Search

Ionically conducting oxide heterostructures provide ideal geometries for revealing interfacial effects in solid state ionics. Beyond that, they can provide a high density of heterointerfaces and thus allow for reliable studies of interfacial phenomena with respect to fundamental understanding as well as with respect to nanoionic applications. Substituting an entire single layer rather than random single ions by aliovalent impurities enables the possibility of 2D doping rather than conventional 0D doping. To the best of the authors' knowledge, this technique has never been applied to ionically conducting systems. Acceptor‐doped barium zirconate has been widely investigated, particularly for applications in clean energy technologies, most importantly for intermediate temperature fuel cells. Here, epitaxial BaZrO3–BaYOx heterostructures and in particular 2D doping of BaZrO3 by yttrium are reported. The effects on proton conductivity are studied. The results not only lead to fundamental insight into the pros and cons of 2D doping of ion conductors, but may also help improve protonic devices. [ABSTRACT FROM AUTHOR]

We apply density functional theory to estimate the energetics and charge carrier concentrations and, in turn, the resistance across the (210)[001] and (111)[11¯0] grain boundaries (GBs) in proton conducting Y-doped BaZrO3, assessing four commonly used approximations in space charge modelling. The abrupt core approximation, which models the GB core as a single atomic plane rather than a set of multiple atomic planes, gives an underestimation of the GB resistance with around one order of magnitude for both GBs. The full depletion approximation, which assumes full depletion of effectively positive charge carriers in the space charge layers, has negligible effect on the GB resistance compared to a more accurate model with decaying depletion. Letting protons redistribute in the continuity between atomic planes gives a GB resistance up to 5 times higher than the case where protons are restricted to be located at atomic planes. Finally, neglecting trapping effects between the acceptor doping and the defect charge carriers gives a higher GB resistance with a factor of roughly 2. [ABSTRACT FROM AUTHOR]

Acceptor-doped BaZrO3 is currently the most promising ceramic proton conductor for application in electrolyzers and reactor membranes. Its overall proton conductivity is, however, limited by space-charge formation due to defect segregation to grain boundaries (GBs). In this contribution, we determine the vibrational contribution to the free energy of GB formation and defect segregation in the symmetric Σ3 (112)[1̅10] tilt GB in BaZrO3 in order to elaborate on the high temperature GB defect chemistry. We consider segregation of several different defect species: oxygen vacancies UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/, protons UGRAPHIC DISPLAY="INLINE" ID="UGT2" SRC="UGT2"/ and the four trivalent acceptor dopants Sc, In, Y and Gd UGRAPHIC DISPLAY="INLINE" ID="UGT3" SRC="UGT3"/. The calculations reveal that the segregation free energy of UGRAPHIC DISPLAY="INLINE" ID="UGT4" SRC="UGT4"/ decreases with temperature while it remains fairly constant for UGRAPHIC DISPLAY="INLINE" ID="UGT5" SRC="UGT5"/, which increase the relative stability of UGRAPHIC DISPLAY="INLINE" ID="UGT6" SRC="UGT6"/ with respect to UGRAPHIC DISPLAY="INLINE" ID="UGT7" SRC="UGT7"/ with increasing temperature. As a consequence the onset for hydration of the GB core is shifted towards lower temperatures. For the dopants we find that both the segregation energy and entropy correlate with the ionic radius, where they are positive for the smaller dopants (Sc and In) while negative for the larger (Y, Gd). In turn, this leads to similar segregation free energies for all species, which are close to zero, at high temperatures where dopants are mobile, implying that dopant segregation is almost entirely driven by segregated positively charged defects. Neglecting the phonon contribution erroneously leads to the conclusion that the thermodynamic driving force for dopant segregation increases with increasing dopant radii, and it is therefore important to consider free energies when predicting the high temperature defect chemistry of BaZrO3 GBs. [ABSTRACT FROM AUTHOR]

Differentiating chemical and thermal expansion is virtually impossible to achieve experimentally. While thermal expansion stems from a softening of the phonon spectra, chemical expansion depends on the chemical composition of the material. In the present contribution, we, for the first time, completely decouple thermal and chemical expansion through first principles phonon calculations on BaCeO3, providing new fundamental insights to lattice expansion. We assess the influence of defects on thermal expansion, and how this in turn affects the interpretation of chemical expansion and defect thermodynamics. The calculations reveal that the linear thermal expansion coefficient is lowered by the introduction of oxygen vacancies being 10.6 × 10−6 K−1 at 300 K relative to 12.2 × 10−6 K−1 for both the protonated and defect-free bulk lattice. We further demonstrate that the chemical expansion coefficient upon hydration varies with temperature, ranging from 0.070 to 0.115 per mole oxygen vacancy. Ultimately, we find that, due to differences in the thermal expansion coefficients under dry and wet conditions, the chemical expansion coefficients determined experimentally are grossly underestimated – around 55% lower in the case of 10 mol% acceptor doped BaCeO3. Lastly, we evaluate the effect of these volume changes on the vibrational thermodynamics. [ABSTRACT FROM AUTHOR]

In the present contribution, we assess the efficiency of the alkali metals (Na, K, Rb and Cs) as A-site acceptor dopants in proton conducting BaZrO3 by first principles calculations. The calculated acceptor–proton complexes become weaker with increasing dopant size, with binding energies ranging from −0.33 eV for Na to −0.10 eV for Cs, which is in the range of, or even lower than, those found for B-site doped BaZrO3. By mapping out all relevant minimum energy pathways for the proton, we reveal that the highest migration energy barrier for most of the alkali metals is comparable or even lower than that of Y. Further, all A-site dopants display more exothermic hydration enthalpies compared to that of Y-doped BaZrO3, ranging from −131 kJ mol−1 to −83 kJ mol−1 for Na and Cs, respectively. The calculated dopant solubility increases in the order Na ＜ Cs ＜ Rb ＜ K, with the predicted solubilities of the two latter being in the range of that of e.g. Y. Although Cs would lead to the highest proton mobility, the higher solubility of K and Rb renders them more attractive A-site dopants for BaZrO3. Overall, our results suggest that alkali metals as A-site dopants may enhance the bulk proton conductivity of BaZrO3, compared to Y-doped BaZrO3. [ABSTRACT FROM AUTHOR]

First-principles calculations were utilized to elucidate the complete defect equilibria of surfaces of proton conducting BaZrO3, encompassing charged species adsorbed to the surface, defects in the surface layer as well as in the subsurface space-charge region and bulk. Defect calculations were performed for the BaZrO3 (0 0 1) surface with focus on protons, oxygen vacancies and Y-acceptor dopants as well as adsorbed hydroxide and oxide adions. Protons were found to exhibit a particularly strong tendency to segregate to the surface with a segregation energy of −1.3 eV. While the concentration of negatively charged Y-acceptors and hydroxide species on the outer surface can be quite high, they do not fully charge compensate the protons, yielding a net positive charge of the surface. The resulting surface potential can exceed 1 V, resulting in significant depletion of charge carriers in the subsurface space-charge region. Moreover, the results are discussed in relation to surface adsorption of water, and computational approaches for treating charged point defects in periodic slab cells are evaluated with respect to symmetry and charge compensation. [ABSTRACT FROM AUTHOR]

In this contribution, we study possible confinement effects on the atomic and electronic structure, and phonon properties of neutral UGRAPHIC DISPLAY="INLINE" ID="UGT3" SRC="UGT3"/ and fully charged UGRAPHIC DISPLAY="INLINE" ID="UGT4" SRC="UGT4"/ oxygen vacancies in BaZrO3 (001) ultra-thin films. First principles phonon calculations were performed as a function of film thickness (from 3 to 7 atomic planes) with two complementary DFT methods. The calculations reveal that for both types of vacancies the confinement effect is very short-range; for films containing 5 planes or more, the oxygen vacancy properties are predicted to be similar to those observed in the bulk material. [ABSTRACT FROM AUTHOR]

The perovskite BaZrO3 has attracted considerable attention in the recent decade due to its high temperature proton conducting properties, and possible application as electrolyte in intermediate temperature fuel cells and electrolyzers. In this contribution, we performed, for the first time, first-principles calculations of the phonon contribution to the defect thermodynamics of the ZrO2 terminated (001) surface of BaZrO3. The approach allows us to determine both the segregation enthalpy and entropy of defects, which we apply to two fundamental defects in BaZrO3; fully charged oxygen vacancies (vO••) and protonic defects (OHO•). The calculations show that both defects exhibit favorable segregation enthalpies of -65 and -125 kJ/mol, respectively. Further, the vibrational formation entropy of the surface vO•• is significantly higher than that of the bulk vO••, due to smaller local structural relaxations of the surface defect, leading to a finite surface segregation entropy of 53 J/mol K. OHO•, on the other hand, displays nearly identical vibrational spectra at the surface and in the bulk, and the segregation entropy is therefore negligible. Hence, phonons not only stabilize the surface vO•• compared to the bulk defect thermodynamically at high temperatures, but also affect the relative stability of vO•• and OHO• at the surface. Finally, we apply a simplified space charge model to the (001) surface, and show that neglect of phonons results in strongly underestimated surface concentrations of vO••. [ABSTRACT FROM AUTHOR]

The impact of phonons on the hydration and defect thermodynamics of undoped and acceptor (Sc, In, Y and Gd) doped BaZrO3 is addressed by means of first principles supercell calculations. In contrast to previous, similar investigations, we evaluate contributions from all phonon modes, and also pressure/volume effects on the phonon properties. The calculations are performed at the GGA-level with the PBE and RPBE functionals, both of which predict for BaZrO3 a stable cubic perovskite structure. For all dopants, the vibrational formation entropy of the doubly positively charged oxygen vacancy UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/ is significantly lower than that of the protonic defect UGRAPHIC DISPLAY="INLINE" ID="UGT2" SRC="UGT2"/, which therefore also is the dominant contribution to the entropy of hydration, in addition to loss of H2O(g). The large, negative contribution from phonons to the formation entropy of UGRAPHIC DISPLAY="INLINE" ID="UGT3" SRC="UGT3"/ stems both from large local structural relaxations, and contraction of the entire supercell and corresponding blue-shift of the phonon spectrum. Neglect of the phonon contribution to the vacancy free formation energy leads to a considerable error (100 kJ mol−1 at 1000 K). For the acceptor doped systems, the calculated hydration entropy becomes more negative in the order Y ＜ Gd ＜ In ＜ Sc, which stems from that both the formation volume and vibrational formation entropy of UGRAPHIC DISPLAY="INLINE" ID="UGT4" SRC="UGT4"/ and UGRAPHIC DISPLAY="INLINE" ID="UGT5" SRC="UGT5"/ become more negative with increasing dopant ion size. Both functionals also give similar defect entropies for undoped, and Sc- and In-doped BaZrO3. However, for the larger Y- and Gd-ions, PBE underestimates (too negative) the vibrational formation entropies due to a poor representation of low frequency modes at the R point of the Brillouin zone. The calculated hydration entropies are in good agreement with experimental results, lending support to the applicability of the adopted method. [ABSTRACT FROM AUTHOR]

Abstract: Defect segregation of oxygen vacancies, protons, titanium vacancies, and the mutual complex between the two latter (i.e. Ruetschi type defects) at the (1 1 0) surface of rutile TiO2 has been studied through ab initio Density Functional Theory calculations (DFT). Oxygen vacancies and protons form mainly on the 2-fold coordinated oxygen sites, while titanium vacancy related defects are expected to be most prominent at the 5-fold titanium sites. All included defects are shown to be more stable at the surface than in the bulk oxide, and the relative tendency to segregate is most pronounced for oxygen vacancies. Based on the obtained defect segregation energies and previously calculated bulk defect chemistry at lower temperatures, our estimated surface defect formation energies indicate that the surface of TiO2 can be expected to be dominated by titanium vacancies, protons and their mutual complexes at lower temperatures under wet, oxidizing conditions. Finally, possible routes of dissociative water adsorption on the surface, involving the aforementioned defects are discussed. [Copyright &y& Elsevier]

The behaviour of hydrogen in both defect free and acceptor doped bulk rutile TiO2is investigated through defect calculations performed within the density functional theory formalism. Both interstitial and substitutional hydrogen defects are shown to behave as shallow donors in the material, thereby existing as effectively positive hydroxide defects, , and substitutional hydrogen defects, . Of the investigated isolated hydrogen defects, the defect is shown to have the lowest formation energy over the whole Fermi level range under both oxidising and reducing conditions. However, the formation energy of is only 0.35 eV higher under conditions where TiO2is in equilibrium with Ti2O3, indicating that it exists as a minority defect in rutile TiO2under highly reducing conditions such as under growth of TiO2on a Ti metal surface. The enthalpy of hydration of oxygen vacancies is calculated to be −1.64 and −1.60 eV for the 2 × 2 × 3 and 3 × 3 × 3 supercells, indicating that defects will prevail in wet atmospheres, even at high temperatures. Hydrogen incorporated in the material can furthermore be expected to associate significantly with various acceptor centres (e.g., substitutional N and Al acceptors and Ti vacancies). The binding energies of the substitutional NH×O(imide) defect and of the association complexes between H and fully ionised substitutional Al, , and Ti vacancies, and (i.e., Ruetschi type defects), are calculated to be −0.36, −0.12, −0.47 and −0.82 eV, respectively. [ABSTRACT FROM AUTHOR]