  In contrast to bulk materials, nanoscale crystal growth is critically
influenced by size- and shape-dependent properties. However, it is challenging
to decipher how stoichiometry, in the realm of mixed-valence elements, can act
to control physical properties, especially when complex bonding is implicated
by short and long-range ordering of structural defects. Here, solution-grown
iron-oxide nanocrystals (NCs) of the pilot wustite system are found to convert
into iron-deficient rock-salt and ferro-spinel sub-domains, but attain a
surprising tetragonally distorted local structure. Cationic vacancies within
chemically uniform NCs are portrayed as the parameter to tweak the underlying
properties. These lattice imperfections are shown to produce local
exchange-anisotropy fields that reinforce the nanoparticles magnetization and
overcome the influence of finite-size effects. The concept of atomic-scale
defect control in subcritical size NCs, aspires to become a pathway to
tailor-made properties with improved performance for hyperthermia heating over
defect-free NCs.
