Direct visualization of spatially correlated displacive short-range ordering in NbCoSb.

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.