arXiv subjects

Ulrich Aschauer

Publications and source records attributed to Ulrich Aschauer.

At least 37 records · Page 2Linked to original sources

arXiv2020

Local polarization in oxygen-deficient LaMnO$_3$ induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO$_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn--O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn--O bond lengths, orbital order and consequently the charge localization and polarity are tunable via strain.

cond-mat.mtrl-sci↗

arXiv2020

Self-consistent DFT+$U$+$V$ study of oxygen vacancies in SrTiO$_3$

Contradictory theoretical results for oxygen vacancies in SrTiO$_3$ (STO) were often related to the peculiar properties of STO, which is a $d^0$ transition metal oxide with mixed ionic-covalent bonding. Here, we apply, for the first time, density functional theory (DFT) within the extended Hubbard DFT+$U$+$V$ approach, including on-site as well as inter-site electronic interactions, to study oxygen-deficient STO with Hubbard $U$ and $V$ parameters computed self-consistently via density-functional perturbation theory. Our results demonstrate that the extended Hubbard functional is a promising approach to study defects in materials with electronic properties similar to STO. Indeed, DFT+$U$+$V$ provides a better description of stoichiometric STO compared to standard DFT or DFT+$U$, the band gap and crystal field splitting being in good agreement with experiments. In turn, also the description of the electronic properties of oxygen vacancies in STO is improved, with formation energies in excellent agreement with experiments as well as results obtained with the most frequently used hybrid functionals, however at a fraction of the computational cost. While our results do not fully resolve the contradictory findings reported in literature, our systematic approach leads to a deeper understanding of their origin, which stems from different cell sizes, STO phases, the exchange-correlation functional, and the treatment of structural relaxations and spin-polarization.

cond-mat.mtrl-sci↗

arXiv2019

The role of metastability in enhancing water-oxidation activity

While metastability enhanced water-oxidation activity was experimentally reported, the reason behind this effect is still unclear. We determine here, using density functional theory calculations, oxygen evolution reaction overpotentials for a variety of defective (001) surfaces of three different perovskite materials. For all three, we find a large range of overpotentials for different reaction sites including also overpotentials at the top of the activity volcano. Assuming that these sites dominate the apparent catalytic activity, this implies that a large number of geometrically different reaction sites, as they occur when a catalyst is operated at the border of its stability conditions, can lead to a strong enhancement of the apparent activity. This also implies that a pre-treatment of the catalyst creating a variety of different reactive sites could lead to superior catalytic activities for thermodynamically stable materials.

cond-mat.mtrl-sci↗

arXiv2019

Density functional theory study of the oxygen evolution activity on Sr$_2$TaO$_3$N surfaces

Solar water splitting has attracted much attention as a clean and renewable route to produce hydrogen fuel. Since the oxygen evolution half-reaction (OER) requires high overpotentials, much research has focused on finding catalyst materials that minimize this energy loss. Oxynitrides with a layered perovskite structure have the potential to combine the superior photocatalytic properties of layered perovskite oxides with enhanced visible-light absorption caused by the band gap narrowing due to less electronegative nitrogen ions. In this paper, we study the OER on the (001) and (100) surfaces of the layered oxynitride Sr$_2$TaO$_3$N using density functional theory (DFT) calculations to obtain the OER free energy profiles and to determine the required overpotentials at various sites on each surface. We find that the reconstructed grooved (100) surface is most relevant for photocatalysis due to suitable band-edge positions combined with a low overpotential and good carrier mobility perpendicular to the surface.

cond-mat.mtrl-sci↗

arXiv2019

Water oxidation catalysis on reconstructed NaTaO$_3$ (001) surfaces

Polar perovskite oxide surfaces are subject to structural reconstruction as a possible stabilisation mechanisms, which changes the surface structure and hence the surface chemistry. To investigate this effect, we study the oxygen evolution reaction (OER) on the reconstructed (001) surface of NaTaO$_3$, by means of density functional theory (DFT) calculations and compare it with the non-polar (113) surface of the same material. For the clean surface the reconstruction has a beneficial effect on the catalytic activity, lowering the minimal overpotential from 0.88 V to 0.70 V while also changing the most active reaction site from Na to Ta. Under photocatalytic conditions, the Ta sites are covered by oxygen adsorbates, rendering a lattice oxygen site on the NaO terrace the most active with a very low overpotential of 0.32 V. An alternative surface reconstruction stable in contact with water leads to an oxygen coupling mechanism with an overpotential of 0.52 V. Our results show that terraced surface reconstructions enable novel reaction pathways with low overpotentials that do not exist on other non-polar NaTaO$_3$ surfaces nor on non-polar surfaces of chemically similar materials such as SrTaO$_2$N.

cond-mat.mtrl-sci↗

arXiv2019

Nitrogen loss and oxygen evolution reaction activity of perovskite oxynitrides

Perovskite oxynitride photocatalysts were reported by experiment to evolve small amounts of N$_2$ due to the self-oxidation of nitrogen ions by photo-generated holes. The N$_2$ evolution rate was observed to decrease with increasing reaction time and was found to be correlated with a decrease in O$_2$ evolution (OER) activity, the origin of this latter effect however being unknown. Here we investigate, by means of density functional theory calculation, anion vacancies at the TaON-terminated (001) surface of the perovskite oxynitride SrTaO$_2$N. We find an energetic preference for oxygen and nitrogen vacancies to reside at the surface, where they are spontaneously healed by *O and *OH adsorbates under OER conditions. For nitrogen vacancies, this self-healing leads to an altered stoichiometry Ta$_4$O$_{8+x}$N$_{4-x}$ that is accompanied by electron doping. Substitution of N by O at the surface also leads to tensile strain, which confines the excess charge to the very surface layer, affecting the binding energy of reaction intermediates and significantly increasing the OER overpotential. This peculiar change in electronic structure thus provides an atomic scale explanation for the experimentally observed drop in OER activity of perovskite oxynitrides.

cond-mat.mtrl-sci↗

arXiv2019

Self-consistent site-dependent DFT+$U$ study of stoichiometric and defective SrMnO$_3$

We propose a self-consistent site-dependent Hubbard $U$ approach for DFT+$U$ calculations of defects in complex transition-metal oxides, using Hubbard parameters computed via linear-response theory. The formation of a defect locally perturbs the chemical environment of Hubbard sites in its vicinity, resulting in different Hubbard $U$ parameters for different sites. Using oxygen vacancies in SrMnO$_3$ as a model system, we investigate the dependence of $U$ on the chemical environment and study its influence on the structural, electronic, and magnetic properties of defective bulk and strained thin-film structures. Our results show that a self-consistent $U$ improves the description of stoichiometric bulk SrMnO$_3$ with respect to GGA or GGA+$U$ calculations using an empirical $U$. For defective systems, $U$ changes as a function of the distance of the Hubbard site from the defect, its oxidation state and the magnetic phase of the bulk structure. Taking into account this dependence, in turn, affects the computed defect formation energies and the predicted strain- and/or defect-induced magnetic phase transitions, especially when occupied localized states appear in the band gap of the material upon defect creation.

cond-mat.mtrl-sci↗

arXiv2019

Band-gap engineering in AB(O$_x$S$_{1-x}$)$_{3}$ perovskite oxysulfides: A route to strongly polar materials for photocatalytic water splitting

Polar heterogeneous photocatalysts were shown to lead to enhanced charge-carrier separation that results in superior activity for example for photocatalytic water splitting. Promising photocatalyst materials such as oxynitrides can be rendered polar by epitaxial strain, which however also increases their band gap, making them unsuitable for visible light absorption. This suggests a trade-off between small band gaps and polar distortions - both being crucial for the catalyst's efficiency. In this paper we investigate, using density functional theory calculations, the suitability of strained AB(O$_x$S$_{1-x}$)$_{3}$ perovskites for photocatalytic water splitting. These materials normally have band gaps too small for water splitting but inducing polar distortions via epitaxial strain can increase the band gap to the suitable range. We find perovskite BaZr$_y$Ti$_{1-y}$O$_2$S compounds to be highly promising for photocatalytic water splitting due to large polar distortions and suitable band gaps.

cond-mat.mtrl-sci↗

arXiv2019

LaTiOxNy thin film model systems for photocatalytic water splitting: physicochemical evolution of the solid-liquid interface and the role of the crystallographic orientation

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. The intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention. One is the critical issue of the compositional/structural surface modifications occurring during operation and how these affect the photoelectrochemical performance. The second concerns the relation between the electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance since it is exactly at the surface where the visible light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art of oxynitride thin film fabrication and characterization before focusing on LaTiO2N selected as representative photocatalyst. We report the investigation of the initial physicochemical evolution of the surface. Then we show that, after stabilization, the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations and be up to 5 times larger than for polycrystalline samples.

cond-mat.mtrl-sci↗

arXiv2018

Anion-order driven polar interfaces at LaTiO$_2$N surfaces

Perovskite oxynitrides have recently attracted attention for their ability to photocatalytically split water. Compared to oxides the arrangement of anions in the material represents a further structural degree of freedom. The bulk oxynitride LaTiO$_2$N prefers a bonding-dominated \textit{cis} nitrogen arrangement, while we have previously shown that the (001) surface prefers a non-polar \textit{trans} order to compensate polarity. Here we consider, using density functional theory calculations, the polar/non-polar interface that would necessarily be present between the two anion orders. We show that the Ti-terminated surface will adopt up to two \textit{trans} ordered surface layers, which has a beneficial effect on the oxygen evolution efficiency. We then consider the hypothetical case of a polar \textit{cis} ordered surface layer atop a non-polar \textit{trans} bulk and show that similar electronic reconstructions as in the LaAlO$_3$/SrTiO$_3$ interface can be expected when interfaces between different anion orders are engineered in one and the same oxynitride material.

cond-mat.mtrl-sci↗

arXiv2018

Strain-induced heteronuclear charge disproportionation in EuMnO$_3$

Charge disproportionation transitions in complex oxides most commonly link high-temperature phases containing identical cations with the same oxidation state and crystallographic site, to low-temperature phases in which charge transfer between these ions results in unequal oxidation states. Here we propose, based on density functional calculations, a related concept that we term \textit{heteronuclear} charge disproportionation, in which charge transfer occurs between different elements on different crystallographic sites. We show for the case of EuMnO$_3$ that such a transition from the experimentally observed Eu$^{3+}$Mn$^{3+}$O$_3$ phase to the so far unknown Eu$^{2+}$Mn$^{4+}$O$_3$ phase can be triggered by pressure and epitaxial strain. We then identify measurable signatures to aid in an experimental exploration of the complex pressure- and biaxial strain-dependent phase stability of EuMnO$_3$ that we hope to motivate with our predictions. We suggest other candidate crystal chemistries for heteronuclear charge disproportionation, in which novel physics could emerge from the coexistence of instabilities.

cond-mat.mtrl-sci↗

arXiv2018

Epitaxial Strain-Dependence of Band Gaps in Oxynitrides compared to Oxides

Perovskite oxynitrides are a promising class of material for photocatalytic water-splitting due to their small band gaps and suitably aligned band edges. Recently, epitaxially strained oxynitrides started to attract interest due to the possibility to engineer the anion order and ferroelectric distortions. However, contrary to oxides there have been no studies on the band-gap evolution in oxynitrides with epitaxial strain. Here, we investigate, using density functional theory calculations, the influence of epitaxial strain on the band gap of two different oxynitrides and compare our results with oxides. As opposed to cubic oxides, where both compressive and tensile strain narrow the band-gap, we find that the anion order leads to non-degenerate N \textit{2p} bands already at zero strain, which leads to opposite and anion order-dependent evolutions of the band gap under compressive and tensile strain. Further, we find that in oxynitrides polar distortions increase the band gap by up to 1 eV for 4% strain whereas for oxides the increase is only about 0.1 eV for the same amount of strain. The reason for this difference is the larger ionic radius of nitrogen leading to larger ferroelectric distortions and a stronger dependence of the band gap on the distortions amplitude. These results imply that ferroelectric distortions are strongly detrimental to light absorption and that their suppression, for example above a critical temperature, should result in a marked decrease in band gap and enhanced absorption of visible light.

cond-mat.mtrl-sci↗

arXiv2018

Water Oxidation Chemistry of Oxynitrides and Oxides: Comparing NaTaO$_3$ and SrTaO$_2$N

The oxygen evolution reaction (OER) plays an important role in evaluating a photocatalyst and to understand its surface chemistry. In this work we present a comparative study of the OER on the oxide NaTaO$_3$ (113) surface and the oxynitride SrTaO$_2$N (001) surface. Oxynitrides are highly promising photocatalysts due to their smaller band gap and resulting better visible light absorption compared to oxides but our knowledge about their surface structure and chemistry is still very limited. With the goal to compare the surface chemistry of oxides and oxynitrides, we perform density functional theory calculations to obtain the free energy changes associated with the OER reaction steps. For the OER at the Ta site of the clean surfaces, our results predict the rate-limiting step for both materials to be the formation of the *OOH intermediate, with a larger overpotential for the oxide than the oxynitride (1.30 V vs 1.01 V). The Na site is found to be more active than the Ta site on the oxide surface with an OER overpotential of 0.88 V, whereas the OER at the Sr site on the oxynitride has an overpotential of 1.14 V. For the A sites, contrary to the Ta site, the deprotonation of *OH was found to be the rate-limiting step. Computed Pourbaix diagrams show that at relevant (photo)electrochemical conditions all surfaces are covered with oxygen adsorbates. Oxygen adsorbates at A (Na, Sr) sites are however found to couple and desorb as O$_2$, leaving these sites empty under typical operating conditions. Following this desorption, we find the OER to proceed by the conventional *OOH mechanism on the SrO termination of the oxynitride but by a direct coupling of neighbouring *O at Na sites on the oxide surface. This coupling mechanism on the oxide has the smallest overpotential of 0.79 V compared to 0.88 V for the oxynitride, implying that the oxide is a better OER catalyst.

cond-mat.mtrl-sci↗

arXiv2018

Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2)- at fivefold-coordinated Ti sites was transformed to peroxo (O2)2- and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.

cond-mat.mtrl-sci↗

arXiv2018

(Sub)surface mobility of oxygen vacancies at the TiO$_2$ anatase (101) surface

Anatase is a metastable polymorph of TiO$_2$. In contrast to the more widely-studied TiO$_2$ rutile, O vacancies (V$_\mathrm O$'s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V$_\mathrm O$'s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures $\geq$ 200 K. After an initial decrease of the V$_\mathrm O$ density, a temperature-dependent dynamic equilibrium is established where V$_\mathrm O$'s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, DFT calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously-distributed subsurface defects in the reduced sample.

cond-mat.mtrl-sci↗

arXiv2017

Bulk and surface properties of the Ruddlesden-Popper oxynitride Sr$_2$TaO$_3$N

Oxynitrides with the perovskite structure are promising candidates for photocatalysis under visible light due to their appropriate optical and electronic properties. Recently, layered perovskites have attracted attention for their improved performance with respect to the bulk perovskites in photocatalytic water splitting. In this paper, we investigate the structural and electronic properties of the layered Ruddlesden-Popper oxynitride Sr$_2$TaO$_3$N and its (001) surfaces using density functional theory (DFT) calculations. We find that the energetically favoured configuration of the bulk has an in-plane \textit{cis} anion order and exhibits rotations of the TaO$_6$ octahedra. Furthermore, we show that the TaON-terminated (001) surface suppresses exciton recombination due to higher-energy surface states, giving a potential explanation for the good photocatalytic performance.

cond-mat.mtrl-sci↗

arXiv2017

Anion order and spontaneous polarization in LaTiO\textsubscript{2}N oxynitride thin films

The perovskite oxynitride LaTiO\textsubscript{2}N is a promising material for photocatalytic water splitting under visible light. One of the obstacles towards higher efficiencies of this and similar materials stems from charge-carrier recombination, which could be suppressed by the built-in electric field in polar materials. In this study, we investigate the spontaneous polarization in epitaxially strained LaTiO\textsubscript{2}N thin films via density functional theory calculations. The effect of epitaxial strain on the anion order, resulting out-of-plane polarization, energy barriers for polarization reversal and the corresponding coercive fields are studied. We find that for compressive strains larger than 4\% the thermodynamically stable anion order is polar along the out-of-plane direction and has a coercive field comparable to other switchable ferroelectrics. Our results show that strained LaTiO\textsubscript{2}N could indeed suppress carrier recombination and lead to enhanced photocatalytic activities.

cond-mat.mtrl-sci↗

arXiv2016

Coupling and competition between ferroelectricity, magnetism, strain and oxygen vacancies in AMnO3 perovskites

We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size / tolerance factor, epitaxial strain, ferroelectricity and magnetism in the perovskite manganite series, AMnO3 (A=Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic over antiferromagnetic ordering due to suppressed antiferromagnetic superexchange as the polar distortion bends the Mn-O-Mn bond angles away from the optimal 180deg. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for ferromagnetic than antiferromagnetic orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure and magnetism can be modified using chemical potential, strain, and electric or magnetic fields.

cond-mat.mtrl-sci↗