Fig. 4

Enhanced conductivity in tetragonal ceria results from electron transfer during the phase transition. a Temperature variation of electron paramagnetic resonance (EPR) spectra of 5 nm ceria. The left inset is an example of the integral signal for the paramagnetic defect (\(V_o^ -\)) and the right inset is an example of the integral signal for Ce³⁺. The corresponding integration ranges are depicted with black dashed lines. b The relative concentration of the Ce³⁺ and the \(V_o^ -\) species as a function of temperature extracted from integrating the EPR signals. c The current–voltage (I–V) curves for the 5 nm sample under varying temperature conditions. All the curves are normalized by the room temperature current (I_RT) at −10 V. The inset depicts the expanded low current region. d Temperature dependence of the log-scaled relative currents, \({\mathrm{lg}}\left| {\frac{I}{{I_{{\mathrm{RT}}}}}} \right|\), with the applied voltage of 10 V for both 5 nm and bulk sample. The arrows correlate to the local structures of 5 nm ceria