Ni Sublattice Research Articles

Antisite Defects of the L12 Structure Determined by the Phase Field Microelasticity Model

A phase field microelasticity simulation is performed to examine the antisite defect of L12-Ni3Al in Ni75 Al5.3 V19.7 ternary alloy. Combinimg strain energy with the phase field model leads to an atom configuration change as time proceeds. For the Ni sublattice, the antisite defect AlNi, the equilibrium occupancy probability (OP) of which declines, precedes NiNi and VNi in reaching equilibrium; subsequently, NiNi and VNi present a phenomenon of symmetrical rise and decline individually. Similarly, for the Al sublattice, the antisite defect NiAl, the OP of which eventually rises, takes fewer time steps than AlAi and VAl to attain equilibrium. Thereafter, AlAl rises while VAl declines symmetrically at the axes of the NiAlMAI curve. Furthermore, the OP for the Al sublattice is much more sensitive to strain energy than that for the Ni sublattice.

Application of mechanical spectroscopy to studies of atomic diffusion in ordered compounds

An example of useful application of mechanical spectroscopy to diffusion measurements is presented. A thermally activated anelastic relaxation effect has been observed in Ni 3Ga, an intermetallic compound of the L1 2 structure. The relaxation is of similar nature to that reported for Ni 3Al [H. Numakura, K. Nishi, Mater. Sci. Eng. A 442 (2006) 59–62], and is attributed to stress-induced reorientation of Ga atoms in the Ni sublattice. By assuming the diffusion mechanism proposed for these compounds [H. Numakura, T. Ikeda, M. Koiwa, A. Almazouzi, Phil. Mag. A 77 (1998) 887–909], the diffusion coefficient of Ga has been evaluated from the relaxation rate. It agrees fairly well with the extrapolation of the high-temperature data of the tracer diffusion coefficient in the literature.

Thermal magnetic properties of the Ni sublattice in half-metallic NiMnSb: A theoretical study based on first-principles calculations

Thermal magnetic properties of the Ni sublattice in half-metallic NiMnSb: A theoretical study based on first-principles calculations

The effect of minor elements on the growth and electrical properties of NiO on Ni

In this study, the effects of Cu additions and external doping with CeO 2 on the oxidation of nickel were evaluated. The materials studied were pure Ni, Ni–5 wt.% Cu, and Ni with the surface doped with CeO 2 by pulsed laser deposition (PLD). The oxidation kinetics were measured using thermogravimetric analysis (TGA). The oxidation microstructures were observed by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS). A 4-point probe was used to measure the area-specific resistance (ASR) of the NiO scales. The Cu additions had a negligible effect on the oxidation kinetics but Cu was found to be present in the outer portions of the scale in significant concentrations. Doping with CeO 2 resulted in a significant decrease in the NiO growth rate. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO 2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. This suggests that the poisoned interface model, proposed by Pieraggi and Rapp, does not describe the effect of the CeO 2 dopant. The ASR values decreased commensurate with the scale thicknesses indicating that there was insignificant doping of Ce + 4 ions onto the Ni sublattice in the scales. Mechanisms are presented to attempt to describe the above observations and a theoretical model for calculating ASR is presented.

Chromium and tantalum site substitution patterns in Ni3Al(L12) γ′-precipitates

The site substitution behavior of Cr and Ta in the Ni3Al(L12)-type γ′-precipitates of a Ni–Al–Cr–Ta alloy is investigated by atom-probe tomography (APT) and first-principles calculations. Measurements of the γ′-phase composition by APT suggest that Al, Cr, and Ta share the Al sublattice sites of the γ′-precipitates. The calculated substitutional energies of the solute atoms at the Ni and Al sublattice sites indicate that Ta has a strong preference for the Al sites, while Cr has a weak Al site preference. Furthermore, Ta is shown to replace Cr at the Al sublattice sites of the γ′-precipitates, altering the elemental phase partitioning behavior of the Ni–Al–Cr–Ta alloy.

Charge disproportionation and collinear magnetic order in the frustrated triangular antiferromagnetAgNiO2

We report a high-resolution neutron diffraction study of the crystal and magnetic structure of the orbitally degenerate frustrated metallic magnet ${\text{AgNiO}}_{2}$. At high temperatures the structure is hexagonal with a single crystallographic Ni site, low-spin ${\text{Ni}}^{3+}$ with spin 1/2 and twofold orbital degeneracy, arranged in an antiferromagnetic triangular lattice with frustrated spin and orbital order. A structural transition occurs upon cooling below 365 K to a tripled hexagonal unit cell containing three crystallographically distinct Ni sites with expanded and contracted ${\text{NiO}}_{6}$ octahedra, naturally explained by spontaneous charge order on the Ni triangular layers. No Jahn-Teller distortions occur, suggesting that charge order occurs in order to lift the orbital degeneracy. Symmetry analysis of the inferred Ni charge order pattern and the observed oxygen displacement pattern suggests that the transition could be mediated by charge fluctuations at the Ni sites coupled to a soft oxygen optical phonon breathing mode. At low temperatures the electron-rich Ni sublattice (assigned to a valence close to ${\text{Ni}}^{2+}$ with $S=1$) orders magnetically into a collinear stripe structure of ferromagnetic rows ordered antiferromagnetically in the triangular planes. We discuss the stability of this uncommon spin order pattern in the context of an easy-axis triangular antiferromagnet with additional weak second-neighbor interactions and interlayer couplings.

Diffusion of Re and Ru in the γ′ Phase of Ni Based Alloys

The diffusion of Re and Ru in the � 0 -Ni3Al phase has been investigated at a range of temperatures 1423–1523 K by using the pseudo-binary diffusion couples. It was found that the concentration gradient of Re appeared to promote the uphill diffusion of Al, whereas that of Ru did not promote the Al uphill diffusion. The cross interdiffusion coefficients of ~ D Ni and ~ D Ni were independent of the matrix ordering since they were comparable qualitatively between those in the � 0 -Ni3Al and in the � -Ni phases. The tracer diffusion coefficients for Re and Ru in Ni3Al were estimated by extrapolation and the obtained activation energies were consistent with the site preferences of the elements reported in the literature. Further, the results of this work suggest that the diffusion of Ru in Ni3Al is mostly governed by the diffusion in Ni sublattice and the diffusion of Re is mainly controlled by the formation of anti-site defects in the Ni sublattice with negligible contribution of the anti-site bridge mechanism. [doi:10.2320/matertrans.MRA2007632]

Magnetic structure of theS=1Ni5(TeO3)4Br2layered system governed by magnetic anisotropy

Magnetization, neutron diffraction, and antiferromagnetic resonance measurements were employed to investigate the magnetic ground state of Ni-5(TeO3)(4)Br-2 single crystal. Despite the layered topology of the Ni2+ sites, typical for frustrated antiferromagnets, the system orders antiferromagnetically below T-N=29 K. Noncollinear arrangements of the Ni sublattices having a very complex temperature dependence were found from the neutron diffraction and magnetization measurements. Single-ion anisotropy, associated with the strongly distorted Ni-centered octahedra, has the same magnitude as the antiferromagnetic exchange interaction. The effect of these anisotropies prevails over the geometrical frustration leading to a long-range magnetic ordering below T-N.

Site preference of transition-metal elements in B2 NiAl: A comprehensive study

First-principles supercell calculations based on density functional theory were performed to study the T = 0 K site preference of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition-metal elements in B2 NiAl. By adopting a statistical-mechanical Wagner–Schottky model within the canonical ensemble, the effects of finite temperature on site preference were further considered. The calculations showed that, at all alloy compositions and temperatures, Co, Tc, Ru, Rh, Re, Os, Ir and Pt have a consistent preference for the Ni sublattice, while Ti, Zr, Nb, Hf and Ta have a consistent preference for the Al sublattice. In contrast, the site preference of V, Cr, Mn, Fe, Cu, Mo, Pd, Ag, W and Au was found to depend on both composition and temperature. The present calculated results compare favorably with existing theoretical and experimental studies in the literature.

Atomic Partitioning of Platinum and Ruthenium in Advanced Single Crystal Ni-Based Superalloys

The elemental partitioning characteristics of advanced single crystal Ni-base superalloys containing additions of both Pt and Ru have been investigated using atom probe tomography. Detailed microanalysis revealed Ru additions partitioned preferentially to the disordered matrix, whereas Pt additions tended to partition to the ordered intermetallic γ′ precipitates. The partitioning characteristics of three nominally similar alloys with systematic variations in the levels of Cr, Ru and Pt were investigated. For this particular set of experimental alloys, minor changes in the partitioning characteristics of the constituent elements could be attributed to additions of Cr and Ru. The preferential site occupancy of Pt within the L12 lattice was also statistically quantified with ALCHEMI (atomic site location by channelling enhanced microanalysis). In contrast to the atomic partitioning characteristics of Ru, Pt exhibited a tendency to occupy the Ni sublattice sites in the γ′ structure.

Magnetothermopower in Nd1−xEuxNiO3 compounds

We have measured magnetization M(T,H), thermal conductivity κ(T,H), and thermopower S(T,H) of polycrystalline samples of Nd1−xEuxNiO3, 0⩽x⩽0.35, as a function of temperature and external magnetic field. The data indicate a metal-insulator (MI) transition in a wide range of temperature (200<TMI<325K). The magnetic susceptibility χ(T) data, after the subtraction of the rare-earth contribution, exhibit a Curie-Weiss-like behavior at temperatures above TMI. Although a clear antiferromagnetic AF transition of the Ni sublattice is observed at TN⩽TMI, χ(T) still increases down to 5K, suggesting a heterogeneous ground state. The thermal conductivity of the NdNiO3 compound is not affected by an external magnetic field of 90kOe in a wide range of temperature, and its temperature dependence below 15K is approximately quadratic, strongly suggesting the presence of disorder. S(T) is negative above TMI and varies linearly with temperature. Below TMI, there is a minimum close to 120K, and S(T) changes its sign at T∼30K, indicating a competition between two types of charge carriers. A pronounced peak in S(T) at TS∼20K is observed and the peak remains unaltered under magnetic fields up to 90kOe. However, its magnitude is enhanced by ∼25% with applied magnetic field, exhibiting a clear magnetothermopower effect. The combined results indicate a coexistence of ordered and disordered phases below TN and that an applied magnetic field is suitable for enhancing the thermoelectric properties close to TS.

Identification of lattice vacancies in the B2-phase region of Ni–Al system by positron annihilation

The point defects of B2-type intermetallic compound Ni–Al have been investigated as a function of different Ni compositions ranging from 47 to 55 at.% by means of positron lifetime and coincidence Doppler-broadening (CDB) measurements. The position lifetime of the annealed samples, expressed by single-component lifetime of τ = 170–180 ps, indicates positron saturation trapping at single vacancies in Ni–Al samples. The CDB measurements have been applied to identify these vacancy-type defects formed on the Ni sublattice in the B2-phase Ni–Al alloys.

Searching for switchable molecular conductors: Salts of [M(dcbdt) 2] (M = Ni, Au) anions with [Fe(sal 2-trien)] + and [Fe(phen) 3] 2+

Salts of [Fe III(sal 2-trien)] +and [Fe II(phen) 3] 2+ cations and M[(dcbdt) 2] − anions with M = Ni and Au (dcbdt = dicyanobenzenedithiolate) with formula [Fe(sal 2-trien)] [M(dcbdt) 2] and [Fe(phen) 3] [M(dcbdt) 2] 2 were obtained and characterized by single X-ray diffraction and magnetic measurements. None of these salts shows a clear spin crossover behaviour and their magnetic properties are due essentially to the cations in a high spin S = 5/2 and low spin states for the Fe III and Fe II salts respectively. The magnetic Ni sublattices in both compounds appear to have a negligible direct contribution to the magnetization but enhance the AF interactions in the cation sublattice.

Atomistic simulations of diffusion mechanisms in off-stoichiometric Al-richNi3Al

This paper presents dynamics simulation results of diffusion in off-stoichiometric Al-richNi3Al (Ni73Al27) attemperature ranging from 1300 to 1550 K. The interatomic forces are described by the Finnis–Sinclairtype N-body potentials. Particular attention is devoted to the effect of the extra2% of Al atoms sitting on the Ni sublattice as antisite point defects(AlNi) on diffusion. Simulation results show that Ni atoms mainly diffuse through theNi sublattice at the temperatures investigated. Al atoms diffuse via both theintrasublattice and antistructure bridge (ASB) mechanisms. The contributionto Al diffusion from the ASB mechanism decreases at the lower temperature(T<1400 K). The presence of antisite point defects(AlNi) enhances both Aland Ni diffusion in Ni73Al27. The Ni–Al coupled diffusion effect is observed and understood at the atomic level for thefirst time.

Magnetic phase diagrams of R 3(Co:Ni) 13B 2, R = Y and Nd intermetallic compounds

The magnetization, magnetic susceptibility, heat capacity and neutron powder diffraction (NPD) data of the series R 3Ni 13− x Co x B 2, R = Y and Nd are reviewed, and the magnetic phase diagrams are given for the two R substitutions. The influence of the Y substitution by Nd is to increase the cell constants, and the average Co moment. The Ni substitution by Co modifies the magnetic properties from weak itinerant behavior due to the Ni sublattice to strongly coupled Co moments near to the rigid band model. In the Co rich side of the solid solution Nd 3Ni 13− x Co x B 2 a spin reorientation transition (SRT) is induced.

On the propensity of magnetism in 3d transition-metal-MgCNi 3 alloys

The effects of disorder and incipient magnetism in MgC ( Ni 1 - x T x ) 3 ( T ≡ Fe , Co or Cu) alloys are studied using coherent-potential approximation and Ginzburg–Landau coefficients. The first-principles, local-density-functional-based calculations for substitutionally disordered Fe and Co impurities in the Ni sub-lattice of MgCNi 3, in low concentrations, show that incipient magnetism resides in these materials. The overestimation of the calculated magnetic properties points to the limitations of the local-density approximation. However, using a phenomenological approach based on Ginzburg–Landau coefficients and the fixed-spin moment method, we show that MgC ( Ni 1 - x T x ) 3 alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC ( Ni 0.95 Fe 0.05 ) 3 and MgC ( Ni 0.90 Co 0.10 ) 3 alloys.

Magnetic properties of LiNi 0.5Mn 1.5O 4 spinels prepared by wet chemical methods

We present the magnetic properties of LiNi 0.5Mn 1.5O 4 spinels prepared by the sol–gel and pyrolysis techniques. Structural properties show that the material synthesized by pyrolysis method exhibit an ordered spinel structure. Characterization methods include SQUID magnetometry and ESR spectroscopy. Magnetic measurements have evidenced the ferromagnetic ordering below T c = 129 K in LiNi 0.5Mn 1.5O 4. Results show that actually no impurity phase is detected in LiNi 0.5Mn 1.5O 4, thus the ferrimagnetic behavior is attributed to an intrinsic property of this material. The magnetic order in this case is trivially a collinear ferrimagnetic ordering in which both the Ni sublattice and the Mn sublattice are ferromagnetic. ESR measurements show a two-component signal with a complex shape. The dominant band is assigned to Mn 4+ ions that are the only paramagnetic entities in this compound.

Atomistic simulations of diffusion mechanisms in stoichiometricNi3Al

The first molecular dynamics (MD) simulation of diffusion mechanisms in ordered stoichiometricNi3Al with the Finnis–Sinclair interatomic potential is reported in this paper. Results show thatNi atoms mainly diffuse through Ni sublattices via the vacancy mechanism; Al atomsdiffuse via both the antistructure bridge and the intrasublattice mechanism. Molecularstatics (MS) simulations are carried out to estimate the migration energy barriers for theelementary jumps and the different diffusion mechanisms with both the ‘embedded atommethod’ and Finnis–Sinclair interatomic potentials. It is proven that the six-jump cyclemechanism is not energetically favourable because of the high migration energy. Thus, MDand MS simulations yield the same results on the diffusion mechanism in stoichiometricNi3Al. Ni diffusion mechanisms obtained in this work agree with experimental results andtheoretical calculation.

A combined first-principles and experimental study of the lattice site preference of Pt in B2 NiAl

First-principles supercell calculations were undertaken to determine the site preference of Pt in the B2 NiAl lattice structure. The results from these calculations were compared to structural analysis results from high-energy X-ray diffraction experiments. The calculations showed that, at T = 0 K, Pt always has a preference for the Ni sublattice in B2 NiAl, with such a preference being quite strong in Al-rich NiAl, but relatively weak in Ni-rich and stoichiometric NiAl. To predict the site occupation of Pt in B2 NiAl at finite temperatures, a statistical–mechanical Wagner–Schottky model was used in which the formation enthalpies of point defects were determined from the present first-principles calculations. Calculations at T = 1273 K again showed a consistent preference of Pt for the Ni sublattice in Al-rich, Ni-rich and stoichiometric B2 NiAl. The results from these latter calculations were found to be in very good agreement with those from experiment.

Strain field fluctuation effects on lattice thermal conductivity of ZrNiSn-based thermoelectric compounds

We present a model calculation of the lattice thermal conductivity of ZrNiSn-based half-Heusler thermoelectric compounds for temperatures where phonon scattering is dominated by Umklapp and point defect scattering. The difference in mass between impurity and host atoms dominates point defect scattering for alloying Hf on the Zr sublattice, whereas differences in size and interatomic coupling forces between impurity and host atoms dominate point defect scattering for alloying Pd on the Ni sublattice. Because Pt is heavier and larger than Pd, we predict that Pt will further reduce lattice thermal conductivity when alloyed on the Ni sublattice of these half-Heusler compounds.