Eui-Chol Shin

Oxide ion conduction anisotropy deconvoluted in polycrystalline apatite-type lanthanum silicates

Recent efforts to understand the anisotropic oxide ion conduction mechanism in apatite-type La9.33+x(SiO4)6O2+3x/2 (−0.5 ≤ x ≤ 1.25) by the sophisticated structural investigation and atomistic simulations could not definitely be corroborated nor refuted by the experimental conductivity measurements, since no measurements on single crystalline lanthanum silicates are available. In the present work the strong frequency dispersions in the bulk impedance of the polycrystalline silicates were successfully described by a hierarchical ladder network of two resistors and non-ideal capacitor elements with the smaller resistance element representing the contribution from the grains whose c-axes are oriented parallel to the transport and the larger resistance element from the grains in unfavorable orientations. All nominally lanthanum excess compositions (0.5 ≤ x ≤ 1.25) exhibited similar bulk conduction behavior which is consistent with the solubility limit at x = 0.43 ± 0.12 determined by microprobe analysis. A high-conductivity component attributable to the grains with c-axis transport exhibited an activation energy of 0.33 ± 0.02 eV. The process with a higher activation energy of 0.70 eV is ascribed to the grains oriented in the perpendicular direction with respect to transport direction. For lanthanum-deficient compositions with x < 0 the conductivity values plummeted from those of the lanthanum-excess ones and the activation energies increased up to 0.59 eV and 0.87 eV for the parallel and perpendicular direction, respectively. Oxide ion conduction anisotropy deconvoluted from the polycrystalline specimens suggests a consistent description of the conduction mechanism and defect chemistry of the lanthanum silicates. With the activation energy values of 0.33 eV for the migration of oxygen interstitials along the c-axis, the difference between the lanthanum excess and the lanthanum deficit composition can be attributed to the energy for generating oxygen interstitials which also appears to amount to ca. 0.3 eV. As the c-axis transport pathways are non-linear, the parallel and perpendicular transport are not completely separable and the migration energy for the perpendicular transport can be ascribed to the energy for the inter-channel reaction in addition to the energy for the parallel transport.

A comprehensive treatment of universal dispersive frequency responses in solid electrolytes by immittance spectroscopy: low temperature AgI case

Motivated by the recent work on β″-alumina polycrystalline ceramics, we revisit the frequency dispersion behavior of AgI. Series of admittance and capacitance Bode plots at different temperatures revealed the presence of well-defined parallel capacitance effects and power-law frequency dependencies. Non-trivial bulk dispersion is thus successfully described by the bulk conductance with activation energy of 0.262 eV in parallel connection to several capacitive effects: (i) a mobile-charge contribution, universally observed in many solid electrolytes, approximated by a Cole-Davidson model with Δ𝜖C ≅ 28, γC ≅ 0.388, and (τC)−1 thermally activated by 0.237 eV, (ii) a dipolar and vibratory contribution represented by another Cole-Davidson model of the dielectric strength of Δ𝜖D ≅ 9.0 with γD ≅ 0.110, τD ≅ 0.003 s, corresponding to Nearly-Constant-Loss behavior, and (iii) high frequency limit capacitance corresponding to the dielectric constant 𝜖S ≅ 6.4. Electrode effects are described by an ideal Warburg response and the coefficient activated by 0.165 eV, which is connected in parallel to the bulk capacitive elements (i) to (iii). The modeling allows a simulation of ac behavior of AgI as a function of temperature as well as frequencies, addressing all of the universally observed dispersive responses.

Positive temperature coefficient resistor behavior in praseodymium-doped ZnO (0001¯)∣(0001¯) boundaries

Clear positive temperature coefficient resistor (PTCR) dc behavior has been shown in Pr-doped ZnO (0001¯)∣(0001¯) bicrystals by electrical characterization over an unprecedentedly wide temperature range between 40 and 1070 K. With subtraction of the PTCR dc, the admittance can be described by a deep trap level at 0.26 eV but no clue to the origin of the PTCR behavior is provided. Capacitance-voltage characteristics revealed a maximum in the Schottky barrier heights consistent with the PTCR behavior. The PTCR behavior in Pr-doped ZnO c-axis oriented bicrystals is thus phenomenologically analogous to that of the ferroelectric BaTiO3.