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Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and electronic conducting materials. Previous atomistic analysis has widely covered the effects of substitution on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations has rarely been explored. Here, we investigate soluble monovalent (Li+, Na+, K+, Rb+), divalent (Fe2+, Co2+, Mn2+, Mg2+, Ni2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+), trivalent (Al3+, Fe3+, Sc3+, In3+, Lu3+, Yb3+, Y3+, Er3+, Gd3+, Eu3+, Nd3+, Pr3+, La3+) and tetravalent (Si4+, Ge4+, Ti4+, Sn4+, Hf4+, Zr4+) cation substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on the diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cation-substituted systems follows the order Gd3+Ca2+Na+. With the same charge, a larger size mismatch with the Ce4+ cation yields a lower oxygen ionic diffusivity, i.e., Na+K+, Ca2+Ni2+, Gd3+Al3+. Based on these trends, we identify species that could tune the oxygen ionic diffusivity: we estimate that the optimum oxygen vacancy concentration for achieving fast oxygen ionic transport is ≈2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values as the temperature is increased. We find that co-substitutions can enhance the impact of the single substitutions beyond that expected by their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrate the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that govern the oxygen diffusion coefficient and migration energy would enable design criteria to be defined for tuning the ionic properties of the material, e.g., by co-substitutions.

Giant electrostrictors based on oxygen deficient fluorites are of great technological and scientific interest due to their unusual electromechanical properties. It is, however, still not clear whether solid solutions between different fluorites can affect the electrostrictive properties.In this work, we investigated the electromechanical response of four different solutions of 10 mol% gadolinium-doped ceria (GDC10) and 8mol% yttria-stabilized zirconia (8YSZ), corresponding to 1, 5, 10 and 20 wt% of 8YSZ into GDC10. An increase in the electrostriction coefficient with the 8YSZ content (maximum between 10 and 20 wt%) was registered. This effect suggests that the development of complex solid solutions could represent a new tool to develop electromechanical materials with superior properties.

The phase transition process of pure and 0.5 at% Fe-doped 0.95(Na1/2Bi1/2)TiO3–0.05BaTiO3 (NBBT5) single crystals under in-situ electric-field was investigated using the Raman spectroscopy. The pure NBBT5 exhibited R3c (a−a−a− octahedral tilting) symmetry when subjected to an external electric field (E) up to 70 kV/cm. While after the Fe-doping, the crystal underwent a transition from P4bm (a0a0c+ octahedral tilting) to P4mm (a0a0c0 octahedral tilting) phase under E of 30–40 kV/cm. The results indicated that the octahedral tilting in the Fe-NBBT5 was substantially suppressed by the electric field, which was suggested to contribute to the enhanced piezoelectric response compared to the pure one.

An ion-conducting crystal possessing non-cubic crystal symmetry and a non-dilute composition will in general display a multiplicity of different migration paths and different site energies for the mobile ions. Predicting the macroscopic rate of ion transport, and in particular, identifying substituent cations that optimise this rate, is therefore challenging. In this study, molecular dynamics simulations employing pair potentials were used to calculate the oxygen tracer diffusion coefficient as a function of temperature in various substituted systems based on the rhombohedral perovskite oxide bismuth ferrite, BiFeO3. Substituent cations that maximise (or minimise) are identified. The results also reveal the limits of the standard crystal-chemical approach to maximising by matching the size of the substituent cation to that of the host's. The implications for the use of BiFeO3 as a high-performance oxide-ion conductor or as a multiferroic medium are discussed.

A nanolaminated structure that is predominated by nanoscale lamellar boundaries with misorientations

Oxygen defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cations and oxygen vacancies (VO) pairs (Ce-VO), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-Vo pairs in a grain boundaries-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV/cm. The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on states' density and orbitals at the valence band edge. Our research indicates that the electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise of one order of magnitude, i.e., from 5·10-4 at room temperature to 5·10-3 at -193 °C, due to an increase in the population of the electrically active pairs. At a constant VO concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not.

$(\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2})\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}x\mathrm{BaTi}{\mathrm{O}}_{3}(\mathrm{NBT}\text{\ensuremath{-}}x\mathrm{BT})$ single crystal, as one of the most promising candidates to replace lead-based ferroelectrics, is a typical and unique lead-free system with morphotropic phase boundary (MPB) by far. In this paper, the phase transition process of NBT-0.06BT crystal in a wide temperature range of $20\ensuremath{\sim}550{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ was analyzed at atomic scale by theoretical fitting to the measured synchrotron radiation x-ray absorption fine structure spectra quantitatively. Detailed analysis revealed a gradually transition of local structure at the point of phase transition from rhombohedral to tetragonal phase in NBT-0.06BT crystal on the bond lengths of Bi-O and Bi-Ti. The continuous evolution of local structure with no obvious barrier to overcome is one of the intrinsic characteristics of MPB at atomic scale and is responsible for the high piezoelectric properties. These results could promote the understanding of the origin of the high piezoelectric performance around the MPB and meanwhile guide new lead-free material designs.

Recent investigations have shown that highly oxygen defective cerium oxides generate non-classical electrostriction that is superior to lead-based ferroelectrics. In this work, we report the effect of field-assisted spark plasma sintering (SPS) on electro-chemo-mechanical properties of calcium doped ceria (CDC). Nanometric powders of Ca.10 nm are rapidly consolidated to form polycrystalline nanostructures with a high degree of crystalline disorder. Remarkably, the resultant material demonstrates a large electromechanical strain without a frequency-related relaxation effect. We conclude that electromechanical activity in CDC materials strictly depends on the Ca2+- V o ¨ interaction, while disorder at the crystalline boundaries has a minor effect.

Single crystals of x at.% Nb + 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 (x%-Nb:NBT-6BT, x = 0.12, 0.23, and 0.48) show improved ferroelectric and piezoelectric properties. In particular, the large piezoelectric constant (d33 = 532 pC/N) and high field-induced strains (Smax = 0.57%) in 0.23%-Nb:NBT-6BT single crystal demonstrate its promising application potential. The underlying mechanisms were investigated directively by atomistic scale simulations. We found that, Nb5+ cations prefer to substitute Ti4+ cations on B-site and will first fill up the oxygen vacancies generated due to the loss of Bi2O3 and/or Na2O. Higher concentration of Nb2O5, however, will be compensated by Ti and/or Bi vacancies. The ions positions surrounding the defect center(s) were identified.

Ion migration in Na0.5Bi0.5TiO3 (NBT) was investigated by molecular dynamics (MD) simulations and molecular static (MS) simulations. In MD simulations (>104 ions), performed for simulation times of tsim = 0.3 ns and at simulation temperatures in the range 1200 ≤ T/K ≤ 2400, oxide ions exhibited appreciable diffusivity, with derived activation enthalpies lower than 0.8 eV, whereas sodium and bismuth cations displayed no diffusion at the temperatures and on the time scale of the simulations. MS calculations yielded high activation barriers (>2 eV) for the migration of these cations. These results strongly suggest that the contribution of A-site cations to the total ionic conductivity is negligible. Bismuth vacancies and sodium vacancies were found to be detrimental to the rate of oxygen diffusion. The effect of ordered arrangements of Bi3+/Na+ cations on oxygen tracer diffusion was also examined by MD simulations. The (111)-ordered structure was seen to display higher rates of oxygen diffusion than the (100)- and (110)-ordered structures. MS calculations of the individual barriers for oxygen-ion jumps provided insights into this behavior. Our simulation results thus suggest design rules for developing superior low-temperature oxide ion conductors based on NBT perovskites and related materials.

Density-functional-theory (DFT) calculations within the generalised gradient approximation (GGA) were used to examine the behaviour of point defects in the cubic BVIO3 perovskite-type oxide, ReO3. Energies of reduction and of hydration were calculated, and the results are compared with literature data for ABO3 perovskite oxides. The activation energies of migration were determined for O2−, H+, Li+, Na+, K+ and H3O+. An occupied A site in ReO3 is found to be beneficial to oxide-ion migration by a vacancy mechanism as well as to proton migration by a Grotthuss mechanism. Na+, K+ and H3O+ exhibit activation energies of migration higher than 2 eV, whereas Li+ is characterised by a very low migration barrier of 0.1 eV. Reasons for this behaviour are discussed. Our results suggest that H+, O2−, and especially Li+, are highly mobile ions in ReO3.

The understanding of surface chemistry changes on oxygen electrodes is critical for the development of reversible solid oxide fuel cell (RSOFC). Here, we report for the first time that the electrochemical potentials can drastically affect the surface composition and hence the electrochemical activity and stability of PrBaCo2 O5+δ (PBCO) electrodes. Anodic polarization degrades the activity of the PBCO electrode, whereas the cathodic bias could recover its performance. Alternating anodic/cathodic polarization for 180 h confirms this behavior. Microstructure and chemical analysis clearly show that anodic bias leads to the accumulation and segregation of insulating nanosized BaO on the electrode surface, whereas cathodic polarization depletes the surface species. Therefore, a mechanism based on the segregation and incorporation of BaO species under electrochemical potentials is considered to be responsible for the observed deactivation and recovery process, respectively.

A combination of polarized Raman technique, infrared reflectance spectra, and first-principles density-functional theoretical calculations were used to investigate structure transformation and lattice vibrations of Na0.5Bi0.5TiO3, Na0.5Bi0.5TiO3–5%BaTiO3, and Na0.5Bi0.5TiO3–8%K0.5Bi0.5TiO3 single crystals. It was found that Na0.5Bi0.5TiO3 is of a two-phase mixture with rhombohedral and monoclinic structures at room temperature. Correspondingly, three Raman-active phonon modes located at 395, 790, and 868 cm−1, which were previously assumed as A1 modes of rhombohedral phase have been reassigned as A′′, A′, and A′ modes of monoclinic phase in the present work. In particular, a strong low-frequency A′′ mode at 49 cm−1 was found and its temperature dependence was revealed. Two deviations from linearity for the abrupt frequency variation in the A′′ mode and Ti–O bond have been detected at temperatures of ferroelectric to antiferroelectric phase transition TF–AF and dielectric maximum temperature Tmax. The appearance of Na–O vibrations at 150 cm−1 was found below Tmax, indicating the existence of nanosized Na+TiO3 clusters. The observed Raman and infrared active modes belonging to distinct irreducible representations are in good agreement with group-theory predictions, which suggests 9A1+9E and 36A′′+24A′ modes for the rhombohedral and monoclinic phases of Na0.5Bi0.5TiO3, respectively.

Crystalline polar metallocenes are potentially useful active materials as piezoelectrics, ferroelectrics, and multiferroics. Within density functional theory (DFT), we computed structural properties, energy differences for various phases, molecular configurations, and magnetic states, computed polarizations for different polar crystal structures, and computed dipole moments for the constituent molecules with a Wannier function analysis. Of the systems studied, Mn2(C9H9N)2 is the most promising as a multiferroic material, since the ground state is both polar and ferromagnetic. We found that the predicted crystalline polarizations are 30&ndash, 40% higher than the values that would be obtained from the dipole moments of the isolated constituent molecules, due to the local effects of the self-consistent internal electric field, indicating high polarizabilities.

A lead-free single crystal 0.92(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3 (NBT-6BT-2KNN) with dimension of Φ35 mm × 10 mm is grown by a precisely controlled top seeded solution growth method. The 〈001〉 oriented single crystals have excellent piezoelectric properties with a giant strain of 0.83% at 28 kV/cm. Application of an electric-field ≥14 kV/cm leads to a phase transition from pseudocubic to coexistence of tetragonal and pseudocubic. A strong ferroelectric domain texture occurs during the phase transition. Furthermore, the variation of tetragonal phase fraction agrees well with the macroscopic strain curve, demonstrating that the induced tetragonal phase plays a critical role in the high strain property.

Gd-doped ceria (CGO) fluorite exhibits prominent electro-chemo-mechanical properties and giant-electrostriction at room temperature and has been recently disclosed in both CGO polycrystalline films and bulk. The electromechanical properties of CGO depend on the oxygen vacancy defects of the fluorite lattice. Early experiments suggest that defects along the [111] crystallographic direction promote high atomic distortion. These factors result in the largest electrostriction response ever measured. However, only out-of-plane electrostriction (i.e. M13) in (111) CGO oriented thin films has been reported so far, and several questions remain open about the electrostriction mechanism in the oxygen-defective fluorite. Here, we present electromechanical performances along different crystallographic directions. We grow thin films by pulsed laser deposition on single crystal substrates to obtain longitudinal and shear deformations (i.e. M11 and M12) of highly coherent (100), (110) and (111) oriented CGO thin films. As a result, we find an order of magnitude higher electrostriction coefficient along [100]. Such an analysis gives a new insight into the mechanism of CGO electrostriction.

Gadolium doped ceria, Gd:CeO2 (CGO), have recently been shown to possess an exceptional high electrostriction coefficient (Q), which is at the least three orders of magnitude larger than the best performing lead-based electrostrictors, e.g. Pb(Mn1/3Nb2/3)O3. Herein, we show that CGO thin films fabricated by a pulsed laser deposition method can be directly integrated onto the Si substrate by using TiN films of few nanometers as functional electrodes. The exceptional good coupling between TiN and Ce0.8Gd0.2O1.9 yields a high electrostriction coefficient of Qe = 40 m4 C−2 and a superior electrochemomechanical stability with respect to the metal electrodes.