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Structural voids are a defining feature of the half-Heusler structure. Using temperature-dependent neutron diffraction, the disordering of these vacancies is followed across the C1$_b$-L2$_1$ transition in samples of Ni$_{1+x}$MnSb for various Ni excesses $x$, demonstrating the second-order nature of this transition. Structure solution on high-resolution room-temperature neutron diffraction as well as X-ray diffraction data yields a vacancy content in excess of $1-x$, the concentration of constitutional vacancies in the ideal model of site occupations. This is mirrored by the increased lattice constant during temperature-dependent diffraction on cooling, implying an annihilation of the initial vacancy excess at elevated temperatures. The reason for this very slow time-scale of vacancy concentration equilibration as compared to typical systems will be discussed, and consequences for the materials properties will be pointed out.
P. Neibecker et al., phys. status solidi B 259, 2100174 (2022)
