BaO Termination Research Articles

Ferroelectric polarization switching induced from water adsorption in BaTiO3 ultrathin films

The influence of water on the out-of-plane polarization of ${\mathrm{BaTiO}}_{3}$ (BTO) ultrathin films is studied by means of density functional theory calculations. The adsorption is investigated for different coverages on both terminations of BTO with, for each case, all possible states of polarization, namely, paraelectric, polarized upward, and polarized downward. We thus demonstrate different behavior as a function of the termination. While ${\mathrm{H}}_{2}\mathrm{O}$ adsorbs only dissociatively on the BaO termination, with a reinforced interaction compared to BTO without out-of-plane polarization, only molecular adsorption is observed on the ${\mathrm{TiO}}_{2}$ termination. In addition, the presence of water is able to switch the polarization. Whatever the initial state of polarization is, water induces a downward state on the BaO termination and an upward polarization on the ${\mathrm{TiO}}_{2}$ one. A detailed analysis of this phenomenon is given.

First‐principles study of the atomic structures, electronic properties, and surface stability of BaTiO3 (001) and (011) surfaces

The atomic structures, electronic properties, and surface stability of (001) and (011) surfaces of BaTiO3 are studied by first‐principles calculations. Four differently terminated BaTiO3 surfaces are considered in this study, including (001)‐BaO, (001)‐TiO2, (011)‐BaTiO, and (011)‐O2 terminations. The relaxations and rumplings are calculated and discussed, finding that the first layer relaxes inwards, while the second layer relaxes outwards for (001) and (110) surfaces. The data obtained for electronic properties show that O2p states in (001)‐BaO/(001)‐TiO2 termination shift to the lower/higher energy region, leading to a wide/narrow band gap. And the new produced surface states are observed in (011) surface terminations, which is mainly attributed to the supplied electrons from outermost surface atoms, even O atoms are oxidized. Furthermore, the (001) surface of BaTiO3 is found to be more stable than the (011) surface according to the predicted surface energy which is 0.86 and 2.92 J/m2 for (001) and (011) surfaces, respectively. Of which, BaO termination is predicted to be more likely to cleavage from the (001) direction than the TiO2 termination is.

Adsorption of CO2 and Facile Carbonate Formation on BaZrO3 Surfaces

The adsorption of CO2 and CO on BaZrO3 (0 0 1) was investigated by first-principles calculations with a focus on the BaO termination. CO2 was found to strongly chemisorb on the surface by formation of carbonate species with an adsorption enthalpy of up to −2.25 eV at low coverage and −1.05 eV for a full monolayer. An adsorption entropy of −8.8 × 10–4 eV K–1 was obtained from the vibrational properties of the adsorbates. Surface coverages were evaluated as a function of temperature and CO2 partial pressure, and the obtained coverage under 1 bar CO2 was more than 0.8 at 1000 K (conditions relevant for steam methane reforming). The fully saturated surface was stable up to about 400 K under ambient atmosphere, i.e., 400 ppm of CO2. The initial stage of BaCO3 formation was evaluated according to migration of barium to the carbonate overlayer, which was found to result in a significant stabilization of the system. The barium migration was found to be essentially unobstructed with a barrier of only ∼5 meV. In li...

Surface Relaxations, Surface Energies and Electronic Structures of BaSnO3 (001) Surfaces: Ab Initio Calculations

The surface relaxations, surface energies and electronic structures of BaO- and SnO2-terminated BaSnO3 (001) surfaces have been studied by employing the first-principles density functional theory. For both terminations, we find that the upper-layer Ba and Sn atoms move inward, whereas upper-layer O atoms move outward from the surface. Moreover, the largest relaxations are occurred on the first-layer atoms of both terminations. The surface rumpling of BaO-terminated BaSnO3 (001) is slightly less than that of the SnO2-terminated BaSnO3 (001) surface. The surface energies show that both terminated surfaces are energetically stable and favorable. Finally, the surface band gap is slightly decreased for the BaO termination, while it is dramatically decreased for the SnO2 termination.

First‐principles Study of Perovskite Ultrathin Films: Stability and Confinement Effects

AbstractIn this first‐principles study we investigate the atomic, electronic, and vibrational structure of BaZrO3(001) ultrathin films and surfaces, using a hybrid functional and a local Gaussian‐like basis set. The low‐index nonpolar (001) surface is known to be the most stable. We considered both possible kinds of nonpolar terminations (BaO and ZrO2) for the (001) surface. The systems were studied using a slab model. Ultrathin films were modeled using slabs with the number of atomic planes ranging from three to seven, whereas surfaces were modeled with much thicker slabs composed of 15 atomic planes. In order to estimate the Gibbs free energy at finite temperatures, lattice vibrational frequencies were also calculated. We found that phonons noticeably affect the relative thermodynamic stability of the two termination layers: while at room temperature the BaO termination has the lowest energy, at intermediate temperatures (500 K) both terminations can coexist, and at higher temperatures (900 K) the ZrO2‐terminated surface becomes the most stable. We considered the effect of two‐dimensional confinement on the structural, electronic, and vibrational properties of these ultrathin films. We found these confinement effects to be short ranged, with the properties of three‐plane films to be the only ones that noticeably differ from the bulk material. Finally, we briefly consider confinement effects in such ultrathin films containing neutral and fully charged oxygen vacancies (charge states 0 and +2). We show, in particular, how lattice vibrations affect the Gibbs formation energy of a neutral oxygen vacancy making it completely independent of the film thickness at high temperatures (1000 K), due to cancellation of enthalpy and entropy contributions.

Strong Surface‐Termination Effect on Electroresistance in Ferroelectric Tunnel Junctions

Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin‐film devices requires atomic‐scale band‐structure engineering based on depolarization‐field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO3, it is demonstrated that the surface termination of the ferroelectric in contact with a simple‐metal electrode critically affects properties of electroresistance. BaTiO3 barrier‐layers with TiO2 or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance‐switching ratio in the junctions can be remarkably enhanced up to 105% at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.

Surface structure and polarization of cubic and tetragonal BaTiO3: An ab initio study

Cubic and tetragonal surfaces of BaTiO3 are investigated using the density functional theory (DFT) within the new generalized gradient approximation of Wu and Cohen (GGA-WC) to the exchange and correlation functional. The cubic and tetragonal surfaces of BaTiO3 are compared. For the tetragonal phase, we take into account two surface orientations (001) and (100) noted Tz and Tx, respectively. The atomic rumpling and the difference between interlayer distances are determined and compared to the experimental results as well as to previous theoretical ones. Atomic displacements are also highlighted by comparing the variation of c/a ratio along the selected surface slabs. Considerable tetragonality reduction is found for (001) surface termination of the tetragonal structure. This represents an important atomic redistribution compared to the other surfaces. Therefore, an important release of surface stress is noted for Tz termination. Stress is released for all BaO terminations in comparison with TiO2 terminations. Anisotropy of stress is noted for Tx terminations. The perpendicular polarization to the surface is cancelled due to electrostatic conditions of supercell construction. A parallel polarization is retained for BaO termination and not for TiO2 termination of Tx surface. Our findings are in good agreement with recent experimental ones.

First-principles study of the (001) surface of half-metallic cubic BaCrO3

First-principles calculations have been performed to study the (001) surfaces of half-metallic cubic BaCrO3 with BaO and CrO2 terminations by employing density function theory under the generalized gradient approximation (GGA)+U (Hubbard parameter). Surface relaxations reveal that the CrO2-terminated surface could be rougher than the BaO-terminated surface and an oscillatory (−+−) damping (|Δd12|>|Δd23|>|Δd34|) relaxation phenomenon appears for both the BaO-termination and the CrO2-termination. Meanwhile, the grand thermodynamic potential F indicates that only the BaO-terminated surface can exist steadily in the range of the accessible values of the CrO2 chemical potential. The electronic structure calculations predict that BaO-terminated surface preserves the bulk's half-metallic property. For the CrO2-terminated surface, surface states can be identified which result in the deterioration of its half-metallicity.

Strong effect of substrate termination on Rashba spin-orbit splitting: Bi onBaTiO3(001)from first principles

We demonstrate by first-principles calculations that the Rashba spin-orbit splitting in the $6p$ states of a Bi adlayer on ${\text{BaTiO}}_{3}(001)$ is strongly affected by the substrate termination. For the ${\text{TiO}}_{2}$ termination the absolute splitting is very large (about $0.23\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$); in contrast, the splitting becomes strongly reduced for the BaO termination (less than $0.07\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$). Our findings are explained by the termination-dependent hybridization of the Bi surface states with electronic states in the substrate.

Adsorption of CO, CO 2 and NO molecules on a BaTiO 3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO 2 and NO molecules on a BaTiO 3 surface following a first preliminary calculation of O and O 2 adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO 2-termination. Consequently, we extended our study to CO, CO 2 and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO 2 molecule directly located above an O surface atom (called O s) is of the order of −0.18 eV. The O–C distance length is then 1.24 Å and the O–C–O and O–C–O s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N–O bond is broken when a NO molecule is absorbed on a Ba–O s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O–Ba–O s and N–O s–Ba angles are 106.5° and 63.0°, respectively. The N–O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O s atom, the N–O axis being tilted toward the Ba atom. The N–O s–Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.

First-principle study on electronic structure of BaTiO3 (001) surfaces

Electronic structures of the (001) surface of cubic phase barium titanate have been calculated by using plane-wave pseudopotential method within the density functional theory. Geometry optimization indicates that the largest atomic relaxation occurs to metal atoms in surface layers where all atoms are displaced inward, and the interlayer distances vary alternately. Total energy calculation shows that the TiO2-terminated surface is unstable compared with that with BaO termination. One reason is that surface state of O-2p orbital observed in band structure of TiO2 termination helps the electron state shift towards higher energy region in both valence band and conduction band. Redistribution of electrons near surface region in TiO2 termination also provides evidence that intensity of covalent bonding between Ti and O atoms differs and hence TiO2 termination is more suitable for surface adsorption. In contrast, such difference was eliminated in BaO termination, accounting for its superiority in surface stability.

Surface rumples and band gap reductions of cubic BaZrO3 (001) surface studied by means of first-principles calculations

Electronic properties of the (001) surface of cubic BaZrO3 with BaO and ZrO2 terminations have been studied using first-principles calculations. Surface structure, partial density of states, band structure and surface energy have been obtained. We find that the largest relaxation appears in the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO2-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO2-terminated surface. Results of surface energy calculations reveal that the BaZrO3 surface is likely to be more stable than the PbZrO3 surface.

DFT study of BaTiO3 (001) surface with O and O2 adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO3 (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO3 (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO2 termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O2 molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O2 molecule adsorbed in various geometrical configurations are also analyzed. For O2, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 A. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results.

First-principles study of the (001) surface of cubic BaZrO3 and BaTiO3

First-principles density functional calculations are employed to study the (001) surface of BaZrO3 with both BaO and ZrO2 termination. Surface structure and band structure have been obtained. For the ZrO2-terminated surface, the large displacement of the O atoms in the first layer results in the small surface rumpling. In addition, the largest relaxation appears on the second layer atoms but not on the very first layer ones. The analysis of the structure relaxation parameters reveals that the rumpling of the (001) surface for BaZrO3 with the BaO termination is stronger than that of BaTiO3. The origin of the different surface relaxation behavior between the two materials is discussed. The surface state appears in the band structure of the ZrO2-terminated surface of BaZrO3.