Origin of Semiconductor–Metal–Semiconductor-Like Transition in LaFeO3

A comprehensive investigation of the semiconductor–metal–semiconductor-like transition (SMST) in LaFeO ₃ nanoceramics, synthesized via a modified sol-gel auto-combustion method, was conducted at low temperatures. Scanning electron microscopy (SEM) of the fracture surface reveals a porous microstructure with an average grain size of approximately 60 nm. X-ray photoelectron spectroscopy (XPS) confirms the presence of Fe ²⁺ , Fe ³⁺ , and Fe ⁴⁺ ions and oxygen vacancies in the LaFeO ₃ matrix. Impedance analysis indicates that the observed SMST behavior is associated with valence state transitions among Fe ^+ 3 , Fe ^+ 2 , and Fe ^+ 4 ions. The Kohlrausch-Williams-Watts (KWW) parameter is less than unity, suggesting non-Debye-type relaxation, as supported by modulus spectroscopy. The temperature-dependent frequency exponent indicates that the conduction mechanism is governed by both correlated barrier hopping (CBH) and non-overlapping small polaron tunnelling (NSPT). A high dielectric constant in the low-frequency region confirms the dominant contribution of electrode polarization. Moreover, the temperature-dependent dielectric constant and tangent loss further validate the SMST behavior.