Effects of Oxygen on Lattice Defects in Single-Crystalline Mg2Si Thermoelectrics.

Lattice defect engineering has attracted attention due to its ability to develop thermoelectric materials with low thermal conductivity. For Mg2Si single crystals (SCs), Si vacancy (VSi) defects can be introduced and consequently result in the formation of dislocation cores. These lattice defects confer Mg2Si SCs with a lower thermal conductivity compared to Mg2Si polycrystals. To reveal a mechanism for the stabilisation of VSi in the Mg2Si SCs, we investigated the effects of oxygen (O) on lattice defects by performing electronic structure calculations, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and photoelectron holography. On the basis of these calculations, we predicted that O stabilised the formation of VSi when it was located at the Si site or at an interstitial site. All experiments confirmed the presence of O inside the Mg2Si SCs. However, O was suggested to be located not at the specific site in the crystal lattice of Mg2Si but at dislocation cores. The interaction between O and the dislocation cores in the Mg2Si SC is expected to immobilise dislocation cores, leading to the stabilisation of VSi formation.

Bi-Deficiency Leading to High-Performance in Mg3 (Sb,Bi)2 -Based Thermoelectric Materials.

Mg3 (Sb,Bi)2 is a potential nearly-room temperature thermoelectric compound composed of earth-abundant elements. However, complex defect tuning and exceptional microstructural control are required. Prior studies have confirmed the detrimental effect of Mg vacancies (VMg ) in Mg3 (Sb,Bi)2 . This study proposes an approach to mitigating the negative scattering effect of VMg by Bi deficiency, synergistically modulating the electrical and thermal transport properties to enhance the thermoelectric performance. Positron annihilation spectrometry and Cs -corrected scanning transmission electron microscopy analyses indicated that the VMg tends to coalesce due to the introduced Bi vacancies (VBi ). The defects created by Bi deficiency effectively weaken the scattering of electrons from the intrinsic VMg and enhance phonon scattering. A peak zT of 1.82 at 773 K and high conversion efficiency of 11.3% at ∆T = 473 K are achieved in the optimized composition of Mg3 (Sb,Bi)2 by tuning the defect combination. This work demonstrates a feasible and effective approach to improving the performance of Mg3 (Sb,Bi)2 as an emerging thermoelectric material.

Relevance of Solidification Kinetics for Enhanced Thermoelectric Performance in Al-Doped Higher Manganese Silicides.

The solidification kinetics of an alloy from its liquid state forms an underlying basis for microstructural engineering, wherein the state of thermodynamic equilibrium associated with the melt-grown crystal and the quenched amorphous solid denotes the two limits for crystallinity in the alloy synthesis. In this study, we report the implication of the crystalline state on the thermal and electrical transport properties of partially substituted Mn(Si1-xAlx)γ by comparing the single crystals melt-grown by the Bridgman method, and polycrystals synthesized from melt spinning (MS) and subsequent spark plasma sintering (SPS). The rapidly solidified alloys exhibited nanocrystalline microstructures in MS ribbons, while melt-grown single crystals displayed characteristics evolution of MnSi striations with limited solubility of Al. It was observed that Al as a p-type dopant enhances the carrier concentration and electrical conductivity, while nanocrystallinity in MS + SPS polycrystals and secondary phases in monocrystals were effective in enhancing the phonon scattering. Maximum zT values of ∼0.54 (±0.05) at 823 K and 0.75 (±0.05) at 873 K were attained for the single crystal (directed perpendicular to the c-axis) and melt-spun polycrystals (along the in-plane direction), respectively. These results present the efficacy of aliovalent Al substitution and demonstrate the critical role of the solidification kinetics in optimizing the carrier concentration and enhancing the phonon scattering in higher manganese silicide crystals for thermoelectric applications.

Low Lattice Thermal Conductivity in a Wider Temperature Range for Biphasic-Quaternary (Ti,V)CoSb Half-Heusler Alloys.

Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m-1 K-1 within a wide temperature range (300-873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications.

Incommensurately modulated crystal structure of α' (O'3)-type sodium cobalt oxide NaxCoO2 (x ∼ 0.78).

A single-phase sample of α' (O'3)-type layered sodium cobalt oxide NaxCoO2 (x ∼ 0.78) was prepared and its incommensurately modulated crystal structure was analyzed using the (3+1)-dimensional superspace approach to the powder neutron diffraction data. The crystal structure of the cobaltate is accurately described based on the superspace group C2/m(α0γ)00, wherein the positions of Na atoms are most significantly modulated in the monoclinic a direction to form an ordered arrangement. Such a displacive modulation causes a quasi-periodic shift of Na atoms from the centers of the NaO6 polyhedra between undulated CoO2 sheets, changing the form of the NaO6 polyhedron from an octahedral coordination (O) to a trigonal prismatic (P) one, via an intermediate capped trigonal prismatic NaO7 coordination (C). At the positions where the Na atoms are most significantly shifted, the neighboring Na atoms are located at almost touching distances. However, the occupation factor of Na atoms becomes zero at such positions, yielding Na-deficient sites VNa, sandwiched either between C and P, or C and C-type polyhedra.

Enhancing the Thermoelectric Performance of Mg2Sn Single Crystals via Point Defect Engineering and Sb Doping.

Mg2Sn is a potential thermoelectric (TE) material that exhibits environmental compatibility. In this study, we fabricated Sb-doped Mg2Sn (Mg2Sn1-xSbx) single-crystal ingots and demonstrated the enhancement of TE performance via point defect engineering and Sb doping. The Mg2Sn1-xSbx single-crystal ingots exhibited considerably enhanced electrical conductivity because of the donor-doping effect in addition to high carrier mobility. Moreover, the Mg2Sn1-xSbx single-crystal ingots contained Mg vacancy (VMg) as a point defect. The introduced VMg and doped Sb atoms formed nanostructures, both acting as phonon-scattering centers. Consequently, lower lattice thermal conductivity was achieved for the Mg2Sn1-xSbx single-crystal ingots compared with polycrystalline counterparts. Owing to the significant enhancement in the electrical conductivity and the reduction in the lattice thermal conductivity, the maximum power factor of 5.1(4) × 10-3 W/(K2 m) and the maximum dimensionless figure of merit of 0.72(5) were achieved for the Mg2Sn0.99Sb0.01 single-crystal ingot, which are higher than those of single-phase Mg2Sn1-xSb polycrystals.

Control of the Thermoelectric Properties of Mg2Sn Single Crystals via Point-Defect Engineering.

Mg2Sn is a potential thermoelectric (TE) material that can directly convert waste heat into electricity. In this study, Mg2Sn single-crystal ingots are prepared by melting under an Ar atmosphere. The prepared ingots contain Mg vacancies (VMg) as point defects, which results in the formation of two regions: an Mg2Sn single-crystal region without VMg (denoted as the single-crystal region) and a region containing VMg (denoted as the VMg region). The VMg region is embedded in the matrix of the single-crystal region. The interface between the VMg region and the single-crystal region is semi-coherent, which does not prevent electron carrier conduction but does increase phonon scattering. Furthermore, electron carrier concentration depends on the fraction of VMg, reflecting the acceptor characteristics of VMg. The maximum figure of merit zTmax of 1.4(1) × 10-2 is realised for the Mg2Sn single-crystal ingot by introducing VMg. These results demonstrate that the TE properties of Mg2Sn can be optimised via point-defect engineering.

Reducing Lattice Thermal Conductivity of MnTe by Se Alloying toward High Thermoelectric Performance.

Lead-free manganese telluride has been considered to be a promising candidate for mid-temperature thermoelectric materials. In this work, we report point defect scattering-induced reduction of thermal conductivity in MnTe with Se alloying, fabricated by a facile method combining mechanical alloying and spark plasma sintering. A low lattice thermal conductivity of 0.56 W/mK was obtained for MnTe0.92Se0.08, which is quite close to the amorphous limits. A detailed Debye model analysis reveals the underlying mechanism of phonon scattering and well predicts the thermal conductivity with different contents of Se. Meanwhile, a slight increase of carrier concentration was also observed after Se alloying, accompanied by a variation of energy gap that may be associated with the competition among anions in trapping charges. Further Na doping leads to enhanced electrical transport properties, achieving a maximum ZT value of 1.03 at 873 K. An average ZT of 0.52 and a calculated efficiency of more than 9% also suggest the promising application of MnTe at medium temperatures.

Optimised thermally driven molecular stability of an SCO metal complex for TEC Seebeck generation enhancement.

The thermoelectricity effect allows the generation of electrical potential in an electrolyte upon application of a thermal gradient. In the previous work, the spin crossover effect in metal complexes was shown to be beneficial for generating a high Seebeck coefficient due to the high entropy associated with the conformational change accompanying the spin state change. In this study, we examine the diamagnetic stability of a spin crossover material through optimisation of the ligand chain length. We show that the diamagnetic stability of the spin crossover material can enhance the thermoelectrochemical Seebeck effect through ligand optimisation of the octahedral structure. The increase of carbon chain length from C14 to C16 in the long alkyl chain of the N-donor ligand increased Seebeck generation in a Co(iii)L16 complex to 1.94-fold that of a previously studied paramagnetic Co complex, and in a Fe(iii)L16 complex to 3.43-fold that of a less diamagnetic Fe complex. We show with DSC studies of an Fe based octahedral complex that an endothermic absorption accompanies the spin crossover transition, which enhances the Seebeck coefficient of this metal complex. Thus, we can correlate the diamagnetic stabilisation with temperature. We therefore indicate a molecular design strategy for optimisation of a spin crossover metal complex.

Crystal Structure and Thermoelectric Properties of Lightly Substituted Higher Manganese Silicides.

The dissipation of MnSi layered precipitates during solidification is critical for further enhancement of the thermoelectric properties of the higher manganese silicides. We have investigated the effects of partial substitution of V in Mn sites and of Ge in Si sites on the crystal structures and thermoelectric properties of these silicides in detail. As previously reported, a small amount of V-substitution is quite effective in completely dissipating the MnSi striations; in contrast, a small proportion of these MnSi striations always remains present in the Ge-substitution case, even in the vicinity of the Ge solubility limits. For completely MnSi-dissipated samples, domain separation of the regular and highly strained arrangements of the Si atoms is realized. This domain separation suppresses the deterioration of the carrier mobility of the partially V-substituted samples and maintains even higher electrical conductivity to yield a high thermoelectric power factor of ∼2.3 mW/K 2 m at higher temperatures.

High-Performance Thermoelectric Bulk Colusite by Process Controlled Structural Disordering.

High-performance thermoelectric bulk sulfide with the colusite structure is achieved by controlling the densification process and forming short-to-medium range structural defects. A simple and powerful way to adjust carrier concentration combined with enhanced phonon scattering through point defects and disordered regions is described. By combining experiments with band structure and phonons calculations, we elucidate, for the first time, the underlying mechanism at the origin of intrinsically low thermal conductivity in colusite samples as well as the effect of S vacancies and antisite defects on the carrier concentration. Our approach provides a controlled and scalable method to engineer high power factors and remarkable figures of merit near the unity in complex bulk sulfide such as Cu26V2Sn6S32 colusites.

Theoretical and experimental investigation of the excellent p-n control in yttrium aluminoborides.

First-principles calculations were carried out to elucidate the excellent control of p-n characteristics recently reported for yttrium aluminoborides YxAlyB14 [Formula: see text] with different occupancies of Al sites [Formula: see text]. Such control of the occupancy of metal sites in borides is unusual. Calculations based on detailed x-ray diffraction data reveal a stable configuration of the atomic sites, indicating that such variation in occupancy is possible. A shift from positive through zero to negative values of the Seebeck coefficient is also clearly illustrated by determining the density of states for different configurations.