Spectroscopic insight into the structural and microstructural properties of La2Ce2O7 ceramics

Abstract

Ceria-based compounds with trivalent dopant ions are currently being investigated as electrolytes for solid oxide fuel cells operating at low to intermediate temperatures. In these materials, the dependence of the ionic conductivity on the dopant content, the relative cation size and the dopant/vacancy or vacancy/vacancy interactions are the subject of intense research, but there is scarce information about a possible correlation between these properties and microstructural ones, such as texture. Here, we address this question in the heavily doped fluorite-like La₂Ce₂O₇ oxide, using a combination of X-ray diffraction, electron microscopy, optical absorption, Raman scattering and impedance spectroscopy. In all cases, materials with varying degree of 〈1 1 1〉 texture are compared with untextured samples. In this context, the most relevant Raman signatures are the fluorite-like mode at ∼448 cm⁻¹ and a vacancy-related band at ∼578 cm⁻¹. Although remarkable changes were observed upon varying the sintering temperature or the excitation wavelength, both textured and untextured materials displayed similar evolution. A selective enhancement of the vacancy-related band was observed upon decreasing the excitation wavelength from 647 to 488 nm, which is attributed to a resonance phenomenon involving vacancy-related electronic transitions. An indication of incipient vacancy ordering was given by the observation of an intense band at ∼346 cm⁻¹, denoting C-R₂O₃-like short-range order; its enhancement with the annealing temperature is attributed to the growth of the C-like regions. Conductivity was measured in textured and untextured, fully dense pellets, in the former along and across the texture direction. Impedance plots suggest that the conductivity is dominated by oxide ions and do not allow to separate a grain boundary contribution in these materials with grain size around 1–2 μm. The conductivity amounts to around 10⁻⁵ S/cm at 650 K, with activation energy of 1.08–1.16 eV in the temperature range from around 470 to 870 K. Results were similar in textured and untextured materials. We conclude that there is no significant correlation between texture and vacancy concentration or vacancy ordering. A similar conclusion – no influence of texture – is reached concerning the electrical properties.