Filling faults and vacancy swap occupation in half-Heusler compounds.

Defects play a vital role in understanding and elucidating the structure-property relationship in materials science. Here, we report the existence of a new kind of planar defect, 'filling fault', in half-Heusler (HH) compounds-a structurally and functionally diverse family of materials. The ideal HH structure is composed of three occupied and one vacant sublattices. The two-dimensional filling fault layer tends to form with the originally vacant 4d sites occupied, fully or partly, which results in a vacancy swap occupation between the 4c and 4d sublattices. It is found that the filling faults are apt to form fully in Ni-based ZrNiSn and ScNiSb, moderately in Co-based TiCoSb and hardly in Fe-based NbFeSb and VFeSb. The unique defect configurations and the formation rule clarify the long-standing structure puzzles in HH compounds. Furthermore, we find that the filling faults and the associated vacancy swap occupation can significantly enhance the piezoelectricity and change the temperature dependence of electrical conductivity. These results deepen the understanding of precise crystallographic structures and defect-property relationships in solids, and facilitate the design of advanced materials and functional devices.