Ni Sublattice Research Articles

Phase separation and antisite defects in the thermoelectric TiNiSn half-Heusler alloys

The half-Heusler TiNiSn alloys have recently gained an attention as promising candidates for thermoelectric applications. Improvement of these alloys for such applications can be obtained by both electronic and compositional optimizations. The latter can result in a miscibility gap, allowing a phase separation in the nano-scale and consequently a thermal conductivity reduction. Combination of ab initio calculations and statistical thermodynamics was applied for studying the relative stability of a number of superstructures in TiNiSn based alloys. The quasi-binary phase diagram beyond T=0 K for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated using energy parameters extracted from the total energy calculations for ordered structures in the Ni sublattice. We demonstrated that a decomposition of the off-stoichiometric Ni-rich half-Heusler alloy into the stoichiometric TiNiSn phase and into Ni deficient Heusler TiNi{sub 2}Sn phase occurs at elevated temperatures—an effect which recently had been observed experimentally. Furthermore, favorable energetic conditions for antisite defects formation were deduced, based on calculations of the energy of formation, an effect which was explained as a cooperative process of partial disordering on the Ni sublattice. The influence of these two effects on improvement of the thermoelectric performance of TiNiSn based half Heusler compounds is discussed. - Graphical abstract: Phase separationmore » and antisite defects in the thermoelectric TiNiSn alloy, are covered as methods for nanostructuring and thereby enhancement of the thermoelectric potential. - Highlights: • Ab initio calculations/statistical thermodynamics was applied for studying the TiNiSn system. • The phase diagram for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated. • Decomposition of the Ni-rich HH into TiNiSn and Ni deficient TiNi{sub 2}Sn phases was observed. • Favorable energetic conditions for antisite defects formation were deduced.« less

Structural and magnetic properties of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4−δ spinels: A first-principles study

Structural and magnetic properties of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4−δ are investigated using density-functional theory calculations. Results indicate that nonstoichiometric LiNi0.5Mn1.5O4−δ and stoichiometric LiNi0.5Mn1.5O4 exhibit two different structures, i.e., the face-centred cubic (Fd-3m) and primitive, or simple, cubic (P4332) space groups, respectively. It is found that the magnetic ground state of LiNi0.5Mn1.5O4 (P4332 and Fd-3m) is a ferrimagnetic state in which the Ni and Mn sublattices are ferromagnetically ordered along the [110] direction whereas they are antiferromagnetic with respect to each other. We demonstrate that it is the presence of an O-vacancy in LiNi0.5Mn1.5O4−δ with the Fd-3m space group that results in its superior electronic conductivity compared with LiNi0.5Mn1.5O4 with the P4332 space group.

Phase transformations in Ni + Al and Ni + Al + Cr mixtures during mechanochemical synthesis and subsequent heating

Mechanochemical processing of elemental mixtures with the compositions Ni75Al25, Ni70Al25Cr5, and Ni75Al20Cr5 (5 at % Cr in the mixtures instead of the equivalent amount of Ni or Al) leads to the formation of nanocrystalline nickel-based solid solutions (crystallite size in the range ≃ 7–12 nm). Comparison of experimentally determined lattice parameters of the solid solutions with Vegard’s law values and with the lattice parameters evaluated using the Bozzolo-Ferrante rule, which takes into account the bulk moduli of constituent elements, suggests that the atoms in the solid solutions are bonded more strongly. Heating the synthesized ternary solid solutions in a calorimeter to 1000°C leads to the formation of an ordered γ′-phase (L12). Analysis of the relative intensity ratio of superlattice and fundamental reflections indicates that the Cr atoms always reside in the Al sublattice, independent of the composition of the starting mixture. When 5 at % Cr is incorporated instead of Ni, the chromium atoms force out aluminum from the Al sublattice, and the Ni deficiency in the Ni sublattice is compensated by the Al atoms. The ordered phases remain nanocrystalline (crystallite size in the range ≃ 40–70 nm).

Site occupation evolution of alloying elements in L12 phase during phase transformation in Ni75Al7.5V17.5

Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.

First-principles study of self- and solute diffusion mechanisms inγ′-Ni3Al

Multiple, competing diffusion mechanisms have been postulated in L1${}_{2}$ ordered compounds such as Ni${}_{3}$Al due to the presence of two distinct sublattices for self- and solute diffusion. They involve intrasublattice, intersublattice, two-pair vacancies, the six-jump cycle, and the antisite assisted jumps. In this work we systematically study these diffusion paths in L1${}_{2}$ ordered Ni${}_{3}$Al, postulated from three decades of experiments, by coupling density functional theory and the nudged elastic band methods. We determine activation barriers for self-diffusion (Ni and Al) atoms and solute diffusion (Cr, Co, and Ti) atoms, and also explore how solutes influence self-diffusion activation barriers. These calculations reveal that Ni vacancies mediate both self- and solute diffusion in ordered Ni${}_{3}$Al. Other findings include: (i) Ni atoms diffuse in their own sublattice via nearest neighbor jumps, (ii) Al atoms also preferentially diffuse in the Ni sublattice, (iii) solutes in the proximity of vacancies strongly affect the formation and also the migration energies in problems studied here, and (iv) diffusion mechanism(s) of solutes depend strongly on whether they occupy a Ni or Al site. This site occupancy is predicted by our activation barrier calculations and agrees with experiments. Our calculations reveal that antisite assisted solute diffusion has the lowest activation barrier. However, experiments report a higher activation barrier that correspond to the barriers for the six jump cycle mechanism determined by our calculations. Thus, our calculations together with published experimental work point to multiple, competing mechanisms driving the diffusion of ternary elements in Ni${}_{3}$Al.

Local structure of NiAl compounds investigated by extended X-ray absorption fine-structure spectroscopy

The local structures of pure NiAl and Ti-, Co-doped NiAl compounds have been obtained utilizing extended X-ray absorption fine-structure (EXAFS) spectroscopy. The results provide experimental evidence that Ni antisite defects exist in the Ni-rich NiAl compounds. The site preference of Ti and Co has been confirmed. Ti occupies the Al sublattice, while Co occupies the Ni sublattice. The structure parameters obtained by EXAFS were consistent with the X-ray diffraction results. Owing to the precipitation of α-Cr, the local structure of NiAl-Cr has not been obtained, making the site preference of Cr unclear.

Disorder trapping during crystallization of theB2-ordered NiAl compound

Using molecular dynamics simulations, disorder trapping associated with solidification is studied for the (100), (110), and (111) growth directions in the B2 NiAl ordered alloy compound. At the high interface velocities studied we observe pronounced disorder and defect trapping, i.e., the formation of antisite defects and vacancies in the crystal at higher than equilibrium concentrations upon rapid solidification. The vacancies are located primarily on the Ni sublattice and the majority of antisite defects are Ni atoms on the Al sublattice, while the concentration of Al on the Ni sublattice is negligibly small. The defect concentration is found to increase in an approximately linear relationship with increasing the interface velocity. Further there is no significant anisotropy in the defect concentrations for different interface orientations. Our results suggest that the currently available models of disorder trapping should be extended to include both antisite defects and vacancies.

First—principles study of stacking fault energies in Ni3Al intermetallic alloys

The first-principles method based on the projector augmented wave method within the generalized gradient approximation was employed to calculate the superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF) energies of the binary Ni3Al alloys with different Al contents and the ternary Ni3Al intermetallic alloys with addition of alloying elements, such as Pd, Pt, Ti, Mo, Ta, W and Re. The results show that the energies of SISF and CSF increase significantly with increase of Al contents in Ni3Al. Addition of Pd and Pt occupying the Ni sublattices does not change the SISF and CSF energies of Ni3Al markedly in comparison with the Ni–23.75Al alloy. While addition of alloying elements, such as Ti, Mo, Ta, W and Re, occupying the Al sublattices dramatically increases the SISF and CSF energies of Ni3Al. The results suggest that the energies of SISF and CSF are dependent both on the Al contents and on the site occupancy of the ternary alloying element in Ni3Al intermetallic alloys.

Site preference of magnetic atoms in Ni–Mn–Ga–M (M = Co, Fe) ferromagnetic shape memory alloys

AbstractSite preference of cobalt and iron atoms substituted in Ni–Mn–Ga alloys is examined using the compositional dependence of saturation magnetization determined at 5 K. Variations in magnetization of Ni50Mn25+xGa25−x−yMy(M = Co, Fe) alloys are systematically compared with the values computed for different possible configurations of site occupancies. The study reveals that the dopant atoms occupy the regular Mn site, rather than the vacant Ga site, with ferromagnetic exchange relative to the moments on Ni and Mn sublattices. The excess and displaced Mn atoms occupy the vacant Ga site with antiferromagnetic exchange. Site preference appears to have a correlation to the atomic volume of the elements.

Site preference and elastic properties of Fe-, Co-, and Cu-doped Ni2MnGa shape memory alloys from first principles

The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni${}_{2}$MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density $n$ and the shear modulus ${C}^{\ensuremath{'}}$ can be considered as predictors of the composition dependence of the martensitic transition temperature ${T}_{M}$ of the alloys. The physics underlying the composition-dependent ${C}^{\ensuremath{'}}$ are discussed based on the calculated density of states.

Interface Effect of Magnetic Properties in Ni Nanoparticles with a hcp Core and fcc Shell Structure

We have fabricated hexagonal close-packed (hcp) Ni nanoparticles covered by a face-centered cubic (fcc) Ni surface layer by polyol method. The magnetic properties have been investigated as a function of temperature and applied magnetic field. The magnetic behavior reveals that the system should be divided magnetically into three distinct phases with different origins. The fcc Ni phase on the shell contributes to the superparamagnetism through a wide temperature range up to 360 K. The hcp Ni phase at the core is associated with antiferromagnetic nature below 12 K. These observations are in good agreement with the X-ray absorption spectroscopy and magnetic circular dichroism measurements. In our particular case, the unique hcp core and fcc shell structure gives rise to an additional anomaly at 20 K in the zero-field-cooled magnetization curve. Its position is barely affected by the magnetic field but its structure disappears above 30 kOe, showing a metamagnetic transition in the magnetization versus magnetic field curve. This new phase originates from the magnetic exchange at the interface between the hcp and fcc Ni sublattices.

Diffusion and Growth of the μ Phase (Ni6Nb7) in the Ni-Nb System

Incremental diffusion couple experiments are conducted to determine the average interdiffusion coefficient and the intrinsic diffusion coefficients of the species in the Ni6Nb7 (μ phase) in the Ni-Nb system. Further, the tracer diffusion coefficients are calculated from the knowledge of thermodynamic parameters. The diffusion rate of Ni is found to be higher than that of Nb, which indicates higher defect concentration in the Ni sublattice.

Microscopic phase field study of the antisite defect of Ni3Al in binary Ni-Al alloys

The temporal evolution feature of a microscopic phase field model is utilized to study the antisite defects of L12-Ni3Al; this is quite different from other physicist’ interests. There are mainly two points in brief. Firstly, antisite defects NiAl and AlNi, which are caused by the deviation from the stoichiometric Ni3Al, coexist in the Ni3Al phase. The surplus Ni atom in the Ni-rich side is prone to substitute Al thus producing the antisite defect NiAl that maintains the stability of the L12 structure. In other case, the surplus Al atom in the Al-rich side is accommodated by a Ni sublattice consequently giving rise to antisite defect AlNi. The calculated equilibrium occupancy probability of NiAl is much higher than that of AlNi. This point is generally in line with other theoretical and experimental works. Additionally, both NiAl and AlNi have a strong negative correlation to time step during the disorder-order transformation. Since the initial value of NiAl and AlNi on each site of the matrix is right at the concentration that we set, we can observe the decrease process of NiAl and AlNi from the initial disordered high anti-structure state to their respective equilibrium state, i.e. to the result of the ordering process further coarsening.

Magnetism-driven dislocation dissociation, cross slip, and mobility in NiAl

We have examined the interplay between magnetism and dislocation core properties in NiAl alloys using quantum mechanics/molecular mechanics simulations. We observe the magnetism-driven site preference of Fe and Co impurities at the dislocation core. When occupying the Ni sublattice, Fe and Co impurities are found to induce spontaneous dislocation cross slip. When occupying the Al sublattice, the impurities render the originally undissociated dislocation to split into two partials. Finally we observe the magnetism-driven dislocation mobility when Fe impurity occupies the Al sublattice, which has also been discovered experimentally. The magnetic interaction between the impurities and the host atoms is responsible for the significant changes in the dislocation core structure and mobility.

Room temperature Mott metal-insulator transition and its systematic control in Sm1−xCaxNiO3 thin films

We report an epitaxial growth and electronic transport properties of Ca-doped SmNiO3 (Sm1−xCaxNiO3, 0≤x≤0.1) thin films deposited on (001)-oriented LaAlO3 substrates by the pulsed laser deposition method. Due to strong electron correlations of the Sm1−xCaxNiO3 films, the Mott metal-insulator (MI) transition appears around 370 K, which decreases to room temperature only by the 1%–2% Ca doping, and dramatically shifts to lower temperatures by increasing the Ca content. For x≥0.1, the film reveals metallic conductivity down to the lowest temperature measured. In the insulating phase of x<0.04, we observe another resistivity anomaly around 200 K corresponding to an antiferromagnetic ordering of the Ni sublattice of SmNiO3 matrix. A complete electronic phase diagram of the Sm1−xCaxNiO3 thin film is unveiled by this work.

Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti 50Ni 30Pt 20 (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.

Effect of Nb on electronic structure of NiTi intermetallic compound: A first-principles study

The electronic structure of pure and Nb doped B2 NiTi have been calculated with the first-principle ultrasoft pseudopotential approach of the plane wave based on the density functional theory aiming at examining the effect of Nb on the electronic structure and bonding characteristic of NiTi. The point defect formation energy calculation results show that the introduction of Nb largely enhances the energy required to form the vacancies and anti-sites of both Ni and Ti sublattice. Furthermore, the calculation results on the partial density of states (PDOS) suggest that the addition of Nb increase the s-s, p-p and d-d electronic interactions between Nb and neighboring Ni and Ti atoms. The electron density of NiTi was also increased with the introduction of Nb. All these Nb-inuced results on electronic structure and bonding characteristic may hamper the hop of Ni and Ti atoms, which may facilitate to improve of oxidation resistance of NiTi intermetallic compound in the experimental findings.

Solute diffusion in the γ′ phase of Ni based alloys

We systematically investigate the diffusion mechanisms of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition metal solutes in γ′-Ni3Al phase using first-principles calculations. The results reveal that the diffusion of Ni-substituting solute is mainly controlled by the sublattice diffusion mechanism via Ni vacancies and the diffusion of Al-substituting solute is mostly governed by the formation of the anti-structure defects on the Ni sublattice with negligible contribution of the anti-structure bridge mechanism. The elements which occupy both the Al and Ni sites show a diffusion behavior similar to that of the Ni-substituting solutes. Our calculations show that larger atoms can move much faster than smaller atoms, which disprove the traditional view that larger atoms move slower than smaller atoms.

Energies and Lifetimes of Magnons in Complex Ferromagnets: A First-Principle Study of Heusler Alloys

The energies and lifetimes of magnons in several Mn-based Heusler alloys are studied using linear response density functional theory. The number of the spin wave branches in Co(2)MnSi corresponds to the number of its magnetic sublattices in contrast with the NiMnSb case in which the induced Ni sublattice cannot support optical magnons. The half-metallicity of these systems results in long-living acoustic spin waves. The example of non-half-metallic Cu(2)MnAl shows that the hybridization with Stoner continuum leads not only to the damping of magnons but also to a renormalization of their energies.

Electron energy losses by channelling-enhanced microanalysis in a γ′-strengthened Ni-base alloy with Ti additions

The addition of Ti to a γ′-strengthened Ni-base model alloy shows a strong preference of Ti for the γ′-phase. Ti strongly influences the diffusion behaviour of constituents of the Ni 3Al (γ′) phase, although the mechanism is uncertain. Electron energy loss channelling experiments were therefore performed to determine the electronic structure in the vicinity of the Ti atom. The experimental results showed an increase in the Ti L 3/L 2 ratio when probing the Ni sublattice planes compared to the Al sublattice planes, indicating a strong charge transfer from the Ti to the Ni atoms. This finding was supported by first-principles calculations, indicating an increase in electron density in the vicinity of the Ti atom and a strengthening of the chemical bonds.