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Disappearance of Localized Valence Band Maximum of Ternary Tin Oxide with Pyrochlore Structure, SnNbO 2

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Yoshihiro Aiura, Kenichi Ozawa, Izumi Hase, Kyoko K. Bando, Hiroto Haga, Hirofumi Kawanaka, Akane Samizo, Naoto Kikuchi, and Kazuhiko Mase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12572 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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The Journal of Physical Chemistry

Disappearance of Localized Valence Band Maximum of Ternary Tin Oxide with Pyrochlore Structure, Sn2Nb2O7 Yoshihiro Aiura*, 1, Kenichi Ozawa2, Izumi Hase1, Kyoko Bando3, Hiroto Haga1, 4, Hirofumi Kawanaka1, Akane Samizo1, 5, Naoto Kikuchi1, Kazuhiko Mase6,7 1

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science

and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan 2

Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

3

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan 4

Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan

5

Department of Materials Science and Technology, Faculty of Industrial Science and

Technology, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan 6

Institute of Materials Structure Science, High Energy Accelerator Research Organization

(KEK), Tsukuba, Ibaraki 305-0801, Japan

1 ACS Paragon Plus Environment


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SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801,

Japan

ABSTRACT: The electronic structure of ternary tin oxide with pyrochlore structure, Sn2Nb2O7, which has the contribution of the Sn 5s orbitals in the valence band, is examined by synchrotronradiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy together with ab initio calculations. The empirical spectra are qualitatively consistent with the calculated density of states with the exception of a striking discrepancy in the electronic structure around the valence band maximum (VBM). The band calculation suggests the presence of a sharp peak around the VBM, mainly due to dispersionless bands which are derived from hybridization of Sn 5s orbitals and O 2p orbitals close to the Sn atom. However, the photoelectron spectra show no such characteristic spectral feature. From the theoretical prediction about the orbital origin of the localized VBM and the experimental estimation of the elemental composition by chemical analysis, it seems reasonable to conclude that the discrepancy of the electronic structure around the VBM is caused by the off-stoichiometric effect due to the point defects such as oxygen vacancies close to Sn2+, Sn2+ vacancies and Sn4+-on-Nb5+ substitutional defects. Oxides with such localized VBM are not considered to be suitable for practical p-type oxide semiconductors. Therefore, the disappearance of the theoretically-predicted localized VBM suggests the possibility to improve the mobility of hole carriers, and is considered to be closely related to our success in the development of the p-type oxide semiconductor Sn2Nb2O7.


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1. INTRODUCTION The development of high-performance p-type transparent conductive oxides (TCOs) has been strongly desired to create a new market of innovative transparent electronic devices.1 A key material design principle for realizing p-type TCOs, which was initially proposed by Kawazoe et al.,2 is to delocalize the valence band maximum (VBM) by forming hybridization between localized O 2p orbitals and metal bands or by using delocalized chalcogen orbitals such as S 3p bands and Se 4p bands. The delocalization of the VBM is expected to reduce the effective mass of the doped holes and increase the mobility. To narrow down the promising candidates for practical p-type oxide semiconductors, Hautier et al.3 and Sarmadian et al.4 conducted a computational search on thousands of binary and ternary oxides. Among these oxides, binary oxide SnO was reported to exhibit p-type conductivity and has recently been attracting attention.5-7 Based on the characteristic valence band structures associated with the presence of divalent Sn, it is hypothesized that the p-type conductivity of SnO arises from the delocalization of the VBM associated with the strong hybridization between the dispersionless O 2p bands and the Sn 5s bands.7,8 In other words, the localized nature of the O 2p orbitals is weakened by the hybridization with the Sn 5s bands. Because of its relatively isotropic carrier transport path, moreover, it is expected that the carrier mobility is insensitive to the structural randomness as shown for the typical s-orbital-based n-type TCOs, such as indium oxide doped with tin, ZnO and In-Ga-Zn-O.9 Therefore, compared to conventional p-type oxide semiconductors, an s-orbital-based p-type oxide semiconductor such as SnO is considered to be advantageous for practical use. However, binary tin oxide SnO exhibits a practical problem as a p-type TCO. The Sn 5sO 2p hybridization is prominent at the Brillouin zone boundary, whereas the conduction band minimum (CBM) due to Sn 5p orbitals is located at the zone center. As a result, the optical band



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gap is indirect and becomes small (0.7 eV),6 which is undesirable in practical applications. To overcome the specific band gap issue found in SnO, Walsh et al.10 proposed a theoretical material design to align the CBM with the VBM by forming functionalized multinary oxides. In 2008, Hosogi et al.11 reported the role of Sn2+ in the band structure of ternary tin oxides SnM2O6 and Sn2M2O7 (M=Nb and Ta) that have been studied as candidates for visible light responsive photocatalysts. Density functional calculations revealed that the CBM is composed of dispersionless Nb 4d or Ta 5d bands and that the indirect band gap magnitude is approximately the same as that of the direct gap.11-13 As a result, the optical band gap of ternary tin oxides SnM2O6 and Sn2M2O7 becomes large and tunable compared with that of binary tin oxide SnO. The reported band gaps of SnNb2O6, SnTa2O6, Sn2Nb2O7, and Sn2Ta2O7 are 2.3, 3.1, 2.3, and 3.0 eV, respectively.11 Therefore, the ternary tin oxides should be expected to become transparent to visible light.

Figure 1. (a) Electronic band structure and (b) density of states near the VBM of Sn2Nb2O7 calculated by the density functional method. The energy position of the VBM is shown by the dotted lines.


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As shown in Figure 1, however, our band calculation of Sn2Nb2O7 suggests the presence of a sharp peak or dispersionless bands around the VBM, which is consistent with previous reports.11,13 Since the localized VBM predicted by the band calculations is expected to increase the effective mass of hole carriers and to decrease the mobility, it is considered from the theoretical point of view that ternary tin oxide with pyrochlore structure, Sn2M2O7 (M=Nb or Ta), is not suitable as the p-type semiconducting oxide. In spite of the theoretical prediction, recently, we successfully demonstrated a p-type semiconductor of Sn2M2O7.14 The hole mobility observed for Sn2Nb2O7 is considerably low (0.19 cm2V-1s-1)14 compared with that of SnO (2.4 cm2V-1s-1) 7. Since the top of the valence band for SnO is considerably dispersive,8 the low hole mobility for Sn2Nb2O7 may be caused by the localized nature of the VBM if the actual electronic structure near the VBM is in accordance with the band calculation. However, the hole mobility of Sn2Nb2O7 was measured using low density polycrystalline samples, and so that the measured mobility may not reflect the intrinsic (maximum) value. To elucidate the limitation of the semiconductor characteristics and to determine material design guidelines for the optimization, it is desired to study the electronic structure near the VBM and the CBM of Sn2M2O7. In the present study, we have performed photoelectron spectroscopy (PES) and X-ray absorption fine structure (XAFS) measurements. In addition, we have calculated the electronic structure of Sn2Nb2O7 with ideal pyrochlore structure within the full-potential augmented plane wave with local orbitals method and the generalized gradient approximation for the exchange-correlation energy functional. A sharp peak around the VBM predicted by the band calculations was not observed in the PES spectra. Combining the theoretical prediction about the orbital origin of the sharp peak around the VBM and the experimental estimation of the elemental composition by chemical analysis, it seems reasonable to conclude that the disappearance of the sharp peak around the VBM is caused by the



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off-stoichiometric effect. Our results suggest that the ternary tin oxides with pyrochlore structure, Sn2M2O7 (M=Nb, Ta) has the potential for a promising candidate material of practical p-type TCO. 2. EXPERIMENTAL METHODS Synchrotron-radiation-excited PES, near-edge XAFS (NEXAFS) and extended XAFS (EXAFS). Using high-resolution soft X-ray, PES measurements and NEXAFS measurements of the Sn M4,5-edge and O K-edge were performed at the 13B beamline of the Photon Factory, High Energy Accelerator Research Organization (KEK).15,16 The NEXAFS spectra were acquired in the partial electron yield mode. Clean surfaces were prepared by fracturing the single-crystal samples in an ultra-high vacuum (UHV) preparation chamber at a pressure of 10-8 Pa.17 The cleanliness of the surfaces was checked by the core-level PES measurement (see Section 1 of Supporting Information). The binding energy of the PES spectra was referenced to the Fermi cutoff position in the spectra of the Ta sample holders. To examine whether the spectra reveal the bulk or intrinsic electronic structure, the Sn K-edge NEXAFS measurement acquired in the transmission mode was performed at beamline NW10A of the Photon Factory Advanced Ring, KEK.18 In order to study the influence of the point defects on the local structure, moreover, EXAFS spectra of Sn K-edge and Nb K-edge were taken using single crystal and polycrystalline samples with various conducting characteristics. All measurements were performed at room temperature. Materials. The Sn2Nb2O7 single crystal was prepared by the floating zone (FZ) method. The powder X-ray diffraction pattern could be indexed clearly by a pyrochlore structure and sharp Laue spots were shown. Using X-ray fluorescence spectrometer, the atomic ratio of Sn to Nb was estimated to be 0.98. Mössbauer spectra showed that our single crystal used here is composed of 95.5% Sn2+ state and 4.5% Sn4+ state (see Section 2 of Supporting Information). The isomer shift of a singlet ascribed to a Sn4+ species showed no notable difference from that of the reference


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sample CaSnO3. This suggests that oxidized tins from divalent state to tetravalent state reside in a regular octahedral site with all equivalent Sn-O distances, that is, the Nb site.14,19,20 By comparing the relative intensity of Sn 3d and O1s core-level PES speaks of Sn2Nb2O7 and that of the reference sample SnO2, the actual oxygen content of Sn2Nb2O7 was estimated to be 3.63 per a Sn atom (see Section 3 of Supporting Information). From the empirical results, it is concluded that the single crystal used here contains many O vacancies (V •• in Kröger-Vink notation), Sn2+ vacancies V and Sn4+-on-Nb5+ substitutional defects Sn the non-stoichiometric composition Sn

.

Nb

, and that the actual composition is expressed as .

Sn

.

O

.

.

The Sn2Nb2O7 polycrystal samples were prepared by a solid-state reaction. The semiconducting properties were able to be controlled by the annealing condition such as the temperature, the time and the atmosphere.14 A SnO2 single crystal (SurfaceNet GmbH) was also used as a reference for the PES measurement and the Sn M4,5-edge and O K-edge NEXAFS measurements. Sn2+ in Sn2Nb2O7 has a formal electronic configuration of [Kr] 4d105s2 while Sn4+ in SnO2 is different, [Kr] 4d105s0. Commercial SnO (Kojundo Chemical Laboratory; purity, 99.9%) that has the same formal electronic configuration as that of Sn2Nb2O7 was also used as a reference for the Sn K-edge NEXAFS measurement. Electronical properties. Hall measurement showed that carrier density of Sn2Nb2O7 polycrystal samples is 2.5 × 1017 cm-3 and the carrier type is positive.14 Sign of the Seebeck constant of the polycrystalline sample also indicated positive (+5.9 × 106 cm-3). The mobility of hole carrier was estimated to be 0.19 cm2V-1s-1. Since it is considered that the low mobility is caused by low density of the polycrystal (approximately 60 % with respect to the ideal crystal or the single crystal) due to low annealing temperature (1023-1173 K), the improvement of the mobility will be expected by preparing high-density film.



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Contrary to the p-type Sn2Nb2O7 polycrystal, the Sn2Nb2O7 single crystal used here had a negative Seebeck coefficient. Since the single crystal prepared by the FZ method was annealed at high temperature (above the melting point) in a reducing atmosphere, the n-type semiconducting properties is considered to be caused by V •• . However, judging from the high electronic resistivity and the absence of a noticeable state in the band gap region of the PES (Figures 3-5), it is considered that the electron carriers doped by V •• was nearly, though not completely, compensated by the hole carriers due to V crystal, Sn

.

Nb

.

Sn

.

and Sn O

.

. Also from the observed actual composition in the single

, the amount of the actual electron carriers contributing to the

electronical properties was estimated to be very slight. Although the semiconducting properties are different between the polycrystal and the single crystal, the Sn K-edge and Nb K-edge NEXAFS spectra acquired in the transmission mode are almost the same between them (see Section 4 of Supporting Information). Therefore, it is considered that the basic framework of the electronic structure is not affected observably by slight carrier doping in the polycrystal and the single crystal. Band calculation. The electronic structure of Sn2Nb2O7 was calculated using the WIEN2k package21 within the full-potential augmented plane wave with local orbitals (FLAPW+lo) method and the generalized gradient approximation for the exchange-correlation energy functional.22 For the lattice parameter, we used the value of a=10.581 Å reported in the literature.23 Since the atomic position of the O in the Nb2O6 tetrahedron (u, 1/8, 1/8) is not available for Sn2Nb2O7, we used the u-value (0.425) of Sn2Ta2O7 as the initial crystal parameter. 24 The optimized atomic positon of the O in the Nb2O6 tetrahedron after the relaxation was estimated to be (0.435, 1/8, 1/8). The density of states (DOS) was obtained using the modified tetrahedron method.25 The calculated band gap was estimated to be 0.9 eV, which is considerably smaller than the optical band gap of 2.3 eV.11,14


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In order to compare the calculated electronic structure with the empirical spectra (Figures 4, 5, and 6) directly, the VBM in the band calculation is aligned with that in the PES spectra, and the CBM in the band calculation is set to the Fermi energy. The calculated DOS is broadened using a Gaussian function with a full width at half maximum of 100 meV, corresponding to the total energy resolution of the PES spectra measured at the photon energy of 100 eV. 3. RESULTS and DISCUSSION Figure 2(a) shows the Sn K-edge NEXAFS spectra of Sn2Nb2O7, SnO and SnO2 acquired in the transmission mode using hard X-ray of approximately 29 keV. The Sn K-edge NEXAFS spectra of Sn2Nb2O7 and SnO with formally Sn2+ are similar to each other. In contrast, a clear chemical shift of approximately 4 eV is observed for SnO2 with the formally Sn4+. Previous electron energy-loss spectroscopy carried out in a transmission electron microscopy (TEM-EELS) has also reported a similar chemical shift of the Sn M4,5-edge (of approximately 3.5 eV) between SnO and SnO2.26 Although the Sn K-edge NEXAFS spectra and the TEM-EELS spectra reflect the bulk or intrinsic electronic structures, the energy resolution with several eVs is not sufficient to discuss the semiconducting properties. Using high-resolution soft X-ray, PES and NEXAFS are powerful tools to elucidate the electronic structures near the VBM and the CBM in detail. However, since PES27 and NEXAFS acquired in partial electron yield mode28 are surface-sensitive techniques, we must pay attention to the off-stoichiometric, that is, the extrinsic influence in the spectra. To elucidate the intrinsic electronic structure, it is desired to use well-defined surfaces without contamination and atomic defects, such as a cleaved surface29 or a fractured surface17 of a single crystal. As shown in Figure 2(b), a similar chemical shift (3.7 eV) between Sn2Nb2O7 and SnO2 is also confirmed in the Sn M4,5-edge NEXAFS spectrum measured using the fractured surface of the single crystals. In addition, the Sn M4,5-edge NEXAFS spectrum of SnO2 using the



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soft X-ray is very similar to the Sn M4,5-edge TEM-EELS spectrum.26 That is, the surface Sn valence state near the fractured surface of single crystal is essentially the same as that in the bulk. Therefore, it seems reasonable to suppose that the NEXAFS and PES spectra of the fractured surface of single crystals measured using the high-resolution soft X-ray as shown below reflect the bulk or intrinsic electronic structure of Sn2Nb2O7.

Figure 2. (a) Sn K-edge NEXAFS spectra of Sn2Nb2O7, SnO and SnO2. The spectra are acquired in the transmission mode. Sn2Nb2O7 and SnO2 powder samples were obtained by grinding the single crystals; commercial SnO powder was used. (b) Sn M4,5-edge NEXAFS spectra of Sn2Nb2O7 and SnO2. The spectra were acquired in the partial electron yield mode. Clean surfaces were prepared by fracturing Sn2Nb2O7 and SnO2 single-crystals under UHV. The Sn M4,5-edge NEXAFS spectra are derived from optical transitions between the Sn 3d core levels and the conduction bands near the surface. Since the chemical shift of the Sn 3d core


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level measured under the same experimental condition is shown to be considerably small between Sn2Nb2O7 and SnO2 (about 0.2 eV, see Section 5 of Supporting Information), the observed chemical shift is considered to be mainly caused by the difference in the conduction bands, which are closely related the Sn valence state. The photon energy dependence of the PES spectra of Sn2Nb2O7 and SnO2 is shown in the bottom panels of Figure 3. PES spectra were normalized by the area of the valence bands from the Fermi energy to the binding energy of 14 eV. Because the Sn2Nb2O7 single crystal used here has a negative Seebeck coefficient, the CBM is located at the Fermi level, and the binding energy of the VBM, which is nearly consistent with the optical adsorption edge size,11 corresponds to the band gap (EG). The PES spectra of SnO2 measured at the photon energies of 50 and 100 eV (red lines in the bottom panel of Figure 3(b)) are similar to previous O K-edge X-ray emission (XES) spectra.30,31 Since the XES spectra show the O 2p partial density of states (PDOS) in accordance with the electric dipole selection rule, the valence PES at 50 and 100 eV should reflect the O 2p PDOS. This is supported by the fact that the relative cross section of the O 2p to that of Sn 5sp is dramatically enhanced with decreasing photon energy, as shown in the middle panel of Figure 3(b). On the other hand, the photon energy dependence of the PES spectra of Sn2Nb2O7 is more complicated by the presence of the Nb atom. The cross section of Nb 4d is strongly enhanced in the photon energy range between 30 and 50 eV but almost disappears at a photon energy of approximately 90-100 eV due to the Cooper minimum, as shown by the yellow line in the top left panel of Figure 3(a).32,33 As a result, while the PES spectrum at 100 eV is mainly caused by the O 2p orbital contribution, the PES spectrum at 50 eV is considered to contain not only an O 2p orbital contribution but also the Nb 4d orbital contribution.



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Figure 3. (top) Atomic subshell photoionization cross section of elements contained in Sn2Nb2O7 and SnO2 by Yeh and Lindau32, (middle) relative ratio of the cross section for the photon energies used in the PES measurements, and (bottom) photon energy dependence of PES spectra of (a) Sn2Nb2O7 and (b) SnO2. Energy gaps between the VBM and the Fermi energy correspond to the band gap (EG = 2.3 eV11 for Sn2Nb2O7, while 3.6 eV34 for SnO2). Dotted lines in the top panels mark the photon energies used in the PES measurements of 50 eV, 100 eV, 600 eV, and 1210 eV. Figure 4(a) shows the PES spectra at 50 eV (yellow) and 100 eV (red) after subtracting the Shirley-type backgrounds, as shown by the dotted lines in the PES spectra of Figure 3(a), and then normalized by the spectral intensity at the binding energy of ~5 eV. Figure 4(b) shows that the PES spectrum at 100 eV is qualitatively similar to the calculated O 2p PDOS except for the


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characteristic peak feature around the VBM. As shown in the middle panel of Figure 3(a), the PES spectrum at 50 eV contains almost no Sn 5sp orbital contribution and consists of the O 2p and the Nb 4d orbital components. Therefore, the difference spectrum obtained by subtracting the PES spectrum at 100 eV from the PES spectrum at 50 eV is expected to show mainly the Nb 4d orbital contribution. Figure 4(c) shows that the difference spectrum resembles the Nb PDOS except for the disappearance of the characteristic peak feature around the VBM. The difference spectrum was normalized to the photoionization cross section of O 2p (red line in the top panel of Figure 3(a)). As shown in Figure 4(d), the sharp peak that is not observed in the PES spectra is primarily derived from the O orbitals.

Figure 4. (a) PES spectra of Sn2Nb2O7 measured at photon energies of 50 eV and 100 eV and (b) O 2p and (c) Nb 4d orbital contributions to the PES spectra and the corresponding calculated PDOS. (d) Calculated total DOS and PDOS of Sn, Nb and O near the VBM.



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Batzill et al. have reported a detailed and comprehensive study about the valence band structure for the stoichiometric and reduced surfaces of the single crystalline SnO2.35 Characteristic surface bands caused by oxygen vacancies were shown for the reduced surface, which was prepared by sputtering and annealing under UHV. The surface bands, which consist of Sn 5s orbitals in the Sn2+ state, are very similar to the electronic structure near the VBM of Sn2Nb2O7 in Figure 4. From the band calculation, the localized state of Sn2Nb2O7 is predicted due to the hybridization between Sn 5s and O 2p orbitals in the Sn2O tetrahedral structure, which is inherent for the ideal and stoichiometric pyrochlore structure. For both the fractured Sn2Nb2O7 surface and the reduced SnO2 surface, however, it must be noted that the localized state was not observed near the VBM. The reason about the disappearance of the localized VBM is considered to be derived from the Sn and oxygen vacancies and/or the crystal structure issue pointed out by previous report,36 as discussed later. Figure 5(a) shows the occupied and unoccupied O 2p PDOS deduced from the PES spectra measured at the photon energy of 100 eV and O K-edge NEXAFS spectra, respectively. Previous band calculation39 and hard X-ray PES measurement7 of SnO2 showed that the predominant peak structure at the binding energy of ~5 eV corresponds to the O 2p non-bonding (NB) bands, which hardly include Sn 5sp contributions. The VBM of SnO2 consists of the O 2p NB bands. As shown in Figure 5 (a), a similar characteristic peak structure at ~5 eV has been observed also in the PES spectra of Sn2Nb2O7. In order to facilitate the interpretation of those spectral features of Sn2Nb2O7, the calculated electronic structure was decomposed to the Nb 4d and O(1) orbital contributions from the Nb2O6 octahedral skeleton structure and the Sn 5s and the oxygen orbital (O(2), orange balls in Figure 5(c)) contributions in the Sn2O tetrahedral structure. From our band calculation in Figure 5(b), it was suggested that the characteristic peak at ~5 eV is mainly caused by the


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contribution from the oxygen in Nb2O6 octahedral skeleton (O(1), red balls in Figure 5(c)), whereas it was shown that the VBM of Sn2Nb2O7 is derived from an another weak structure appearing at the low binding side of the O(1) 2p NB bands. As shown in Figure 6, it is clear that the VBM of Sn2Nb2O7 is mainly derived from the anti-bonding (AB) bands of the Sn2O(2) tetrahedral structure by hybridization between Sn 5s and O(2) 2p orbitals. Here, one may notice that the electronic structure of the original p-type TCO, that is, ternary copper oxide with delafossite structure CuAlO2 is also able to be decomposed to the contributions from the Cu2O conducting tetrahedral structure and the Al2O3 octahedral skeleton structure.2 The electronic structure around the VBM is derived from the hybridization with Cu 3d orbital for CuAlO2 delafossite, whereas from that with Sn 5s orbital for Sn2Nb2O7 pyrochlore.



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Figure 5. (a) PES spectra of Sn2Nb2O7 and SnO2 measured at a photon energy of 100 eV (lefthand side) and O K-edge NEXAFS (right-hand side), (b) calculated PDOS of O(1) atoms (red) and O(2) atoms (yellow) and the sum of them (O, black), and the expanded view near the VBM (inset), (c) Schematic view of the pyrochlore structure. The O K-edge NEXAFS spectrum is acquired in the partial electron yield mode. The binding energy of the O K-edge NEXAFS spectrum is rescaled to align the PES spectrum with the previously obtained SXES spectrum.30 The binding energy of Sn2Nb2O7 in the NEXAFS spectrum refers to the calculated Nb 4d DOS just above EF.13 Next, we turn our attention to the unoccupied electronic structure around the CBM. The spectral intensity of the O K-edge NEXAFS of SnO2 gradually increases from the Fermi energy to the binding energy of 3 eV above EF (red dotted line in Figure 5(a)). In contrast, the spectral intensity of Sn2Nb2O7 rapidly increases just above EF, and a sharp peak appears at EB=~-1 eV (red solid line). The difference of the slope near the CBM can be interpreted using the difference in the orbital components. Whereas the CBM of SnO2 is caused by the itinerant Sn 5s bands (or Sn 5s-O 2p AB bands),30,37 the CBM of Sn2Nb2O7 is caused by the dispersionless Nb 4d bands (or Nb 4dO(1) 2p AB bands) as shown in Figure 6. It should be noted that the O K-edge NEXAFS spectrum of Sn2Nb2O7 is qualitatively similar to the calculated unoccupied O 2p PDOS shown by the black line in Figure 5(b). The empirical unoccupied electronic structure around the CBM is consistent with the electronic image based on previous theoretical material design as mentioned in the indtroduction.10 We will discuss the disappearance of the sharp peak around the VBM in the PES spectra of Sn2Nb2O7. In Figure 5(b), the calculated oxygen PDOS (O, black) was decomposed to the contributions from O(1) in the Nb2O6 octahedral structure and O(2) adjacent to the Sn2+ atom in


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the Sn2O tetrahedral structure. As shown in the expanded view (inset), the sharp peak around the VBM predicted by the band calculation contains much contribution of O(2) compared with the contribution of O(1). This can be clearly seen from the density contour maps for the top of the valence band, or the VBM. The density contour map for the lattice plane at c=1/8, which contains only oxygens at O(1) and O(2) sites, is shown in the bottom left panel of Figure 7. It is clear that the contribution of O(2) to the VBM is dominant, whereas that of O(1) is faint. Here, we should notice that our band calculation was performed based on the assumption of an ideal and stoichiometric pyrochlore structure represented by the chemical formula of Sn2Nb2O7. However, it is found that the actual single crystal used here contains many point defects such as V •• , V Sn Sn

and

and that the composition has to be expressed as the non-stoichiometric composition .

Nb

.

Sn

.

O

.

, as shown previously. In order to identify the oxygen vacancy site,

EXAFS measurements of Sn and Nb K-edges were performed using the Sn2Nb2O7 samples with various conducting properties, which are derived by the amount of the point defects. The conducting properties and the composition amount of the samples used here, which were estimated by Hall, Seebeck, XRF and Mössbauer measurements, are summarized in Table 1. The oxygen amount of the polycrystal was calculated assuming the charge compensation. As shown in Nb Kedge EXAFS spectra (Figure 8 (b)), there was no essential difference in the local atomic structure around the Nb atoms, meaning that the Nb2O6 octahedral structure is stable or robust against the point defects. Since there is no O(2) oxygen atoms adjacent to the Nb atoms (the shortest Nb-O(2) distance, dNb-O(2), is 4.41 Å), the Nb K-edge EXAFS peak in the primary coordination sphere (red area in Figure 8 (b)) is derived from scattering with six O(1) oxygen atoms adjacent to Nb in the Nb2O(1)6 octahedral skeleton structure (dNb-O(1)= 2.10 Å). Since there is no noticeable difference, the observed oxygen vacancies should not be derived from the O(1) oxygens. As shown in the Sn



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K-edge EXAFS spectra, on the other hand, a drastic difference in the local atomic structure around the Sn atoms was shown depending on the conducting properties of the samples, meaning that the Sn2O tetrahedral structure is unstable or fragile against the point defects. There is two oxygen sites adjacent to the Sn2+; one is the O(2) oxygen site in the Sn2O tetrahedral structure (dSn-O(2)=2.30 Å, the nominal coordination number is two), and the other is the O(1) site in the Nb2O6 octahedral skeleton structure (dSn-O(1)=2.55 Å, the nominal coordination number is six). Therefore, the Sn Kedge EXAFS peak positon in the primary coordination sphere (red area in Figure 8 (a)) is estimated from the average distance of dSn-O(1) and dSn-O(2) weighted by each actual coordination number, and the peak intensity is in proportion to the average number of the actual oxygen coordination number at the O(1) and O(2)sites. Since there is no change in the local structure of oxygen atoms at the O(1) sites with respect to the conducting properties, as mentioned above, the noticeable difference in the Sn-O EXAFS spectral shape is able to be explained by the change in the local structure of O(2) oxygen atoms (see Section 6 of Supporting Information). This means that the observed oxygen vacancies are considered to be mainly derived from the O(2) oxygen atoms. This was also supported by our preliminary calculation of defect formation energy of the O(1) and O(2) vacancies (see Section 7 of Supporting Information). That is, it seems reasonable to suppose that V •• of 0.23 per the unit cell, which are estimated by subtracting the actual oxygen composition (6.77) from the nominal oxygen composition (7), are considered to be derived from V •• at the O(2) site (V •• ). This means that the sharp peak around the VBM, which contains a large O(2) contribution, is drastically affected by V •• .


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Figure 6. Schematic illustration of the electronic structure of ternary tin oxide with pyrochlore structure, Sn2Nb2O7. The electronic structure was decomposed to the Nb 4d and O(1) 2p orbital contributions in the Nb2O6 octahedral skeleton (left) and the Sn 5s and O(2) orbital contributions in the Sn2O tetrahedral structure (right).



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Figure 7. Two-dimensional density contour maps for the top of the valence band of Sn2Nb2O7 as a function of c position. Lattice planes at c=0 and 1/4 (top left) contain only cations, whereas one at c=1/8 contains only oxygens at O(1) and O(2) sites. Finally, we will consider the driving force of the p-type semiconducting properties in Sn2Nb2O7.14 As shown in Table 1, many point defects included in the samples represented nominally by Sn2Nb2O7 serve as carrier sources; V •• the V

and Sn

generates the n-type (electron) carriers, and

generate the p-type (hole) ones. Since the Sn 5s orbitals are directly bonded with

the O(2) oxygen orbitals (the left-hand panel of Figure 6), V ••

will easily cause V

in order to

maintain the charge neutrality of the system. On the other hand, there is no the O(2) oxygens atoms adjacent to Sn

(see Figure S6 in Supporting Information). The shortest distance between Sn4+

atoms and the O(2) oxygen atoms is estimated to be 4.41 Å, which is too large compared with the bond length from Sn4+ atom to the nearest O(1) oxygen atoms (2.10 Å). Therefore, it is considered that Sn

is insensitive to V ••

and serve as the hole carrier source for the p-type semiconducting

properties. Actually, we showed that the hole carriers were successfully generated in Sn2M2O7 (M=Nb and Ta) by controlling the formation of Sn

.14 As shown in Figure 6, the p-type

conduction pathway is derived from the hybridization of Sn2+ 5s orbital and O(2) 2p orbitals in the Sn2O tetragonal structure. Therefore, V ••

and the V

will be undesirable in order to function

the p-type carriers for the semiconducting properties effectively. Since the defect formation energy is different depending on the type of the point defects (V •• , V , Sn

), it is expected that the

semiconducting properties of Sn2Nb2O7 can be controlled by carefully varying the sample preparation conditions, such as annealing temperature and annealing atmosphere.14 When the sample is prepared at high temperature under the absence of oxygen such as N2 flow or vacuum


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atmosphere, V ••

and V

increase, whereas Sn

decreases, and the n-type semiconducting

characteristics is realized. In the case that the sample is annealed at relatively low temperature under an extremely low oxygen atmosphere, conversely, V ••

and V

decrease, the Sn

increases, and Sn2Nb2O7 shows p-type semiconducting characteristics.

polycrystal

single crystal

Carrier type

Formula

p-type

5+ 4+ 2Sn2+ 1.48 Nb1.74 Sn0.26 O6.36

0.52±0.16

0.26±0.03

0.64±0.06

Insulating

5+ 4+ 2Sn2+ 1.68 Nb1.84 Sn0.16 O6.60

0.32±0.18

0.16±0.02

0.40±0.10

n-type

5+ 4+ 2Sn2+ 1.39 Nb1.89 Sn0.11 O6.34

0.61±0.15

0.11±0.01

0.66±0.06

n-type

5+ 4+ 2Sn2+ 1.79 Nb1.92 Sn0.08 O6.77

0.21±0.20

0.08±0.01

0.23±0.13

V

Sn

V ••

Table 1. Actual composition and defect contents of Sn2Nb2O7 used in EXAFS measurements (Figure 8). The amount of V •• was estimated assuming that there is no V •• .

Figure 8. k3-weighted Fourier transforms obtained from (a) Sn K-edge EXAFS spectra and (b) Nb K-edge EXAFS spectra of polycrystal samples with p-type semiconducting properties (p-type, red), insulating properties (O.R., black), n-type semiconducting properties (n-type, blue) and single crystal with n-type semiconducting properties (S.C., green).



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4. CONCLUSIONS In order to elucidate the origin of a p-type oxide semiconductor of ternary tin oxide with pyrochlore structure, Sn2Nb2O7, synchrotron-radiation-excited PES and NEXAFS measurements were performed and were compared with the electronic structure by ab-initio calculations. To overcome the issue of the surface sensitivity of the PES and soft X-ray NEXAFS acquired in the partial electron yield mode, well-defined surfaces without contamination produced by fracturing the single crystal samples in the UHV atmosphere were used in the measurements. O 2p and Nb 4d contributions to the PES spectra of Sn2Nb2O7 were semi-quantitatively estimated by means of the photon energy dependence of the cross section32 and the Cooper minimum33. The sharp peak around the VBM predicted by the band calculation was not observed in the PES spectra. Since high mobility of hole carriers cannot be expected in the localized VBM, it is considered from the theoretical point of view that ternary tin oxide with pyrochlore structure, Sn2M2O7, is not suitable as the p-type semiconducting oxide. From the empirical point of view, however, the disappearance of the localized VBM suggests the possibility of improvement of the mobility of hole carriers, and is considered to be closely related to our success in the development of the p-type semiconducting oxide Sn2Nb2O7.14 The band calculations showed that the dispersionless bands around the VBM are composed mainly of the O(2) 2p orbitals. Combining with the experimental estimation of the elemental composition by chemical analysis, it is concluded that the disappearance of the localized VBM is caused by the off-stoichiometric effect due to the point defects such as V •• , V Sn

and

. The carrier source and the conduction pathway of p-type semiconducting Sn2Nb2O7 are

summarized in Figure 9. As shown in our study,14 the semiconducting properties of Sn2Nb2O7 is , but not to V . That is, the hole carriers generated by V

do not contribute to

the electric conduction effectively. This means that the hole carriers by V

are considered to be

sensitive to Sn


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compensated by the electron carriers generated by V ••

in order to maintain the charge neutrality,

since the Sn 5s orbitals are directly bonded with the O(2) oxygen orbitals in the same Sn2O tetrahedral structure. In other words, the p-type semiconducting Sn2Nb2O7 contains many (electron-doped) V •• Sn

due to the existence of the counter (hole-doped) defects such as V

and

. The p-type conducting properties of Sn2Nb2O7 are considered to be caused by doping the

hole carriers generated by Sn

in the Nb2O6 octahedral skeleton structure into the Sn2O

conduction pathway. Our results impose two constrains to realize p-type semiconductor of multinary oxides with pyrochlore structure, Sn2M2O7 (M=Nb, Ta): to increase Sn creating the hole carriers, and to reduce V ••

and V

in the Nb2O6 octahedral structure for in the Sn2O tetragonal structure for

protecting the p-type conduction pathway, which is closely related to the hole mobility. In addition to the issues on the doping mechanism of hole carriers and on the electronic structure near the VBM related closely with the hole mobility, it is critical to elucidate the doping limit to hole carrier to judge whether Sn2Nb2O7 is promising as a practical semiconducting material. Since the p-type doping limit has been suggested to be closely related to the energy position of the VBM against the energy criteria such as the vacuum level,38-40 it is strongly desired to estimate the upward shift of the VBM caused by the hybridization with Sn 5s orbitals. Moreover, in order to elucidate the limit of the p-type semiconducting properties of Sn2Nb2O7 quantitatively, further experimental and theoretical investigations on the microscopic relationship between the VBM and the actual crystal structure36,41,42 are also desired.



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Figure 9. Carrier source and conduction pathway of semiconducting Sn2Nb2O7.


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ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI: Wide-scan PES spectra of Sn2Nb2O7 and SnO2; 119Sn Mössbauer spectra of Sn2Nb2O7; O1s and Sn 3d core spectra of Sn2Nb2O7 and SnO2; Sn K-edge and Nb K-edge NEXAFS spectra of n-type Sn2Nb2O7 single crystal and p-type Sn2Nb2O7 polycrystal; Sn 3d core-level PES spectra of Sn2Nb2O7 and SnO2; Average Sn-O distance in Fourier transforms of Sn K-edge EXAFS spectra; Formation energy of O(1) and O(2) oxygen vacancies (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.A., A.S. and N.K. acknowledge Prof. Ko Mibu for fruitful discussion about the Mössbauer results. Figures 5, 6, 7, 9 and TOC image were drawn using the program VESTA.43 The Mössbauer measurements were conducted in Nagoya Institute of Technology, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The PES and NEXAFS measurements were performed



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under the approval of the Photon Factory Advisory Committee (Proposal Nos. 2015G666 and 2015P014).

REFERENCES (1) Wang, Z.; Nayak, P. K.; Caraveo-Frescas, J. A.; Alshareef, H. N. Recent Developments in p-Type Oxide Semiconductor Materials and Devices. Adv. Mater. 2016, 28, 3831-3892. (2) Kawazoe, H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H. P-type Electrical Conduction in Transparent Thin Films of CuAlO2. Nature 1997, 389, 939-942. (3) Hautier, G.; Miglio, A.; Ceder, G.; Rignanese, G.-M.; Gonze, X. Identification and Design Principles of Low Hole Effective Mass p-Type Transparent Conducting Oxides. Nat. Commun. 2013, 4, 2292. (4) Sarmadian, N.; Saniz, R.; Partoens, B.; Lamoen, D.; Volety, K.; Huyberechtsc G.; Paulc, High Throughput First-Principles Calculations of Bixbyite Oxides for TCO Applications. Phys. Chem. Chem. Phys. 2014, 16, 17724-17733. (5) Pan, X. Q.; Fu, L. Tin Oxide Thin Films Grown on the (1 012) Sapphire Substrate. J. Electroceram. 2001, 7, 35-46. (6) Ogo, Y.; Hiramatsu, H.; Nomura, K.; Yanagi, H.; Kamiya, T.; Hirano, M.; Hosono, H. pChannel Thin-Film Transistor using p-Type Oxide Semiconductor, SnO. Appl. Phys. Lett. 2008, 93, 032113.


Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Ogo, Y.; Hiramatsu, H.; Nomura, K.; Yanagi, H.; Kamiya, T.; Kimura, M.; Hirano, M.; Hosono, H. Tin Monoxide as an s-Orbital-Based p-Type Oxide Semiconductor: Electronic Structures and TFT Application. Phys. Status Solidi A 2009, 206, 2187–2191. (8) Togo, A.; Oba, F.; Tanaka, I.; Tatsumi, K. First-Principles Calculations of Native Defects in Tin Monoxide. Phys. Rev. B 2006, 74,195128. (9) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Room-Temperature Fabrication of Transparent Flexible Thin-Film Transistors using Amorphous Oxide Semiconductors. Nature 2004, 432, 488-492. (10) Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S.-H. Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals. Chem. Mater. 2009, 21, 547–551. (11) Hosogi, Y.; Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7 (M = Nb and Ta) and Their Photocatalytic Properties. Chem. Mater. 2008, 20, 1299-1307. (12) Katayama, S.; Hayashi, H.; Kumagai, Y.; Oba, F.; Tanaka, I. Electronic Structure and Defect Chemistry of Tin(II) Complex Oxide SnNb2O6. J. Phys. Chem. C 2016, 120, 9604-9611. (13) Velikokhatnyi, O. I.; Kumta, P. N. Ab-initio Study of Doped Tin Niobates and Tantalates as Prospective Catalyst Supports for Water Electrolysis. ECS Trans. 2010, 28, 37-48. (14) Kikuchi, N.; Samizo, A.; Ikeda, S.; Aiura, Y.; Mibu, K.; Nishio, K., submitted.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

(15) Toyoshima, A.; Kikuchi, T.; Tanaka, H.; Mase, K.; Amemiya, K.; Ozawa, K. Performance of PF BL-13A, a Vacuum Ultraviolet and Soft X-ray Undulator Beamline for Studying Organic Thin Films Adsorbed on Surfaces. J. Phys.: Conf. Ser. 2013, 425, 152019. (16) Ozawa, K.; Suzuki, M.; Tochikubo, R.; Kato, H.; Sugizaki, Y.; Edamoto, K.; Mase, K. Electron-Donor Dye Molecule on ZnO(101 0), (0001), and (0001 ) Studied by Photoelectron Spectroscopy and X‑ray Absorption Spectroscopy. J. Phys. Chem. C 2016, 120, 8653-8662. (17) Aiura, Y.; Hase, I.; Bando, H.; Yasue, T.; Saitoh, T.; Dessau, D. S. Photoemission Study of the Metallic State of Lightly Electron-Doped SrTiO3. Surf. Sci. 2002, 515, 61-74. (18) Nomura, M.; Koike, Y.; Sato, M.; Koyama, A.; Inada, Y.; Asakura, K. A New XAFS Beamline NW10A at the Photon Factory. AIP Conf. Proc. 2007, 882, 896-898. (19) Stewart, D. J.; Knop, O.; Meads, R. E.; Parker, W. G. Pyrochlores. IX. Partially Oxidized Sn2Nb2O7 and Sn2Ta2O7: A Mössbauer Study of Sn(II,IV) Compounds. Can. J. Chem. 1973, 51, 1041-1049. (20) Cruz, L. P.; Savariault, J.-M.; Rocha, J.; Jumas, J.-C.; Pedrosa de Jesus, J. D. Synthesis and Characterization of Tin Niobates. Solid State Chem. 2001, 156, 349-354. (21) Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties, Vienna University of Technology, Vienna, 2001. (22) Perdew, J. P.; Bruke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.


Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Bodiot, D. Comparison of Elements Niobium and Tantalum from Compounds Yielded by Solid State Reactions between their Hemipentoxides and Oxides SnO, CeO2, UO2, or M2O3 (M= Lanthanoid). Rev. Chim. Miner. 1968, 5, 569. (24) Inorganic Crystal Structure Database #27119. (25) Blöchl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouinzone Integrations. Phys. Rev. B 1994, 49, 16223-16233. (26) Moreno, M. S.; Egerton, R. F.; Midgley, P. A. Differentiation of Tin Oxides using Electron Energy-Loss Spectroscopy. Phys. Rev. B 2004, 69, 233304. (27) Hüfner, S. Photoelectron Spectroscopy; Springer-Verlag: Berlin Heidelberg, 1995. (28) Isomura, N.; Soejima, N.; Iwasaki, S.; Nomoto, T.; Murai, T.; Kimoto, Y. Depth-selective X-ray Absorption Spectroscopy by Detection of Energy-Loss Auger Electrons. Appl. Surf. Sci. 2015, 355, 268-271. (29) Aiura, Y.; Hase, I.; Bando, H.; Yagi-Watanabe, K.; Ozawa, K.; Iwase, T.; Nishihara, Y.; Shiino, O.; Oshima, M.; Kubota, M.; Ono, K. Ta 5d Band Symmetry of 1T-TaS1.2Se0.8 in the Commensurate Charge-Density-Wave Phase. Phys. Rev. Lett. 2003, 91, 256404. (30) Chang, G. S.; Forrest, J.; Kurmaev, E. Z.; Morozovska, A. N.; Glinchuk, M. D.; McLeod, J. A.; Moewes, A.; Surkova, T. P.; Nguyen Hoa, H. Oxygen-Vacancy-Induced Ferromagnetism in Undoped SnO2 Thin Films. Phys. Rev. B 2012, 85, 165319. (31) Kurganskii, S. I.; Manyakin, M. D.; Dubrovskii, O. I.; Chuvenkova, O. A.; Turishchev, S. Yu. Domashevskaya, E. P. Theoretical and Experimental Study of the Electronic Structure of Tin Dioxide. Phys. Solid State 2014, 56, 1748-1753.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

(32) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1 ⩽ Z ⩽ 103. At. Data Nucl. Data 1985, 32, 1-155. (33) El-Batanouny, M.; Strongin, M.; Williams, G. P. Site Determination for Palladium on Niobium using Angle-Resolved Photoemission. Phys. Rev. B 1983, 27, 4580-4585. (34) Reimann, K.; Steube, M. Experimental Determination of the Electronic Band Structure of SnO2. Solid State Commun. 1998, 105, 649-652. (35) Batzill, M.; Katsiev, K.; Burst, J. M.; Diebold, U.; Chaka, A. M.; Delley, B. Gas-phasedependent properties of SnO2 (110), (100), and (101) single-crystal surfaces: Structure, composition, and electronic properties, Phys. Rev. B 2005, 72, 165414. (36) Cruz, L. P.; Savariault, J.-M.; Rocha, J. Pyroclore Tin Niobate. Acta Crystallogr. C 2001, 57, 1001. (37) Robertson, J. Electronic structure of SnO2, GeO2, PbO2, TeO2 and MgF2. J. Phys. C: Solid State Phys. 1979, 12, 4767- 4776. (38) Zhang, S. B.; Wei S.-H; Zunger A. A phenomenological model for systematization and prediction of doping limits in II–VI and I–III–VI2 compounds. J. Appl. Phys. 1998, 83, 3192-3196. (39) Robertson, J.; Clark, S. J. Limits to doping in oxides. Phys. Rev. B 2011, 83, 075205. (40) Sarmadian, N.; Saniz, R.; Partoens, B.; Lamoen, D. Easily doped p-type, low hole effective mass, transparent oxides. Sci. Rep. 2016, 6, 20446. (41) Birchall, T.; Sleight, A. W. Nonstoichiometric Phases in the Sn-Nb-O and Sn-Ta-O Systems Having Pyrochlore-Related Structures. J. Solid State Chem. 1975, 13, 118-130.


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(42) Subramanian, M. A.; Aravamudan, G.; Rao, G. V. S. Oxide Pyrochlores – a Review. Prog. Solid State Chem. 1983, 15, 55-143. (43) Momma K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272-1276.



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TOC Graphic


Disappearance of Localized Valence Band ... - ACS Publications

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