Volume 236, Issue 6-8 - Special Issue dedicated to Paul Heitjans on the occasion of his 75th birthday; Guest Editors: Bernhard Gadermaier and Martin Wilkening

Nanocrystalline mullite-type bismuth-bearing complex oxides Bi 2 (M 0.5 Al 0.5 ) 4 O 9 (M=Fe 3+ , Ga 3+ ) are prepared by high-energy ball milling of the corresponding microcrystalline counterparts. An unusual five-fold coordination of metal cations is revealed in nanostructured Bi 2 (M 0.5 Al 0.5 ) 4 O 9 by means of 27 Al magic angle spinning nuclear magnetic resonance and 57 Fe Mössbauer spectroscopies. The concentration of five-fold coordinated cations increases with decreasing crystallite size of a material at the expense of octahedrally coordinated ones. In addition to the nuclear spectroscopic methods, Rietveld analyses of the X-ray diffraction data of the as-prepared nanooxides show that the constituent tetrahedra, octahedra, and the newly formed structural units with five-fold cation coordination are strongly distorted. With decreasing crystallite size of mullites, the average volume of their octahedra increases whereas this parameter decreases for tetrahedra. The macroscopic behaviour of the non-equilibrium nanomullites is characterised by SQUID magnetometry. The Fe-containing mullites exhibit a superposition of a dominant antiferromagnetism and a weak ferromagnetism. The increase in both the remanent magnetization and the coercive field with decreasing crystallite size is attributed to the effect of spin canting. The latter is confined to the interfacial and surface regions of the nanomaterials, and arises due to both the mechanically induced deformation of constituent structural units and the formation of cation sites with the unusual five-fold coordination.

For the first time, a spinel-type high entropy oxide (Zn 0.25 Cu 0.25 Mg 0.25 Co 0.25 )Al 2 O 4 as well as its derivative lithiated high entropy oxyfluoride Li 0.5 (Zn 0.25 Cu 0.25 Mg 0.25 Co 0.25 ) 0.5 Al 2 O 3.5 F 0.5 and oxychloride Li 0.5 (Zn 0.25 Cu 0.25 Mg 0.25 Co 0.25 ) 0.5 Al 2 O 3.5 Cl 0.5 are prepared in the nanostructured state via high-energy co-milling of the simple oxide precursors and the halides (LiF or LiCl) as sources of lithium, fluorine and chlorine. Their nanostructure is investigated by XRD, HR-TEM, EDX and XPS spectroscopy. It is revealed that incorporation of lithium into the structure of spinel oxide together with the anionic substitution has significant effect on its short-range order, size and morphology of crystallites as well as on its oxidation/reduction processes. The charge capacity of the as-prepared nanomaterials tested by cyclic voltammetry is found to be rather poor despite lithiation of the samples in comparison to previously reported spinel-type high entropy oxides. Nevertheless, the present work offers the alternative one-step mechanochemical route to novel classes of high entropy oxides as well as to lithiated oxyfluorides and oxychlorides with the possibility to vary their cationic and anionic elemental composition.

Two new copper bismuth sulfide halides, CuBi 2 S 3 Cl and CuBi 2 S 3 Br, were synthesized by a two-step process of ball milling followed by annealing. Both compounds are obtained as dark grey powders and crystallize in the monoclinic space group C 2/ m with lattice parameters a  = 12.9458(11) Å, b  = 3.9845(3) Å, c  = 9.1024(8) Å and β  = 91.150(3)° for the sulfide chloride and a  = 13.3498(8) Å, b  = 4.1092(2) Å, c  = 9.4173(6) Å and β  = 90.322(4)° for the sulfide bromide. Also known for related compounds, the copper atoms are strongly disordered. Quantum-chemical calculations suggest that modelling the structure with fixed copper positions does not satisfactorily describe all structural features, which insinuates copper ion mobility at elevated temperatures.

Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) powders were prepared from different NO x -free precursors using an aqueous-based solution-assisted solid-state reaction (SA-SSR). The sintering behavior, phase formation, microstructure and ionic conductivity of the powders were explored as a function of sintering temperature. The powders showed a relatively narrow temperature windows in which shrinkage occurred. Relative densities of 95% were reached upon heating between 900 and 960 °C. Depending on the morphological features of the primary particles, either homogeneous and intact microstructures with fine grains of about <2 µm in size or a broad grain size distribution, micro-cracks and grain cleavages were obtained, indicating the instability of the microstructure. Consequently, the ceramics with a homogeneous microstructure possessed a maximum total ionic conductivity of 0.67 mS cm −1 , whereas other ceramics reached only 0.58 mS cm −1 and 0.21 mS cm −1 .

Beta-detected NMR is a type of nuclear magnetic resonance that uses the asymmetric property of radioactive beta decay to provide a “nuclear” detection scheme. It is vastly more sensitive than conventional NMR on a per nuclear spin basis but requires a suitable radioisotope. I briefly present the general aspects of the method and its implementation at TRIUMF, where ion implantation of the NMR radioisotope is used to study a variety of samples including crystalline solids and thin films, and more recently, soft matter and even room temperature ionic liquids. Finally, I review the progress of the TRIUMF β NMR program in the period 2015–2021.

The diffusive behavior in a spinel-type Li + ion battery material, Li[Ni 1/2 Mn 3/2 ]O 4 , has been studied with positive and negative muon spin rotation and relaxation ( μ ± SR) measurements in the temperature range between 200 and 400 K using a powder sample. The implanted μ + locates at an interstitial site near O 2− ion so as to form a O–H like bond, while the implanted μ − is mainly captured by an oxygen nucleus, resulting in the formation of muonic oxygen. This means that local magnetic environments in Li[Ni 1/2 Mn 3/2 ]O 4 were investigated from the two different sites in the lattice, i.e., one is an interstitial site for μ + SR and the other is an oxygen site for μ − SR. Since both μ + SR and μ − SR detected an increase in the fluctuation rate of a nuclear magnetic field for temperatures above 200 K, the origin of this increase is clearly confirmed as Li diffusion. Assuming a random walk process with the hopping of thermally activated Li + between a regular Li site and the nearest neighboring vacant octahedral sites, a self-diffusion coefficient of Li + was found to range above 10 −11  cm 2 /s at temperatures above 250 K with an activation energy of about 0.06 eV.

Various nuclear magnetic resonance (NMR) methods are combined to study the structure and dynamics of Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) samples, which were obtained from sintering at various temperatures between 650 and 900 °C. 6 Li, 27 Al, and 31 P magic angle spinning (MAS) NMR spectra show that LATP crystallites are better defined for higher calcination temperatures. Analysis of 7 Li spin-lattice relaxation and line-shape changes indicates the existence of two species of lithium ions with clearly distinguishable jump dynamics, which can be attributed to crystalline and amorphous sample regions, respectively. An increase of the sintering temperature leads to higher fractions of the fast lithium species with respect to the slow one, but hardly affects the jump dynamics in either of the phases. Specifically, the fast and slow lithium ions show jumps in the nanoseconds regime near 300 and 700 K, respectively. The activation energy of the hopping motion in the LATP crystallites amounts to ca. 0.26 eV. 7 Li field-gradient diffusometry reveals that the long-range ion migration is limited by the sample regions featuring slow transport. The high spatial resolution available from the high static field gradients of our setup allows the observation of the lithium ion diffusion inside the small (<100 nm) LATP crystallites, yielding a high self-diffusion coefficient of D  = 2 × 10 −12  m 2 /s at room temperature.

Polyhydroborate-based salts of lithium and sodium have attracted much recent interest as promising solid-state electrolytes for energy-related applications. A member of this family, sodium dicarba- nido -undecahydroborate Na-7,9-C 2 B 9 H 12 exhibits superionic conductivity above its order-disorder phase transition temperature, ∼360 K. To investigate the dynamics of the anions and cations in this compound at the microscopic level, we have measured the 1 H and 23 Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 148–384 K. It has been found that the transition from the low- T ordered to the high- T disordered phase is accompanied by an abrupt, several-orders-of-magnitude acceleration of both the reorientational jump rate of the complex anions and the diffusive jump rate of Na + cations. These results support the idea that reorientations of large [C 2 B 9 H 12 ] − anions can facilitate cation diffusion and, thus, the ionic conductivity. The apparent activation energies for anion reorientations obtained from the 1 H spin-lattice relaxation data are 314 meV for the ordered phase and 272 meV for the disordered phase. The activation energies for Na + diffusive jumps derived from the 23 Na spin-lattice relaxation data are 350 and 268 meV for the ordered and disordered phases, respectively.

A comprehensive multinuclear ( 7 Li, 31 P, 75 As, 77 Se, 127 I) NMR study has been conducted to characterize local structural configurations and atomic distributions in the crystallographically ordered solid solutions of composition Li 6 PS 5- x Se x I (0 ≤  x  ≤ 1) and in Li 6 AsS 5 I. Throughout the composition range, structural ordering between the atoms on the Wyckoff sites 4a and 4c is maintained, with the I − ions exclusively occupying the 4a sites. 31 P magic-angle spinning nuclear magnetic resonance (MAS NMR) can serve to differentiate between the various possible PS 4- n Se n 3− tetrahedral units in a quantitative fashion, indicating a preference of P-S relative to P-Se bonding. Each individual PS 4- n Se n 3− tetrahedron is represented by a peak cluster containing up to five resonances, representing the five different configurations in which the PCh 4 3− units are surrounded by the four closest chalcogenide anions occupying the 4c sites; the distribution of S 2− and Se 2− over these sites is close to statistical. Non-linear 7 Li chemical shift trends as a function of x are interpreted to indicate that the Coulombic traps created by sulfur-rich PS 4- n Se n 3− ions ( n  ≥ 2) within the energy landscape of the lithium ions are deeper than those of the other anionic species present (i.e. selenium-richer PCh 4 3− tetrahedra, isolated chalcogenide or iodide ions), causing the Li +  ions to spend on average more time near them. Temperature dependent static 7 Li NMR linewidths measured on Li 6 PS 5 I and Li 6 AsS 5 I indicate a two-step motional narrowing process characterized by a clear dynamic distinction between a more rapid localized intra-cage process and a slower, long-range inter-cage process. In the solid solutions this differentiation gradually disappears, leading to an overall increase of lithium ionic mobility with increasing selenium content, which can be attributed to the influences of higher anionic polarizability and a widening of the lithium migration pathways caused by lattice expansion. Furthermore, the low-temperature phase transition in Li 6 PS 5 I, which tends to immobilize the lithium ions below 170 K, is suppressed in the solid solutions. The results offer interesting new insights into the -structure/ionic mobility correlations in this new class of compounds.

A comprehensive multinuclear ( 7 Li, 31 P, 35 Cl, 77 Se, 79 Br) nuclear magnetic resonance (NMR) study has been conducted to characterize local structural configurations and atomic distributions in the crystallographically disordered solid solutions of composition Li 6 PS 5− x Se x X (0 ≤  x  ≤ 1, X = Cl, Br) with the Argyrodite structure. In contrast to the situation with the corresponding iodide homologs, there is no structural ordering between the 4a and 4 c sites, with the halide ions occupying both of them with close to statistical probabilities. Nevertheless, throughout the composition range, the 16e Wyckoff sites of the Argyrodite structure are exclusively occupied by the chalcogen atoms, forming PY 4 3− (Y = S, Se) tetrahedra, indicating the absence of P-halogen bonds. 31 P magic-angle spinning (MAS)-NMR can serve to differentiate between the various possible PS 4− n Se n 3− tetrahedral units in a quantitative fashion. Compared to the case of the anion-ordered Li 6 PS 5− x Se x I solid solutions, the preference of P–S over P–Se bonding is significantly stronger, but it is weaker than in the halide free solid solutions Li 7 PS 6− x Se x . Each individual PS 4− n Se n 3− tetrahedron is represented by a peak cluster of up to five resonances, representing the five different configurations in which the PY 4 3− ions are surrounded by the four closest chalcogenide and halide anions occupying the 4c sites; this distribution is close to statistical and can be used to deduce deviations of sample compositions from ideal stoichiometry. Non-linear 7 Li chemical shift trends as a function of x are interpreted to indicate that the Coulombic traps created by sulfur-rich PS 4− n Se n 3− ions ( n  ≤ 2) within the energy landscape of the lithium ions are deeper than those of the other anionic species present (i.e., selenium-richer PY 4 3− tetrahedra, isolated chalcogenide or iodide ions), causing the Li + ions to spend on average more time near them. Temperature dependent static 7 Li NMR linewidths indicate higher mobility in the present systems than in the previously studied Li 6 PS 5− x Se x I solid solutions. Unlike the situation in Li 6 PS 5− x Se x I no rate distinction between intra-cage and inter-cage ionic motion is evident. Lithium ionic mobility increases with increasing selenium content. This effect can be attributed to the influences of higher anionic polarizability and a widening of the lithium ion migration pathways caused by lattice expansion. The results offer interesting new insights into the structure/ionic mobility correlations in this new class of compounds.

The use of independent component analysis (ICA) for the analysis of two-dimensional (2D) spin-alignment echo– T 1 7 Li NMR correlation data with transient echo detection as a third dimension is demonstrated for the superionic conductor Li 10 GeP 2 S 12 (LGPS). ICA was combined with Laplace inversion, or discrete inverse Laplace transform (ILT), to obtain spectrally resolved 2D correlation maps. Robust results were obtained with the spectra as well as the vectorized correlation maps as independent components. It was also shown that the order of ICA and ILT steps can be swapped. While performing the ILT step before ICA provided better contrast, a substantial data compression can be achieved if ICA is executed first. Thereby the overall computation time could be reduced by one to two orders of magnitude, since the number of computationally expensive ILT steps is limited to the number of retained independent components. For LGPS, it was demonstrated that physically meaningful independent components and mixing matrices are obtained, which could be correlated with previously investigated material properties yet provided a clearer, better separation of features in the data. LGPS from two different batches was investigated, which showed substantial differences in their spectral and relaxation behavior. While in both cases this could be attributed to ionic mobility, the presented analysis may also clear the way for a more in-depth theoretical analysis based on numerical simulations. The presented method appears to be particularly suitable for samples with at least partially resolved static quadrupolar spectra, such as alkali metal ions in superionic conductors. The good stability of the ICA analysis makes this a prospect algorithm for preprocessing of data for a subsequent automatized analysis using machine learning concepts.

Li-bis(trifluoromethylsulfonyl)imide based ionic liquids with either butyl-trimethylammonium or N,N-dimethyl-N-(2-(propionyloxy)-ethyl)butan-1-ammonium as the anion were studied using proton and fluorine relaxometry as well as using field-gradient diffusometry to gain separate access to cation and anion dynamics in these compounds. The transport parameters obtained for these ionic liquids are compared with the estimates based on the conductivity data from literature and from the present work. The impact of cation size on correlation effects, the latter parameterized in terms of various Haven ratios, is mapped out.

This review gives an overview of current trends in the investigation of confined molecules such as higher alcohols, ethylene glycol and polyethylene glycol as guest molecules in neat and functionalized mesoporous silica materials. All these molecules have both hydrophobic and hydrophilic parts. They are characteristic role-models for the investigation of confined surfactants. Their properties are studied by a combination of solid-state NMR and relaxometry with other physicochemical techniques and molecular dynamics techniques. It is shown that this combination delivers unique insights into the structure, arrangement, dynamical properties and the guest-host interactions inside the confinement.

A systematic structural study on the intercalation of the radical cation 2-(4- N -methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1 H -imidazol-1-oxyl-3- N -oxide ( p -MPYNN) into a synthetic fluoromica clay named Somasif ® ME 100, Na 2 x Mg 3.0− x Si 4 O 10 (F y OH 1− y ) 2 ( x  = 0.33, y  = 0.98), has been carried out. The guest–host interactions in these intercalation compounds have been characterized by X-ray powder diffraction and solid-state NMR of the constituent nuclei ( 23 Na, 19 F, 27 Al and 29 Si) of the Somasif lattice. The intercalation process can be quantitatively monitored using 23 Na MAS-NMR. As a function of radical ion loading, different kinds of stacking arrangements can be realized, which modify the intermolecular interactions between the electron spins of the guest species, as probed by electron spin resonance (EPR) and 1 H solid state nuclear magnetic resonance (NMR). The results are discussed in relation to previous literature data on similar systems.

The compound LiNi 0.5 Mn 1.5 O 4 is used as novel cathode material for Li-ion batteries and represents a variant to replace conventional LiMn 2 O 4 . For a further improvement of battery materials it is necessary to understand kinetic processes at and in electrodes and the underlying diffusion of lithium that directly influences charging/discharging times, maximum capacities, and possible side reactions. In the present study Li tracer self-diffusion is investigated in polycrystalline sintered bulk samples of near stoichiometric LiNi 0.5 Mn 1.5 O 4 with an average grain size of about 50–70 nm in the temperature range between 250 and 600 °C. For analysis, stable 6 Li tracers are used in combination with secondary ion mass spectrometry (SIMS). The tracer diffusivities can be described by the Arrhenius law with an activation enthalpy of (0.97 ± 0.05) eV, which is interpreted as the sum of the formation and migration energy of a thermally activated Li vacancy.

Ionic redistribution at solid interfaces in ionic materials is the keystone of nanoionics. An experimental master piece has been provided by CaF 2 -BaF 2 heterolayers. Meanwhile this system and the involved heterojunctions are extraordinarily well-understood. The present paper gives an account of this model system by reviewing not only transport experiments and defect-chemical modeling as a function of temperature and spacing of the individual layers, but also transition from semi-infinite to mesoscopic conditions, transition from Mott–Schottky to Gouy–Chapman behavior as well as the impact of ionic redistribution on the electronic minority carriers. Owing to the availability of bulk transport data, the analysis works well for in-plane and out-of-plane measurements with only the space charge potential as fit parameter. Space charge effects are able to provide an interpretation of the annealing behavior, too. The experiments are corroborated by molecular dynamics simulations. Extrapolating the ionic redistribution effects down to the atomic level may even explain homovalent doping effects in non-equilibrium mixtures of the two fluorides.

The ionic conductivity of alkali aluminum germanium phosphates (MAGP) has been investigated by two different techniques, i.) a fs-Plasma-Charge Attachment Induced Transport (CAIT) approach and ii.) a classical two electrode DC approach. Amorphous MAGP samples of the composition M 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 M=(Li–Cs) have been synthesized by the melt-quenching technique. Comparison of fs-Plasma-CAIT and DC data reveal that the ionic conductivities as well as the activation energies for ion transport agree within the error margins of the experiment. While conventional expectation suggests that a DC approach should fail because of spontaneous charge carrier blocking, this work demonstrates that DC measurements are a simple tool for quantifying ionic conductivities provided that only a small amount of charge has been transported in total.

The chemical expansion of Pr 0.1 Ce 0.9 O 2– δ (PCO) and CeO 2– δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm’s with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption.

For ion transport in network glasses, it is a great challenge to predict conductivities specifically based on structural properties. To this end it is necessary to gain an understanding of the energy landscape where the thermally activated hopping motion of the ions takes place. For alkali borophosphate glasses, a statistical mechanical approach was suggested to predict essential characteristics of the distribution of energies at the residence sites of the mobile alkali ions. The corresponding distribution of site energies was derived from the chemical units forming the glassy network. A hopping model based on the site energy landscape allowed to model the change of conductivity activation energies with the borate to phosphate mixing ratio. Here we refine and extend this general approach to cope with minimal local activation barriers and to calculate dc-conductivities without the need of performing extensive Monte-Carlo simulations. This calculation relies on the mapping of the many-body ion dynamics onto a network of local conductances derived from the elementary jump rates of the mobile ions. Application of the theoretical modelling to three series of alkali borophosphate glasses with the compositions 0.33Li 2 O–0.67[ x B 2 O 3 –(1 −  x )P 2 O 5 ], 0.35Na 2 O–0.65[ x B 2 O 3 –(1 −  x )P 2 O 5 ] and 0.4Na 2 O–0.6[ x B 2 O 3 –(1 −  x )P 2 O 5 ] shows good agreement with experimental data.

The increasing demand for batteries forced the development of energy storage systems that rely on materials consisting of abundant elements in the Earth’s crust. Switching from Li + to K + as the main ionic charge carrier needs highly conducting potassium-bearing electrolytes to realize K + ion batteries (PIBs). The knowledge gained from the design of Li-ion batteries (LIBs) and Na-ion batteries (NIBs) may conceptually inspire also the establishment of PIBs. Considering, for instance, the hexatitanates Na 2 Ti 6 O 13 , Li 2 Ti 6 O 13 , and H 2 Ti 6 O 13 , which were previously investigated as components for LIBs and NIBs, here we investigated ion dynamics in the K-analog K 2 Ti 6 O 13 . Ionic transport in polycrystalline samples of K 2 Ti 6 O 13 was studied in a moisture-free atmosphere by broadband impedance spectroscopy in a temperature range from 20 °C to 450 °C. The current study aims at establishing a correlation between structural features of K 2 Ti 6 O 13 and long-range ionic transport. As expected for K + transport in K 2 Ti 6 O 13 with its geometrically obstructed structure, the overall activation energy of ion transport in the ternary oxide takes a rather high value of 0.97(2) eV. Almost the same result (0.95(3) eV) is obtained for the migration activation energy, which we extracted from the analysis of crossover frequencies of the corresponding conductivity isotherms. By comparing our results with those of Na 2 Ti 6 O 13 (0.82 eV), Li 2 Ti 6 O 13 (0.65 eV), and H 2 Ti 6 O 13 , we clearly see how the size of the mobile cation correlates with both specific conductivities and activation energies. This comparison points to K + being the main charge carrier in K 2 Ti 6 O 13 . It also helps in laying the foundations to derive the relevant structure-property relationships in this class of materials.

Nanostructured materials can show considerably different properties as compared to their coarse-grained counterparts. Especially prepared by high-energy ball milling they are to be characterized by a large fraction of point defects in the bulk and structurally disordered interfacial regions. Here, we explored how the overall conductivity of SmF 3 can be enhanced by mechanical treatment and to which degree aliovalent substitution is able to further enhance anion transport. For this purpose nanocrystalline (hexagonal) SmF 3 was prepared by high-energy ball milling; mechanosynthesis helped us to replace Sm 3+ in SmF 3 by Ba 2+ and to create vacancies in the F anion sublattice. We observed a remarkable increase in total (direct current) conductivity when going from nano-SmF 3 to Sm 1− x Ba x F 3− x for x  = 0.1. Electrical modulus spectroscopy was used to further characterize the corresponding increase in electrical relaxation frequencies.

In this overview, we summarize the phenomenon of thermotransport (the close coupling of mass transport and heat transport) in two fast ion conductors: yttria-doped zirconia and gadolinia-doped ceria. We focus on two recent molecular dynamics calculations using the Green-Kubo formalism. We show that the Onsager thermotransport cross coefficient (mass-heat coupling) is negative, meaning that oxygen ions would drift, in principle, to the hot side in a temperature gradient. Simulation results presented in this overview show reasonable agreement with available experimental data for thermal conductivity. Results of this study suggest that the coupling between mass and heat transport in oxygen ion electrolytes could have significant effect for practical applications.