Effective control of conductivity in lutetium orthoferrite with cobalt doping measured by terahertz time-domain spectroscopy.

The effective control of conductivity in LuFeO (LFO) with Co doping is explored by terahertz (THz) time-domain spectroscopy. It is demonstrated that the conductivity of 5% Co-doped LFO (LFO:Co 5%) is lower than that of LFO, while that of 15% Co-doped LFO (LFO:Co 15%) is significantly higher than LFO. Furthermore, LFO exhibits two lattice vibration peaks at 0.58 and 1.61 THz, LFO:Co 5% shows only one lattice vibration peak at 1.61 THz, while no distinct vibration peak is observed in LFO:Co 15%. The disappearance of lattice vibration at 0.58 THz is attributed to the shortened Fe (Co)-O bond length resulting from Co doping, thus suppressing magnetic resonance effect of Fe. With 15% Co doping, structural stability is enhanced, and the asymmetric vibration of Lu at surface/interface/boundary is suppressed, resulting in the disappearance of vibration peak at 1.61 THz. The conductivity of LFO:Co 5% is lower than that of LFO, mainly because the lattice vibration at 1.61 THz and oxygen vacancy defects introduced by doping jointly increase the degree of carrier back-scattering, which decreases carrier movement, while the enhancement of conductivity by electronegativity at 5% Co doping is very limited. The significantly higher conductivity of LFO:Co 15% compared to LFO is due to the obvious increase in overall electronegativity and suppression of lattice vibration by 15% Co doping, thereby improving carrier mobility. The insights of this investigation provide important experimental data and theoretical basis for design and production of high-conductivity and stable solid oxide fuel cells cathode.