a Ratio in Bi

Jun 2, 2016 - A nearly single monoclinic phase with space group Cc, which was the same as the low-temperature ... Monoclinic phases with Cm, Pm, and C...
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High-Temperature Monoclinic Cc Phase with Reduced c/a Ratio in Bibased Perovskite Compound Bi2ZnTi1−xMnxO6 Runze Yu,†,∥,⊥ Narumi Matsuda,†,∥ Ken Tominaga,† Keisuke Shimizu,† Hajime Hojo,*,† Yuki Sakai,‡ Hajime Yamamoto,† Kengo Oka,§ and Masaki Azuma*,† †

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, 226-8503, Japan Kanagawa Academy of Science and Technology, KSP, 3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa 213-0012, Japan § Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan ‡

S Supporting Information *

ABSTRACT: Monoclinic phases with Cm, Pm, and Cc space groups are indispensable to understand the high performance of electromechanical properties at the morphotropic phase boundary (MPB) of lead-based perovskite oxides Pb(ZrxTi1−x)O3 (PZT), [Pb(Mg1/3Nb2/3)O3]1−x-(PbTiO3)x (PMN−PT), and [Pb(Zn1/3Nb2/3)O3]1−x-(PbTiO3)x (PZN−PT). Here, a nearly single monoclinic phase with space group Cc was observed in the Bi-based lead-free perovskite compound Bi2ZnTi1−xMnxO6 at x = 0.4. This phase was the same as the low-temperature phase of the MPB composition of PZT but existed at a much higher temperature. Despite the reduced pseudo c/a ratio of 1.065, which is the same as that of PbTiO3 at room temperature, ionic model calculation based on the Rietveld refinement data indicated the polarization of Bi2ZnTi0.6Mn0.4O6 is 95.8 μC/cm2. The tilting and significant anisotropic distortion of the octahedron were found to cause the c/a ratio to reduce. Accordingly, the effective piezoelectric constant d33 of Bi2ZnTi0.6Mn0.4O6 thin film was found to be 12 pm/V.

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PZT at very low temperature.20−22 This monoclinic phase can be regarded as a result of anti-ferrodistortive distortion of the Cm phase leading to the doubling of the c-axis. The increasing success of PZT and related compounds resulted in lead being released into the environment, mainly in the form of either lead oxide or lead zirconate titanate. The concern about lead toxicity is therefore growing, and the search is on for a new lead-free compound, such as (K, Na)NbO3 or (Bi, Na)TiO3.23−26 Considering the 6s2 electronic configuration of Bi3+, which is essentially the same as that of Pb2+, much attention is being paid to Bi-based perovskite compounds in the search for the lead-free piezoelectric materials.27−30 Recently, several Bi-based PbTiO3-type compounds have been synthesized, such as BiCoO3,27 Bi2ZnTiO6,28 and Bi2ZnVO629 with pronounced tetragonal distortion. The monoclinic phase with a Cm space group, the same as that of the MPB composition of PZT, was found in a solid solution between tetragonal BiCoO3 and rhombohedral BiFeO3,31,32 and the polarization rotation was observed as functions of composition and temperature.32 The same structure was also observed in the BiAl0.75Ga0.25O3 system.33 However, because of the giant tetragonal distortion of c/a ≈ 1.2, polarization has not been able to be reversed in these compounds. During the systematic study of pressure-induced structural change of Bi2ZnTiO6, we found that the structure of Bi2ZnTiO6 was sensitive to pressure and that the c/a ratio

INTRODUCTION The ultrahigh piezoelectric, dielectric, and electromechanical responses in lead-based perovskite oxides, such as Pb(Zr x Ti 1−x )O 3 (PZT), [Pb(Mg 1/3 Nb 2/3 )O 3 ] 1−x -(PbTiO 3 ) x (PMN−PT), and [Pb(Zn1/3Nb2/3)O3]1−x-(PbTiO3)x (PZN− PT), have long been known to be related to the nearly vertical phase boundary between the rhombohedral and the tetragonal phases, which is called the morphotropic phase boundary (MPB).1−6 However, the crystal structures of these compounds near MPB were unclear until Noheda et al. observed a monoclinic phase with space group Cm and a (√2)a × (√2)a × a unit cell, where a is the lattice constant of a cubic perovskite in PZT (designated as MA phase by Vanderbilt and Cohen7).8,9 The enhancement of piezoelectric response is attributed to the lack of a symmetry axis in the monoclinic structure, which induces the rotation of the ferroelectric polarization vector between the polar axes of the tetragonal and rhombohedral phases.7,10,11 Such a finding in PZT was followed by studies on the structures of MPB phases in PZN− PT12−14 and PMN−PT15−18 systems, and two new monoclinic phases were reported named MB and MC phases. The MB phase has the same space group as MA but with the lattice parameters of c < a and b.19 The MC phase has the Pm space group with the polarization lying between [101] and [001] directions. These results successfully explained the high piezoelectric performance of MPB phases in lead-based perovskite oxides. Besides the phases mentioned above, a new monoclinic phase with space group Cc (No. 9, cell choice 1) was found in the © XXXX American Chemical Society

Received: March 17, 2016

A

DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry decreased from 1.21 to 1.10 under pressure.29 This fact suggested that c/a ratio of this compound can be suppressed by another perturbation, namely, chemical substitution. As the large c/a ratio partly originates from the second-order Jahn− Teller (SOJT) effect of Ti4+, substitution of Ti4+ with no SOJT active Mn4+ with a fully occupied t2g orbital is expected to largely reduce the c/a ratio, preserving the insulating character. In this study, we investigated the Mn substitution of Ti for the Bi 2 ZnTiO 6 . We found the pseudo c/a ratio of Bi2ZnTi1−xMnxO6 monotonously decreased with Mn substitution for Ti and reached 1.065 at x = 0.4. The crystal symmetry changed from tetragonal P4mm to monoclinic Ia (No. 9, cell choice 3) with a (√2)a × (√2)a × 2a unit cell. This is the same structure as that of the low-temperature phase of PZT, but we chose Ia space group instead of Cc (both are No. 9 with different cell choice) for easier comparison with other distorted perovskite structures. The structure stability under high pressure and high temperature as well as the piezoelectric property of Bi2ZnTi1−xMnxO6 thin films were systematically investigated.

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EXPERIMENTAL SECTION

Polycrystalline samples of Bi2ZnTi1−xMnxO6 (x = 0, 0.2, 0.3, 0.4, 0.45, and 0.5) were prepared from mixtures of Bi2O3, ZnO, TiO2, and MnO2 with a stoichiometric composition. The Bi2O3 powder was preheated in a box furnace at 673 K for 6 h. The mixture was carefully ground using an agate mortar in a glovebox, sealed into a gold capsule with a diameter of 3.6 mm and a height of 5 mm, and then treated at 6 GPa and 1473 K for 30 min in a cubic-anvil-type high-pressure apparatus. Bi2ZnTiO6 was prepared as previously reported.28 Laboratory X-ray diffraction (XRD) data at different temperatures were collected with a D8 ADVANCE diffractometer (Bruker) with Cu Kα radiation, and the lattice parameters and monoclinic angles were refined by the Rietveld method using TOPAS software. Synchrotron XRD (SXRD) data under ambient conditions were collected with the large Debye−Scherrer camera installed on the BL02B2 beamline of SPring-8 (wavelength of 0.419732 Å). The diffraction data were analyzed with the Rietveld method using the RIETAN-FP program.34 Selected-area electron diffraction (SAED) patterns were taken using a JEOL JEM-2100F microscope. The energy-dispersive SXRD data for Bi2ZnTi1−xMnxO6 at high pressure were collected at beamline BL14B1 of SPring-8 using a cubic-anvil-type high-pressure apparatus with a solid-state detector fixed at 2θ = 4.5°. The lattice parameters were calculated by profile fitting. Bi2ZnTi1−xMnxO6 thin films were grown on SrRuO3/SrTiO3 (001) substrates using pulsed laser deposition with a KrF excimer laser (λ = 248 nm) followed by postannealing. Ceramic targets were prepared by sintering the mixture of Bi2O3, 50% mol excess ZnO, TiO2, and Mn2O3 at an ambient pressure. The thin films were grown in an oxygen partial pressure of 30 Pa with the substrate temperature of 300 °C. After the deposition, the thin films were annealed at 450−500 °C under O2 flow. The thickness of the thin films was determined to be 100 nm using a stylus thickness gauge. The composition of the thin films was studied using energy-dispersive X-ray spectroscopy (EDS). The crystal structure of the thin films was investigated by XRD with a Cu Kα radiation (Rigaku SmartLab). Local displacement versus electric field curves was measured by detecting the vertical motion of an atomic force microscope (Agilent 5420) cantilever with a conducting tip connected to a ferroelectric test system (Toyo FCE1E).

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Figure 1. (a) Laboratory PXRD patterns of Bi2ZnTi1−xMnxO6 (x = 0, 0.2, 0.3, 0.4, 0.45, and 0.5). Filled triangles stand for peaks from the second phase. (b) Composition dependence of the lattice parameters and the monoclinic angles. The a/√2 and c/2 are plotted for Bi2ZnTi1−xMnxO6 with Ia space group to match the perovskite cell of Bi2ZnTiO6.

cell of a = 3.8210 Å and c = 4.6250 Å as previously reported.28 The splitting between 001 and 100 peaks decreases with Mn substitution for Ti, indicating the suppression of c/a ratio. In addition, 101 and 110 peaks of the tetragonal phase split into two, suggesting the lowering of the symmetry. All the peaks were indexed assuming the monoclinic (√2)a × (√2)a × 2a unit cell. Doubling of the c-axis is necessary to explain the ED pattern with 101 reflection as discussed later. The composition dependence of lattice parameters was plotted in Figure 1b. The increase in a-axis and decrease in c-axis resulted in the shrinkage of pseudo c/a ratio, which reached 1.065 at x = 0.4, comparable to that of PbTiO3. The appearance of second phase at x = 0.5 indicated the solubility limit is x = 0.45. Figure 2 shows the SAED patterns of Bi2ZnTi0.6Mn0.4O6 at room temperature viewed along the [1−10], [001], and [−111] zone axes. The presence of supercell reflections that can be indexed as 101 and 011 (see Figure 2c) indicates the doubling of the unit cell along the c axis. The crystal structure of this compound was further investigated by Rietveld analysis of the SXRD data. Since the (√2)a × (√2)a × 2a unit cell with Ia space group (No. 9) was reported for the lowtemperature phase of PbTi0.48Zr0.52O3, and the reflection condition of h + k + l = 2n was consistent with Ia, this structure was chosen as the initial structure model. Figure 3a shows the SXRD pattern of Bi2ZnTi0.6Mn0.4O6 and the results of the Rietveld fitting. The occupation factors of Bi, Zn, Ti, and

RESULTS AND DISCUSSION

Crystal Structure and Polarization. Figure 1a shows the laboratory powder X-ray diffraction (PXRD) patterns for Bi2ZnTi1−xMnxO6 (x = 0, 0.2, 0.3, 0.4, 0.45, and 0.5). The pattern for x = 0 sample can be indexed with a tetragonal unit B

DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) [1−10], (b) [001], and (c) [−111] zone axis SAED patterns for Bi2ZnTi0.6Mn0.4O6 at room temperature.

Figure 3. (a) Result of the Rietveld refinement of SXRD data of Bi2ZnTi0.6Mn0.4O6 at room temperature. Observed (+), calculated (line), and their difference (bottom line) profiles are shown. Bragg reflections are indicated by tick marks. (b) The refined crystal structure of Bi2ZnTi0.6Mn0.4O6 at room temperature. Dashed and solid lines represent the monoclinic and pseudo cubic unit cells, respectively. The red arrow denotes the polarization vector.

Table 1. Crystallographic Parametersa of Bi2ZnTi0.6Mn0.4O6 at Room Temperature

Mn were refined in the initial fitting, but these converged to 2:1:0.6:0.4 within the standard deviations. All the occupation factors were therefore fixed to these values in the final refinement. Table 1 lists the refined structural parameters with satisfactorily low R factors.34 The refined crystal structure is illustrated in Figure 3b. It can be regarded as an a−b−c− tilt system according to the Glazer notation.35 This tilting of the octahedron is one of the reasons of the unit cell doubles in the c-direction. Meanwhile we also noticed that the average B−O bond distance (∼2.08 Å) of Bi2ZnTi0.6Mn0.4O6 is almost unchanged from what is observed in BZT (∼2.05 Å). However, incorporation of Mn in the B site had strongly affected the distribution of these B−O bonds distances; for example, the four in-plane bonds are no longer equivalent, and the longest and shortest B−O bond lengths are 2.435 and 1.794 Å, respectively, while these are 2.156 and 1.936 Å for the PZT. These indicate that there is a significant anisotropic distortion of the BO6 octahedron for Bi2ZnTi0.6Mn0.4O6 compared with BZT. Both the tilting and anisotropic distortion of the BO6

atom

site

x

y

z

B (Å2)

g

Bi Zn Ti Mn O1 O2 O3

4a 4a 4a 4a 4a 4a 4a

0.015(3) 0.969(3) 0.969 0.969 0.935(5) 0.186(5) 0.168(5)

0.747(1) 0.237(2) 0.237 0.237 0.322(3) 0.577(4) 0.986(4)

0.008(1) 0.228(1) 0.228 0.228 0.951(2) 0.636(2) 0.718(3)

1.24(3) 0.44(12) 0.44 0.44 0.34(30) 0.34 0.34

1.0 0.5 0.3 0.2 1.0 1.0 1.0

a Space group Ia (No. 9, choice 3), Z = 4, a = 5.5318(4) Å, b = 5.4881(2), c = 8.3269(10) Å, β = 89.29(6)°, ρcalc = 8.2775 g/cm3,V = 252.78(3)Å3. R factors (%): Rwp = 5.618, RI = 1.291. Occupation factors of all sites are fixed to unity.

octahedron result in the reduced c/a ratio and monoclinic distortion. Despite the reduced c/a ratio of 1.065 comparable to that of PbTiO3, transition metals shift largely in the oxygen C

DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

atures. On the one hand, in the case of monoclinic BiFe0.7Co0.3O3, a gradual change to the tetragonal phase accompanied by the polarization rotation was observed.32 On the other hand, the monoclinic β angle of Bi2ZnTi0.6Mn0.4O6 was almost independent of the temperature (see Figure 4b). No Cm phase as observed for PZT appeared on heating.36 Instead, a first-order transition to a tetragonal phase with c/a ≈ 1.10 was observed at 800 K (see Figure 4a). One possible reason is that the step of heating (100 K/step) is too large to observe the Cm phase. This needs to be carefully investigated in the future work. We performed the Rietveld refinement for the SXRD data of Bi2ZnTi0.6Mn0.4O6 at 900 K (The refined data can be found at Table S2). The calculated polarization is ∼134 μC/cm2, which is greatly larger than that of Cc phase and even larger than that of Bi2ZnTiO6 at room temperature.28 As stated above, the Ia (Cc) crystal structure is the result of the tilting of the octahedron, which is the reason the unit cell doubles in the c-direction, so the monoclinic to tetragonal transition should be attributed to the release of the octahedron tilting. On further heating, the sample decomposed into Bi12TiO20, Mn2O3, ZnO, and TiO2 without a transition to the paraelectric phase. Next, we studied the structure stability of Bi2ZnTi1−xMnxO6 under pressure, since pressure-induced phase transition from tetragonal to orthorhombic with a 1.3% volume collapse had been observed for Bi2ZnTiO6.29 Figure 5a shows energy-

octahedron. The difference in two M−O bonds (M = Zn, Ti, Mn) in the c-direction is as large as 2.435−1.794 = 0.641 Å. This value is much larger than that for Cc phase of PbZr0.52Ti0.48O3 at 20 K, 2.156−1.996 = 0.160 Å.22 This means Bi2ZnTi0.6Mn0.4O6 has a larger polar distortion than PbZr0.52Ti0.48O3. The atomic displacements in BO6 octahedron are found in Table S1. It can be seen that nearly all the atomic positions of Bi2ZnTi0.6Mn0.4O6 have larger displacements than those in PbZr0.52Ti0.48O3. The large distortion and displacements resulted in the calculated ionic polarization of 95.8 μC/ cm2, which is ∼2.5 times as large as that reported for the lowtemperature phase of PbZr0.52Ti0.48O3 (39 μC/cm2).22 The polarization is in the (110) plane of the pseudo cubic perovskite cell and tilted from the [001] direction of a pseudo cubic perovskite ([001]c) by ∼29.7° (See Figure 3b), while the corresponding tilting angle for the Cc phase of PZT is ∼23°.22 This means the polarization of Bi2ZnTi0.6Mn0.4O6 projected to the c-direction is also larger than that of PZT. This is the first observation of the monoclinic Ia (Cc) phase in the Bi-based perovskite compound. Moreover, this Ia (Cc) phase is preserved up to 700 K as discussed later and in the wide composition range from x = 0.2−0.45 for Bi2ZnTi1−xMnxO6, while the Cc phase is found only below 20 K and in the narrow composition range in the vicinity of PbZr0.5Ti0.5O3 for PZT.36 Temperature and Pressure Stability of Monoclinic Phase of Bi2ZnTi1−xMnxO6. Since temperature-induced phase transition from monoclinic to tetragonal had been observed for PZT9 and BiCo1−xFexO3,32 we also investigated the structural stability of the Bi2ZnTi0.6Mn0.4O6 on heating. Figure 4a shows the SXRD patterns of Bi2ZnTi0.6Mn0.4O6 at elevated temper-

Figure 5. (a) Energy-dispersive SXRD patterns of Bi2ZnTi0.6Mn0.4O6 under various pressures. (b) Pressure dependence of the perovskite volumes of Bi2ZnTiO6, Bi2ZnTi0.8Mn0.2O6, and Bi2ZnTi0.6Mn0.4O6.

dispersive SXRD patterns of Bi2ZnTi0.6Mn0.4O6 at various pressures indicating a monoclinic to orthorhombic phase transition at ∼2.5 GPa. The same transition was also observed for Bi2ZnTi0.8Mn0.2O6 at ∼3.5 GPa (the energy-dispersive SXRD patterns are shown in Figure S1). Figure 5b shows the pressure dependence of the perovskite volume for Bi2ZnTi1−xMnxO6 (x = 0.0, 0.2, and 0.4; The lattice parameters

Figure 4. (a) Temperature variation of the SXRD patterns of Bi2ZnTi0.6Mn0.4O6. (b) Temperature dependence of the lattice parameters and the β angle. D

DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

that of the bulk sample, ∼1.07 for Bi2ZnTi0.6Mn0.4O6 thin film. Because the in-plane lattice constant of Bi2ZnTi0.6Mn0.4O6 bulk sample is 0.38% larger than that of STO, slight compressive strain should be imposed on the Bi2ZnTi0.6Mn0.4O6 thin film, which should increase the c/a ratio of the film. Figure 6b shows the dependence of displacement on applied voltage for these films. The observed butterfly loops are typical of ferroelectric materials. The effective piezoelectric constant d33 estimated from the slope of the displacement curve at ∼0 V is ∼31 and 12 pm/V for x = 0.2 and 0.4 Bi2ZnTi1−xMnxO6 thin films, respectively. The d33 is small, because the c/a is reduced due to the tilting of the octahedron, not the suppression of the polar distortion, as was clarified by the structure refinement of bulk Bi2ZnTi1−xMnxO6 samples.

were obtained using the energy-dispersive profile fitting method). The transition pressure decreases as the c/a ratio decreases. Interestingly, the magnitudes of the volume collapse, 9.9% and 7.2% for x = 0.2 and 0.4 samples, respectively, are much larger than that of Bi2ZnTiO6 29 and nearly the same as that of BiCoO3.37 We had reported that the pressure-induced volume collapse of BiCoO3 takes place by heating at a moderate pressure leading to a negative thermal expansion (NTE).37 Moreover, 3% volume collapse of BiNiO3 under pressure takes place on heating at ambient pressure in Bi1−xLnxNiO3 (Ln: lanthanides) and BiNi1−xMxO3.38−43 Similar NTE is expected in Bi2ZnTi1−xMnxO6 derivatives with further reduced c/a ratio. Crystal Structure and Piezoelectric Property of Bi 2 ZnTi 1−x Mn x O 6 Thin Films. Since the monoclinic Bi2ZnTi0.6Mn0.4O6 has a polar structure with a suppressed c/a ratio of 1.065, comparable with that of PbTiO3, and the monoclinic Ia (Cc) space group allows a polarization rotation, a good ferroelectric and piezoelectric property could be expected. Therefore, Bi2ZnTi1−xMnxO6 thin films were prepared to study the ferroelectric/piezoelectric properties. Figure 6a shows

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CONCLUSIONS We synthesized a series of Bi2ZnTi1−xMnxO6 (x = 0.0−0.5) compounds at 6 GPa and 1573 K. The pseudo c/a ratio was dramatically reduced by Mn substitution, and monoclinic phase with a (√2)a × (√2)a × 2a unit cell and Ia (Cc) space group, the same as the low-temperature phase of the MPB composition of PZT, was found in a wide composition range. The c/a ratio of Bi2ZnTi0.6Mn0.4O6 was as small as 1.065, comparable with that of PbTiO3. Structure analysis indicated that the polarization in the monoclinic Ia (Cc) phase was in a (110)c plane, with the tilting angle of 29.7° from the [001]c direction. Despite the reduced c/a ratio, the shift of the cation in the oxygen octahedron was large, and the calculated ionic polarization of Bi2ZnTi0.6Mn0.4O6 was as large as 95.8 μC/cm2. Consequently, the effective piezoelectric constant d33 of Bi2ZnTi0.6Mn0.4O6 thin film was found to be 12 pm/V. A monoclinic to tetragonal phase transition owing to the release of the octahedral rotation was observed at ∼800 K on heating. A pressure-induced monoclinic-to-orthorhombic phase transition accompanied by a 7.2% volume collapse was observed for Bi2ZnTi0.6Mn0.4O6. These results will stimulate further research into lead-free piezoelectric and negative thermal expansion compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00618. Energy-dispersive SXRD patterns of the Bi2ZnTi0.8Mn0.2O6 and Bi2ZnTiO6 at various pressures. Comparison of the atomic displacements in BO 6 octahedron for Bi 2 ZnTi 0.6 Mn 0.4 O 6 (BZTM) and PbTi0.48Zr0.52O3 (PZT). Crystallographic parameters of Bi2ZnTi0.6Mn0.4O6 at 900 K (PDF)

Figure 6. (a) XRD θ-2θ patterns of Bi2ZnTi1−xMnxO6 (BZTMO) thin films with x = 0.2 and 0.4 on SrRuO3 (SRO)/SrTiO3 (STO)(001) substrates. The peaks are indexed on the basis of pseudocubic indices. (b) Displacement as a function of applied voltage for these thin films.

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typical θ-2θ XRD patterns of Bi2ZnTi1−xMnxO6 thin films with x = 0.2 and 0.4, indicating that c-axis orientated epitaxial perovskite phases are obtained. We confirmed using EDS that the thin films are stoichiometric in cation ratio within the experimental error. The c-axis lengths decrease with x from 4.23 Å at x = 0.2 to 4.19 Å at x = 0.4. Such behavior is consistent with that of bulk Bi2ZnTi1−xMnxO6. Reciprocal space mappings around STO 113 and 103 (not shown) indicated that the inplane lattice constants of the thin films are locked to that of STO substrate (3.91 Å). The c/a ratio is slightly larger than

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (H.H.) *E-mail: [email protected]. (M.A.) Present Address ⊥

Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.

Author Contributions ∥

E

These authors contributed equally. DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge J. Chen at Univ. of Science and Technology Beijing for fruitful discussions. This work was supported by the Scientific Research on Innovative Areas (26106507), Young Scientists (B) (26820291 and 26800180), and Challenging Exploratory Research (15K14119) from the Japan Society for the Promotion of Science (JSPS). The synchrotron-radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (2014A1732, 2014A3615, 2014B1731, 2015A1788, 2015A3615, and 2015B1730) and under the Shared User Program of JAEA Facilities (Grant Nos. 2014A-E21 and 2015A-E11) with the approval of the Nanotechnology Platform project supported by the Ministry of Education, Culture, Sports, Science and Technology (Grant Nos. A-14-AE-0015 and A-15-AE-0011).

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DOI: 10.1021/acs.inorgchem.6b00618 Inorg. Chem. XXXX, XXX, XXX−XXX