  The present work puts forward a concept that the thermostable O1s XPS peaks
with energy of about 531 eV in negative charge-transfer SrFeO_{3-\delta}
perovskite are determined by the peroxo-like oxygen species. The peroxo group
forms via coupling two oxygen anions coordinated to iron cations with
d^5\bar-under{L} (\bar-under{L}-oxygen electron hole) configuration. By means
of plane-wave DFT+U approach there have been shown that the peroxo group
represents a metastable state in the absence of oxygen vacancies nearby. The
O-O bonding confines two electron holes freezing the 3+ oxidation state for two
iron cations bridged by peroxide. Increasing the peroxo group numbers makes the
ferrite a semiconductor with charge-transfer gap of about 0.6 eV.
