Structural voids are a defining feature of the half-Heusler structure. Using temperature-dependent neutron diffraction, complemented by room-temperature X-ray diffraction and high-resolution neutron diffraction as well as calorimetry and macroscopic magnetization measurements, the disordering of these vacancies is followed across the Clb–L21 transition in samples of (Formula presented.) for various Ni excesses x. The thermal vacancy concentration increases toward high temperatures, and excess vacancy concentrations large enough to increase the low-temperature macroscopic magnetization can be retained by quenching. Finally, the consequences of the strongly differing relaxation rates of order and vacancy concentrations are demonstrated, showing that in these and related systems the thermal history potentially determines material properties such as ordering temperatures much more strongly than usually.
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