new metal–nitrogen–hydrogen compounds like magnesium amide Mg(NH2)2. The interaction results in the synthesis of the nanopowders where hydrogen and nitrogen atoms become incorporated into the crystal lattices of the intermetallic alloys. The nitrogenated materials have the smallest particle size, down to 40nm, and a specific surface area close to 20m2/g.

Peculiar features of interaction of intermetallic compounds based on Ti-Al, Ni-Al with Ti and Ni metallic substrates in the mode of self-propagating high-temperature synthesis

Self-propagating high-temperature synthesis (SHS) is applied for production of titanium and nickel aluminides with various operating properties, which is important for deformation of items in the SHS mode and application of coatings on intermetallic and metallic substrates (for instance, Ti, Ni, Al, and others). The possibilities of application of intermetallic coatings and joining (welding) of such materials in the SHS mode are studied.

Synthesis, Crystal Structure, Atomic Hirshfeld Surfaces, and Physical Properties of Hexagonal CeMnNi4

The hexagonal polymorph of CeMnNi(4) has been synthesized using cold crucible, high-frequency induction melting with subsequent Czochralski crystal pulling. Single-crystal X-ray diffraction, multitemperature synchrotron powder X-ray diffraction (90 to 600 K), and neutron powder diffraction data have been measured to establish the detailed crystal structure and in particular the location of the Mn atoms. The neutron diffraction data provide sufficient scattering contrast between Mn and Ni to establish that the 2c site has an occupancy of 13% Mn atoms, while the 3g site has an occupancy of 25% Mn atoms. Thus, the crystal structure is complex with considerable disorder. Rietveld refinement of the multitemperature synchrotron data establishes a near linear thermal expansion coefficient of 13.9(3) × 10(-6) K(-1) and 14.9(3) × 10(-6) K(-1) for the a and c axes, respectively. Atomic Hirshfeld surfaces are introduced as a new tool to investigate the atomic coordination and interactions in intermetallic compounds. The atomic displacement parameters (ADPs) are observed to be much larger for the heavy Ce atom than for the lighter Mn and Ni atoms, and this correlates with the large atomic Hirshfeld volume of Ce relative to Mn and Ni. The fit of a Debye model to the ADPs gives θ(D) = 312(3) K. Magnetic susceptibility data measured between 2 and 350 K indicate ferromagnetic ordering at 122(2) K (Weiss constant) based on a linear fit of the inverse magnetic susceptibility in the paramagnetic region. Transport properties were measured on a polycrystalline sample containing CeO(2) (2.8%) and Ni (7.7%) impurities. The electrical conductivity is observed to be metallic with a distinct kink in the data around 120 K coinciding with the observed Curie temperature. The lattice thermal conductivity (κ(L)) increases from 0.5 W/Km at 2 K to 8 W/Km at 50 K, and the relatively moderate value of κ(L) probably reflects the significant structural disorder.

Interaction of intermetallic compounds of rare-earth metals, nickel, and aluminum with hydrogen

AtT=293 K andpH2≤10 MPa. we synthesized hydridesR3Ni8Al-H7.2–15.2 (R=Nd. Sm. Gd. Tb. Dy, Ho, Er, Tm, and Lu). The X-ray analysis revealed an isotropic deformation of the original crystal lattice due to hydrogen penetration. For the hydrides obtained, the lattice constanta exceeds that of the original intermetallide by 3.9–6.0% and the constantc and the cell volumeV are increased by 6.0–12.6% and 15.6–26.4%, respectively. The volume expansion per absorbed hydrogen atom is (2.4–3.6)·10−3 nm−3. By construction of the isotherms of hydrogen desorption for the systemsR3Ni>8Al-H, we established the existence of the α-solid hydrogen solution and the region of its coexistence with β-hydride, the region of coexistence of β- and γ-hydrides, and the β- and γ-hydride phases. Heats of the phase transitions were determined as follows: ΔH=31.4±0.8 kJ/(mole H2) in the system Gd3Ni8Al-H for the transition γ→β, ΔH=39.8±1.1 kJ/(mole H2) in the system Tb3Ni8Al−H for the transition β→α. and ΔH=37.1±5.1 kJ/(mole H2) in the system Tm3Ni8Al−H for the transition γ→β. Among the products of hydrogenation, intermetallic compounds Lu3Ni8Al and Tm3Ni8Al. we revealed the β′- and γ-phases. We also found that, atpH2≤10 MPa andT=400–430 K, the phases of Sm3Ni8Al and Gd3Ni8Al decay into two hydridesRH2–3 andR(Ni.Al)5Hx.

Interaction of intermetallic compounds with hydrogen at pressures up to 250 MPa: the LaCo 5− xMn x-H 2 and CeNi 5−H 2 systems

A new method has been proposed for the synthesis and thermodynamic study of intermetallic hydrides at pressures of gaseous hydrogen up to 250 MPa. From preliminary experiments on hydrogen compressibility and consideration of available p- V- T data for hydrogen at high pressures we have chosen a modified van der Waals equation giving a calculation error of 0.1%–0.5% or less. Absorption and desorption isotherms for the LaCo 5− x Mn x -H 2 ( x−0.05) and CeNi 5−H 2 systems were measured at temperatures from 195 K up to room temperature. The highest hydrogen content obtained in a hydride composition of LaCo 5− x Mn x does not exceed 8 at. H per formula unit, and for the high pressure plateau ΔH abs= ΔH des=14.4 kJ (mol H 2) −1 and ΔS abs= ΔS des=110.2 J K −1 (mol H 2) −1. In the CeNi 5-H 2 system the existence of a single plateau and considerable hysteresis were observed.

NMR-analysis of indirect magnetic interactions in intermetallic rare-earth compounds

NMR Spectroscopy has given several pieces of information with regard to the indirect magnetic interactions in intermetallic compounds of gadolinium or europium. These comprise information on the contribution of non-s conduction electrons, on the predominance of short-range interactions, on the anisotropy of the transferred hyperfine interaction and on the range of d electron spin polarization.

Hyperfine interactions in intermetallic compounds between Gd and 3 d transition metals

Measurements of hyperfine interactions at 155Gd nuclei in metallic compounds between Gd and 3 d transition metals and at 61Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4 f moments is directly related to that caused by 3 d moments.

Magnetic interactions in intermetallic compounds

The magnetic interactions in intermetallic compounds containing 4 f or 3 d electrons are discussed. The magnetism of rare-earth intermetallic compounds can adequately be described by the RKKY model. An evaluation of the validity of this model is given by means of experimental data presented for the compounds Gd 2Al, Gd 3Al 2, GdAl 2, GdPdIn, and GdCuAl, together with the changes in the magnetic properties observed when Gd is gradually replaced by La, Y (no change in conduction electron concentration) or by Th (increase of the conduction electron concentration). In GdAl 2 it also proved possible to replace Gd by non-magnetic Yb, which decreases the conduction electron concentration. The magnetic properties and the magnetovolume effects of various intermetallic compounds of Fe, Co, and Ni, with the rare earth, Y or Th are discussed in terms of a model involving itinerant 3 d electrons.

Proton Magnetic Resonance Studies of Structure, Diffusion, and Resonance Shifts in Titanium Hydride

Measurements of the proton magnetic resonance at ∼26.9 Mc in titanium hydride samples, ranging in composition from TiH1.61 to TiH1.97, have been made in the temperature range from —196° to about 200°C. The second moment of the proton resonance at the lower temperatures shows that the hydrogen atoms are randomly distributed among the lattice sites which are located tetrahedrally with respect to the titanium atoms. Self-diffusion of the hydrogen atoms narrows the proton resonance above room temperature. The temperature dependence of the correlation frequency for the proton motions, obtained from the linewidths, leads to diffusional activation energies which increase with hydrogen content from 9.4 kcal/g atom for TiH1.607 to 10.2 for TiH1.923. Moreover, the diffusion rate is directly proportional to the number of unfilled tetrahedral holes in the metallic lattice, which indicates that the self-diffusion takes place via a vacancy mechanism. Proton resonance shifts to higher applied magnetic fields were observed. They were measured at room temperature for all specimens and were found to increase from 0.01% for TiH1.607 to about 0.032% for TiH1.969. For these two extreme compositions, the temperature dependence of the shift was measured between —95°C and 190°C and was found to be similar to the bulk susceptibility, the shifts for TiH1.969 exhibiting an anomaly at about 13°C as does the susceptibility. These results are interpreted semiquantitatively in terms of exchange interactions which pair spins of electrons in the conduction band with those of electrons localized on the hydrogen. The results suggest that the hydrogen is held in the lattice by a combination of covalent and ionic bonding, the latter involving a net positive charge on the hydrogen. The general importance of exchange interactions in intermetallic compounds is commented upon.