Evaluation of the Oxygen Migration Energy in Doped Ceria by Terahertz Spectroscopy

We measure the terahertz conductivity of doped ceria by terahertz time-domain spectroscopy at different temperatures to characterize the shape of the potential associated with microscopic ion hopping. The experimental results for $\mathrm{Gd}$- and $\mathrm{La}$-doped ceria show that the activation energy in doped ceria is larger than that in stabilized zirconia. This result can be explained by a simple rigid-body model with the assumption of doping-induced lattice expansion, i.e., by considering the gap between cations. Our results for conventional fluorite-type solid-oxide electrolytes show that terahertz spectroscopy can be used to investigate fast microscopic ionic conduction in various electrolytes, which cannot be accessed through conventional impedance measurements.

Figure 1

Schematic of the crystal structure of cubic ceria and zirconia in the stick-and-ball scheme.

Figure 2

(a) XRD patterns of nondoped ${\mathrm{Ce}\mathrm{O}}_2$, GDC, LDC, and GSZ sintered pellets. All patterns are characteristic of the cubic fluorite structure. Data are offset for clarity. XRD data of ${\mathrm{Ce}\mathrm{O}}_2$ (ICSD180955) and ${\mathrm{Zr}\mathrm{O}}_2$ (ICSD173962) are added with vertical violet bars. (b) Infrared reflection spectra of the four samples. Large noise around 400 cm⁻¹ (denoted by the asterisk) is due to low sensitivity of the two infrared interferometers.

Figure 3

(a) Experimental setup for THz time-domain spectroscopy. (b) Time-domain profiles of the THz pulses obtained without (black curve) and with the GDC sintered pellet inside the electric furnace (red and blue curves). Time-domain profiles in this plot can be compared because we remove the time delay caused by the change in the optical path length due to the heated gas and silica tube inside the electric furnace.

Figure 4

THz conductivity spectra of (a) GDC, (b) LDC, and (c) nondoped ceria sintered pellets at different temperatures. Solid smoothing curves are guides to the eye. Dashed line in (c) is the fitted curve analyzed with the two Lorentz oscillators from the mid-infrared reflection, as shown in Fig. S2 within the Supplemental Material [13].

Figure 5

(a) Optical conductivity at 0.34 THz as a function of temperature for GDC, LDC, nondoped ceria, and GSZ sintered pellets. Accuracy of the measured conductivity is 0.02 S/cm. Black lines are those fitted through the least-squares method. (b) THz activation energy as a function of dopant radius for doped ceria and stabilized zirconia.

Figure 6

(a) Illustration at the top shows the local structure around the mobile oxygen ion in the fill-space scheme. Graph at the bottom shows the potential on the migration path along the [001] direction. (b) THz activation energy as a function of g.