Thermally robust negative dielectric response in strontium-doped lanthanum ferrite: high-temperature structural and electrical transport insights

Abstract Materials exhibiting a negative dielectric constant, commonly observed in metals, have recently attracted significant interest due to their unusual electromagnetic properties. Although this behaviour has been reported in composites and meta-materials, studies on single-phase perovskite oxides, particularly at high temperatures, remain limited. In the present work, temperature-dependent structural, dielectric, and electrical properties were investigated over the range from room temperature (RT) to 600 °C and a frequency range from 20 Hz to 2 MHz in La 1− x Sr x FeO x ( x = 0–0.5) ceramics. High-temperature (RT to 700 °C) XRD has been done for all the compositions, and Rietveld refinement confirms an increase in symmetry from orthorhombic ( Pnma ) to cubic ( Fm 3 ¯ c ) crystal structure with increasing temperature. Dielectric measurements indicate a transition from positive to negative dielectric constant in undoped LaFeO 3 ( T > 150 °C), while all Sr-doped samples exhibit a negative dielectric constant over the entire temperature range. Dielectric response is well described by the Drude model, enabling the estimation of plasma frequency ( ω p ) and damping frequency ( ω τ ), confirming the role of thermally activated charge carriers in driving the negative dielectric behaviour. Electrical conductivity follows Arrhenius behaviour with reduced activation energy upon Sr doping as supported by four-probe measurements, impedance analysis and transference number studies.