Temperature-Dependent Soft X-ray Photoemission and Absorption Studies of Charge Disproportionation in La1-xSrxFeO₃

We have measured the temperature dependence of the photoemission and x-ray absorption spectra of La₁₋ₓSrₓFeO₃ (LSFO) epitaxial thin films with $x=0.67$, where charge disproportionation (3Fe3.67+→ 2Fe³⁺+ Fe⁵⁺) resulting in long-range spin and charge ordering is known to occur below TCD=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 TCD the intensity at the Fermi level (EF) was relatively high compared to that below TCD 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 TCD 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.