Strain Engineering for Anion Arrangement in Perovskite Oxynitrides

∇Department of Physical Science and Engineering and ○Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Gokiso, Sh...
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Strain Engineering for Anion Arrangement in Perovskite Oxynitrides Daichi Oka,†,‡ Yasushi Hirose,*,†,§ Fumihiko Matsui,# Hideyuki Kamisaka,† Tamio Oguchi,⊥ Naoyuki Maejima,# Hiroaki Nishikawa,# Takayuki Muro,∥ Kouichi Hayashi,∇,○ and Tetsuya Hasegawa†,§ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza Aoba, Aoba, Sendai, Miyagi 980-8578, Japan § Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado, Takatsu, Kawasaki, Kanagawa 213-0012, Japan # Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan ⊥ Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan ∥ Japan Synchrotron Radiation Research Institute (JASRI), Kouto 1-1-1, Sayo, Hyogo 679-5198, Japan ∇ Department of Physical Science and Engineering and ○Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan ‡

S Supporting Information *

ABSTRACT: Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1−xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LPXANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements. KEYWORDS: perovskite, oxynitride, epitaxial thin film, coordination chemistry, strain engineering, X-ray absorption near-edge structure

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leads to the formation of characteristic layered structures due to their larger ionic radii. This implies that oxynitrides, oxyfluorides, and oxyhydrides have much broader degrees of freedom in either the local anion configuration or long-range order of anions,5,6 which would enable material design based on coordination chemistry, analogous to that for metal complexes. For example, first-principles calculations predict ferroelectricity in ATaO2N (A = Sr or Ba) compounds, provided the nitrogen ions are arranged in a trans configuration.7 It is reasonable to expect that other physical properties, such as magnetic and

ontrol of the atomic arrangement in crystals would be a promising approach to realize favorable functional materials, although it still remains as one of the most challenging issues in solid-state chemistry. To date, this concept has been mainly applied to alloys and mixed-cation ionic crystals. For instance, the effects of cation order on the electronic properties of perovskite oxides have been intensively studied over the last decades.1,2 In contrast, perovskite-related oxides with mixed-anions have become experimentally available only recently, and the influence of the anion order on the physical properties is an emerging research field.3,4 Among the anions, H−, N3−, and F− can occupy O2− sites to preserve the three-dimensional framework, while the introduction of thirdor higher-row anions, such as P3−, As3−, S2−, Se2−, and Cl−, © 2017 American Chemical Society

Received: January 8, 2017 Accepted: March 17, 2017 Published: March 28, 2017 3860

DOI: 10.1021/acsnano.7b00144 ACS Nano 2017, 11, 3860−3866

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ACS Nano optical properties, could also be controlled by the anion arrangements in these mixed-anion compounds, as in the case of metal complexes.8,9 Despite the significant potential of this approach, only a few trials have been reported on the control of anion arrangement in mixed-anion compounds, either locally or with a long-range scale. This is partly due to a lack of established procedures to synthesize mixed-anion compounds with metastable anion configurations. For example, in perovskite oxynitrides, the N 2p orbital strongly prefers local cis coordination, in which the orbital overlap between N 2p and empty d orbitals is maximized.3 In addition, the local cis-unit tends to align with random orientation over a long-range due to the similar bond lengths of metal−nitrogen and metal−oxygen bonds.5,10 For example, in the case of SrTaO2N, cis-type local geometry is thermodynamically more stable than the trans-type,11−14 and the long-range arrangement of cis units is completely random or only partially ordered.11,12 The trans-type configuration was reported only in ref 15, in which the refinement of the neutron diffraction data may be incomplete, as suggested in ref 12. In a previous study, we attempted to control the anion arrangement in SrTaO2N thin films by utilizing epitaxial strain from the substrate.14 First-principles calculations suggested that compressive biaxial stress stabilizes the metastable ferroelectric trans structure relative to the most stable cis structure, and classical ferroelectric behavior was observed in tetragonally strained SrTaO2N epitaxial thin films grown on Nb:SrTiO3 (NSTO) substrates. However, no experimental evidence for the trans-type anion configuration was obtained.16,17 Neutron diffraction analysis, which has been used to determine the anion arrangements in bulk mixed-anion compounds, is not applicable to thin-film samples, because of their small volumes. Consequently, the concept of strain engineering for anion arrangement in mixed-anion compounds is still a controversial issue. In this report, site-selective elemental analyses were conducted to determine the anion configurations in compressively strained epitaxial thin films of Ca1−xSrxTaO2N perovskite oxynitrides. Linearly polarized X-ray absorption near-edge structure (LP-XANES) and electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) were employed as alternative approaches to conventional diffraction-based structural analysis. The results demonstrated that the epitaxial strain-induced systematic variation of the site occupancies for anions that originated from the formation of trans-type anion configurations in Ca1−xSrxTaO2N thin films.

Figure 1. Tetragonal distortion of Ca1−xSrxTaO2N epitaxial thin films. (a) Reciprocal space maps around the 103 peaks of Ca1−xSrxTaO2N epitaxial thin films coherently grown on NSTO substrates and partially relaxed SrTaO2N epitaxial film. (b) The caxis lattice constants and pseudotetragonal distortion (Dpt) as a function of nominal Sr content, x.

maps (RSMs) of the obtained Ca1−xSrxTaO2N films. Within the range of 0 ≤ x ≤ 0.5, the films grew coherently on NSTO substrates, and the out-of-plane lattice parameter increased monotonically with x (Figure 1b). On the other hand, higher Sr contents of x ≥ 0.6 caused lattice relaxation. As a result, the largest tetragonal distortion was attained at the composition of Ca0.5Sr0.5TaO2N. The pseudotetragonal distortion of this film (Dpt ≡ c / ab = 1.050) was almost twice that of the partially relaxed SrTaO2N epitaxial thin film on an NSTO substrate (Dpt = 1.026). All of the coherently grown films had atomically flat surfaces and sharp film/substrate interfaces (Figure S1). The Dpt value of the Ca0.5Sr0.5TaO2N film is notably comparable to that of the ferroelectric trans-ordered SrTaO2N structures (Figure 2b and c; Dpt ≈ 1.06) predicted by theoretical calculations (see ref 20 and supplemental note for details of the

RESULTS AND DISCUSSION Preparation of Highly Strained Perovskite Oxynitride Epitaxial Thin Films. We have previously14 grown epitaxial SrTaO2N thin films (a = 4.03 Å; pseudocubic approximation)18 on NSTO substrates (a = 3.905 Å), in which the epitaxial strain was partially relaxed due to the relatively large lattice mismatch of −3.1%. To achieve a larger strain effect in the present study, the magnitude of lattice mismatch was precisely controlled by adjustment of the lattice parameters of the solid solution between SrTaO2N and CaTaO2N (a = 3.95 Å; pseudocubic approximation),18 Ca1−xSrxTaO2N. Epitaxial Ca1−xSrxTaO2N thin films (x = 0−0.7; 1) were synthesized using a nitrogen plasma-assisted pulsed laser deposition technique, the details of which have been described in our previous reports.14,19 Figure 1a shows reciprocal space

Figure 2. Optimized structures of anion-ordered SrTaO2N. (a) Pbnm cis, (b) I4cm trans, and (c) P4bm trans structures. Lattice constants and pseudotetragonal distortion (Dpt) are listed in the boxes. Drawings were constructed using VESTA.24 3861

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Figure 3. Analysis of anion distribution in Ca1−xSrxTaO2N epitaxial thin films using LP-XANES. (a) Schematic illustration of site-selective absorption of linearly polarized X-rays by anions in a perovskite lattice. OPP- and IPP-active sites are indicated by orange and purple outlines of the p orbitals, respectively. (b) Experimental setup for the LP-XANES measurements. (c−f) LP-XANES spectra around the (c and e) N and (d and f) O K-edges of SrTaO2N (c and d) and Ca0.5Sr0.5TaO2N (e and f) epitaxial thin films grown on NSTO substrates. The graphs show OPP and IPP components (upper panel) and XLD (lower panel). The XLD spectra were calculated as the difference between the OPP and IPP components. The yellow shaded areas indicate π* excitation peaks. (g) Normalized XLD as a function of Dpt. The normalized XLD values were calculated as (IOPP − IIPP)/(IOPP + IIPP) from the peak areas of the OPP and IPP components around the peak positions (397.8−398.2 eV and 531.8−532.2 eV for N and O K-edge spectra, respectively). Normalized XLD values evaluated from TFY spectra (Figures S5 and S6) are also shown for nitrogen.

theoretical calculation), and much higher than those of the cisordered SrTaO2N (Figure 2a; Dpt = 0.97 or 1.01, depending on the definition of a pseudotetragonal lattice). Thus, we propose that the ferroelectric trans-type N configuration should be more stabilized in the Ca0.5Sr0.5TaO2N film than in the partially relaxed SrTaO2N films. Ca1−xSrxTaO2N films were also coherently grown on (110) planes of DyScO3 (DSO; a = 3.94 Å, pseudotetragonal approximation) for comparison. The x = 0.5 films (Ca0.5Sr0.5TaO2N) grown on DSO had a smaller tetragonal distortion of Dpt = 1.031 (Figure S2) than that on NSTO (Dpt = 1.050), in accordance with the smaller lattice mismatch between DSO and the film. Evaluation of Anion Site Occupations. The anion arrangements in the epitaxial Ca1−xSrxTaO2N thin films were evaluated from N K-edge and O K-edge LP-XANES. These spectra represent electron excitations from the occupied 1s orbital to the unoccupied states, and thus the spectral shape reflects the partial density of states (PDOS) of the unoccupied 2p orbitals hybridized with Ta 5d t2g orbitals. Therefore, with the use of linearly polarized X-ray, site-selective excitation is possible for single-crystalline films (Figure 3a): Under an inplane polarized (IPP) setup, where the electric vector of the incident X-rays is parallel to the [100] direction of the pseudotetragonal cell (E // [100]pt), π* excitation is allowed for anions at axial sites along the surface normal through 2px−

5dzx hybridization and half of the anions at in-plane equatorial sites through 2px−5dxy hybridization. On the other hand, with an out-of-plane polarized (OPP; E // [001]pt) setup, π* excitation is allowed only for anions at equatorial sites (2pz− 5dzx and 2pz−5dyz hybridization). Therefore, the difference in occupancies between the axial and the equatorial sites can be assessed from the intensity of X-ray linear dichroism (XLD) at the π* excitation peak, for either O or N anions. From the LP-XANES spectra of the Ca1−xSrxTaO2N thin films, which were obtained under horizontal (100% IPP) and pseudovertical (21% IPP + 79% OPP) polarization conditions (Figure 3b), the IPP and OPP components of the N K-edge and O K-edge XANES and the XLD (IOPP − IIPP) spectra were calculated. The LP-XANES spectra were collected with either the total electron yield (TEY) or total fluorescence yield (TFY) modes. While the bulk-sensitive TFY mode measurement was applicable to films grown on insulating DSO substrates, analysis of the O K-edge spectra was difficult for the TFY spectra due to severe interference signals that originated from the oxide substrates. Therefore, we hereafter focus on the TEY spectra, unless otherwise noted. The N K-edge and O K-edge XANES spectra of the Ca1−xSrxTaO2N thin films on NSTO substrates showed π* excitation peaks ranging from 0 to 4 eV from the edges (Figures S3 and S4). For the partially relaxed SrTaO2N film on NSTO 3862

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ACS Nano (Dpt = 1.026), only a small XLD was observed in both the N Kedge and O K-edge spectra (Figure 3c and d, respectively), which indicates a random distribution of the anions. This observation is consistent with our previous hypothesis that the SrTaO2N epitaxial thin film is composed of a trace amount of trans-type domains and the surrounding cis-type matrix.14 The macroscopically random anion arrangement in the cis-type matrix in the SrTaO2N epitaxial thin film is inconsistent with that in the reported bulk specimens with partial ordering.11,12 This is plausibly because the epitaxial thin film consists of a mixture of small domains with the unique axis in in-plane and out-of-plane directions, which were statistically averaged in the LP-XANES measurements. On the other hand, the highly distorted Ca0.5Sr0.5TaO2N on NSTO (Dpt = 1.050) exhibited clear XLD in both the N K-edge and O K-edge spectra (Figure 3e and f, respectively). The negative XLD, i.e., larger absorption under the IPP condition, around the π* excitation peaks in the N K-edge spectrum indicates higher nitrogen occupancy of the axial sites according to the site selectivity shown in Figure 3a. Although positive XLD is expected for the O K-edge spectrum based on the site selectivity, the observed XLD spectrum has not only positive but also negative components around the π* excitation peak. The presence of two components is due to the splitting of the Ta t2g orbitals, and the observed XLD is consistent with higher oxygen occupancy of the equatorial sites, as discussed later. The normalized XLD intensities ([IOPP − IIPP]/[IOPP + IIPP]) at the π* excitation peaks were monotonically enhanced as Dpt of the Ca1−xSrxTaO2N thin films increases (Figure 3g, open circles). The same Dpt dependence of XLD was confirmed in the N K-edge spectra obtained with the bulksensitive TFY mode (Figure 3g, filled triangles). In addition, the Ca1−xSrxTaO2N epitaxial thin films grown on DSO substrates showed similar XLD against the epitaxial strain Dpt, not against the chemical composition x (Figure 3g, filled inverse triangle), which confirms that neither A-site composition nor cell volume had a detectable influence on the XLD intensity. From these results, it was concluded that the increase of N and decrease of O at the axial sites were induced by compressive epitaxial strain. The behavior of the π* excitation peak in the N K-edge XLD spectrum of the epitaxial Ca0.5Sr0.5TaO2N thin film indicates that the occupancy of the axial sites by N, OaxN, is approximately 46%, provided that the IOPP and IIPP are proportional to the number of nitrogen atoms at the active sites and that the effect of octahedral rotation can be ignored. This value is obviously larger than that for random anion arrangement, 1/3. Site-specific EELS, combined with high-angle annular darkfield (HAADF) STEM, provided further evidence for the increased (decreased) occupancy of the axial sites by N (O) in this film. The HAADF-STEM image clearly indicated atomic columns of a perovskite lattice (Figure 4a). From the highresolution image (Figure 4b), we assigned several points that represented the axial and equatorial anion sites, at which the EELS spectra were averaged. As shown in Figure 4c, the averaged spectra at the axial sites showed stronger N K-edge and weaker O K-edge peaks relative to the equatorial sites. From the peak areas for the O K- and N K-edges, OaxN was estimated to be ca. 50%, which agrees well with that obtained from LP-XANES. Local Anion Arrangement. The nitrogen occupancy at the axial site in the Ca0.5Sr0.5TaO2N thin film grown on NSTO is as high as ca. 50%, which provides evidence of a strain-

Figure 4. Direct evaluation of anion distribution in Ca0.5Sr0.5TaO2N epitaxial thin film using STEM-EELS. (a) Low- and (b) highmagnification HAADF-STEM images of Ca0.5Sr0.5TaO2N epitaxial thin film grown on NSTO substrate. (c and d) EELS spectra around the (c) N and (d) O K-edges measured at axial and equatorial sites. EELS spectra were constructed by averaging the spectra obtained at several atomic columns. The selected points used for the calculation are shown in Figure S7.

induced change in the anion distribution at a macroscopic scale. However, there are several possible local anion arrangements that are consistent with this site occupancy: the partial formation of an ordered trans-type structure (Figure 5a, OaxN = 100%), the formation of a perfectly ordered cis-type structure where the axial sites are preferentially occupied by nitrogen (Figure 5b, OaxN = 50%),11,12 and their intermediates. In this section, we determine the local anion arrangement of the Ca0.5Sr0.5TaO2N film by comparison of the experimental LPXANES spectra with those simulated using first-principles and assuming model structures. The simulation of LP-XANES spectra was conducted for simple model configurations of ordered trans (Figure 5a) and ordered cis (Figure 5b) structures. It is reasonable to assume that the observed spectra were expressed as linear combination of these two extreme models because XANES is known to reflect the localized circumstance around an atom, and the peaks are assignable to (anti-) bonding states of it.21 Here, we focus on the shape of the π* excitation peak (0−4 eV from the edge) because the contribution of the A site cation to this peak is almost negligible; the PDOS of each model structure indicates that the conduction band minimum is composed of Ta 5d t2g, O 2p, and N 2p orbitals (Figure S8). The π* peaks in both of the calculated N K-edge absorption spectra for trans- and cis-ordered (Figure 5e and g, respectively) structures show negative XLD signals due to the high nitrogen occupancy at the axial sites, which is in accordance with the experimental spectra. However, the precise spectral features differ between the two model structures. The spectra calculated for the trans structure have two common features with the experimental spectra: (1) The N K-edge spectrum shows large negative XLD in the region of 2−4 eV from the edge (region B) with a tail in the region of 0−2 eV (region A) (Figure 5c and e), and (2) XLD of the O K-edge spectrum has a small negative 3863

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Figure 5. Theoretical simulations of LP-XANES spectra for ordered SrTaO2N. (a and b) Schematic images of (a) trans- and (b) cis-oriented structures used for the simulation. (c and d) Experimentally observed LP-XANES spectra around the (c) N and (d) O K-edges of a Ca0.5Sr0.5TaO2N epitaxial thin film grown on NSTO substrate (same data as Figure 3e and f, respectively, with a magnified scale). (e−h) Simulated LP-XANES and XLD spectra around the (e and g) N and (f and h) O K-edges for I4cm trans- (e and f) and Pbnm cis-type (g and h) SrTaO2N.

peak in region A and a large positive peak in region B (Figure 5d and f). These features are observed in the other trans-type structures investigated here (P4bm and P4mm, Figure S9). In contrast, the theoretical spectra for the cis-type structure have distinct differences from the experimental spectra: (1) Negative XLD in the N K-edge spectrum has no tail component in region A (Figure 5g), and (2) the O K-edge spectrum shows only positive XLD with a peak around the border of regions A and B (ca. 2 eV) (Figure 5h). Thus, the experimental spectra can be rationalized consistently by assuming the partial formation of an ordered trans structure in the Ca0.5Sr0.5TaO2N thin film grown on NSTO, rather than the formation of an ordered cis structure. As already mentioned, the shapes of the XLD spectra can be understood in terms of the splitting of the Ta t2g orbitals. Figure 6 shows the calculated PDOS of the Ta 5d t2g, N 2p, and O 2p orbitals for the trans ordered structure. The Ta dyz and dzx orbitals are located at higher energies than the Ta dxy orbital, although the energy ranges of these orbitals overlap with each other (Figure 6d). The p orbitals of axial N can mix only with dyz and dzx orbitals to form py−dyz and px−dzx hybrid orbitals with π* antibonding character, respectively (Figure 6c). Thus, the N K-edge spectrum shows strong absorption in region B with a weaker tail in region A under the IPP condition (E // x), which can be used to measure the PDOS of the dzx orbital through px−dzx hybridization. In contrast, the N 2pz orbital does not form a π* antibonding state with Ta 5d t2g orbitals (Figure 6c); therefore, no absorption is allowed under the OPP (E // z) condition, which results in negative XLD for both regions A and B (Figure 5e). In the O K-edge spectrum, the weak absorption observed in region A of the O K-edge IPP spectrum can be ascribed to px−dxy hybridization between two of the equatorial O orbitals along the y-axis and the Ta dxy orbital. In contrast, under the OPP condition, region B shows a strong absorption through the hybridization between either of the equatorial O orbitals and either of the dyz and dzx orbitals.

Figure 6. Splitting of the Ta t2g orbitals in trans-type SrTaO2N. (a) Schematic images of antibonding states composed of Ta t2g and anion 2p orbitals. (b−d) PDOS of (b) O, (c) N, and (d) Ta for I4cm trans-type SrTaO2N. The PDOS of O is a projection to the atom labeled as O(1) in Figures 5a and 6a. The energies indicated by the black triangles (2.13 and 5.64 eV) represent the peak positions shown in Figure 5e and f.

These two contributions change the sign of XLD in the O Kedge spectrum from negative in region A to positive in region B (Figure 5f). The splitting of the Ta t2g orbitals is partly attributable to the ligand field in a manner analogous with metal complex molecules. When an isolated octahedral Ta complex has four O2− at equatorial sites and two N3− at axial sites (Figure 6a), the dxy orbital is energetically differentiated from the others due 3864

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Discover). Surface morphologies were observed using atomic force microscopy with a scanning probe microscope (SII NanoTechnology, SPI4000 with SPA400). Polarized X-ray Absorption Spectroscopy. Polarized X-ray absorption spectroscopy measurements were performed at the BL27SU beamline of SPring-8. The incident angle of the polarized X-rays was set at 75° from the surface normal. In-plane and out-ofplane components were calculated from spectra obtained with horizontally and pseudovertically polarized X-rays. The XANES spectra were normalized with respect to the areas integrated in the range of 395−412 eV (N K-edge) and 528−546 eV (O K-edge). The nitrogen occupancies of the axial (OaxN) and equatorial (OeqN) sites were calculated by solving the following simultaneous equations:

to the tetragonal symmetry. Strong electron-donating ligands such as N3− generally destabilize the anti-bonding states of the host metal ion,22 so that the Ta dyz and dzx orbitals hybridized with N3− should be energetically higher than the dxy orbital, as shown in the calculated PDOS (Figure 6d). The PDOSs of dyz and dzx orbitals show further splitting into 2.3 and 3.6 eV (Figure 6d). While the former component is hybridized more strongly with the O pz orbital than with N orbitals, the latter component is hybridized preferentially with the N px or py orbital. This splitting would also be a consequence of the difference in electron-donating ability of N3− and O2−. These considerations suggest that spectral shapes of LP-XANES can be used as fingerprints for the local coordination of oxynitrides.

IIP/IOPP = (Oax N + Oeq N)/2Oeq N

O

CONCLUSIONS The strain engineering of anion arrangements was demonstrated for a series of Ca1−xSrxTaO2N thin films coherently grown on NSTO and DSO substrates. LP-XANES and STEMEELS measurements revealed that N occupancy at axial sites systematically increased when the imposed compressive strain became stronger. A combined and detailed analysis of the XLD spectra and first-principles calculations revealed that the increase of N occupancy at axial sites originated from the partial formation of a metastable trans-type arrangement, rather than from the perfect ordering of a cis-type local anion arrangement. The rationalized interpretation of the spectral shapes from metal complex molecular-like orbital splitting indicates that LP-XANES analysis is a practical tool for investigation of the local anion geometry in epitaxial thin films of oxynitrides. We consider that the strain engineering of anion arrangement and site-selective spectroscopic analyses are both applicable to oxyfluorides and oxyhydrides23 as well as oxynitrides, and this is expected to stimulate the material design of mixed-anion compounds based on solid-state coordination chemistry.

ax N

+ 2O

eq N

(1)

=1

(2)

where IIP and IOPP are the intensity of the π* peaks in the N K-edge spectra (integrated from 397.8 to 398.2 eV) under in-plane and out-ofplane polarization conditions, respectively. In the former equation, it is assumed that IOPP and IIPP are proportional to the number of nitrogen atoms. Scanning Transmission Electron Microscopy. A 200 kV scanning transmission electron microscope with a spherical aberration corrector (Jeol, JEM-ARM200F) was used for cross-sectional observations of thin-film specimens prepared by the focused ion beam method. The probe convergence semiangle and the inner semiangle for HAADF measurements were ca. 21 and 68 mrad, respectively. Site-selective EELS was performed using an energy filter (Gatan, GIF Quantum ER Model 965). The spectra were accumulated for 0.3 s per point. Base lines of N and O K-edge spectra were corrected by conventional power-law fitting for the pre-edge regions.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00144. X-ray diffraction patterns and an atomic force microscope image of the films grown on NSTO substrates; crystal structure analysis on the films grown on DSO substrates; all the experimental and theoretical LPXANES spectra other than displayed in the manuscript; the HAADF-STEM image used to construct the EELS spectra; DOS calculated for the model structures; and supplemental note about the space group of the calculated model structures (PDF)

MATERIALS AND METHODS First-Principles Calculation. Three model cells of SrTaO2N with different O/N order were analyzed theoretically. The lattice constant and structure of the unit cells were taken from previous research.20 Note that with density functional perturbation theory (DFPT) calculations, rotations of the octahedra are incorporated with no imaginary phonon left in the entire q-vector space (dynamically stable). Theoretical simulations of LP-XANES spectra were conducted for these cells using the full-potential linearized augmented plane-wave (FLAPW) approach, as implemented in the HiLAPW software package. The local density approximation (LDA) functional of Moruzzi−Janak−Williams was employed, and the spectra were calculated by adapting Fermi’s golden rule to the transition dipole operator. Sample Preparation and Characterization. Ca1−xSrxTaO2N epitaxial thin films were synthesized on (100) planes of 0.5 wt % Nbdoped SrTiO3 and (110) planes of DyScO3 single crystals by the nitrogen plasma-assisted pulsed laser deposition method.14,19 Sintered pellets of mixtures of Sr2Ta2O7 and Ca2Ta2O7 with appropriate ratios of Sr and Ca were synthesized, and each pellet was ablated with a KrF excimer laser (λ = 248 nm) and reacted with nitrogen plasma produced using an electron cyclotron resonator (Tectra, Gen2). During the deposition process, the substrates were heated at 600−650 °C, and the nitrogen gas pressure was set at 1.0 × 10−5 Torr. The films had stoichiometric compositions within the experimental error (10%), which were evaluated using X-ray photoemission spectroscopy with stoichiometric SrTaO2N14 and CaTaO2N19 thin films as standard samples. Lattice constants were determined from X-ray diffraction measurements with a four-axis diffractometer (Bruker AXS, D8

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Daichi Oka: 0000-0003-2747-9675 Hideyuki Kamisaka: 0000-0002-2398-4650 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST) and Grants-in-Aid for Scientific Research (nos. 15H01043 and 12J08258) from the Japan Society for the Promotion of Science (JSPS). Synchrotron radiation experiments were performed at SPring8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; proposal nos. 2013B1328, 3865

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(19) Oka, D.; Hirose, Y.; Fukumura, T.; Hasegawa, T. Heteroepitaxial Growth of Perovskite CaTaO2N Thin Films by Nitrogen Plasma-Assisted Pulsed Laser Deposition. Cryst. Growth Des. 2014, 14, 87−90. (20) Zhu, W.; Kamisaka, H.; Oka, D.; Hirose, Y.; Leto, A.; Hasegawa, T.; Pezzotti, G. Stress Stabilization of a New Ferroelectric Phase Incorporated into SrTaO2N Thin Films. J. Appl. Phys. 2014, 116, 53505. (21) Mizoguchi, T.; Tatsumi, K.; Tanaka, I. Peak Assignments of ELNES and XANES Using Overlap Population Diagrams. Ultramicroscopy 2006, 106, 1120−1128. (22) Mingos, D. M. P. The Electronic Factors Governing the Relative Stabilities of Geometric Isomers of Octahedral Complexes with πAcceptor and π-Donor Ligands. J. Organomet. Chem. 1979, 179, C29− C33. (23) Bouilly, G.; Yajima, T.; Terashima, T.; Kususe, Y.; Fujita, K.; Tassel, C.; Yamamoto, T.; Tanaka, K.; Kobayashi, Y.; Kageyama, H. Substrate-Induced Anion Rearrangement in Epitaxial Thin Films of LaSrCoO4−xHx. CrystEngComm 2014, 16, 9669−9674. (24) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276.

2014A1437, and 2015B1338). The authors thank Dr. Hirosuke Matsui, Mr. Masahito Sano, Mr. Naoki Kashiwa, Mr. Kenji Sugita, and Dr. Takuo Ohkochi for assistance with these experiments. The authors appreciate fruitful discussions with Dr. Tomoteru Fukumura.

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DOI: 10.1021/acsnano.7b00144 ACS Nano 2017, 11, 3860−3866