Temperature-dependent soft x-ray photoemission and absorption studies of charge disproportionation in La_1-xSr_xFeO₃

We have measured the temperature dependence of the photoemission and x-ray absorption spectra of La_1-xSr_xFeO₃ (LSFO) epitaxial thin films with x = 0.67, where charge disproportionation (3Fe ^3.67+ → 2Fe³⁺ + Fe⁵⁺) resulting in long-range spin and charge ordering is known to occur below T_CD = 190 K. With decreasing temperature we observed gradual changes of the spectra with spectral weight transfer over a wide energy range of ∼5 eV. Above T _CD the intensity at the Fermi level was relatively high compared to that below T_CD but still much lower than that in conventional metals. We also found a similar temperature dependence for x = 0.4, and to a lesser extent for x = 0.2. These observations suggest that a local charge disproportionation occurs not only in the x = 0.67 sample below T_CD but also over a wider temperature and composition range in LSFO. This implies that the tendency toward charge disproportionation may be the origin of the unusually wide insulating region of the LSFO phase diagram.