Rare Earth Doped Ceria: The Complex Connection Between Structure and Properties

Abstract

The need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials.

Keywords: defects chemistry; diffraction; energy; microscopy; rare earths doped ceria; spectroscopy; structure; theoretical calculations.

Figure 1

i–V curves of the (A) Ni/CGO- and (B) Ni/YSZ-based cells. Experiments were conducted at 850°C. The reformate consisted of gas mixture IV (blue) containing 7% H₂, 7% H₂O, 20% CO, 20% CO₂, and 46% N₂, and the reference mixture V (red) consisted of 7% H₂, 7% H₂O, 86% N₂. i–V curves were recorded with (squares) and without (circles) the addition of 20 ppm of H₂S. Reprinted with permissions from Riegraf et al. (2017). © 2017 American Chemical Society.

Figure 2

Bulk ionic conductivity data of different Ce_0.8RE_0.2O_1.9 solid solutions measured in air. In the inset are shown conductivity data collected at 800°C on Ce_0.8Sm_0.2O_1.9 (black circles) and Ce_0.8Gd_0.2O_1.9 (blue diamonds) vs. pO₂. In the low pO₂ range, also electrons contribute to charge transport (ionic/electronic mixed regime). Data from Eguchi et al. (1992).

Figure 3

(A) Conductivity data of Ce_1−xY_xO_2−x vs. composition x at different T values. (B) Activation energies E_i vs. x for Ce_1−xRE_xO_2−x solid solutions. In the inset are shown the E_i values in the (300 < T < 600 K range) together with σ_i values at 400 K and x ≈ 0.10 as a function of the ionic radii of the 8-fold coordinated RE³⁺ ions. Data from Faber et al. (1989).

Figure 4

Sketch of fluorite (Left) and C-type (Right) structures, the latter displayed as octant of full cell to facilitate comparisons. The corresponding M-M connectivity is displayed below.

Figure 5

x_max values reported in the tabulated data from the literature. Red lines are guides to the eye, related to the largest and smallest x_max observed for each composition. The inset reports the average x_max values obtained for each dopant with at least two entries.

Figure 6

Evolution upon doping of (A) lattice parameter and (B) mean atomic volume for La (Andrievskaya et al., 2011), Nd (Horlait et al., 2011), Sm (Coduri et al., 2018), Eu (Mandal et al., 2006), Gd (Scavini et al., 2015), Y (Coduri et al., 2013a), Er (Horlait et al., 2011), and Lu (Artini et al., 2016).

Figure 7

(A) Interatomic M-O distances (left) and coordination numbers (right) obtained by EXAFS on Sm-doped ceria from different works, listed on the left side. Full symbols stand for M = Ce, empty symbols for M = Sm. Black circles refer to data from Giannici et al. (2014); red circles from Nitani et al. (2004); black triangle from Shirbhate et al. (2016). Interatomic distances are also compared with XRD and PDF, in full and empty blue symbols, respectively. (B) NMR spectra of Y-doped compounds showing different coordinations to O. Labels stand for the 6-, 7-, and 8-fold coordinations. Reprinted with permissions from Kim and Stebbins (2007). © 2012 American Chemical Society. (C) raman spectrum for x(Gd) = 0.375 with labels representing the coordinations involved. Data from Coduri et al. (2017). (D) Experimental PDF for x(Sm) = 0.25 with corresponding atom pairs (left). Fit of the same curve using single fluorite (green) and mix of fluorite and C-type (blue) on the right hand side. Data from Coduri et al. (2018).

Figure 8

(a) Nanodomains within white dashed lines observed in x(Y) = 0.25 by HRTEM and corresponding SAED pattern. Reprinted with permissions from Li et al. (2012). © 2012 American Chemical Society. (b) Characteristic signals of EELS spectra taken at O K-edge for different dopants. Reprinted with permissions from Ou et al. (2008a). © 2008 American Physical Society. (c) Experimental PDF for x(Gd) = 0.344, and (d) corresponding evolution of x(M2) coordinate with interatomic distance r. Data from Coduri et al. (2017). (e) Sketch of cation arrangement as in Figure 4 within the basal plane (z = 0) representing C-type domains, enclosed by black solid line, embedded in a fluorite matrix. Green dashed lines stand for APBs. The concept is discussed in Coduri et al. (2013a).

Figure 9

Spin density of bulk CeO₂ in presence of an oxygen vacancy, showing the formation of two Ce³⁺ centers localized around the vacancy. Reprinted with permission from Plata et al. (2012). © 2012 American Institute of Physics.