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Feb 16, 2017 - Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China. ‡. BCCRD, Key Laboratory of Fu...
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Zero Thermal Expansion and Semiconducting Properties in PbTiO3− Bi(Co, Ti)O3 Ferroelectric Solid Solutions Zhao Pan,† Jun Chen,*,† Xingxing Jiang,‡ Zheshuai Lin,‡ Linxing Zhang,† Longlong Fan,† Yangchun Rong,† Lei Hu,† Hui Liu,† Yang Ren,§ Xiaojun Kuang,∥ and Xianran Xing*,† †

Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China BCCRD, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China § X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Zero thermal expansion (ZTE) behavior is rare but important for both fundamental studies and practical applications of functional materials. Until now, most available ZTE materials are either electrical insulating oxides or conductive metallic compounds. Very few ZTE materials exhibit the semiconductor feature. Here we report a ZTE in a semiconducting ferroelectric of 0.6PbTiO3−0.4Bi(Co0.55Ti0.45)O3−δ. Its unit cell volume exhibits a negligible change over a broad temperature range from room temperature to 500 °C. The ZTE is supposed to be correlated with the spontaneous volume ferroelectronstriction. Intriguingly, the present ZTE material also exhibits the semiconducting characteristic accompanied by negative temperature coefficient of resistance. The mechanism of electric conduction is attributed to the electronic hopping from one ion (Ti3+) to another (Ti4+). The semiconductor nature has also been confirmed by the noticeable visible-light absorption with the relatively lower band gap (Eg) value of 1.5 eV, while the ferroelectric property can be well-maintained with large polarization. The first-principles calculations reveal that the drastically narrowed Eg is related to the Co−Ti substitution. The present multifunctional material containing ZTE, semiconducting, and ferroelectric properties is suggested to enable new applications such as the substrate for solar conversion devices. Y2Fe17C0.61,8 and antiperovskite nitrides (e.g., Mn3Cu0.5Ga0.5N3 and Mn 3 Zn 0 .93 N 9 ), or an insulating one, such as Zr0.4Sn0.6Mo2O87 and TaO2F.10 Very few of the ZTE functional materials exhibit semiconducting properties. The discovery of semiconducting ZTE materials may find new applications for these materials in electric devices, which will further enrich the application range of ZTE materials. In this study we have designed new perovskite-type (ABO3) solid solutions of (1 − x)PbTiO3−xBi(Co0.5Ti0.5)O3 (abbreviated as (1 − x)PT−xBCT) to exhibit an intriguing property of ZTE semiconducting ferroelectrics. The band gap (Eg) was significantly reduced by the formation of oxygen vacancies and defect energy levels through the B-site nonequivalence substitution. The ZTE property was achieved by the chemical substitution with the weak ferroelectric activity cation of Co.11 Furthermore, the cation of Bi which has the unique 6s2 lonepair electron configuration is introduced to ensure wide temperature range of ZTE due to the strong hybridization between Bi and oxygen.4 The crystal structure, thermal expansion, and electric properties have been investigated in

1. INTRODUCTION Materials with zero thermal expansion (ZTE) behavior are of special interest due to the advantages of precise control and avoidance of thermal shock over a certain temperature range.1−23 ZTE is especially important for the applications of functional materials which can prevent or reduce internal stress in materials subject to large temperature fluctuations, such as in aerospace engineering and thermomechanical actuators.4 In most cases, ZTE can be achieved by forming composites with positive and negative thermal expansion (NTE) compounds, or by chemical substitution in single-phase materials.3,5 The former approach usually leads to microstrain within composites, which is not easily obtained for dense materials. It is therefore preferable to achieve ZTE behavior within single-phase materials because of advantages such as freedeom from internal stress, high mechanical performance, and easy preparation. However, single-phase ZTE materials are rare, and have been found in magnetic functional materials such as Invar Fe0.65Ni0.35 alloys,1 antiperovskite nitrides of Mn3Cu0.5Ga0.5N3 and Mn3(Cu0.55Sn0.45)N,6 the framework structure of Zr0.4Sn0.6Mo2O8,7 and PbTiO3 (PT)-based ferroelectrics.4 Most ZTE functional materials exhibit either an electronic conducting property, such as alloys of Fe0.65Ni0.35 and © XXXX American Chemical Society

Received: November 18, 2016

A

DOI: 10.1021/acs.inorgchem.6b02761 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the (1 − x)PT−xBCT-based solid solutions. The multifunctional properties of ZTE, semiconductance, and ferroelectricity have been successfully achieved in the solid solutions of 0.6PbTiO3−0.4Bi(Co0.55Ti0.45)O3−δ (abbreviated as 0.6PT− 0.4BCT5) by substituting Ti with a small amount of Co at the composition 0.6PbTiO3−0.4Bi(Co0.5Ti0.5)O3.

SXRD patterns at RT (Figures S1−S5 and Table S1). Hightemperature XRD measurements were performed to identify the phase transitions and determine the temperature dependent lattice parameters. Over the temperature range from RT to TC, the NTE behavior of (1 − x)PT−xBCT has been weakened with increasing content of BCT (Figure S6). The average volumetric coefficient of thermal expansion (CTE) of 0.7PT− 0.3BCT and 0.6PT−0.4BCT is −1.73 and −0.74 × 10−5 °C−1, respectively (Table S2). With further increasing BCT, the material has a tendency to exhibit positive thermal expansion such as 0.5PT−0.5BCT. Intriguingly, ZTE can be achieved in the 0.6PT−0.4BCT5 (α̅ V = 0.33 × 10−6 °C−1, 25−500 °C) by substituting Ti with a small amount of Co at the composition of 0.6PT−0.4BCT (Figure 1 and Figure S7). The CTE of 0.6PT−

2. EXPERIMENTAL SECTION Samples of (1 − x)PT−xBCT were prepared by the conventional solid state method. The raw materials are PbO, TiO2, Bi2O3, and Co2O3, with purity >99%. Powders were stoichiometrically weighed and mixed by using an agate mortar, ensuring the powders are as homogeneous as possible. Calcination of these mixed powders was performed at 850 °C for 5 h. After calcination, the powders were ground in mortar and pressed into pellets with a diameter of 10 mm and thickness of 2−3 mm, and then the pellets were sintered at 1100 °C for 2 h in a closed crucible. The compacts were embedded by the calcined powders with the same composition in order to reduce the volatilization of Pb and Bi during the sintering process. Temperature dependent X-ray powder diffraction patterns (XRD) of 0.7PT−0.3BCT, 0.6PT−0.4BCT, 0.6PT−0.4BCT5, and 0.5PT− 0.5BCT were collected from room temperature (RT) to 700 °C, operating at 40 kV and 40 mA with Cu Kα1/2 radiation (PANalytical, PW 3040-X’Pert Pro). In order to precisely determine the thermal expansion, high-energy synchrotron powder diffraction (SXRD) studies were performed on 0.6PT−0.4BCT5 from RT to 680 °C at beamline 11-ID-C at the Advanced Photon Source (APS) with the light wavelength of 0.11165 Å. Alternating current impedance spectroscopy (IS) measurements were performed with the Solartron 1260A impedance/gain-phase analyzer over 10−1−107 Hz from room temperature (RT) to 600 °C. Before the IS measurements, the pellets were coated with platinum paste and fired at 800 °C for 30 min to burn out the organic components in order to form dense electrodes. X-ray photoelectron spectroscopy (XPS, AXIS ULTRADLD, Kratos, Japan) measurements were employed to analyze the oxidation state of the elements of PT-BCT. Ultraviolet−visible (UV−vis) absorption spectra of the 0.6PT−0.4BCT5 powders were measured on a UV−vis spectrophotometer (TU-1901). The first-principles calculations were performed by CASTEP,12 a plane-wave pseudopotential total enery package based on a density functional theory (DFT) method. The effective interactions between the atom cores and valence electrons (Bi 5d106s26p3, Co 3d74s2, Ti 3d22s2, and O 2s22p4) were modeled by optimized norm-conserving pseudopotentials,13 which allow us to choose a relatively small planewave basis set without compromising the computational accuracy. The kinetic energy cutoff 550 eV and dense Monkhorst−Pack14 k-point mesh spanning less than 0.05 Å−1 in the Brillouin zone were chosen. Besides, to account for the effect of localized d orbitals on the electron structure, the LDA+U method15 with the on-site orbital dependent Hubbard Ud = 8 eV for both titanium and cobalt was employed in the calculation. In the crystal optimization, the BFGS16 scheme was chosen, and the convergence tolerances were set to 10−5 eV/atom, 0.03 eV Å−1, 0.05 GPa, and 0.001 Å for energy, max force, stress, and displacement, respectively. For the band structure calculation, the random occupation between Co and Ti is modeled by the supercell methodology, and one 2 × 2 × 2 supercell was built with half of the (Ti, Co) position occupied by titanium and half occupied by cobalt atoms. It is true that there is a difference between the real materials and our computational model, since in the real material the (Ti, Co) position is randomly occupied by titanium and cobalt atoms. We think the supercell methodology can be used to investigate the effect of doping atoms on the electron structure, which has been widely performed previously.17−19

Figure 1. Unit cell volumes of 0.7PT−0.3BCT and 0.6PT−0.4BCT5 as a function of temperature. The inset is the 0.6PT−0.4BCT5 sample.

0.4BCT5 is negligible, indicating ZTE behavior. Another merit of the present 0.6PT−0.4BCT5 is that ZTE occurs in a wide temperature range from RT up to 500 °C. For comparison, the thermal expansion properties for representative ZTE materials are summarized (Table S3). It needs to be mentioned that the ZTE property in most materials can be obtained below RT due to a low contribution from lattice vibration, and can normally be in a narrow temperature range window, such as in Invar alloys (α̅ V = 3.6 × 10−6 °C−1, 5 to 30 °C),4 Fe[Co(CN)6] (α̅V = −4.41 × 10−6 °C−1, −268.8 to 27 °C),20 and N(CH3)CuZn(CH3)4 (α̅ V = 0.6 × 10−6 °C−1, −73 to 127 °C).21 Very few ZTEs can be achieved in the high-temperature range. The present ZTE is comparable to that of typical ZTE materials of Zr0.4Sn0.6Mo2O8 (α̅V = −0.18 × 10−6 °C−1, −261 to 327 °C),7 TaO2F (α̅ V = −3 to 3 × 10−6 °C−1, 25 to 600 °C),10 and (Sc0.85Ga0.05Fe0.10)F3 (α̅ V = −0.70 × 10−6 °C−1, 27 to 627 °C).22 The wide ZTE temperature range in 0.6PT−0.4BCT5 is ascribed to the enhanced spontaneous polarization (PS), which can be calculated through the structural refinement results by assuming a point charge model.23 The present 0.6PT− 0.4BCT5 species exhibit a large PS (59.4 μC cm−2) (Table S4), which will induce a high TC according to Landau theory with the relationship TC = αPS2.23 The variation of PS can directly affect the thermal expansion property, due to the strong correlation between ferroelectricity and NTE in the PT-based ferroelectrics.4 With increasing temperature, PS of 0.6PT− 0.4BCT5 decreases slightly and can be well-maintained at a high value upon the approach to TC (Figure 2), resulting in a little change in unit cell volume as a function of temperature. The NTE behavior of PT-based ferroelectrics can also be well-

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Thermal Expansion Property. The crystal structure for all investigated compositions has been indexed to have tetragonal symmetry according to the B

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Figure 2. Temperature evolution of the spontaneous polarization (PS) in 0.6PT−0.4BCT5. The inset is the schematic illustration of the role of PS (δzPb/Bi and δzTi/Co).

interpreted by the concept of spontaneous volume ferroelectrostriction (SVFS),4 defined as ωS =

Vexp −Vnm Vnm

× 100%

(1)

where Vexp indicates the unit cell volume of the experimental one, and Vnm indicates the nominal one which can be obtained by extrapolation from the paraelectric to the ferroelectric phase. The SVFS can quantitatively describe how ferroelectricity contributes to the abnormal change in the ferroelectric phase volume. Strong ferroelectricity induces a large value of ωS and thus a strong NTE, while a weakened one induces a small value of ωS and produces low thermal expansion. In the present study, the composition of 0.6PT−0.4BCT5 exhibits a smaller ωS (1.6%) than that of 0.7PT−0.3BCT (1.8%), but it is much smaller than that of PT (3.1%).4 It corresponds well with the change in CTEs of 0.6PT−0.4BCT5 (−0.033× 10−5 °C−1), 0.7PT−0.3BCT (−1.73× 10−5 °C−1), and PT (−1.99 × 10−5 °C−1).4 3.2. Electrical Property. Electrical behavior of 0.6PT− 0.4BCT and 0.6PT−0.4BCT5 has been characterized over a wide temperature range as a function of frequency by using ac impedance methods (Figures S8 and S9). Details of the experiments are provided in the Supporting Information. Upon substitution of Ti with a small amount of Co, the resistance of 0.6PT−0.4BCT5 has been dramatically reduced at RT (Figure S8). The temperature dependence of selected complex impedance spectrum Z′ and Z″ (called Nyquist plot) of 0.6PT−0.4BCT5 is shown in Figure 3a. The effect of temperature on impedance behavior of 0.6PT−0.4BCT5 is clearly visible with a rise in the selected temperatures. The presence of two semicircular arcs in the impedance spectrum of the prepared ceramics indicates the presence of both bulk (Rg) and grain boundary (Rgb) contributions to its overall electrical property. The values of Rg and Rgb can be obtained from the intercept on the Z′-axis. It can be noticed that the Rg and Rgb of 0.6PT−0.4BCT5 decrease with the increase in temperature, which suggests the semiconducting feature because of the negative temperature coefficient of resistance (NTCR).24 As temperature increases to 200 °C, the overall resistance of 0.6PT−0.4BCT5 sharply reduces by about 3 orders of magnitude.

Figure 3. (a) Complex impedance plots of 0.6PT−0.4BCT5 at selected temperatures of RT and 200 °C. The inset is the equivalent circuit. (b) Temperature dependence of electrical conductivity for 0.6PT−0.4BCT5 from RT to 500 °C.

The temperature dependent ac electrical conductivity (σac) of 0.6PT−0.4BCT5 has also been investigated (Figure 3b), which obeys the Arrhenius equation24 σac = σ0 exp( −Ea /kBT )

(2)

where σ0 is the pre-exponential factor, Ea is the electrical activation energy of conduction, and kB is the Boltzmann constant. It is observed that the ac conductivity of 0.6PT− 0.4BCT5 shows a good linear relationship with increasing temperature. The conductivity of 0.6PT−0.4BCT5 is approximately 7 × 10−8 S cm−1 at RT while it dramatically increases to about 1.0 × 10−2 S cm−1 at 500 °C. A semiconducting property has been achieved in the present material 0.6PT−0.4BCT5. In the previous NTE/ZTE materials, an insulating property was observed in NTE oxides of ZrW2O8 (σac < 10−8 S cm−1 at 500 K)25 and PbTiO3 ferroelectric ceramics,26 while there is a conducting property in the alloys of LaFe10.5Co1.0Si1.5 (6−7 × 103 S cm−1 at 77−340 K)27 and YbGaGe (2.2−3.7 × 103 S cm−1 at 77−400 K).25 The activation energy (Ea) of 0.6PT− 0.4BCT5 extracted from the σ−T plots has been determined to be 0.56 eV. There are certain studies suggesting that the oxide ion conductors normally have Ea higher than 1 eV; the Ea value range 0.2−0.9 eV corresponds to p-type polaronic conductors, and for n-type polaronic conductors it is less than 0.2 eV.28 It suggests that the present ZTE 0.6PT−0.4BCT5 could be a ptype semiconductor. Electric conduction is in general composed of electronic conduction and ionic conduction. In perovskite oxides, the only possible ionic conductivity comes from the migration of oxygen C

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between peaks corresponding to oxygen vacancy and OH groups was not possible. The fit of the O 1s spectrum for 0.6PT−0.4BCT and 0.6PT−0.4BCT5 assigned BEs of ∼531 eV to oxygen in the lattice, and ∼529.5 eV for oxygen vacancy and OH groups. The BEs are close to previous studies of the O 1s spectrum in perovskite.31 Note that the proportion of oxygen vacancies (S/M) increases by changing the stoichiometric ratio of Co and Ti of 0.6PT−0.4BCT (Figure S12), which corresponds well with the increased amount of Ti3+ (Figure 4 and Figure S11). The presence of Ti3+ may lead to electron hopping conduction from Ti3+ to Ti4+, which was already known in spinel materials.32 In the present perovskite system, both Ti3+ and Ti4+ occupy the octahedral site, and there is one oxygen atom in between Ti3+ and Ti4+. The direct hopping of electrons from Ti3+ to Ti4+ is blocked by an oxygen atom. Nevertheless, such hopping becomes possible when oxygen vacancies are present, which act as a “bridge” between Ti3+ and Ti4+. The oxygen vacancies carrying positive charge attract electrons from Ti3+, but they are not effective electron traps. Electrons can easily escape from the oxygen vacancies traps and hop to Ti4+. Therefore, conductivity is enhanced. To confirm the semiconductor feature of the ZTE ferroelectric of 0.6PT−0.4BCT5, the UV−vis spectroscopy measurement has been investigated (Figure S13). A considerably narrowed band gap is observed in 0.6PT−0.4BCT5 with a noticeable visible-light absorption and a small band gap Eg value of 1.5 eV. The relatively lower Eg is comparable with the recently reported (Sc0.85Ga0.05Fe0.1)F3 (Eg = 1.9 eV),22 and much lower than the typical NTE material of ZrW2O8 (Eg = 4.0 eV),33 ScF3 (Eg = 5.5−6.0 eV),22 and PT (Eg = 2.8 eV) (Figure S13). The color change also indicates the change in band gap. Pure PT exhibits a light yellow color, while the present sample of 0.6PT−0.4BCT5 is dark brown (see the inset of Figure 1). 3.4. First-Principles Calculations. In order to further investigate the influence of Co−Ti substitution on the formation of oxygen vacancies, first-principles calculations34 have been performed, in which a 2 × 2 × 2 Bi(Co0.5Ti0.5)O3 supercell (40 atoms) has been built. In this model the parent PbTiO3 lattices were not considered because they are not involved in the Ti−Co substitution. As shown in Figure 5, the Ti and Co atoms are located on the different (001) planes, coordinated to the neighbor “bridging” or “dangling” oxygen atoms. The electron density difference (EDD) maps in the (010) plane (as shown in Figure 5) clearly show that many more electrons are accumulated on the Ti−O bonds as compared with the Co−O bonds, revealing the stronger bond strength of the former. This conclusion is also demonstrated by the Mulliken population35 analysis, i.e., the bond orders of Ti− O bonds are much larger than those of Co−O bonds. Therefore, the oxygen atoms connected to Co atoms are easier to remove to form the oxygen vacancies; the formation energies for oxygen vacancies at the sites around Co atoms are smaller than those around Ti atoms (also labeled in Figure 5). This explains the experimental observations that the content of oxygen vacancies increases as more Co atoms are incorporated into the crystal lattice (Figure S12). Moreover, since the valence state of Co is equal to +2, the electrons left by the formation of oxygen vacancies cannot be completely saturated by the Co2+ cations. Some charges are transferred to the Ti cations, inducing the valence state of Ti from +4 to +3. Thus, the existence of the Ti3+ cation was revealed by the XPS measurement.

vacancies, which, if present, would lead to a spike on the complex impedance plots at low frequency.29,30 However, no such spike is observed in 0.6PT−0.4BCT5. Therefore, the ionic contribution from the migration of oxygen vacancies to the electric conduction of the 0.6PT−0.4BCT5 ceramics is not significant. On the basis of this consideration, it is reasonable to believe that the electronic conduction plays a more important role in the electric conduction. The mechanism for the electric conduction can be explained by the hopping conductivity model. It is proposed that the electric conduction of NTCR materials is attributed to the electronic hopping from one ionic site to another. Further discussions follow. 3.3. XPS and UV−Vis Spectroscopy of (1 − x)PT−xBCT. According to the XPS data, one can see that the cobalt is stabilized in the bivalent state (Figure S10). However, the Ti 2p peaks of ZTE 0.6PT−0.4BCT5 can be deconvoluted into four peaks as Ti3+ 2p3/2 at 457.5 eV, Ti4+ 2p3/2 at 458.0 eV, Ti3+ 2p1/2 at 463.5 eV, and Ti4+ 2p1/2 at 465.2 eV (Figure 4). The

Figure 4. X-ray photoelectron spectroscopy patterns (XPS) of Ti 2p for (a) PbTiO3 and (b) 0.6PT−0.4BCT5, respectively.

Ti3+ can be evidenced by the peak splitting approximately at 465 eV for both 0.6PT−0.4BCT and 0.6PT−0.4BCT5, while for pure PT there is only a singlet at the same position (Figure 4 and Figure S11). In the O 1s spectra shown in Figure S12, the fit of the spectrum was resolved using two peaks where binding energy (BE) of the higher intensity corresponds to oxygen in the PT−BCT lattice (M), including Pb−O, Bi−O, Ti−O, and Co−O bonding, whereas the BE of the lower intensity was attributed to the presence of oxygen vacancy and hydroxyl (OH) groups on the surface (S). A fit for distinguishing D

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ZTE materials serve as a particular class of functional materials, which are used typically in fields of precision engineering, such as microelectronic and optical devices. The present multifunctional material of 0.6PT−0.4BCT5, combining ZTE and semiconducting and ferroelectric characteristics, could be promising for new applications. For example, semiconducting ferroelectric is a new kind of functional property of materials, which would be an effective way to enhance solar energy convention efficiency.36 The present ZTE and controllable thermal expansion property of semiconducting ferroelectrics may be helpful for the substrate of solar conversion devices due to avoidance of thermal fluctuation in a certain temperature range.

4. CONCLUSIONS In conclusion, a wide temperature range ZTE has been found in (1 − x)PbTiO3−xBi(Co, Ti)O3 by the chemical substitution of low-valence Co2+ for high-valence Ti4+. Intriguingly, the present ZTE material exhibits semiconducting characteristics with the NTCR behavior, which is also confirmed by the narrow band gap (1.5 eV). The semiconducting nature is attributed to the increased oxygen vacancies induced by the nonequivalence substitution, evidenced by both of the XPS experiments and first-principles calculations. The present study combining functional properties of ZTE, semiconductance, and ferroelectricity would promise new applications to be enabled.

Figure 5. Map of electron density change before and after the formation of chemical bonds (electron density difference, EDD) in the (010) plane. The Ti, Co, and O atoms are represented by gray, blue, and red balls, respectively, and M indicates the Mulliken population.

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ASSOCIATED CONTENT

S Supporting Information *

In fact, the introduction of the complexes of Co and oxygen vacancies not only affects the valence states on Ti cations, but also influences the energy band gaps of the (1 − x)PT−xBCT compounds. Figure 6 displays the electronic density of states close to the forbidden bands in the Bi(Co0.5Ti0.5)O3 supercell with and without the oxygen vacancies. It is clear that the presence of oxygen vacancies on both “bridging” and “dangling” sites results in the occurrence of defect levels in the forbidden band, which leads to a significant decrease of the energy band gap. The increase of oxygen vacancies induced by more Co substitution would introduce more defect levels, and further decrease the band gap such as in 0.6PT−0.4BCT5.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02761.

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SXRD patterns, structure refinements, thermal expansion properties, and spectroscopy results (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Figure 6. Density of states (DOS) of the (a) perfect lattice, (b) the lattice with an oxygen vacancy at the dangling (O1 site), and (c) bridging (O2 site) oxygen connected to cobalt atoms. The oxygen vacancy is indicated by the red circle. E

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Jun Chen: 0000-0002-8693-2508 Xianran Xing: 0000-0003-0704-8886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Grants 21322102, 21590793, 91422301, 21231001), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), and the Fundamental Research Funds for the Central Universities, China (Grant FRF-TP-14-012C1). Z.L. and X.J. acknowledge the Special Foundation of the Director of Technical Institute of Physics and Chemistry, CAS. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357.

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DOI: 10.1021/acs.inorgchem.6b02761 Inorg. Chem. XXXX, XXX, XXX−XXX