The properties of solid materials are determined by their crystal structures, in which microscopic crystal defects—such as vacancies, dislocations, and interfaces—play a critical role and significantly affect macroscopic physical properties. A deep understanding of crystal defects and their configurations, as well as the precise atomic occupation, is therefore of great importance for tuning the functional properties of materials.
Half-Heusler compounds are a class of ternary intermetallic compounds with a cubic crystal structure, with the general chemical formula XYZ , where X and Y are typically transition metals and Z is a main-group element. Owing to their rich elemental combinations and tunable electronic structures, these materials have attracted considerable attention in fields such as thermoelectrics, topological insulators, and piezoelectrics. In the ideal crystal structure, the three atomic species X , Y , and Z occupy three distinct cubic sublattices—4a, 4c, and 4b, respectively—while the 4d sublattice remains vacant. However, due to thermodynamic and kinetic factors, in actual synthesized half-Heusler compounds, Y atoms that should occupy the 4c site may partially occupy the 4d vacancies, giving rise to intrinsic defects. Previous studies have generally classified such defects as point defects or local nanoclusters. The presence of intrinsic defects significantly influences electrical, thermal, and thermoelectric properties, making the defect structures and atomic occupations in half-Heusler compounds a long-standing focus of the international thermoelectric community. Nevertheless, limited by characterization techniques and computational capabilities, the specific types of intrinsic defects and the exact atomic occupations have remained controversial.
In earlier studies on Ni‑based and Co‑based half‑Heusler compounds (i.e., where Y is Ni or Co), spectroscopic methods (such as X‑ray and neutron diffraction) and electron microscopy have yielded drastically different results for Y ‑atom occupancy: spectroscopic analyses indicated that Y atoms predominantly occupy the 4c site, with occupancies close to 100%, and only a small fraction (less than 10%) randomly and disorderly occupying the 4d site; in contrast, electron microscopy typically showed nearly equal occupancy of Y atoms at the 4c and 4d sites (roughly 50% vs. 50%), exhibiting correlated disorder. These two characterization approaches produced contradictory conclusions on atomic occupation in the same materials, leading to persistent debates over the intrinsic defect types in Ni‑ and Co‑based half‑Heusler compounds. Interestingly, for Fe‑based half‑Heusler compounds, spectroscopic and electron microscopy analyses gave consistent results—both observed a small amount of Y atoms occupying the 4d site in random disorder.
To address these controversies regarding defect structures and atomic occupations in half‑Heusler compounds, the team carried out a systematic investigation, focusing on the typical Ni‑based half‑Heusler compound ZrNiSn. By combining first‑principles calculations with Monte Carlo simulations, they found that within certain regions of the crystal, Ni atoms occupy only a single sublattice—either 4c or 4d. To alleviate the high‑energy interface formed by direct contact between the 4c‑occupied and 4d‑occupied regions, within the interfacial layer between adjacent 4c/4d regions, the originally vacant 4d or 4c sublattices are simultaneously filled by Ni atoms. This results in a local atomic configuration resembling that of a full‑Heusler phase at the interface, forming a distinctive new kind of planar defect, named “filling fault”. The filling faults act as low‑energy transition interfaces between the 4c‑ and 4d‑occupied regions, ensuring relative structural stability (as illustrated in the image). It is important to note that the filling fault is fundamentally different from the conventional stacking fault: a stacking fault refers to a planar defect formed by a local interruption of the normal periodic stacking sequence of atomic planes in close‑packed crystals, without involving changes in atomic species or deviations from stoichiometry. In contrast, the formation of a filling fault does not disrupt the normal stacking order of the matrix atoms (Zr/Sn); instead, it introduces extra atomic planes at the interface by locally filling originally vacant sublattices. The key features are the presence of vacant sublattices that can be filled within the crystal structure and the inevitable local stoichiometric deviation introduced by the filling fault.
To directly verify these defects experimentally, the team employed four‑dimensional scanning transmission electron microscopy combined with ptychographic reconstruction. Through nanoscale‑thickness reconstructions, they directly observed the existence of filling faults in ZrNiSn, as well as the vacancy swap occupation on both sides of the filling fault (as shown in the image). Furthermore, systematic experimental studies on Ni‑based, Co‑based, and Fe‑based half‑Heusler compounds revealed that the degree of filling fault formation varies significantly among different compositions and changes with the type of Y element. Specifically, filling faults are apt to form hardly in Fe‑based compositions, moderately in Co-based compositions, and fully in Ni‑based compositions. These differences are mainly related to the stability of the corresponding full‑Heusler phases in each composition. This finding provides a reasonable explanation for the aforementioned structural contradictory: regardless of whether in 4c‑ or 4d‑dominated regions, Y atoms actually occupy only one set of sublattices; under diffraction methods, the overall average structure is equivalent to Y atoms predominantly occupying the 4c site with a small amount of 4d disorder, whereas electron microscopy directly resolves the 4c and 4d positions, but due to projection averaging along the sample thickness direction, it ultimately yields an apparent nearly equal occupancy.
The filling fault and the associated vacancy swap occupation have important effects on the physical properties of half‑Heusler compounds. The team discovered that filling faults are intrinsically linked to changes in carrier scattering mechanisms: in Fe‑based compositions, where filling faults are rarely observed, carrier transport exhibits typical phonon‑scattering dominance; in Co‑ and Ni‑based compositions, as the probability of filling‑fault formation increases, the temperature dependence of electrical conductivity gradually weakens, indicating that filling faults significantly suppress carrier transport. In addition, theoretical calculations show that filling faults and vacancy swap occupation can alter the piezoelectric response charges and electrostatic potentials in ZrNiSn, suggesting that these defects can enhance the piezoelectricity of half‑Heusler compounds. Further experimental studies confirmed that several Ni‑based half‑Heusler compounds exhibit substantial piezoelectric responses.
National Science Review
Experimental study