  Although crystalline solids are characterized by their periodic structures,
some are only periodic on average and deviate on a local scale. Distinct local
orderings can exist with identical periodic structures making their differences
hidden to normal diffraction methods. Using x-ray scattering we investigate the
thermoelectric half-Heusler, Nb1-xCoSb, with high vacancy concentrations, where
signs of local order have been attributed to differences in stoichiometry. We
show that the composition is actually always very close to x = 1/6 irrespective
of nominal sample composition, and that the synthesis method controls the local
order. The vacancy distribution is shown to follow a vacancy repulsion model,
and local structural relaxations around the vacancies are characterized.
Control over the local structure provides a new frontier for tuning material
properties.
