  Basing on peculiarities of local potentials, the two principal trends in
vacancy formation are revealed. The proximity to the threshold of local ionic
stability due to a giant potential contribution of electronic delocalization
accounts for thermal anion vacancies typical of intrinsic semiconductors of AB
type. On the other hand, the tendency towards equalizing the potential field
results in a high concentration of structural cation vacancies observed in Ni_3
Sb and relative compounds.
