Atomic Mechanism of Hybridization-Dependent Surface

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Atomic Mechanism of Hybridization Dependent Surface Reconstruction with Tailored Functionality in Hexagonal Multiferroics Shiqing Deng, Shaobo Cheng, Changsong Xu, Binghui Ge, Xuefeng Sun, Rong Yu, Wenhui Duan, and Jing Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08055 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Atomic Mechanism of Hybridization Dependent Surface Reconstruction with Tailored Functionality in Hexagonal Multiferroics Shiqing Deng,† Shaobo Cheng,†,¶ Changsong Xu,§ Binghui Ge,‡ Xuefeng Sun,☨,

⊥,ǁ

Rong Yu,†

Wenhui Duan,§ and Jing Zhu†,¶,* †

National Center for Electron Microscopy in Beijing, School of Materials Science and

Engineering, The State Key laboratory of New Ceramics and Fine Processing, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China ¶

Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P. R. China

§

State Key Laboratory of Low-Dimensional Quantum Physics and Collaborative Innovation

Center of Quantum Matter, Department of Physics, Tsinghua University, Beijing 100084, P. R. China ‡

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, P. R. China ☨

Hefei National Laboratory for Physical Sciences at Microscale, University of Science and

Technology of China, Hefei, Anhui 230026, P. R. China ⊥

Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of

Sciences, Hefei, Anhui 230026, P. R. China ǁ

Collaborative Innovation Center of Advanced Microstructures, Nanjing, Jiangsu 210093, P.

R. China

KEYWORDS: improper ferroelectrics, surface reconstruction, ferroelectric anomaly, oxygen vacancy, multiferroic materials

ACS Paragon Plus Environment

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ABSTRACT The broken symmetry along with anomalous defect structures and charging conditions at multiferroics surface can alter both crystal structures and electronic configurations, bringing in emergent physical properties. Extraordinary surface states are induced into original mutually-coupled order parameters in such strongly correlated oxides, which flourish in diverse properties, however remain less explored. Here, we report the peculiar surface ferroelectric states and reconfigurable functionalities driven by the relaxation of surface and consequent changes in O-2p and Y-4d orbital (p-d) hybridization within a representative hexagonal multiferroics YMnO3. An unprecedented surface reconstruction is achieved by tailored p-d hybridization coupling with in-plane oxygen vacancies, which is atomically revealed based on the advantages of state-of-the-art aberration-corrected (scanning) transmission electron microscopy. Further ab-initio density functional theory calculations verify the key roles of in-plane oxygen vacancies in modulating polarization properties and electronic structure, which should be regarded as the atomic multiferroic element. This surface configuration is found to induce tunable functionalities, such as surface ferromagnetism and conductivity. Meanwhile, the controversial origin of improper ferroelectricity that unexpectedly free from critical size has also been atomically unraveled. Our findings provide new insights into design and implementation of surface chemistry devices by simply controlling the oxygen stoichiometry, greatly advance our understandings of surface science in strongly correlated oxides, and enable exciting innovations and new technological functionality paradigms.

1. INTRODUCTION Delicate surfaces make an intriguing and challenging research subject because of embedded novel physical phenomena and exotic properties in designing promising devices.1-3


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The core of surface researches lies in the recognition of the beneficial or deleterious reconstruction of structural or electronic configurations.3-4 Surface structure-activity relationships and structure controls, which are crucial for materials design and service, rely on the accurate characterization of the active surface stoichiometry and local geometry for the exposed facets of materials.5 For multiferroic material surfaces, broken translation symmetry with dangling bonds and atomic deficiency is introduced to original ferroic orders, posing exciting new challenges for solid state physicists to understand the potential coupling.6-8 This distinct abnormal nature in surface skin layer associates with peculiar properties, such as suppressed polarization or unexpectedly emerged spins, which can significantly influence the surface response or even dominate the entire performance.3, 8-9 One typical paradigm is the sufferance of critical size due to the presence of depolarization fields, induced by the insufficient screening of surface charges especially in conventional ferroelectrics.8,

10-11

Example includes the recently

reported polarization-controlled surface reconstruction mechanism in Pb(Zr0.2Ti0.8)O3 film.8 In contrast to the chemical mechanism in conventional ferroelectrics, the critical thickness is surprisingly absent in in hexagonal multiferroics YMnO3, YMnO3-type RFeO3 and RMnO3 (R=rare earth elements), due to a nontrivial coupling to a zone-boundary lattice instability.12-15 The nature of non-sensitivity to depolarization field and surface charge states is thus obtained, potentially implying the peculiar surface ferroelectric states and reconfigurable functionalities, which however remain as a veiled and less explored realm. Especially, considering the mutually-coupled ferroic orders and novel magnetoelectric coupling

in

hexagonal

multiferroics,16-18

question

arises

whether

a

similar

polarization-dependence could be vital for surface reconstruction and how is the surface broken symmetry coupled to spin-charge-orbital degrees of freedom in giving rise to emergent phenomena.


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Details of the polarization and charge distribution in the surface layers of multiferroics and their relationship to the physical properties of materials have been studied for decades and are largely unresolved.19 Several possible mechanisms have been proposed to explain the different nature of surface layers with bulk, including nonuniform vacancy distributions, compensation of polarization-induced charges by band bending (intrinsic field effect) or adsorption (extrinsic field effect), and the existence of Schottky depletion regions.8, 19 These considerations have motivated a number of surface investigations using X-ray characterization,20 scanning tunneling microscopy (STM),21 low-energy electron diffraction (LEED),22 etc.. However, limitations, such as restricted spatial resolution, required electrical conductivity or complexity of tip-surface interactions, are naturally embedded in these methods. Thus, the link between the observed reconstructed surface configuration, the changes of electronic structure, and the underlying physical phenomena is missing. Recently, the advancements of state-of-the-art spherical aberration (Cs) corrected (scanning) transmission microscopy make it possible to simultaneously determine both atomically resolved lattice and electronic structure, allowing us to relate dedicate surface structure directly to reconstructed properties.23-24 In the present study, close correlations between reconstructed lattice, orbital hybridization and polarization are atomically unraveled at YMnO3 surface. Here, we reveal the atomic mechanisms of surface anomalies in a prototype hexagonal multiferroic YMnO3 and propose a new concept regarding the oxygen vacancy as an atomic-scale multiferroic element. The analysis builds upon our systematic exploration of delicate surface structures in combination with possible point defects using atom-resolved techniques and density-functional theories. Atomic inspections reveal that it is oxygen vacancy tuned p-d orbital hybridization, rather than bound charge (presence of depolarization fields), that couples with surface and gives rise to ferroelectric anomalies, along with tailored


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electronic states and ferromagnetism. This work demonstrates new insights into surface structure of hexagonal multiferroics, where tunable functionalities are embedded, and simultaneously resolves the controversial origin of improper ferroelectricity. The variations of manganese valence give chances to induce Mn3+ (d4)-Mn2+ (d5) double exchange interactions, suggesting the possible surface ferromagnetism and metal conductivity.25 These phenomena enable exciting innovations and technological functionality paradigms, leading to new exploitable realms, such as novel surface chemistry devices, where the oxygen vacancy acts as a control node.

2. RESULTS AND DISCUSSION 2.1. EXPERIMENTAL RESULTS Hexagonal yttrium manganate YMnO3 possesses layered structure, where the Y ion layers are separated by the MnO5 trigonal bipyramid layers.12,

26-27

The trimerization

behaviors of these bipyramids induce the displacement of Y ions along c direction.12 Three-dimensional (3D) YMnO3 model with downward polarization state is displayed in Figure 1a. According to the symmetry (P63cm), YMnO3 contains two kinds of symmetry sites for yttrium (Y1 and Y2). Oxygens can be classified as in-plane oxygens (OP) and on-top oxygens (OT).26, 28 Considering different Wyckoff positions, in-plane oxygens can be sorted as OP1 and OP2, and on-top oxygens as OT1 and OT2,27 as shown in Figure 1b. The MnO5 bipyramids, with a D3h site symmetry, consist of one Mn ion surrounded by three planar (2OP1-1OP2) and two apical oxygen atoms (1OT1-1OT2).27 The YO8 cages, with a D3d site symmetry, contain two planar and six apical oxygen atoms (Figure 1c). The down-down-up configuration of Y ions can be clearly seen from the [100] zone axis (Figure 1d), where the projection distance δ represents the magnitude of displacement, used for determining


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ferroelectric polarization.29-30 Projection along [001] direction is also shown in Figure 1e, where in-plane positions of Mn ions and OT are easily distinguished.

Figure 1. Atomic model of YMnO3. a) Hexagonal model of YMnO3 unit cell. Different symmetry sites of Y ions (Y1 and Y2) and oxygen atoms (OP and OT) are indicated. b) Enlarged MnO5 trimerization unit, where three bipyramids are corner-linked by OP1 atoms. c) The YO8 cages, demonstrating the chemical environment of Y ions in two kinds of symmetry sites. Bond lengths are presented in b and c in angstrom.27 d) Projection along [100] direction. δ is the relative displacement of Y ions. e) [001] projection of the unit cell in a).

Figure 2a shows a HRTEM (high-resolution transmission electron microscope) image of a YMnO3 single crystal viewing from [100] zone axis. For clarity, the original gray scale image is adapted to pseudo-color image. Down-down-up arrangements of Y ions in this region correspond to the downward polarization (along [00 1] direction). None-paralleling between surface and polarization directions can induce bound charges at surface. In the present case, the surface is positively poled. Three Y ions, containing two lower ones and one upper one, are defined as a polarization unit, outlined by a white dashed rectangle frame in Figure 2a. The first unbroken polarization unit from surface is regarded as the outmost layer


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(labeled as number 1). Accordingly, other layers can be consecutively numbered. Distinct reductions of displacement magnitude δ are found at the first three to four layers from surface (separated by white dashed lines), indicating the notably suppressed ferroelectric polarizations. To reveal the details, some quantitative works have been done based on the Gaussian-fitted atomic positions (Details can be found in Supporting Information).31 We calculate relative displacements for each polarization unit, obtaining the mosaic mapping results in Figure 2b, which demonstrate the magnitude of such decrease. The plot of the average displacements, extracted after averaging the magnitudes of each layer with the same distance from surface, shows a significant decrease at the forth layer (Figure 2c). Noting that the spontaneous polarization is proportional to the displacement, the value can be approximately determined as 6.5 µC/cm2 for the bulk region, whereas, averagely reduce to around 1.2 µC/cm2 for the outmost layer, yielding a decrease of 81.5%.

Figure 2. Characterizations of positively and negatively poled surface structure. a) HRTEM image of a downward polarization region with positively poled surface. b) Mosaic mapping results of displacement magnitude, showing a decrease of the displacement near surface. c) The average displacement from the outmost layer to the ninth layer, showing a distinct


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decrease at the topmost four layers. d) HRTEM image of negatively poled surface. e) Polarization magnitude map, showing three layers of the decrease in the displacement near surface. f) The plot of the average displacements from the outmost layer to the ninth layer, a distinct decrease at the topmost three layers can be distinguished. A white dashed rectangle frames a polarization unit of Y ions. Atomic layers are marked with consecution natural numbers in a), b), d) and e). White dashed lines in a) and d) and black dashed lines in b) and e) demarcate the bulk region and the ferroelectric deterioration surface layer, respectively. The errors in c) and f) are estimated by the standard deviation.

To examine the possibility of charge dependence, we further investigate the negative poled surface. A downward polarization region with negatively poled surface is characterized in Figure 2d, where the analogous scenario occurs. Likewise, the ferroelectric polarization relaxes at two to three outmost layers, which can be clearly seen in the polarization mapping in Figure 2e. The polarization anomalies at the outmost three layers can be reaffirmed from the plot of average displacement (Figure 2f). Previous work has reported similar suppression of polarization near surface in some conventional ferroelectrics, showing dependence on the surface charging conditions.8 Surface structures are controlled by the local bound charge conditions induced by negative or positive poled surface, which are determined by the polarization directions. Such behavior roots in the prominent influences of depolarization field on polarizations in conventional ferroelectrics, increasing the polarized state energy and suppressing the ferroelectric transition.13, 32 Thus, the surface charges must be compensated by various providers, such as free charges, oxygen vacancies and other forms of charged defects, contributing to screening the depolarization field. While, case can be quite different for improper ferroelectrics, such as YMnO3. They lack such sensitivity to depolarization field according to previous work,13 implying the possibilities of different reconstructed


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polarization geometries near surface. Our experimental results presented above verify such differences, refuting the polarization dependence of surface reconstruction in conventional ferroelectrics, like the case of Pb(Zr0.2Ti0.8)O3 film.8 We also confirm such claim via investigations of charge neutral surface, where the surface polarization anomaly also exists when absence of any bound charges (Figure S1, Supporting Information). As known to all, displacements of Y ions are companied with tilting and bulking of MnO5 bipyramids,26 thus investigating chemical and structural environment of Mn ions at surface is conducive to better understanding such anomalous behaviors. In particular, vicinal vacancies (e.g., oxygen vacancy) can alter electronic and structural configurations, inducing the valence state variations. The valence states of Mn ions are associated with variations of L2,3 edges, which correspond to the excitations at the spin-orbit split 2p3/2 and 2p1/2 levels to 3d band.33-34 Especially the integrated intensity ratio of two white lines shows monotonic relation with respect to Mn valence states, thus can serve as reliable fingerprints.33 EELS (electron energy loss spectrum) profiles at STEM mode (STEM-EELS) provides valid way for investigating the electronic configurations. A neutral surface with polarization deteriorated layers (demarcated by a white dashed line), was selected for such explorations in order to eliminate contributions of bound charges, as shown in Figure 3a. The polarization direction is parallel to the surface, thus bound charges are absent in such geometry. Utilizing STEM-EELS methods, two line scans at surface layer (red line) and bulk region (green line) are conducted, respectively (Figure 3a). Fifty spectra in total have been collected for each line and then summed up for reasonable signal-to-noise ratio. ELNES (energy-loss near-edge fine structures) of Mn L2,3 edge after background subtraction are correspondingly demonstrated in Figure 3b. Intensities and positions of L3 edges have been normalized and are regarded as the reference peak. Then, we can deduce variations of Mn valence states from L2 edge positions and intensities. The L2


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edge of surface layer shifts towards high energy, with the decreased intensity compared with that of bulk region, indicating the decrease of Mn valence state.33-34 Notably, the white line ratio (L2,3 ratio) monotonously increases with the decrease of Mn valence, thus can accurately serve as a fingerprint for characterizing Mn oxidation state.33 L2,3 ratios for both bulk region and surface layer therefore were calculated for quantitatively determining formal valence. The L2,3 ratio is determined as 2.53 for bulk region and 3.54 for surface layer, accordingly corresponding to the formal valence of +2.8 and +2.2, respectively. A distinct decrease of Mn valence state can be found in the surface layers. To better demonstrate such changes, a line scan from surface layer to bulk region was further conducted, as indicated by a horizontal white arrow in Figure 3a. Fifty spectra have been totally collected and every five spectra are summed up for analysis (from number 1 to 10). That is, spectrum 1 is the sum of the first five raw spectra starting from surface, and spectrum 10 indicates the last five raw spectra (Figures 3c). The L2,3 ratios and the corresponding valence states are then extracted, shown in Figure 3d. The formal valences fluctuate around +2.8 for bulk region, slightly inferior to +3 due to the intrinsic oxygen vacancies,35 and approach to around Mn2+ at utmost surface. Such valence decrease arise from the increase of charge defects, like oxygen vacancies.


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Figure 3. STEM-EELS analysis of Mn L2,3 edge for charge neutral surface. a) STEM-HAADF image of one neutral surface. The bulk region is downward polarized. Two vertical lines (red line with number 1 and green line with number 2) and a horizontal white arrow indicate positions conducting EELS line scans. b) ELNES of Mn L2,3 edge for surface layer (red) and bulk region (green), respectively. c) A sequence of spectra from surface layer (spectrum 1) to bulk (spectrum 10). Each is the sum of five vicinal spectra. d) L2,3 ratios and corresponding Mn valence states variations, quantitatively extracted from c). An abrupt change can be found between surface layer and bulk region.

Oxygen vacancies are common and inevitable in complex oxides, which can act as a control parameter for ferroelectric or (anti)ferromagnetic properties.36-39 Previous work has demonstrated that the oxygen vacancies can be easily stimulated to form simply by the electron beam irradiation, implying the volatility of oxygen atoms in YMnO3.40 In our experiments, we also observed the loss of displacement after long time irradiation.


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Particularly, considering the larger bond length for in-plane Mn-O bond in YMnO3, the OP vacancies ( VO P ) are much easier to form than OT vacancies ( VOT ) according to the Harrison’s rule (bond strength is inversely proportional to seventh power of bond length).40 For surface layer, due to the asymmetric bonding, the formation energy of oxygen vacancies is usually low, VO P is thus much more easily formed. The existence of in-plane VO P at surface layer can also be confirmed by the SAED (selected area electron diffraction) patterns, where redundant diffraction spots ( [1 10] and [2 10] ) show up (Figure S2, Supporting Information).35 Such loss of in-plane oxygen atoms can arouse the lack of the Y-O rehybridization, contributing large anomalies to Born effective charges on the off-centering Y-OP sites,28 and deteriorate the polarization. The polarization deteriorated surface layers coincide with oxygen deficient layers (shaded with light blue in Figure 3d), therefore suggesting that the VO P rather than bound charges should be the primary reason for such ferroelectric anomaly. The roles of in-plane oxygens in preserving polarization can thus be verified. For surface layer, the limited number of unit cells along c axis (about 2-3 unit cell for our experiment) can highlight the oxygen vacancies effects on deteriorating polarizations. Moreover, the symmetrical breaking (three free surfaces) can add the degree of freedom, making paraelectric states energy favored. Additionally, when inducing oxygen vacancies, the electron doping of YMnO3 is expected, resulting in a mixed valence of Mn3+ and Mn2+.25 Therefore, the Mn3+-Mn2+ double exchange system through O-2p orbitals in surface layers is preserved in such configuration, since the occupancy of the d-shell of Mn3+ (3d4) and Mn2+ (3d5) differs by one (Figure S3, Supporting Information).41 One electron can hop from d-shell of Mn2+ ion to that of Mn3+ through O-2p orbital. The overall energy saving in such double exchange interactions can eventually lead to ferromagnetic alignment of neighboring Mn ions. Thus, the electrons are


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itinerant and delocalized, implies possible surface magnetic moments and metallic conductivity, adding the surface physical diversities.25, 41

2.2. THEORETICAL CALCULATIONS DFT (density functional theory) calculations were conducted to better understand the surface behaviors and clarify the roles of in-plane oxygens (Calculation details can be found in Supporting Information). Effects of bound charges on controlling surface reconstruction are first extracted. To magnify surface effects, we start with a slab composed with two non-defective unit cells, which is the minimum thickness for bulk. The unit cells are stacked along c axis with two apical oxygen layer terminated surfaces (positively and negatively poled, respectively), each capped with a vacuum layer of 15 Å, as shown in Figure 4a. Displacement magnitude (δ) of Y ions for the initial polarized unit cell (P63cm) is 50.7 pm, which is identical for each Y ion layer. Corresponding projections along [001] (negative surface) and [001] (positive surface) are demonstrated in Figure 4b and Figure 4c (upward direction is defined as [001] direction), respectively, where the in-plane positions of on-top oxygen atoms can be clearly distinguished. After relaxation, atoms deviate from their original positions, especially for Y ions and on-top oxygen atoms, shown in Figure 4d-f. Careful inspections in the optimized structure (shown in Figure 4d) can find that the displacement of Y ions for negative surface (δ1), determined as 39.1 pm, decreases with respect to bulk value (δ=50.7 pm). While, a slight increase in Y displacement (δ3=52.5 pm) can be discovered for positive-poled surface. Deviations of on-top oxygen atoms (indicated by green arrows in Figure 4e,f) contribute to such variations, stemming from the influence of the unscreened surface bound charge.


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Figure 4. Examinations of bound charge effects based on DFT calculations. a) The initial structure with the thickness of two unit cells. A 15 Å vacuum layer is capped at both negative and positive surface. Three Y ion layers are with the same displacements (indicated by δ). b) and c) Viewing from negative and positive surface, respectively. d) The relaxed structure. Different displacements occur for each Y ion layers. e) and f) Projections from negative and positive surface, respectively. Relaxations of on-top oxygen atoms can be distinguished, indicated by green arrows.

Such results indicate that different charge conditions at surface indeed have asymmetric effects on ferroelectric polarizations. However, as we can see, bound charges have very limited contributions to altering polarization of YMnO3 due to the insensitivity to the depolarization field (though it is fairly large in two-unit-cell thick slab), as consistent with previous results.13 Only 22.9% decrease for negative surface and even a slight increase for positive surface can be induced by bound charges, which is inconsistent with our experimental observations at surface layers, where a distinct decrease of 81.5% is found independent from surface charge conditions. Such discrepancy shows that the bound charge is not the dominant factor for deteriorated polarization at YMnO3 surface.


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Calculations to determine the specific roles of oxygen vacancies are further employed. All eight possible configurations, constructed considering different symmetry sites, with up to two oxygen vacancies in the 30-atom unit cell are explored. The initial unit cell is downward polarized, the same as the case shown in Figure 1a. Optimized defective structures resulted from DFT calculations are shown in Figure 5 ([001] projections can be found in Figure S4, Supporting Information). Figure 5a,b indicate the structures where one out of 18 oxygen atoms is missing in each unit cell, corresponding to the vacancy concentration of 5.6%. This can be formally written as YMnO3-δ with δ=0.17. Figure 5c-f correspond to the unit cells each with two oxygen vacancies of the same type ( V2O P1 or V2O P2 ), as well as those of two different types ( VO P1O P2 ) shown in Figure 5g,h. The oxygen concentration is determined as 11.1% (YMnO3-δ with δ=0.33). Variations of Y layers corrugation can be distinguished for all cases (indicated by green arrows on Y ions), however show different characteristics depending on oxygen vacancy configurations. The Y ions in the same vertical lines with oxygen vacancies will deviate from the initial positions in P63cm symmetry due to the absence of corresponding hybridizations with in-plane oxygens, thus indeed resulting in ferroelectric anomaly. Such movements can be intuitively understood based on the asymmetric bonding of Y ions along z direction and the electrostatic repulsion between positively charged oxygen vacancies and Y ions. Consequently, the original down-down-up configurations of Y ion layers are destroyed or even transformed to up-up-down configurations, as in the case of V2O P2 shown in Figure 5e. Thus, the oxygen vacancies can induce the phase shift of domains, in some sense, acting as anti-phase domain walls. Such results provide theoretical support for previously reported abnormal straight domain walls in non-stoichiometric YMnO3, where ordered oxygen vacancies should exist.42 The migration of oxygen vacancies that are trapped in charged domain walls, can also motivate the propagation of domain walls as well as switch of ferroelectric domains.43 Considering the interlocking of


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ferroelectric and antiferromagnetic domain walls,44-45 it is also expected that the structural defects can lead to anomalous new magnetoelectric phenomena. Noteworthily, the case of V2O P1 keeps the down-down-up configuration and only diminish the magnitude of displacements, accordant with our experimental observations (Y ion layers nearly become flat). Lower oxygen vacancy concentration (around 1.85%) situation was also explored (Figure S5, Supporting Information), where deteriorated polarizations can be very localized. The unit cell volumes expand, while polarization magnitudes deteriorate, when increasing oxygen vacancy concentration, shown in Figure 5i and Figure 5j, respectively. Such tendency shown by DFT calculations is in consistency with measured data that displacements of outmost four layers for negative surface are overlapped. Following the tendency of calculation results, the vacancy concentration for outmost layer is thus estimated as 16%, which can be formally written as YMnO3-δ with δ=0.48. These DFT calculation results provide possible oxygen vacancy configuration for each surface layer, and meanwhile verify the key roles of in-plane oxygen atoms in determining the polarization properties in hexagonal ferroelectrics.


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Figure 5. The relaxed structures with various oxygen vacancy configurations. a) and b) The relaxed unit cell each containing one single oxygen vacancy. c)-f) One unit cell contains two same type oxygen vacancies. g) and h) One unit cell contains two oxygen vacancies with different types. i) The volumes of relaxed unit cells, which expand with the increase of oxygen vacancy concentration j) The polarization displacements of relaxed defective unit cells and negative poled surface in Figure 2d. The tendency of displacements deteriorating with the increase of oxygen vacancy concentration is shaded with light red shadow. Royal blue hollow rhombuses indicate the displacements of outmost four layers for negative surface. The oxygen vacancies are indicated by black (front) and gray (hidden behind) dots. Oxygen vacancy types (OP1, OP2 vacancy or combinations) are shown above each unit cell.


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The first “D” or “S” in parenthesis refer to “Different layer” or “Same layer”, while the second “D” or “S” refer to “Different vertical line” or “Same vertical line”, respectively. Green arrows on Y ions represent the moving directions.

Another intriguing finding is that specific oxygen vacancies can induce ferromagnetism. For instance, VO P2 , V2OP2 (S− D) and VOP1OP2 (S− D) with a ferromagnetic configuration have lower energy than the antiferromagnetism. Such results indicate that ferromagnetism may emerge at surface due to the oxygen vacancies at specific sites, implying the potential surface electromagnetic couplings (See Table S1 in Supporting Information for energy differences between ferromagnetism and antiferromagnetism for all considered defect structures with oxygen vacancies). Further study is based on the analysis and comparison of the electronic structures for both perfect unit cell and defective ones with different type oxygen vacancy. Figure 6 displays the DOS (density of states) of the resolved Y-4d, OP-2p and Mn-3d orbitals for two kinds of unit cells, including the unit cells with OP1 or OP2 vacancy. For perfect unit cell with

P63cm symmetry (shown in Figure S6, Supporting Information), OP-2p orbitals overlap with both neighboring Y-4d and Mn-3d orbitals. Thus, covalence of Y-OP bonds can be expected in this scenario. Such overlap corresponds to the long debated transition mechanisms that whether the ferroelectricity arises from Y-4d-O-2p or Mn 3d-O-2p hybridization.28, 46 As the OP vacancy (OP1 in Figure 6a or OP2 in Figure 6b) is created, Mn-3d-O-2p hybridization exhibits no obvious changes, while Y-4d orbitals (especially the population of out-of-plane degenerate orbitals 3d z 2 ) lower their density to a large extent. Such decrease is perceived for both OP1 and OP2 oxygen vacancy configurations (Figure 6a,b). Consequently, Y-OP bonds become less covalent due to decreased hybridization. It is thus clear that the aforementioned ferroelectricity deterioration at surface skin layer is due to the weakening of Y-4d-O-2p


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hybridization caused by OP vacancy. That is, losing of in-plane oxygens can induce the reduction or absence of such hybridization, followed by the asymmetric covalent bonding strength along z direction that eventually drives the movements of Y ions towards to the side without oxygen vacancies. These results of orbital-resolved DOS also substantiate that Y-4d-O-2p hybridization (or Y-d0-ness) does play a role in the paraelectric-to-ferroelectric transition, clarifying the pending origin of improper ferroelectricity.

Figure 6. Orbital nature of electrons for perfect and defective YMnO3 unit cells. Orbital-resolved densities of states of Y-4d, OP-2p and Mn-3d orbitals for perfect unit cells with a) OP1 vacancy and b) OP2 vacancy, respectively. The overlapping DOSs indicate the hybridization between Y-4d and OP-2p orbitals. The distinct decrease of overlapped DOSs between Y-4d and OP-2p orbitals, compared with the case for perfect unit cell (Figure S6), can be distinguished, manifesting the lowered orbital hybridization, especially for the orbitals along z direction. The dashed gray vertical lines indicate the Fermi level.

2.3. DISCUSSION


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Based on dedicated structural and electronic anomalies, promising electromagnetic couplings and controllable diverse functionalities are potentially embedded in such surface skin layers of this strong correlation system. Contributions of the hybridization between Y-4d and OP-2p orbitals to improper ferroelectricity have been clarified in this study, implying the sensitivity to chemical environment of Y ions, which can be possibly tuned by electron or hole doping. The ferroelectric polarization is induced by Y ions (A site) displacements and influenced by hybridization with in-plane oxygens. While, magnetic moments are contributed by Mn ions (B site). Such scenario can well avoid the contradiction between empty d-shells of transition ions and ferromagnetism, where the partially occupied d-orbitals are expected, as the case for conventional ferroelectrics.47 Furthermore, we have previously testified that the oxygen vacancies (especially the in-plane oxygen vacancy) can tune the in-plane symmetry for MnO5 bipyramid, thus alter the magnetic configurations by tailoring the relative magnitude of inlayer and interlayer exchange interaction constants.18,

35, 38

The 2D spin

frustration structure can thus be broken. Meanwhile, the induced oxygen vacancy can change the electron configurations of Mn ion orbitals from 3d4 to 3d5, adding extra electron to a1g(z2) degenerate orbital, which implies the double exchange interaction and surface ferromagnetism. In this sense, in-plane oxygen vacancies can be well regarded as the control parameter and intermediary for ferroelectricity and (anti)ferromagnetism in improper ferroelectrics, acting as a bridge connecting the polarization and magnetic spins. The easily formed oxygen vacancies at surface layer, as the case demonstrated above, can induce the surface anomaly for both ferroelectricity and antiferromagtism, as well as the couplings. Such abnormalities suggest the diverse functionalities and couplings at surface skin layers, opening a new explorable and exploitable realm.

3. CONCLUSION


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In conclusion, our study presents a convincing evidence that the oxygen vacancy should be considered as a control parameter in designing the surface functionalities of hexagonal multiferroic YMnO3. By investigating the delicate surface structure, functional and ferroelectric anomalies of reconstructed surface layer are revealed, which are driven by the relaxation of of surface and consequent changes Y-O hybridization and coupled with in-plane oxygen vacancies. High-resolution STEM-EELS results confirm the existence of oxygen deficiencies and valence state decrease of manganese in the ferroelectric reconstructed layer, which flourishes in surface ferromagnetism through Mn2+-Mn3+ double exchange system. Density functional calculations based on the imperfect unit cells theoretically support the standpoint that the oxygen vacancy can induce the reduction of the hybridization between Y ions and in-plane oxygens, thus resulting in the deviations of Y ions and anomalies of ferroelectricity. Although YMnO3 has been used as a prototype material for this study, the results presented here can be applied to understand defects related surface property variations in many other strongly correlated oxides and have deep implications for understanding emergent surface functionalities in those complex systems. Our results experimentally clarify the crucial effects of hybridization between yttrium and in-plane oxygens in driving the improper ferroelectricity. The close connections between reconstructed multiferroic properties and chemical compositions at surface, as well as the revealed underlying atomic mechanism foreshadow the chances of engineering coupling effects and applications in surface chemistry devices.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional details of surface structure, oxygen vacancy configurations and corresponding models, including material preparations and characterizations; methods for quantitative analysis of (scanning) transmission electron microscopy image; figures for characterizing charge neutral surface structure; figures of SAED investigations; models of surface ferromagnetism with a mixed valence state of Mn3+ and Mn2+; details of DFT calculations; figures of all eight possible oxygen vacancy configurations, and examination oxygen vacancy concentration dependence. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ORCID Jing Zhu: 0000-0002-2175-9476 Shiqing Deng: 0000-0001-7016-4084

Author Contributions J.Z. proposed and supervised this project, as well as contributed to the result analyses and manuscript writing. S.D. and S.C. conceived and designed this work. W.D. and C.X. carried out density functional theory calculations and assisted the analysis. B.G. conducted scanning transmission electron microscopy experiments. S.D. performed data analysis and prepared the manuscript. X.S. helped grow and provide YMnO3 single crystal sample. All authors discussed the results and commented on the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was financially supported by Chinese National Natural Science Foundation (11374174, 51390471, 51527803), National 973 Project of China (2015CB654902), and National key research and development program (2016YFB0700402). This work made use of the resources of the National Center for Electron Microscopy in Beijing and Tsinghua National Laboratory for Information Science and Technology. The calculations were performed on “Explorer 100” (Tsinghua Univ.) cluster systems. X.S. acknowledges support from the National Natural Science Foundation of China (Grants No. 11374277 and No. U1532147) and the National Basic Research Program of China (Grants No. 2015CB921201 and No. 2016YFA0300103).

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TABLE OF CONTENTS (TOC)


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Atomic Mechanism of Hybridization-Dependent Surface

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