full-Heusler Phases Research Articles

The role of interstitial Cu on thermoelectric properties of ZrNiSn half-Heusler compounds

The density functional theory (DFT) calculations and experiments have confirmed that in the ABC-type half-Heusler compounds, the 3d elements occupying the B position are natural over-stoichiometry. These additional atoms are able to synergistically optimize the electrical and thermal transport properties of half-Heusler compounds. In this work, Cu (3d104s1) is intentionally introduced into the ZrNiSn compound to form Cu interstitial defects. The correlations between the phase structure, microstructure, and thermoelectric properties of ZrNiCuxSn (x = 0–0.20) are investigated with X-ray and neutron diffraction, transmission electron microscopy, atom probe tomography, and band structure and phonon spectra calculations. The diffraction results reveal that Ni/Cu atoms partially occupy the 4d position (3/4, 3/4, 3/4) of the half-Heusler crystal structure, forming interstitial defects. The interstitial Cu defects force the conduction band minimum to gradually move close to the valence band maximum and reduce the bandgap, rather than induce in-gap states as typical Ni interstitials. Besides the interstitial defects, a full-Heusler phase is also formed in the half-Heusler matrix with increasing Cu content. Due to the interstitial defects and interface engineering, the thermal conductivity is suppressed. As a result, a higher figure of merit (ZT) value is achieved (∼1.1 at 950 K) in the ZrNiCu0.05Sn sample. This work analyses the possibility of interstitial defects from a thermodynamic point of view and highlights the defect engineering to positively tune the thermoelectric properties in half-Heusler compounds.

Effect of Sn substitution at Sb site on the magnetic properties of Mn2NiSb full-Heusler alloy

Alignment of spin of Mn-Mn atoms in Heusler structure mainly depends upon the distance between them which can be easily manipulated by substituting the main group Z-elements in the Mn2-based full-Heusler alloys. In this article, we report the effect of Sn doping at the Sb site in the Mn2NiSb1−XSnX (0.02≤x≤0.1) full-Heusler alloys. We have performed the X-Ray diffraction (XRD) measurement of each sample to obtain the structural information of the synthesized full-Heusler phase. Rietveld refinement performed on the each XRD data of Mn2NiSb1−XSnX (0.02≤x≤0.1) revealed that all the samples were crystallized in the Hg2CuTi prototype (Space group-Fm3m) structure. Analysis of selected area diffraction (SAED) pattern obtained from transmission electron microscopy (TEM) measurement revealed that a single phase of full-Heusler was obtained for the highest Sn content (x = 0.1) sample. Magnetization variation with applied magnetic field plot exhibited the ferromagnetic ground state of each sample as evidenced from hysteresis loop and saturation was obtained up to 1.0 Tesla field. Zero field cooled (ZFC) and field cooled (FC) measurements were also investigated and splitting between ZFC and FC were found to be increasing with an increase in the Sn content. This increment in ZFC and FC splitting with Sn content was attributed to the coupling between the ferrimagnetic (FIM) nature of Mn2NiSn phase with ferromagnetic (FM) nature of Mn2NiSb phase at the interfaces. Small shifting of hysteresis curve observed in the virgin M-H loop around the origin also indicated the spontaneous exchange biasing due to the FM/FIM interaction. Compensation of magnetization followed by exchange biasing with two magnetic phase (FM/FIM) governed by the controlled doping of Sn in Mn2NiSb have been alluringly elaborated. Direct interaction between different known magnetic phases/sub-lattices are rarely investigated which can be used to make suitable materials for spintronic application such as magneto resistive read heads and sensors.

High-pressure studies of a biphasic NiTiSn/Ni2TiSn Heusler alloy by in situ X-ray diffraction and first principle calculations.

The present work studies the effect of hydrostatic pressure on a biphasic Heusler alloy using in situ X-ray diffraction. A nanostructured biphasic NiTiSn/Ni2TiSn Heusler alloy was synthesized by four hours of mechanical alloying using a vibrational high-energy mill and characterized by x-ray diffraction measurements and Rietveld method. The sample was submitted to high-pressure conditions (up to 11.9 GPa) using a diamond anvil cell (DAC) loaded with He, and the structural evolutions were followed by in situ synchrotron x-ray diffraction. The results validate the stability of the half and full-Heusler alloys, as no pressure-induced structural phase transition was observed over the entire pressure range. The behavior of the crystallographic parameters, equation of state and the compressibility factors for both the Heusler phases were obtained from the experimental results as well from the density functional theory (DFT) calculations. The second-order Birch–Murnaghan equation of state determined from our experimental results yields bulk moduli of 143(9) GPa and 156(6) GPa for the Half and Full Heusler phases, respectively. According to the DFT calculations, the corresponding values are 131.0(9) GPa and 161.8(8) GPa.

Half-Heusler-like compounds with wide continuous compositions and tunable p- to n-type semiconducting thermoelectrics

Half-Heusler and full-Heusler compounds were considered as independent phases with a natural composition gap. Here we report the discovery of TiRu1+xSb (x = 0.15 ~ 1.0) solid solution with wide homogeneity range and tunable p- to n-type semiconducting thermoelectrics, which bridges the composition gap between half- and full-Heusler phases. At the high-Ru end, strange glass-like thermal transport behavior with unusually low lattice thermal conductivity (~1.65 Wm−1K−1 at 340 K) is observed for TiRu1.8Sb, being the lowest among reported half-Heusler phases. In the composition range of 0.15 < x < 0.50, TiRu1+xSb shows abnormal semiconducting behaviors because tunning Ru composition results in band structure change and carrier-type variation simultaneously, which seemingly correlates with the localized d electrons. This work reveals the possibility of designing fascinating half-Heusler-like materials by manipulating the tetrahedral site occupancy, and also demonstrates the potential of tuning crystal and electronic structures simultaneously to realize intriguing physical properties.

High-temperature phase characterization of AlCrFeNiTi compositionally complex alloys

In this work, alloys composed of the 5 base-elements Al, Cr, Fe, Ni and Ti and elements in minor amounts were produced by arc melting and powder metallurgy. The alloys’ phase distribution was analyzed via high energy X-ray diffraction (HEXRD), neutron diffraction and electron backscatter diffraction (EBSD). The two studied compositions have low concentrations of the elements Al and Ti, medium concentrations of the elements Cr and Fe and high amounts of Ni (chemical composition in at.-%: Al10Cr20Fe20Ni40Ti5, Al10Cr20Fe20Ni35Ti10). The goal then was to analyze the microstructure of these compositions at elevated temperatures in dependence of their chemical composition and the production method. Analysis of both compositions show the presence of B2-phases, L12-phases, Full Heusler-phases (H_L21), C14_Laves-phase and σ-phase. The combination of dilatometric tests with microstructure analysis via HEXRD and neutron diffraction gives the possibility of observing changes in phase amounts in dependence of temperature. A quantification of phase amounts in the compositions is supported by thermodynamic calculations of stable phases, applying the calculation-of-phase-diagrams (CALPHAD) method within the software Thermocalc. Due to its detrimental influence on ductility the existence of σ-phase is undesired in nickel-chromium-iron alloys and hence σ-phase precipitation with an associated dissolution of the phases B2 and L12 at elevated temperatures was studied in detail.

Effects of Doping Ni on the Microstructures and Thermoelectric Properties of Co-Excessive NbCoSn Half-Heusler Compounds.

The half-Heusler (HH) compound NbCoSn with 18 valence electrons is a promising thermoelectric (TE) material due to its appropriate electrical properties as well as its suitable thermal and chemical stability. Nowadays, doping/substitution and tailoring of microstructures are common experimental approaches to enhance the TE performance of HH compounds. However, detailed theoretical insights into the effects of doping on the microstructures and TE properties are still missing. In this work, the microstructure of NbCoSn was tailored through precipitating the full-Heusler phases in the matrix by changing the nominal ratio of Co and Ni on the Co sites, focusing on the resulting TE properties. Further, first-principles calculations were employed to understand the relationship between the microstructure and the TE properties from the thermodynamic point of view. Detailed analysis of the electronic structure reveals that the presence of excess Co/Ni contributes to the increasing carrier concentration. Through an increase in the electrical conductivity and a reduction in the thermal conductivity, the TE performance is improved. Therefore, the present work offers a new pathway and insights to enhance the TE properties by modifying the microstructure of HH compounds via tailoring the chemical compositions.

Magnetic Properties of Intermediate Ni2-xMn1+xSb Full-Heusler Compounds

Investigation of intermediate compounds of Heusler alloys has been the prime focus to study the various magnetic properties. We have successfully synthesized non-stoichiometric full- Heusler intermediate compounds of Ni2-xMn1+xSb (0.25≤x≤0.75) to investigate their magnetic properties. XRD analysis using Rietveld refinement reveals the presence of a single-phase of Cu2MnAl prototype full-Heusler phase. Transmission electron microscopy analysis also confirms the presence of a single phase of best optimized composition (Ni1.5Mn1.5Sb) of full-Heusler alloy. Interestingly, the saturated magnetic moments obtained from the M-H curves were found to be 4.45μB/f.u., 3.16μB/f.u. and 4.65μB/f.u. for Ni2-xMn1+xSb (x= 0.25, 0.5, 0.75) respectively which is the highest reported value (~4.65μB/f.u. for Ni1.25Mn1.75Sb) so far in this family of the compounds. Further, the reversibility of zero-field cooled (ZFC) and field cooled (FC) curves indicate that samples are stabilized in a magnetically homogeneous state. The first principle calculation performed on these materials is well corroborated with the experimental results.

Lattice strain during compressive loading of AlCrFeNiTi multi-principal element alloys

In this work, multi-principal element alloys (MPEAs) with the five base elements Al, Cr, Fe, Ni and Ti plus elements in minor amounts were produced by powder metallurgy and their microstructure and elastic behavior were analyzed via light and scanning electron microscopy, electron backscatter diffraction (EBSD) and synchrotron X-ray diffraction. The two studied compositions are an MPEA with Al, Cr, Fe, Ni and Ti in equimolar ratio as well as a similar composition with a concentration of Ti reduced to 10 mol%. The goal is to analyze the microstructural behavior of these compositions during macroscopic loading in dependence of chemical composition and phases present. Analysis via synchrotron X-ray diffraction predicts the presence of body-centered cubic phases, Full Heusler-phases and C14_Laves-phases in both compositions, MPEA5 and MPEA_Ti10. Synchrotron X-ray diffraction offers the possibility to monitor the deformation of these phases during macroscopic loading of specimens. Thermodynamic calculations of stable phases predicted a microstructure of MPEA5 consisting of body-centered cubic and Full Heusler-phases at room temperature. Further calculation and X-ray diffraction experiments showed the stabilization of minor amounts of C14_Laves-phase (hbox {Fe}_2hbox {Ti}) at room temperature with a decreasing amount of Ti. MPEA5 showed the development of long and un-branched cracks during compressive testing, which resulted in a remarkable decrease in lattice-dependent elastic moduli. MPEA_Ti10 exhibited branched cracks during compression tests. Also, the lattice-dependent elastic moduli of MPEA_Ti10 did not change notably during the compression tests. In both compositions, the Full Heusler-phase showed the lowest lattice-dependent elastic moduli, hence taking the largest share of the overall deformation among all phases present in the materials under macroscopic loading.

Structural and Magnetic Properties of Ni1+x MnSb Bulk Heusler Composite Materials.

Variation in structural and magnetic properties with changing valence electron count (VEC) has been studied well in the family of Heusler compounds, while such changes in VEC resulting in half-Heusler (HH) and full-Heusler (FH) composites have not been reported to observe their effect on the magnetic properties. Herein, we have synthesized the composite of HH and FH phases in Ni1+xMnSb (x = 0.0, 0.3, and 0.6) via changing VEC from 22 to 28 in order to investigate the structural and magnetic properties. Interestingly, a transition from half-metallic ferromagnetic to normal ferromagnetic was revealed in Ni1+xMnSb (x = 0.0, 0.3, and 0.6) materials with increasing VEC. The structural investigations of these materials were performed using a X-ray diffraction technique and analyzed by Rietveld Refinement software for all the samples. Rietveld analysis reveals the presence of a significant amount of the NiSb paramagnetic impurity phase in the HH NiMnSb system while in the case of Ni1+xMnSb (x = 0.3 and 0.6), no such impurity phase was observed. Only FH and HH phases in Ni1+xMnSb (x = 0.3 and 0.6) samples were noticed. The magnetic measurement performed on samples employing a vibrating sample magnetometer reveals the ferromagnetic ordering in all samples. A weak hysteresis loop with saturated magnetic moments ∼2.99 and 2.98 μB at room temperature was observed for NiMnSb and Ni1.3MnSb, respectively, while a strong hysteresis loop with lower magnetic moment of 0.88 μB was observed in the Ni1.6MnSb composite. Furthermore, the observed magnetic moments for the composite Ni1.3MnSb have been explained on the basis of the Slater–Pauling rule in relation to VEC.

Tailoring thermoelectric properties of Zr0.43Hf0.57NiSn half-Heusler compound by defect engineering

The thermoelectric transport properties of Zr0.43Hf0.57NiSn half-Heusler compounds were investigated for samples sintered with different spark plasma sintering (SPS) periods: 8, 32 and 72 min. By means of scanning transmission electron microscopy with a high-angular annular dark-field detector (STEM-HAADF), it was found that sintering time affected the defect concentration, namely the amount of Ni interstitial atoms, and created locally ordered inclusions of full-Heusler phase. The structural information, phase composition and electrical transport properties could be consistently explained by the assumption that Ni interstitials give rise to an impurity band situated about 100 meV below the bottom of the conduction band via a self-doping behavior. The impurity band was found to merge with the conduction band for the sample with intermediate SPS time. The effect was ascribed to the gradual dissolution of full-Heusler phase inclusions and production of interstitial Ni defects, which eventually vanished for the sample with the longest sintering time. It was demonstrated that the modification of the density of states near the edge of the conduction band and enhanced overall charge carrier concentration provided by defect engineering led to overall 26% increase in the thermoelectric figure of merit (ZT) with respect to the other samples.

Determination of structural disorder in Heusler-type phases

A number of Heusler-type phases, e.g. TiNi2Sn, TiNiSb, ZrNiSn, TiFeSb, NbCoSb, which, for a long time, were considered to be stoichiometric with point composition, are characterized by homogeneity regions and/or off-stoichiometry. Traditional high-throughput DFT calculations aim at the discovery of new fully ordered point composition Heusler-type phases and their potential intrinsic and extrinsic defects. In order to extend the application of DFT methods for determination of structural disorder in half-Heusler and full-Heusler phases we propose a new multi-way approach. It is based on evaluation of the crystal structure model that is used in DFT calculations by getting the best agreement between a set of theoretical and available experimental data (e.g. concentration dependencies of the lattice parameter, homogeneity regions, physical and mechanical properties). This approach allows to determine the model of structural disorder in Heusler-type phases, explain or predict the presence of the homogeneity regions or off-stoichiometry, and reveal the mechanisms of solid-solution formation.

A comparative first-principles study of orthorhombic and full-Heusler phases in Ti2AlNb intermetallic

A comparative study on orthorhombic O and cubic full-Heusler phases in Ti2AlNb intermetallic compound was preformed based on the first-principles calculations of the lattice parameters, electronic structures and elastic properties for both phases. The results showed that the full-Heusler phase possesses the nonmagnetic L21 structure at ground-state. Meanwhile, the L21 phase is mechanically stable, and has lower stability, higher melting point, better ductility, lower hardness, more remarkable anisotropy and lower Debye temperature than the O one. The present research suggested that the full-Heusler compound Ti2AlNb is a possible high-temperature structural material for aerospace and automotive applications.

Phase stability and thermoelectric properties of TiCoSb-TiM2Sn (M = Ni, Fe) Heusler composites

Heusler composites have attracted significant attention as a new route towards improving the thermoelectric figure of merit (ZT) through reduction of the lattice thermal conductivity and carrier filtering effects. This work extends this field by investigating TiCoSb-TiM2Sn (M = Ni, Fe) composites. All end-members are stable phases but no clean segregation into half- and full-Heusler phases was observed. Instead, for M = Ni, partial substitution on the Co sublattice and n-type doping occurs, combined with the formation of Ni3Sn2 and full-Heusler phases. For M = Fe, substitution on the Co site occurs, leading to p-type conduction. Rietveld analysis of neutron powder diffraction data reveals no evidence for the presence of metals on the vacant tetrahedral site, signalling the absence of embedded Heusler inclusions. The thermoelectric properties of both series vary systematically with composition. For both n-type (M = Ni) and p-type (M = Fe) series, the highest measured power factors S2/ρ ∼ 0.8 mW m−1 K−2 leading to ZT ∼ 0.12 at 713 K. This work extends knowledge regarding the phase stability and thermoelectric properties of TiCoSb-based Heusler composites.

Spontaneous formation of nanostructures during pulsed laser deposition of epitaxial half-Heusler TiNiSn on MgO(001)

The half-Heusler alloy TiNiSn is a promising material for thermoelectric applications that is inexpensive and non-toxic. We demonstrate the epitaxial growth of smooth TiNiSn thin films on MgO(001) single crystal substrates by pulsed laser deposition, using transmission electron microscopy to investigate nanoscale structural and chemical inhomogeneities. In particular, an interfacial reaction results in the formation of a titanium-rich oxide nanolayer at the substrate interface, and this maintains epitaxy but perturbs film composition. Segregation effects are observed throughout the film, producing a narrow range of off-stoichiometric, half-Heusler compositions. The propensity of titanium to oxidise also leads to the commensurate formation of parasitic titanium oxide nanostructures which may be overlooked in bulk techniques but are a key feature that drives both the formation of Ni-enriched, full-Heusler phases and the local decomposition of the half-Heusler to form additional nickel-stannide phases.

An Ab Initio Study of Thermodynamic and Mechanical Stability of Heusler-Based Fe₂AlCo Polymorphs.

We use quantum-mechanical calculations to test a hypothesis of Glover et al. (J. Mag. Mag. Mater. 15 (1980) 699) that Co atoms in the FeAlCo compound have on average 3 Fe and 3 Co atoms in their second nearest neighbor shell. We have simulated four structural configurations of FeAlCo including the full Heusler structure, inverse Heusler polymorph and two other phases matching this idea. The highest thermodynamic stability at T = 0 K is indeed predicted for one of the phases with the distribution of atoms according to Glover and et al. However, small energy differences among three of the studied polymorphs lead to a disordered CsCl-structure-like (B2-like) phase at elevated temperatures. The fourth variant, the full Heusler phase, is predicted to be mechanically unstable. The global magnetic states are predicted to be ferromagnetic but local magnetic moments of Fe and Co atoms sensitively depend on the composition of the first and second coordination shells.

Structural and electronic properties of molecular beam epitaxially grown Ni1+xTiSn films

This paper demonstrates the molecular beam epitaxial growth of metallic full-Heusler Ni2TiSn precipitates within semiconducting NiTiSn host matrix, and discusses the structural and chemical stability of this biphasic epitaxial composite and its influence on the electronic properties. Structural properties of the epitaxial films were characterized by in-situ reflection high-energy electron diffraction and ex-situ by X-ray diffraction and transmission electron microscopy. The results indicate the presence of a secondary phase with full Heusler ordering within half-Heusler host matrix. Both the parent half-Heusler and the secondary full-Heusler phases in the epitaxial films are strained from their bulk lattice parameters. Electronic properties show an increase in resistivity at low Ni excess.

Phase stability and property evolution of biphasic Ti–Ni–Sn alloys for use in thermoelectric applications

Thermoelectric properties and phase evolution have been studied in biphasic Ti–Ni–Sn materials containing full-Heusler TiNi2Sn embedded within half-Heusler thermoelectric TiNiSn. Materials, prepared by levitation induction melting followed by annealing, were of the nominal starting composition of TiNi1+xSn, with x between 0.00 and 0.25. Phases and microstructure were determined using synchrotron X-ray diffraction and optical and electron microscopy. The full-Heusler phase is observed to be semi-coherent with the half-Heusler majority phase. Differential thermal analysis was performed to determine melting temperatures of the end-member compounds. The thermal conductivity is reduced with the introduction of a dispersed, full-Heusler phase within the half-Heusler material. This leads to an increased thermoelectric figure of merit, ZT, from 0.35 for the stoichiometric compound to 0.44 for TiNi1.15Sn. Beyond x = 0.15 ZT decreases due to a rise in thermal conductivity. Density functional theory calculations using hybrid functionals were performed to determine band alignments between the half- and full-Heusler compounds, as well as comparative energies of formation. The hybrid functional band structure of TiNiSn is presented as well.

Improving the thermoelectric properties of half-Heusler TiNiSn through inclusion of a second full-Heusler phase: microwave preparation and spark plasma sintering of TiNi1+xSn

Half-Heusler thermoelectrics offer the possibility to choose from a variety of non-toxic and earth-abundant elements. TiNiSn is of particular interest and - with its relatively high electrical conductivity and Seebeck coefficient - allows for optimization of its thermoelectric figure of merit, reaching values of up to 1 in heavily-doped and/or phase-segregated systems. In this contribution, we used an energy- and time-efficient process involving solid-state preparation in a commercial microwave oven and a fast consolidation technique, Spark Plasma Sintering, to prepare a series of Ni-rich TiNi1+xSn with small deviations from the half-Heusler composition. Spark Plasma Sintering plays an important role in the process by being a part of the synthesis of the material rather than solely a densification technique. Synchrotron powder X-ray diffraction and microprobe data confirm the presence of a secondary TiNi2Sn full-Heusler phase within the half-Heusler matrix. We observe a clear correlation between the amount of full-Heusler phase and the lattice thermal conductivity of the samples, resulting in decreasing total thermal conductivity with increasing TiNi2Sn fraction. This trend shows that phonons are scattered effectively as a result of the microstructure of the materials with full-Heusler inclusions in the size range of microns to tens of microns. The best performing samples with around 5% of TiNi2Sn phase exhibit maximum figures of merit of almost 0.6 between 750 K and 800 K which is an increase of ca. 35% compared to the zT of the parent compound TiNiSn.

The Role of Nanoparticle Inclusions on High-performance half-Heusler Thermoelectric Materials

AbstractWe report the effect of adding NiO nanoparticles on the transport properties of the half-Heusler alloys Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 (x=0, and 0.2). The half-Heusler matrix materials are prepared by traditional powder metallurgy methods. The resulting bulk matrix is mixed with different volume fractions of nanometer-sized NiO particles, previously synthesized by solution-phase chemical methods. The resulting mixture is densified under uniaxial pressure to form a composite. The resulting material is found to contain both half-Heusler and full-Heusler phases. The corresponding compounds have higher thermal conductivity and electron mobility.