Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1- δ CoSb for the Figure of Merit zT > 1.

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1- δ CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7 Ta0.13 CoSb and Nb0.6 Ta0.23 CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83 CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83 CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.

Tuneable local order in thermoelectric crystals.

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Such disordered crystals with distinct local structures have unique properties arising from both collective and localized behaviour. Different local orderings can exist with identical average structures, making their differences hidden to Bragg diffraction methods. Using high-quality single-crystal X-ray diffuse scattering the local order in thermoelectric half-Heusler Nb1-x CoSb is investigated, for which different local orderings are observed. It is shown that the vacancy distribution follows a vacancy repulsion model and the crystal composition is found always to be close to x = 1/6 irrespective of nominal sample composition. However, the specific synthesis method controls the local order and thereby the thermoelectric properties thus providing a new frontier for tuning material properties.

Mo-Fe/NbFeSb Thermoelectric Junctions: Anti-Thermal Aging Interface and Low Contact Resistivity.

In recent years, high-performance half-Heusler compounds have been developed as promising thermoelectric materials for power generation. Aiming at practical device applications, one key step is to seek suitable metal electrodes so that low interfacial resistivity is guaranteed under long-term thermal aging. In the previous work, the fresh Mo/Nb0.8Ti0.2FeSb junction was found exhibiting low contact resistivity below 1 µΩ cm2; however, it increased by tens of times under long-term thermal aging, mainly originating from the formation of the high-resistivity FeSb2 phase and the appearance of cracks. Here, the Mo-Fe electrodes are employed to build the junctions with Nb0.8Ti0.2FeSb. The interfacial behavior and contact resistance in these junctions were investigated both before and after the thermal aging. Interestingly, no obvious formation of FeSb2 phase and cracks were observed. As a result, the contact resistivity was below ∼1 µΩ cm2 after 15 days' thermal aging, indicating better connection reliability and lower contact resistivity compared to the Mo/Nb0.8Ti0.2FeSb junction. These findings highlight the applicability of Mo-Fe electrodes and pave the way for NbFeSb-based half-Heusler thermoelectric materials for device applications.

Direct visualization of spatially correlated displacive short-range ordering in Nb0.8CoSb.

Whether the atomic arrangement has a long-range order bifurcates solid-state matter into two major categories: crystalline and amorphous, between which lies a short-range order, a frontier research topic of fundamental and application implications. To date, it is still challenging to extract the details of short-range order from the corresponding diffuse diffraction pattern due to the phase problem. Here, we employed the high-angle annular dark field (HAADF) imaging technique to pinpoint the short-range order encoded in the one-of-a-kind diffuse the diffraction bands of defective half-Heusler Nb0.8CoSb. Utilizing a protocol based on two limiting cases, we found that the native Nb vacancies up to 20% are dominantly displacive short-range ordered yet spatially correlated. To the best of our knowledge, this is the first time that a dominantly displacive short-range order is reported at the atomic scale. These results are vital for an in-depth understanding and engineering of the thermodynamics and transport properties of the materials with abundant native defects, including but not limited to defective half-Heusler compounds.

Lanthanide Contraction as a Design Factor for High-Performance Half-Heusler Thermoelectric Materials.

Forming solid solutions, as an effective strategy to improve thermoelectric performance, has a dilemma that alloy scattering will reduce both the thermal conductivity and carrier mobility. Here, an intuitive way is proposed to decouple the opposite effects, that is, using lanthanide contraction as a design factor to select alloying atoms with large mass fluctuation but small radius difference from the host atoms. Typical half-Heusler alloys, n-type (Zr,Hf)NiSn and p-type (Nb,Ta)FeSb solid solutions, are taken as paradigms to attest the validity of this design strategy, which exhibit greatly suppressed lattice thermal conductivity and maintained carrier mobility. Furthermore, by considering lanthanide contraction, n-type (Zr,Hf)CoSb-based alloys with high zT of ≈1.0 are developed. These results highlight the significance of lanthanide contraction as a design factor in enhancing the thermoelectric performance and reveal the practical potential of (Zr,Hf)CoSb-based half-Heusler compounds due to the matched n-type and p-type thermoelectric performance.

Ultrathin Two-Dimensional Pd-Based Nanorings as Catalysts for Hydrogenation with High Activity and Stability.

Despite a few reports on the synthesis of ultrathin 2D nanosheets made of noble metals, it still remains a tremendous challenge to generate their ultrathin hollowed nanostructures, which are of particular interest in highly active catalysis due to their unique structural features. Here, the synthesis of ultrathin 2D Pd nanorings is reported with a hollow interior by selective epitaxial growth of Pd atoms on the periphery of the as-preformed Pd nanosheets in combination with oxidative etching. This approach can be extended to fabricate Pd-based bimetallic ultrathin nanorings such as Pd-Pt. The Pd nanorings exhibit substantially enhanced activity toward the hydrogenation of p-nitrophenol, which is 2.2 and 33.4 times higher than that of the Pd nanosheets and commercial Pd black, respectively. Significantly, the Pd nanorings are highly stable with only less than 11% loss in activity compared to 45.7% loss of the Pd nanosheets and 72.2% loss of the Pd black after ten cycles.