Flux Crystal Growth and the Electronic Structure of ... - ACS Publications

Article pubs.acs.org/JPCC

Flux Crystal Growth and the Electronic Structure of BaFe12O19 Hexaferrite V. V. Atuchin,*,†,‡,§ D. A. Vinnik,∥ T. A. Gavrilova,⊥ S. A. Gudkova,∥,# L. I. Isaenko,§,∇ Xingxing Jiang,○,◆ L. D. Pokrovsky,† I. P. Prosvirin,¶ L. S. Mashkovtseva,∥ and Zheshuai Lin*,○ †

Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia Functional Electronics Laboratory, Tomsk State University, Tomsk 634050, Russia § Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, Novosibirsk 630090, Russia ∥ South Ural State University, 76 Lenin Aven, Chelyabinsk 454080, Russia ⊥ Laboratory of Nanodiagnostics and Nanolithography, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia # Moscow Institute of Physics and Technology (State University), 9 Institutskiy Per., Dolgoprudny 141700, Russia ∇ Laboratory of Crystal Growth, Institute of Geology and Mineralogy, SB RAS, Koptyug Aven, Novosibirsk 630090, Russia ○ BCCRD, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ◆ University of Chinese Academy of Sciences, Beijing 100049, China ¶ Boreskov Institute of Catalysis, SB RAS, Novosibirsk 630090, Russia

Downloaded via UNIV OF SUNDERLAND on October 7, 2018 at 14:01:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The barium hexaferrite, BaFe12O19, microcrystals were obtained by the flux crystal growth method and were characterized by XRD, SEM, and TEM methods. XPS measurements were carried out on a powder sample. The binding energy differences between the O 1s and cation core levels, ΔBa = BE(O 1s) − BE(Ba 3d5/2) and ΔFe = BE(O 1s) − BE(Fe 2p3/2), were used to characterize the valence electron transfer on the formation of Ba−O and Fe−O bonds. The chemical bonding effects were considered on the basis of our XPS results measured for BaFe12O19 and the earlier published structural and XPS data for other Ba- or Fe-containing oxide compounds. The band structure of BaFe12O19 was calculated by spin-polarized DFT methods and compared to the valence band spectrum measured by the XPS method.

1. INTRODUCTION Barium hexaferrite, BaFe12O19, with a magnetoplumbite crystal structure has been actively studied for several decades. This compound is a representative member of the MFe12O19 ferrite family which is used in radio technology, microelectronics, and various energy and engineering applications due to their pronounced magnetic properties, chemical stability, and relatively high Curie temperature.1−4 Current applications of this magnetic material cover different nanocomposites, a wide range of recording and storing components, as well as inductors and microwave communication devices.3−10 The properties of BaFe12O19-based industrial products are influenced by the real defect structure, particle morphology and doping level, and, respectively, the ferromagnetic, chemical, and electrical parameters are strongly dependent on the synthesis conditions.11−19 The complex magnetoplumbite-type structure of BaFe12O19 is depicted in Figure 1.20,21 From the viewpoint of crystal chemistry, in this ferrite, the iron ions are coordinated tetrahedrally (FeO4), trigonal-bipyramidally (FeO5) and © 2016 American Chemical Society

octahedrally (FeO6) by oxygen ions. Thus, this structure possesses a unique possibility to study iron in several coordinations in the same crystal. To estimate the potentials mentioned above, it is important to have a thorough understanding of the electronic structure and chemical bonding effects in BaFe12O19. For the applications, such as electronic devices and catalysts, it is also necessary to determine the surface electronic parameters that control the surface layer chemical properties. One of the most widely used methods for the characterization of surface electronic parameters is X-ray photoelectron spectroscopy (XPS), which can be used for quantitative determination of the electron charge transfer induced by the formation of chemical bonds. Thus, the present investigations are aimed at the synthesis of BaFe12O19 crystals, determination of the microstructural and electronic properties, and comparison of the Received: December 14, 2015 Revised: February 2, 2016 Published: February 3, 2016 5114

DOI: 10.1021/acs.jpcc.5b12243 J. Phys. Chem. C 2016, 120, 5114−5123

Article

The Journal of Physical Chemistry C

samples were thoroughly powdered and attached on a single crystalline silicon holder. The crystal micromorphology was observed by SEM using a LEO 1430 device. The sample preparation and measurement conditions can be found elsewhere.25,26 The selected area electron diffraction was carried out on a BS 513A (Czech Republic) microscope operated at 100 kV. Thin crystal fragments were prepared by gentle dry grinding of the BaFe12O19 crystals and supported on a tungsten grid covered with a holey carbon film. The X-ray photoelectron spectra were recorded on a SPECS (Germany) photoelectron spectrometer using a hemispherical PHOIBOS-150-MCD-9 analyzer and FOCUS-500 (Al Kα radiation, hν = 1486.74 eV, 200 W) monochromator. The binding energy (BE) scale was precalibrated using the positions of the peaks of Au 4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.67 eV) core levels. The powder sample preparation and energy scale calibration methods can be found elsewhere.27,28 A base pressure of a sublimation ion-pumped chamber of the system was less than 6 × 10−10 mbar during the present experiments. The powder samples were loaded onto a conducting double-sided copper scotch. In addition to the survey photoelectron spectra, more narrow spectral regions Fe 3p, Ba 4d, C 1s, O 1s, Fe 2p, Ba 3d and the valence band were recorded. The survey spectra were taken at the analyzer pass energy of 50 eV, and the detailed spectra were registered at 20 eV. The concentration ratios of the elements on the sample surface were calculated from the integral photoelectron peak intensities which were corrected with the theoretical sensitivity factors based on Scofield photoionization cross sections.29 For the peak fitting procedure a mixture of Lorenzian and Gaussian functions was used together with the Shirley background subtraction method. To remove the surface contaminations appeared due to interaction with air, a bombardment of the BaFe12O19 sample surface was carried out using an argon ion gun (SPECS model IQE 11/35). The energy of Ar+ ions, current density and angle of sputter erosion were 1.05 keV, 4−6 mA/cm2 and 45°, respectively.

Figure 1. Crystal structure of BaFe12O19 hexaferrite. The unit cell is outlined. Lone iron and oxygen atoms are omitted for clarity.

Ba−O and Fe−O bonding in BaFe12O19 with other Ba- and Fe-bearing oxides. The barium hexaferrite microcrystals are obtained by the flux crystal growth method, and the electronic structure is studied complementarily by X-ray photoelectron spectroscopy and first-principles calculation.

2. EXPERIMENTAL METHODS Iron oxide (Fe2O3) and barium and sodium carbonates (BaCO3 and Na2CO3) with purity 99.9% (Ural Plant of Chemicals, Russia) were used for crystal synthesis. BaFe12O19 to the flux ratio was chosen according to the literature data.22 Initially, the starting components were treated at 600 °C for 5 h to remove water residuals captured from the air. After that, the components were weighed. The optimal batch composition for pure barium hexaferrite growth is Na2CO3, 26.3 at. %/BaCO3, 10.53 at. %/Fe2O3, 63.17 at. %, as was determined by the previous experiments.12,23 The mixture was gently ground in an agate mortar. Then, it was filled in a 30 mL platinum crucible and placed in a resistive furnace.24 The furnace had a precise temperature regulator. The internal construction allowed the axial temperature gradient tuning to control the crystal growth condition. The furnace was heated from room temperature to 600 °C with the rate of 300 °C/h and from 600 to 1260 °C with the rate of 100 °C/h. The melted mixture was homogenized in an open crucible at 1260 °C for 4 h in the air. After that, the slow cooling to 900 °C with the rate of 4.5 °C/h was carried out. At this stage, the precipitation of the BaFe12O19 crystals appeared from the solution due to the solubility decrease, as induced by the temperature lowering. At 900 °C, the furnace was turned off. The synthesized crystals were separated from the crystallized solution by leaching in hot nitric acid, and, then, they were cleaned with distilled water several times. The obtained powder samples were examined using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). X-ray powder diffraction analyses were performed on a Rigaku Ultima IV diffractometer in the angular range from 10° to 80° with the rate of 0.5°/min using Cu Kα radiation. For this purpose the

3. COMPUTATION METHODS The spin-polarized first-principles electronic structure calculation of BaFe12O19 was performed by the plane-wave pseudopotential method implemented by CASTEP30 based on the density functional theory (DFT).31,32 The func-

Figure 2. Powder XRD pattern of the BaFe12O19 sample. 5115


Article


Figure 3. (a) Most typical SEM pattern of the BaFe12O19 microcrystals and (b) individual plate-like crystal.

4. RESULTS AND DISCUSSION

tionals developed by Ceperley, Alder, Perdew, and Zunger (CA-PZ)33,34 in the local density approximation (LDA) form were adopted to describe the exchange-correlation energy. The effective interaction between the atomic cores and valence electrons were modeled by optimized norm-conserving pseudopotentials35 (with Ba 5s25p66s2, Fe 3d64s2, and O 1s22s22p4 treated as valence electrons), which allow the employment of a relative plane-wave basis set without compromising the computational accuracy. The kinetic energy cutoff of 500 eV and intensive Monkhorst−Pack36 k-point meshes spanning less than 0.04/Å3 were chosen. Moreover, to account for the effect of localized d-orbitals in transition elements, the LDA + U37 method with the on-site orbital dependent Hubbard U38 energy term Ud = 6 eV for the Fe 3d orbital was employed for the electronic structure calculations. The convergence tests revealed that the above computational parameters are accurate enough for the purpose of this study.

The final product of high temperature flux synthesis was black and contained glistening grains. The phase purity of the product was verified by XRD analysis, and the recorded pattern is shown in Figure 2. All the peaks were attributed to the BaFe12O19 phase.20 The diffraction peaks are sharp, and, hence, the BaFe12O19 oxide crystallinity is very good. The micromorphology of the particles without grinding is observed by SEM, as shown in Figure 3. Irregular particles with characteristic dimensions 1−3 μm and without any faceting are evident in the typical image shown in Figure 3a. However, in other SEM patterns, the plate-like crystals ∼3−4 μm thick were occasionally detected, and the example of the image can be observed in Figure 3b. Thus, it can be concluded that the shape/size parameters of the grown microcrystals are inhomogeneous. The particles possess a noticeable charging effect 5116


Article

The Journal of Physical Chemistry C during measurements that indicates its low conductivity. However, this effect, at least partly, may be attributed to bad electrical contacts between the particles with very irregular shapes. The initial TEM observation of as-grown BaFe12O19 microcrystals indicates that these are not transparent for the electron beam. The typical TEM pattern observed from the sample after grinding is shown in Figure S1. Evidently, this is a superposition of several diffraction patterns from the crystalline grains fall into the electron beam. The pattern is completely indexed in reference to the BaFe12O19 structure (PDF 43-0002, space group P63/mmc, a = 5.892 Å, and c = 23.18 Å), and the related d-spacing values can be found in Table S1. The SAED pattern occasionally observed from the individual BaFe12O19 grain is shown in Figure 4. Figure 5. Detailed spectra of the O 1s core level.

Figure 4. SAED pattern of BaFe12O19.

The survey photoelectron spectrum is shown in Figure S1. All spectral features, except one, were attributed to constituent element core levels and Auger lines. A line at 284.8 eV can be attributed to the C 1s signal related, at least partly, to adventitious hydrocarbons adsorbed on the surface of BaFe12O19 particles from the laboratory air. The detailed spectrum of C 1s core level is shown in Figure S2. Besides the main component with the maximum at 284.8 eV, the presence of a wide weakintensity band over 287−289 eV is observed. As can be seen from the comparison of the spectra before and after ion bombardment, the C 1s signal cannot be significantly decreased even by ion sputtering because of a very developed surface of the BaFe12O19 particles. The constituent element core levels are shown in Figures 4−6 and Figures S3 and S4. As to the initial surface state, the binding energy (BE) values of the representative levels Fe 2p3/2 and Ba 3d5/2 fall in the ranges typically observed in iron- and barium-containing oxides.39−44 As is evident from the observation of the spectra recorded from the initial and bombarded surfaces, the spectra of Fe 2p doublet and Fe 3p band remain nearly the same. Comparatively, after the bombardment, the maxima of the Ba 4d and Ba 3d doublets (Figure 7) shift noticeably to higher binding energies. In the spectrum of the O 1s core level, a pronounced shoulder is observed at the higher BE side, and this component at ∼531.8 eV indicates surface hydration during the chemical treatment aimed at the BaFe12O19 microcrystal separation from the flux. However, the hydrate formation due to surface chemical interaction with the air agents cannot be excluded. As a result of the ion bombardment, the main peak of the O 1s band becomes narrower, and the intensity of the higher-BE shoulder

Figure 6. Detailed spectra of the Fe 2p doublet recorded from initial (red) and bombarded (blue) sample.

Figure 7. Detailed spectra of the Ba 3d doublet recorded from initial (red) and bombarded (blue) sample.

at ∼531.8 eV decreases. This verifies partial top-surface species removement by the ion bombardment. The complete surface cleaning, however, is evidently prohibited for the BaFe12O19 powder sample with a somewhat irregular particle surface. 5117


Article


surface charging effects.45−48 The BE values of the constituent element core levels in BaCO3 and Ba(OH)2 were carried out in the literature.40,45,49 For BaCO3, the ΔBa = ΔBE(O 1s − Ba 3d5/2) values were reported in the range −(248.4−248.7) eV.40,45,49 Thus, for the more contaminated initial BaFe12O19 surface, in reference to the BE(Ba 3d5/2) = 779.3 eV value, the O 1s component related to barium carbonate may appear at 530.6− 530.9 eV. For Ba(OH)2, ΔBa = ΔBE(O 1s − Ba 3d5/2) = −(248.9−249.0) eV,40,45 and the related O 1s component may appear at 530.3−530.4 eV. The relations mean that the main O 1s peak at 529.6 should be attributed to the BaFe12O19 compound. Besides, the ΔBE(Ba 3d5/2 − C 1s) = 490.2−490.3 eV can be calculated for BaCO340,45 and, in reference to the BE(Ba 3d5/2) = 779.3 eV value, in the BaFe12O19 spectrum, the C 1s component related to carbonate formation should appear at 289.0−289.1 eV. Indeed, the estimated range is in good relation to the wide C 1s band over 287−289 eV, as shown in Figure S2. Thus, the constituent core levels, including the main O 1s component, are reasonably attributed to BaFe12O19 hexaferrite. It is well-known that core level BE values can be used to obtain information about the chemical bonding in materials.50 However, due to the uncertainties associated with absolute binding energy values (caused by different experimental conditions, e.g., charge, ref 51), chemical bonding effects in a compound can be better described by the BE difference, which is a more robust parameter as it is independent from absolute BE values. For example, ΔSr = BE(O 1s) − BE(Sr 3d5/2) was used successfully for the characterization of Sr−O bonding in oxides.52,53 This quantitative parameter is independent from surface charging effects, and a correlation between Δ(O−Sr) and the mean chemical bond length L(Sr−O) was found for a set of Sr-containing crystals.52,53 In the present study, it is

The chemical composition was estimated by representative element peak areas, only the main component at 529.6 eV is accounted for the O 1s core level, and tabulated atomic sensitivity factors. The calculations were performed without accounting the carbon signal. The ion bombardment effect on the surface chemical composition is insignificant and, respectively, only the results obtained for the initial surface are shown here. The relative element ratio calculated for the powder sample is Ba:Fe:O = 0.06:0.30:0.64, that is in the reasonable consistency with nominal composition Ba:Fe:O = 0.03:0.38:0.59. Nevertheless, the sample surface is noticeably enriched by barium. The sets of the element core levels measured for the initial and ion bombarded BaFe12O19 surfaces are shown in Table 1. Table 1. Binding Energies (eV) of the Constituent Element Core Levels in BaFe12O19 core level

initial surface

bombarded surface

Fe 3p Ba 4d3/2 Ba 4d3/2 C 1s O 1s Fe 2p3/2 Fe 2p1/2 Ba 3d5/2 Ba 3d3/2

55.5 88.5 91.0 284.8 529.6, 531.8 710.4 724.1 779.3 794.5

55.5 88.9 91.5 284.8 529.6, 531.8 710.4 724.1 779.6 794.9

Only barium lines shifted in energy after the ion bombardment, and it is reasonable to suppose that the surface barium excess can be attributed to carbonate or hydrate formation. For this purpose, the BE difference (ΔBE) parameters are suitable because the parameters are insensitive to the BE scale calibration and

Table 2. Core Level and Structural Parameters of Ba-Containing Oxide Crystals Ba 3d5/2, eV

O 1s, eV

ΔBa, eV

ref

L(Ba−O), pm

ref

BaO

779.3

530.3

69

779.9 779.3

529.1 530.8

280.7

70

Ba(OH)2

779.4 779.2

531.0 530.3

289.4

71

780.2 780.0 789.4 778.9 780.4 780.0 780.6 780.0 784.0 778.9 781.5 779.4 779.0 779.2 778.9 779.65 779.3

532.8 531.7 539.9 529.5 531.2 530.5 532.0 530.1 534.5 528.9 531.7 529.7 528.7 529.1 528.9

291.0 295.2 298.5 283.3 301.55 280.15

72 73 74 75 76 77

297.3 274.65 292.3 290.5

78 79 80 81

289.3

82

302.1 290.9

83 20

529.6

−249.7

40 44 54a 40 45 49 40 45 55 56 57 58 59 41 60 61 62 63 64 65 66 67 63 68 present study

277.0

BaCO3

−249.0 −250.9 −250.8 −248.5 −248.7 −248.4 −248.9 −249.0 −247.4 −248.4 −249.5 −249.2 −249.2 −249.5 −248.6 −249.9 −249.5 −250.0 −249.8 −249.7 −250.3 −250.1 −250.0

crystal

Ba(NO3)2 BaSO4 Ba2NaNb5O15 BaTiO3 BaBe2Si2O7 β-BaB2O4 BaTi4O9 BaMoO4 Ba2PbO4 BaSnO3 BaBiO3 BaPbO3 BaFe12O19 a

Private communication from Dr. G. Lupina. 5118


Article


XPS measurements were carried out for powder samples. As to other Ba oxides, a decrease of ΔBa on the L(Ba−O) increase is observed from Figure 8, and this trend is very similar to that found earlier for Sr-containing oxides.49,50 Thus, it can be reasonably supposed that this trend is general for alkaline-earth elements. The prediction, however, should be tested for Ca-containing oxides. In the case of BaFe12O19, Ba is coordinated by 12 oxygens with L(Ba−O) = 290.9 pm, and the value is intermediate among the Ba oxides depicted in Figure 8. As to Ba−O bond ionicity, lower ΔBa value indicates higher bond ionicity. Respectively, lower Ba−O bond ionicity is characteristic for BaO and BaMoO4 crystals. In BaFe12O19, the averaged ionicity of Ba−O bonds is intermediate. The collection of ΔFe and L(Fe−O) data for Fe-containing oxide crystals is presented in Table 3, and the dependence of ΔFe on L(Fe−O) is depicted in Figure 9. There are several

topical to consider the relations for Ba−O and Fe−O bonding using ΔBa = BE(O 1s) − BE(Ba 3d5/2) and ΔFe = BE(O 1s) − BE(Fe 2p3/2) parameters determined by XPS measurements. To obtain the dependence of ΔBa on the averaged bond length L(Ba−O), all available published results on the measured electronic parameters of Ba-bearing oxide crystals with XPS and the related structural data were accumulated and compared. The data collection is presented in Table 2 for 16 Ba-bearing oxide compounds. It should be mentioned that, for BaO, BaCO3, Ba(OH)2, β-BaB2O4, BaSnO3, and BaBiO3, the binding energies of element core levels were measured in several other studies, and different values were reported for the same compounds. Such scatter from different investigators of the same compound is not uncommon, due to differences in sample preparation and spectrometer calibration. The range of mean Ba−O bond lengths listed in Table 2 is similar to that of Sr-containing oxides, and it is much wider than those previously found for Nb5+−O46 and Ti4+−O,47 due to the variety of coordination number (6−14) existing for Ba2+ ion in oxides. It is well-known that the Ba−O bond length increases with coordination number increase.84 In Figure 8, four crystals,

Figure 9. Dependence of ΔFe on L(Fe−O) in oxide crystals.

specific cases. In SrFeO3, iron is in formal valence state Fe4+, and in this oxide crystal, the L(Fe−O) is comparatively short. In FeCr2O4 and Li2FeSiO4, iron is in formal valence state Fe2+, and these oxides evidently possess a higher ΔFe level. In other crystals, the Fe3+ valence state is common, and the compounds form a friable cluster without an evident trend between ΔFe and L(Fe−O). This situation may be controlled by different coordinations possible for Fe3+ ions in oxides. For example,

Figure 8. Dependence of ΔBa on L(Ba−O) in oxide crystals.

including BaCO3, Ba(OH)2, Ba(NO3)2, and BaSO4, possess very high ΔBa values. These oxides, however, are very hygroscopic, and BE(O 1s) could be overestimated when

Table 3. Core Level and Structural Parameters of Fe-Containing Oxide Crystals crystal Fe2O3 YFeO3 LaFeO3 SmFeO3 FeNbO4 BiFeO3 FeBO3 NiFe2O4 LiFeTiO4 SrFe4+O3 Fe2+Cr2O4 Li2Fe2+SiO4 BaFe12O19

Fe 2p3/2, eV

O 1s, eV

ΔFe, eV

ref

L(Fe−O), pm

ref

711.2 711.2 709.7 709.4 710.4 711.2 711.3 711.5 711.5 710.8 710.7 709.3 709.0 709.5 710.4

530.2 529.5 529.0 528.7 528.9 530.2 529.5 530.2 531.1 529.8 529.7 528.1 530.2 530.5 529.6

−181.0 −181.7 −180.7 −180.7 −181.5 −181.0 −181.8 −181.3 −180.4 −181.0 −181.0 −181.2 −178.8 −179.0 −180.8

85 86 85 85 87 88 86 89 90 91 92 85 91 93 present study

202.95

94

201.2 200.7 201.4 201.4 203.2

95 96 97 98 99

202.8 197.9 198.05 192.5 197.4

100 101 102 103 104

200.25

20

5119


Article

The Journal of Physical Chemistry C in BaFe12O19, Fe3+ ions can be found in three different coordinations. In Table 3, the L(Fe−O) values were calculated by simple averaging, and the coordination factor was not specified because of a limited number of compounds. Thus, on further experimental data accumulation, the subclusters specific for the selected Fe3+ ion coordination type may be specified similarly to those previously found in tungstates and molybdates.48,105,106 The calculated electronic partial densities of states (PDOS) projected on the constitutive element in BaFe12O19 are displayed in Figure 10. Because only the outer-shell valence electrons

(ii) The O 2s orbitals are also located at the inner energy level around −17 eV, which is relatively difficult to excite. (iii) The electron energy states near the top of the valence bands are mainly composed of Fe 3p 3d and O 2p orbitals, and the strong hybridization in the wide energy range between these orbitals indicates the stronger covalent interaction between iron and oxygen atoms than that between barium and oxygen atoms in BaFe12O19. To our best knowledge, it is for the first time that the spin-sensitive electronic structure is calculated and analyzed for BaFe12O19. The confirmation of calculated valence band structure by XPS measurements is principally scarce in literature. Thus, the theoretical model could be used for physical properties analysis of BaFe12O19 and other materials from the MFe12O19 ferrite family.

5. CONCLUSIONS In the present study, the BaFe12O19 microcrystals were grown and evaluated by conventional experimental techniques. The electronic structure of BaFe12O19 was carried out by XPS, and the obtained core level parameters open up a possibility for the ionicity analysis for Ba−O and Fe−O bonds. It is shown that the averaged ionicity of Ba−O bonds increases with the mean chemical bond L(Ba−O) increase in oxide crystals. Thus, it is clear that the behavior of Ba−O and the earlier observed Sr−O bonds is similar in oxides, and now, the evaluation of Ca−O bonding by XPS is topical. The behavior of Fe−O bonds is less clear because different coordination of the Fe3+ ions is possible in the Fe3+ ferrites. The present XPS study is a first study where the chemical bonding effects are analyzed for Ba- and Febearing oxide crystals using the binding energy difference parameters. This method is based on the element core level parameters being shifted by valence electron transfer, and the core level parameters can be accurately measured by XPS. Thus, this paves a path to quantitative description of chemical bond ionicity from experimental XPS measurements. Moreover, the calculated electronic partial density of states indicates the absence of obvious spin-splitting in Ba and O atoms. However, the Fe 3d orbitals split strongly in different spin states: the spin-up electrons mostly occupy the valence bands between −15 and −5 eV, while the spin-down electronic states are mostly located at the bottom of the conduction bands and determine the energy bandgap. As other crystals in the MFe12O19 family are in preparation, we will focus on the electronic structure measurements and calculations for these materials in the near future.

Figure 10. Comparison of the experimental XPS spectrum and ab initio electronic structures.

were taken into account in the first-principles calculation, only these electrons are exhibited in the electronic structure. We can see that the DOS and PDOS are in reasonable agreement with the measured XPS spectrum, as the whole experimental spectrum is shifted to the lower binding energy by 3.0 eV. This confirms the validity of the plane-wave pseudopotential method for BaFe12O19. Due to the thermal effect in the measurement condition, the peaks in the measured XPS spectrum are broadened. In the calculated PDOS, no obvious spin-splitting occurs in Ba and O atoms. However, the Fe 3d orbitals split seriously in different spin states: the spin-up electrons mostly occupy the valence bands between −15 to −5 eV, while the spin-down electronic states are mostly located at the bottom of conduction bands and determine the energy bandgap. This implies that BaFe12O19 might possess the strong magnetism, as originated from the existence of iron. Moreover, some bond characteristics can also be deduced from the PDOS: (i) The Ba 5s and 5p orbitals are strongly localized around −25 eV and −10 eV and hybridize little with the orbitals of other elements, manifesting the strong ionicity of barium.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12243.

■

Tables and figures, including d-spacing values, TEM images, and XPS spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5120


Article


■

Co substituted barium hexaferrite nanoparticles. Appl. Phys. A: Mater. Sci. Process. 2015, 119 (4), 1531−1540. (18) Vinnik, D. A.; Zherebtsov, D. A.; Mashkovtseva, L. S.; Nemrava, S.; Yakushechkina, A. K.; Semisalova, A. S.; Gudkova, S. A.; Anikeev, A. N.; Perov, N. S.; Isaenko, L. I.; et al. Tungsten substituted BaFe12O19 single crystal growth and characterization. Mater. Chem. Phys. 2015, 155, 99−103. (19) Shams, M. H.; Rozatian, A. S. H.; Yousefi, M. H.; Valíček, J.; Šepelák, V. Effect of Mg2+ and Ti4+ dopants on the structural, magnetic and high-frequency ferromagnetic properties of barium hexaferrite. J. Magn. Magn. Mater. 2016, 399, 10−16. (20) Obradors, X.; Collomb, A.; Pernet, M.; Samaras, D.; Joubert, J. C. X-ray analysis of the structural and dynamic properties of BaFe12O19 hexagonal ferrite at room temparature. J. Solid State Chem. 1985, 56, 171−181. (21) Ozawa, T. C.; Kang, S. J. Balls&Sticks: easy-to-use structure visualization and animation program. J. Appl. Crystallogr. 2004, 37, 679. (22) Gambino, R. J.; Leonhard, F. Growth of barium ferrite single crystals. J. Am. Ceram. Soc. 1961, 44 (5), 221−224. (23) Vinnik, D. A.; Zherebtsov, D. A.; Mashkovtseva, L. S.; Nemrava, S.; Semisalova, A. S.; Galimov, D. M.; Gudkova, S. A.; Chumanov, I. V.; Isaenko, L. I.; Niewa, R. Growth, structural and magnetic characterization of Co- and Ni-substituted barium hexaferrite single crystals. J. Alloys Compd. 2015, 628, 480−484. (24) Vinnik, D. A. Resistive furnace for single crystal growth. Butlerov Commun. 2014, 39 (9), 153−154 (in Russian). (25) Atuchin, V. V.; Gavrilova, T. A.; Gromilov, S. A.; Kostrovsky, V. G.; Pokrovsky, L. D.; Troitskaia, I. B.; Vemuri, R. S.; Carbajal-Franco, G.; Ramana, C. V. Low-temperature chemical synthesis and microstructure analysis of GeO2 crystals with α-quartz structure. Cryst. Growth Des. 2009, 9 (4), 1829−1832. (26) Atuchin, V. V.; Grossman, V. G.; Adichtchev, S. V.; Surovtsev, N. V.; Gavrilova, T. A.; Bazarov, B. G. Structural and vibrational properties of microcrystalline TlM(MoO4)2 (M = Nd, Pr) molybdates. Opt. Mater. 2012, 34, 812−816. (27) Rubio, E. J.; Atuchin, V. V.; Kruchinin, V. N.; Pokrovsky, L. D.; Prosvirin, I. P.; Ramana, C. V. Electronic structure and optical quality of nanocrystalline Y2O3 film surfaces and interfaces on silicon. J. Phys. Chem. C 2014, 118, 13644−13651. (28) Atuchin, V. V.; Kaichev, V. V.; Korolkov, I. V.; Saraev, A. A.; Troitskaia, I. B.; Perevalov, T. V.; Gritsenko, V. A. Electronic structure of noncentrosymmetric α-GeO2 with oxygen vacancy: ab initio calculations and comparison with experiment. J. Phys. Chem. C 2014, 118, 3644−3650. (29) Scofield, J. H. Hartree-Slater subshell photoionization crosssections at 1254 and 1487 eV. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129−137. (30) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First principles methods using CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220 (5−6), 567−570. (31) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Iterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradients. Rev. Mod. Phys. 1992, 64 (4), 1045−1097. (32) Kohn, W.; Sham, L. J. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140 (4A), A1133− A1138. (33) Ceperley, D. M.; Alder, B. J. Ground state of the electron gas by stochastic method. Phys. Rev. Lett. 1980, 45 (7), 566−569. (34) Perdew, J. P.; Zunger, A. Self-interaction correction to density functional approximations for many electron systems. Phys. Rev. B: Condens. Matter Mater. Phys. 1981, 23 (10), 5048−5079. (35) Rappe, A. M.; Rabe, K. M.; Kaxiras, E.; Joannopoulos, J. D. Optimized pseudopotentials. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41 (2), 1227−1230. (36) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin zone integrations. Phys. Rev. B 1976, 13 (12), 5188−5192.

ACKNOWLEDGMENTS The studies were partly performed using the instrumental equipment from CCU “Nanostructures”. The work was supported in part by the Russian Science Foundation (project no. 14-22-00143) and China “863” project (Grant 2015AA034203). Additionally, this work was partially performed using the equipment from the MIPT Centers of Collective Usage and with financial support from the Ministry of Education and Science of the Russian Federation (Grant No. RFMEFI59414X0009). V.V.A. and L.I.I. were partially supported by the Ministry of Education and Science of the Russian Federation.

■

REFERENCES

(1) Harris, V. G.; Geiler, A.; Chen, Y.; Yoon, S. D.; Wu, M. Z.; Yang, A.; Chen, Z. H.; He, P.; Parimi, P. V.; Zuo, X.; et al. Recent advances in processing and applications of microwave ferrites. J. Magn. Magn. Mater. 2009, 321, 2035−2047. (2) Ö zgür, Ü .; Alivov, Y.; Morkoç, H. Microwave ferrites, part 1: fundamental properties. J. Mater. Sci.: Mater. Electron. 2009, 20, 789− 834. (3) Harris, V. G. Modern microwave ferrites. IEEE Trans. Magn. 2012, 48 (3), 1075−1104. (4) Pullar, R. C. Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics. Prog. Mater. Sci. 2012, 57, 1191−1334. (5) Lemke, M.; Hoppe, W.; Tolksdorf, W.; Welz, F. Magnetically tunable millimetre-wave filter with suingle-crystal barrium ferrite. IEE J. Microwaves, Opt. Acoust. 1979, 3 (6), 253−254. (6) Fu, L.; Liu, X. G.; Zhang, Y.; Dravid, V. P.; Mirkin, C. A. Nanopartening of “hard” magnetic nanostructures via dip-pen nanolithography and a sol-based ink. Nano Lett. 2003, 3 (6), 757−760. (7) Chen, Z. H.; Yang, A.; Gieler, A.; Harris, V. G.; Vittoria, C.; Ohodnicki, P. R.; Goh, K. Y.; McHenry, M. E.; Cai, Z. H.; Goodrich, T. L.; et al. Epitaxial growth of M-type Ba-hexaferrite films on MgO (111)||SiC (0001) with low ferromagnetic resonance linewidths. Appl. Phys. Lett. 2007, 91, 182505. (8) Ö zgür, Ü .; Alivov, Y.; Morkoç, H. Microwave ferrites, part 2: passive components and electrical tuning. J. Mater. Sci.: Mater. Electron. 2009, 20, 911−952. (9) Ting, T.-H.; Wu, K.-H. Synthesis, characterization of polyaniline/ BaFe12O19 composites with microwave-absorbing properties. J. Magn. Magn. Mater. 2010, 322, 2160−2166. (10) Primc, D.; Makovec, D. Composite nanoplatelets combining soft-magnetic iron oxide and hard-magnetic barium hexaferrite. Nanoscale 2015, 7, 2688−2697. (11) Kikkawa, S.; Aiba, K.; Masubuchi, Y.; Saeki, I. Iron source effect on BaFe12O19 preparation through citrate route. J. Ceram. Soc. Jpn. 2009, 117, 15−17. (12) Vinnik, D. A.; Zherebtsov, D. A.; Mashkovtseva, L. S. Growing doped barium ferrite single crystals using the flux method. Dokl. Phys. Chem. 2013, 449 (1), 39−40. (13) Fisher, J. G.; Vu, H.; Farooq, M. U. J. Magmetics 2014, 19, 333− 339. (14) Vinnik, D. A.; Zherebtsov, D. A.; Mashkovtseva, L. S.; Nemrava, S.; Perov, N. S.; Semisalova, A. S.; Krivtsov, I. V.; Isaenko, L. I.; Mikhailov, G. G.; Niewa, R. Ti-substituted BaFe12O19 single crystal growth and characterization. Cryst. Growth Des. 2014, 14, 5834−5839. (15) Cao, H. B.; Zhao, Z. Y.; Lee, M.; Choi, E. S.; McGuire, M. A.; Sales, B. C.; Zhou, H. D.; Yan, J.-Q.; Mandrus, D. G. High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe12O19. APL Mater. 2015, 3, 062512. (16) Wang, L.; Zhang, J.; Zhang, Q.; Xu, N.; Song, J. XAFS and XPS studies on site occupation of Sm3+ ions in Sm doped M-type BaFe12O19. J. Magn. Magn. Mater. 2015, 377, 362−367. (17) Kaur, T.; Kumar, S.; Bhat, B. H.; Want, B.; Srivastava, A. K. Effect on dielectric, magnetic, optical and structural properties of Nd5121


Article

The Journal of Physical Chemistry C (37) Cococcioni, M.; de Gironcoli, S. Linear response approach to the calculation of the effective interaction parameters in the LDA+U method. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71 (3), 035105. (38) Anisimov, V. I.; Zaanen, J.; Andersen, O. K. Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44 (3), 943−954. (39) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corp., Phys. Elect. Div.: Minnesota, 1992. (40) Vasquez, R. P.; Foote, M. C.; Hunt, B. D. Reaction of nonaqueous halogen solutions with YBa2Cu3O7‑x. J. Appl. Phys. 1989, 66 (10), 4866−4877. (41) Atuchin, V. V.; Kesler, V. G.; Kokh, A. E.; Pokrovsky, L. D. Xray photoelectron spectroscopy of β-BaB2O4 optical surface. Appl. Surf. Sci. 2004, 223, 352−360. (42) Aronniemi, M.; Sainio, J.; Lahtinen, J. Chemical state quantification of iron and chromium oxides using XPS: the effect of the background subtractuion method. Surf. Sci. 2005, 578 (1−3), 108− 123. (43) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441−2449. (44) Taran, O. P.; Ayusheev, A. B.; Ogorodnikova, O. L.; Prosvirin, I. P.; Isupova, L. A.; Parmon, V. N. Perovskite-like catalysts LaBO3 (B = Cu, Fe, Mn, Co, Ni) for wet peroxide oxidation of phenol. Appl. Catal. B 2016, 180, 86−93. (45) Fukuda, Y.; Nagoshi, M.; Suzuki, T.; Namba, Y.; Syono, Y.; Tachiki, M. Chemical states of Ba in YBa2Cu3O7−δ studied by X-ray photoelectron spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 39 (16), 11494−11497. (46) Atuchin, V. V.; Kalabin, I. E.; Kesler, V. G.; Pervukhina, N. V. Nb 3d and O 1s core levels and chemical bonding in niobates. J. Electron Spectrosc. Relat. Phenom. 2005, 142 (2), 129−134. (47) Atuchin, V. V.; Kesler, V. G.; Pervukhina, N. V. Zhaoming Zhang, Ti 2p and O 1s core levels and chemical bonding in titaniumbearing oxides. J. Electron Spectrosc. Relat. Phenom. 2006, 152 (1−2), 18−24. (48) Atuchin, V. V.; Aleksandrovsky, A. S.; Chimitova, O. D.; Diao, C.-P.; Gavrilova, T. A.; Kesler, V. G.; Molokeev, M. S.; Krylov, A. S.; Bazarov, B. G.; Bazarova, J. G.; et al. Electronic structure of βRbSm(MoO4)2 and chemical bonding in molybdates. Dalton Trans. 2015, 44 (4), 1805−1815. (49) Schmitz, P. J. Characterization of the surface of BaCO3 powder by XPS. Surf. Sci. Spectra 2001, 8 (3), 190−194. (50) Bagus, P. S.; Illas, F.; Pacchioni, G.; Parmigiani, F. Mechanisms responsible for chemical shifts of core-level binding energies and their relationship to chemical bonding. J. Electron Spectrosc. Relat. Phenom. 1999, 100, 215−236. (51) Swift, P. Adventitious carbon − the panacea for energy referencing? Surf. Interface Anal. 1982, 4 (2), 47−51. (52) Atuchin, V. V.; Grivel, J.-C.; Korotkov, A. S.; Zhang, Z. M. Electronic parameters of Sr2Nb2O7 and chemical bonding. J. Solid State Chem. 2008, 181, 1285−1291. (53) Atuchin, V. V.; Grivel, J.-C.; Zhang, Z. M. Core level photoemission spectroscopy and chemical bonding in Sr2Ta2O7. Chem. Phys. 2009, 360, 74−78. (54) Lupina, G.; Kozlowski, G.; Dabrowski, J.; Wenger, Ch.; Dudek, P.; Zaumseil, P.; Lippert, G.; Walczyk, Ch.; Müssig, H.-J. Thin BaHfO3 high-k dielectric layers on TiN for memory capacitor applications. Appl. Phys. Lett. 2008, 92, 062906. (55) Schmitz, P. J. Characterization of the surface of Ba(NO3)2 powder by XPS. Surf. Sci. Spectra 2001, 8 (3), 185−189. (56) Schmitz, P. J. Characterization of the surface of BaSO4 powder by XPS. Surf. Sci. Spectra 2001, 8 (3), 195−199. (57) Guo, X. L.; Liu, Z. G.; Zhu, S. N.; Xiong, S. B.; Zhu, Y. Y.; Liu, J. M. Ba2NaNb5O15/MgO optical waveguide structure grown on Si wafer by pulsed laser deposition. J. Cryst. Growth 1996, 167, 378−382. (58) Ayouchi, R.; Martín, F.; Ramos-Barrado, J. R.; Leinen, D. Compositional, structural and electrical characterization of barium

titanate thin films prepared on fused silica and Si(111) by spray pyrolysis. Surf. Interface Anal. 2000, 30, 565−569. (59) Atuchin, V. V.; Kesler, V. G.; Sapozhnikov, V. K.; Yakovenchuk, V. N. X-ray photoelectron spectrometry and binding energies of Be 1s and O 1s core levels in clinobarylite, BaBe2Si2O7, from Khibiny massif, Kola peninsula. Mater. Charact. 2008, 59 (9), 1329−1334. (60) Wersand-Quell, S.; Orsal, G.; Thévenin, P.; Bath, A. Growth of beta barium borate (β-BaB2O4) thin films by injection metal organic chemical vapour deposition. Thin Solid Films 2007, 515, 6507−6511. (61) Catalin, N.; Cernea, M. Characterization od BaTi4O9 ceramics by Raman spectroscopy and XPS after ion etching. J. Optoelect. Adv. Mater. 2006, 8 (5), 1879−1883. (62) Xia, C.-T.; Fuenzalida, V. M. Fuenzalida, Room temperature electrochemical growth of polycrystalline BaMoO4 films. J. Eur. Ceram. Soc. 2003, 23, 519−525. (63) Medicherla, V. R. R.; Shripathi, T.; Lalla, N. P. Electronic structure of BaPbO3 and Ba2PbO4. J. Phys.: Condens. Matter 2008, 20, 035219. (64) Manorama, S. V.; Gopal Reddy, C. V.; Rao, V. J. X-ray photoelectron spectroscopic studies of noble metal-incorporated BaSnO3 based gas sensors. Appl. Surf. Sci. 2001, 174, 93−105. (65) Singh, P.; Brandenburg, B. J.; Sebastian, C. P.; Singh, P.; Singh, S.; Kumar, D.; Parkash, O. Electronic structure, electrical and dielectric properties of BaSnO4 below 300 K. Jpn. J. Appl. Phys. 2008, 47 (5), 3540−3545. (66) Guyot, H.; Marcus, J. Charge transfer at the Fe/BaBiO3 interface. Surf. Sci. 1995, 331−333, 1531−1535. (67) Kobayashi, K.; Mizokawa, T.; Ino, A.; Matsuno, J.; Fujimori, A.; Samata, H.; Mishiro, A.; Nagata, Y.; de Groot, F. M. F. Doping dependence of the electronic structure of Ba1‑xKxBiO3 studied by x-rayabsorption spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59 (23), 15100−15106. (68) Pattanayak, R.; Panigrahi, S.; Dash, T.; Muduli, R.; Behera, D. Electric transport properties study of bulk BaFe12O19 by complex impedance spectroscopy. Phys. B 2015, 474, 57−63. (69) Liu, L.-g.; Bassett, W. A. Bassett, Effect of pressure on the crystal structure and lattice parameters of BaO. J. Geophys. Res. 1972, 77 (26), 4934−4937. (70) Holl, C. M.; Smyth, J. R.; Laustsen, H. M. S.; Jacobsen, S. D.; Downs, R. T. Compression of witherite to 8 GPa and the crystal structure of BaCO3 II. Phys. Chem. Miner. 2000, 27, 467−473. (71) Denzinger, W. Die Bestimmung der Kristallstruktur von betaBa(OH)2 bzw. beta-Ba(OD)2 durch Roentgen und Neutronenbeugung an polykristallinen Proben sowie die Lage der Deuteriumatome in alpha-Ba(OD)2. Dissertation Univ. Karlsruhe 1997, 1−97. (72) Nowotny, H.; Heger, G. Structure refinement of strontium nitrate, Sr(NO3)2, and barium nitrate, Ba(NO3)2. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, 39, 952−956. (73) Hill, R. J. A further refinement of the Barite structure. Can. Miner. 1977, 15, 522−526. (74) Foulon, G.; Ferriol, M.; Brenier, A.; Boulon, G.; Lecocq, S. Obtention of good quality Ba2NaNb5O15 crystals: growth, characterization and structure of Nd3+-doped single-crystal fibres. Europ. J. Solid State Inorg. Chem. 1996, 33, 673−686. (75) Buttner, R. H.; Maslen, E. N. Structural parameters and electron difference density in BaTiO3. Acta Crystallogr., Sect. B: Struct. Sci. 1992, 48, 764−769. (76) Krivovichev, S. V.; Yakovenchuk, V. N.; Armbruster, T.; Mikhailova, Y.; Pakhomovsky, Y. A. Clinobarylite, BaBe2Si2O7: structure refinement, and revision of symmetry and physical properties. Neues Jahrb. Mineral. Monatsh. 2004, 8, 373−384. (77) Fröhlich, R. Crystal structure of the low-temperature form of BaB2O4. Z. Kristallogr. 1984, 168, 109−112. (78) Hofmeister, W.; Tillmanns, E.; Baur, W. H. Refinement od barium tetratitanate, BaTi4O9, and hexabarium 17-titanate, Ba6Ti17O40. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, 40, 1510−1512. (79) Panchal, V.; Garg, N.; Sharma, S. M. Raman and X-ray diffraction investigations on BaMoO4 under high pressures. J. Phys.: Condens. Matter 2006, 18, 3917−3929. 5122


Article


(101) Subramanyan, K. N. Neutron and X-ray diffraction studies of certain doped nickel ferrites. J. Phys. C: Solid State Phys. 1971, 4, 2266−2268. (102) Scharner, S.; Weppner, W.; Schmid-Beurmann, P. Cation distribution in ordered spinels of the Li2O-TiO2-Fe2O3 system. J. Solid State Chem. 1997, 134, 170−181. (103) Takeda, T.; Kanno, R.; Kawamoto, Y.; Takano, M.; Kawasaki, S.; Kamiyama, T.; Izumi, F. Metal-semiconductor transition, charge disproportionation, and low-temperature structure of Ca1−xSrxFeO3 synthesized under high oxygen pressure. Solid State Sci. 2000, 2, 673− 687. (104) Shirane, G.; Cox, D. E. Magnetic structures in FeCr2S4 and FeCr2O4. J. Appl. Phys. 1964, 35, 954−955. (105) Atuchin, V. V.; Galashov, E. N.; Khyzhun, O. Yu.; Kozhukhov, A. S.; Pokrovsky, L. D.; Shlegel, V. N. Structural and electronic properties of ZnWO4(010) cleaved surface. Cryst. Growth Des. 2011, 11, 2479−2484. (106) Atuchin, V. V.; Galashov, E. N.; Khyzhun, O. Y.; Bekenev, V. L.; Pokrovsky, L. D.; Borovlev, Yu.A.; Zhdankov, V. N. Low thermal gradient Czochralski growth of large CdWO4 crystals and electronic properties of (010) cleaved surface. J. Solid State Chem. 2015, DOI: 10.1016/j.jssc.2015.05.017.

(80) Rosseinsky, M. J.; Prassides, K. Time-of-flight powder neutron diffraction study of the structure of Ba2PbO4. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, 47, 2519−2522. (81) Megaw, H. D. Crystal structure of double oxides of the perovskite type. Proc. Phys. Soc. London 1946, 58, 133−152. (82) Arpe, R.; Mueller-Buschbaum, H. Ein Beitrag zur Kristallchemie von BaBiO3. Z. Anorg. Allg. Chem. 1977, 434, 73−77. (83) Ivanov, S. A.; Eriksson, S.-G.; Tellgren, R.; Rundloef, H. Neutron powder diffraction study of structural phase transitions in BaPbO3. Mater. Sci. Forum 2001, 378, 511−516. (84) Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides and fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925−946. (85) Bocquet, A. E.; Fujimori, A.; Mizokawa, T.; Saitoh, T.; Namatame, H.; Suga, S.; Kimizuka, N.; Takeda, Y.; Takano, M. Electronic structure of SrFe4+O3 and related Fe perovskite oxides. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45 (4), 1561−1570. (86) Kozakov, A. T.; Kochur, A. G.; Googlev, K. A.; Nikolsky, A. V.; Raevski, I. P.; Smotrakov, V. G.; Yeremkin, V. V. X-ray photoelectron study of the valence state of iron in iron-containing single-crystal (BiFeO3, PbFe1/2Nb1/2O3), and ceramic (BaFe1/2Nb1/2O3) multiferroics. J. Electron Spectrosc. Relat. Phenom. 2011, 184, 16−23. (87) Aono, H.; Sato, M.; Traversa, E.; Sakamoto, M.; Sadaoka, Y. Design of ceramic materials for chemical sensors: effect of SmFeO3 processing on surface and electrical properties. J. Am. Ceram. Soc. 2001, 84 (2), 341−347. (88) Dhak, P.; Dhak, D.; Das, M.; Subashchandrabose, T.; Pramanik, P. A novel synthesis of FeNbO4 nanorod by hydrothermal process. J. Nanopart. Res. 2011, 13, 4153−4159. (89) Mandal, S.; Ghosh, C. K.; Sarkar, D.; Maiti, U. N.; Chattopadhyay, K. K. X-ray photoelectron spectroscopic investigation on the elemental chemical shifts in multiferroic BiFeO3 and its valence band structure. Solid State Sci. 2010, 12, 1803−1808. (90) Postnikov, A. V.; Bartkowski, St.; Neumann, M.; Rupp, R. A.; Kurmaev, E. Z.; Shamin, S. N.; Fedorenko, V. V. Electronic structure and valence-band spectra of FeBO3. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50 (20), 14849−14854. (91) Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. St. C Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717−2730. (92) Arillo, M. A.; López, M. L.; Pico, C.; Viega, M. L.; JiménezLópez, A.; Rodriguez-Castellón, E. Surface characterization of spinels with Ti(IV) distributed in tetrahedral and octahedral sites. J. Alloys Compd. 2001, 317−318, 160−163. (93) Nytén, A.; Stjerndahl, M.; Rensmo, H.; Siegbahn, H.; Armand, M.; Gustafsson, T.; Edström, K.; Thomas, J. O. Surface characterization and stability phenomena in Li2FeSiO4 studied by PES/XPS. J. Mater. Chem. 2006, 16, 3483−3488. (94) Sawada, H. An electron density residual study of alpha-ferric oxide. Mater. Res. Bull. 1996, 31 (2), 141−146. (95) du Boulay, D.; Maslen, E. N.; Strel’tsov, V. A.; Ishizawa, N. A synchrotron X-ray study of the electron density in YFeO3. Acta Crystallogr., Sect. B: Struct. Sci. 1995, 51, 921−929. (96) Dann, S. E.; Currie, D. B.; Weller, M. T.; Thomas, M. F.; Rawwas, A. D. The effect of oxygen stoichiometry on phase relations and structure in the system La1−xSrxFeO3−δ (0 ≤ x ≤ 1, 0 ≤ δ≤0.5). J. Solid State Chem. 1994, 109, 134−144. (97) Maslen, E. N.; Strel’tsov, V. A.; Ishizawa, N. A synchrotron Xray study of the electron density in SmFeO3. Acta Crystallogr., Sect. B: Struct. Sci. 1996, 52, 406−413. (98) Harrison, W. T. A.; Cheetham, A. K. Structural and magnetic properties of FeNbO4-II. Mater. Res. Bull. 1989, 24, 523−527. (99) Sosnovska, I.; Schaefer, W.; Kockelmann, W.; Troyanchuk, I. O. Neutron diffraction studies of the crystal and magnetic structures of BaMnxFe1−xO3 solid solutions. Mater. Sci. Forum 2001, 378, 616−620. (100) Diehl, R. Crystal structure refimenent of ferric borate, FeBO3. Solid State Commun. 1975, 17, 743−745. 5123


Flux Crystal Growth and the Electronic Structure of ... - ACS Publications

Recommend Documents