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Cite This: Inorg. Chem. 2017, 56, 14129-14134
Raman and Density Functional Theory Studies of Li2Mo4O13 Structures in Crystalline and Molten States Songming Wan,*,†,‡ Bo Zhang,† Yanan Yao,†,§ Guimei Zheng,†,§ Shujie Zhang,†,§ and Jinglin You*,‡ †
Anhui Key Laboratory for Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China § University of Science and Technology of China, Hefei 230026, China ‡ School of Material Science and Engineering, Shanghai University, Shanghai 200072, China S Supporting Information *
ABSTRACT: The Li2Mo4O13 melt structure and its Raman spectral characteristics are the key for establishing the composition−structure relationship of lithium molybdate melts. In this work, Raman spectroscopy, factor group analysis, and density functional theory (DFT) were applied to investigate the structural and spectral details of the HLi2Mo4O13 crystal and a Li2Mo4O13 melt. Factor group analysis shows that the crystal has 171 vibrational modes (84Ag + 87Au), including three acoustic modes (3Au), six librational modes (2Ag + 4Au), 21 translational modes (7Ag + 14Au), and 141 internal modes (75Ag + 66Au). All of the Ag modes are Raman-active and were assigned by the DFT method. The Li2Mo4O13 melt structure was deduced from the H-Li2Mo4O13 crystal structure and demonstrated by the DFT method. The results show that the Li2Mo4O13 melt is made up of Li+ ions and Mo4O132− groups, each of which is formed by four corner-sharing MoO3Ø/MoO2Ø2 tetrahedra (Ø = bridging oxygen). The melt has three acoustic modes (3A) and 54 optical modes (54A). All of the optical modes are Ramanactive and were accurately assigned by the DFT method. 2, 3, and 4) melts.11 Voronko et al. deemed that the spectral changes arose from a series of structural changes. When MoO3 was added into a Li2MoO4 melt, monomeric MoO42− groups connected with MoO 3 molecules and stepwise-formed Mo2O72−, Mo3O102−, and Mo4O132− groups that consist of two, three, and four corner-sharing MoO3Ø/MoO2Ø2 (Ø = bridging oxygen) units, respectively.10 The viewpoint is very important for establishing the composition−structure relationship of alkali molybdate melts. However, extracting structural information from a melt Raman spectrum often suffers from limited spectral resolution because of the low-intense, broad, and overlapping Raman bands at high temperature, which often leads to uncertainty in the band assignments and then in structural determination. Therefore, theoretical studies are essential for analyzing the Raman spectra and structures of most melts including the alkali molybdate melts. First-principles calculation provides a means to simulate the Raman spectrum of a melt with an established structure and is an effective tool for aiding the experimental analyses of various melt Raman spectra and structures.12−15 Recently, Wang and co-workers studied the Raman spectra of the K2MonO3n+1 (n = 1, 2, and 3) melts with the restricted Hartree−Fock (RHF) method,16 and we studied the Raman spectra of the
1. INTRODUCTION Alkali molybdates are one kind of old and versatile flux used to produce various single-crystal materials.1 The first reported alkali molybdate flux is lithium molybdate, which was employed to prepare emerald crystals as early as 1888.2 Intense interest in alkali molybdate fluxes was kindled by the emergence of borate nonlinear optical crystals in the 1980s, during which the K2Mo3O10 flux was widely used to grow a large variety of huntite-type crystals with the general formula RM3(BO3)4 (R = rare-earth element; M = Al, Sc, Fe, Ga, or Cr).3 In the following 2 decades, Li2Mo3O10/Li2Mo4O13 and Cs2Mo2O7 fluxes were frequently used to grow LiB3O5 and CsB3O5 crystals of highquality and large size, respectively.4−9 The widespread application of alkali molybdates has attracted great interest in the study of their fluxing behavior, which is believed to be essential in understanding numerous crystal growth processes. The structures of alkali molybdate melts are an important aspect for a full understanding of their fluxing behavior. Recently, Voronko and co-workers investigated the structures of four potassium molybdate melts with the general formula K2MonO3n+1 (n = 1, 2, 3, and 4) by high-temperature Raman spectroscopy.10 Their results showed that the melt spectra changed systematically with an increase in the MoO3 content but subtly upon a further addition of MoO3 into a MoO3-rich melt such as K2Mo3O10. Similar spectral characteristics were also observed as we studied the spectra of Li2MonO3n+1 (n = 1, © 2017 American Chemical Society
Received: September 3, 2017 Published: October 31, 2017 14129
DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
Article
Inorganic Chemistry
Li2Mo4O13 is the most stable phase at both room and high temperatures. The XRD pattern of the synthesized Li2Mo4O13 polycrystalline powder is shown in Figure 1. It matches with the H-Li2Mo4O13 standard pattern (JCPDS 70-1709), indicating that the product is H-Li2Mo4O13. The Raman spectrum of HLi2Mo4O13 is presented in Figure 2.
Li2MonO3n+1 (n = 2 and 3) melts with the density functional theory (DFT) method.11 However, the Raman spectra of alkali molybdate melts rich in MoO3, such as alkali tetramolybdate melts, still remain unexplored, although they are especially important to unraveling the subtle spectral characteristics of MoO3-rich melts and then to determine the composition− structure relationship of alkali molybdate melts. In this paper, the DFT method is used to analyze the structure of the Li2Mo4O13 melt. Prior to this, we study the Raman spectrum of the H-Li2Mo4O13 crystal because of its correlation with that of the Li2Mo4O13 melt.
2. EXPERIMENTAL SECTION Li2Mo4O13 polycrystalline powder was prepared using a two-step solidstate synthesis method. Analytical-grade Li2CO3, H3BO3, and MoO3, purchased from Sinopharm Chemical Reagent Co. Ltd., were used without further purification. In the first step, polycrystalline Li2MoO4 was synthesized by heating a mixture of Li2CO3 and MoO3 with a molar ratio of 1:1 at 500 °C for 12 h and then sintering at 650 °C for 12 h. In the second step, the polycrystalline Li2Mo4O13 was synthesized by heating twice a mixture of Li2MoO4 and MoO3 in a Li2MoO4/MoO3 molar ratio of 1:3 at 550 °C for 12 h. Prior to each heat treatment, the reactants were ground and mixed thoroughly in an agate mortar. The formation of Li2MoO4 and Li2Mo4O13 was confirmed by X-ray diffraction (XRD) analysis, which was performed on a computer-controlled MXPAHF diffractometer with graphitemonochromatized Cu Kα radiation (λ = 1.54056 Å). The Li2Mo4O13 polycrystalline powder was placed in a 2 mm × 5 mm × 10 mm platinum boat and heated by a homemade microfurnace. When the sample began to melt, the temperature was held until a clear and homogeneous melt was obtained. Raman spectra of the polycrystalline powder/melt were recorded on a Jobin Yvon LABRaman HR800 spectrometer equipped with a confocal microscope. The 532 nm line of a Q-switched pulsed SHG−Nd:YAG laser was employed as the excitation source with a power of about 1.0 W. The scattering light was collected in a back-scattering configuration with a total integration time of 200 s for the polycrystalline sample and 100 s for the melt sample. The spectral resolution was less than 1.0 cm−1. All of the DFT calculations were carried out using plane-wave basis sets and the Wu−Cohen functional of the generalized gradient approximation17 available in the Cambridge Sequential Total Energy Package (CASTEP) code.18 Ion−valence electron interaction was represented by norm-conserving pseudopotentials. The valence electron configurations were 2s1 for lithium, 4s24p64d55s1 for molybdenum, and 2s22p4 for oxygen. In these calculations, the tolerances of the self-consistent field, force, and stress were set as 5 × 10−7 eV/atom, 0.05 eV/Å, and 0.1 GPa, respectively. According to the results of convergence tests for the total energies of the Li2Mo4O13 crystal and melt (see Tables S1 and S3 and Figures S1 and S2 for more details), the Brillouin zone integrations were done over a 2 × 1 × 2 Monkhorst−Pack grid for the crystal and over a 3 × 3 × 1 Monkhorst−Pack grid for the melt; a plane-wave cutoff of 800 eV was set for both the crystal and melt. Density functional perturbation theory (DFPT) was employed to probe the lattice dynamics of the Li2Mo4O13 crystal and melt.19−21 Raman frequencies were obtained by diagonalization of the dynamical matrices. Raman intensities were calculated using a hybrid DFPT− finite displacement method and corrected by Bose−Einstein factors corresponding to the experimental temperatures (300 K for the Li2Mo4O13 crystal and 850 K for the Li2Mo4O13 melt) and the wavelength of the excitation source (532 nm). The Raman lines were broadened with a Lorentzian line-shape function using the SWizard software.22
Figure 1. XRD pattern of the synthesized Li2Mo4O13 polycrystalline powder with the standard XRD pattern of H-Li2Mo4O13 (JCPDS 701709).
Figure 2. Experimental and calculated Raman bands of the HLi2Mo4O13 crystal and their corresponding modes (ν, stretching vibration; δ, bending vibration; ω, wagging vibration; τ, twisting vibration).
Much work has been done to show the spectral dependence of structural variations.26−28 Considering that the Li2Mo4O13 melt is evolved from the H-Li2Mo4O13 polycrystalline powder, analysis of the H-Li2Mo4O13 crystal Raman spectrum should be helpful for determining the melt spectrum and structure. Here, we used the factor group analysis method to classify the vibrational modes of the H-Li2Mo4O13 crystal under the assumption that the internal vibrations of the molybdate groups are independent of the lattice vibrations.29,30 The HLi2Mo4O13 crystal structure has been reported by Gatehouse and Miskin.23 It crystallizes in the triclinic P1̅ space group with three formulas per unit cell. All of the atoms occupy the 2i sites except for an oxygen atom, which occupies the 1b site. The factor group of the H-Li2Mo4O13 crystal at the Brillouin zone center is isomorphic to the Ci point group. Table 1 lists the symmetry operations of the Ci point group, the numbers of the atoms invariant under the operation R (NR), and the characters of the irreducible representation under the symmetry operation R (χR). According to factor group analysis, the H-Li2Mo4O13 crystal has 171 vibrational modes (84Ag + 87Au), including three acoustic modes (3Au), 21 translational modes (7Ag +
3. RESULTS AND DISCUSSION Li2Mo4O13 exists in three different crystalline modifications, defined as H, M, and L phases.23−25 Among them, H14130
DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
Article
Inorganic Chemistry Table 1. Factor Group Analysis of H-Li2Mo4O13 Vibrational Modes
a N, s, and v are the numbers of total atoms, molybdate groups plus lithium ions, and lithium ions in the H-Li2Mo4O13 unit cell, respectively. NR(N), NR(s), and NR(s−v) are the numbers of the total atoms whose positions remain invariant, the molybdate groups plus the lithium ions whose centers of mass remain invariant, and the molybdate groups whose centers of mass remain invariant under the symmetry operation R, respectively. bχR(N), χR(T), χR(T′), χR(R′), and χR(n) are the characters of the irreducible representation under the symmetry operation R. N, T, T′, R′, and n denote all of the vibrational modes, the translational modes of the H-Li2Mo4O13 unit cell as a whole, the other translational modes, the librational modes, and the internal modes, respectively. The plus or minus sign is used according to whether R is a pure rotation or a rotation accompanied by a reflection.
Figure 3. Atomic displacements for five selected Raman modes of the H-Li2Mo4O13 crystal.
temperature (see Figure 4b for an example). The phenomenon can be qualitatively interpreted in terms of anharmonic forces
14Au), six librational modes (2Ag + 4Au), and 141 internal modes (75Ag + 66Au). All of the Ag modes are Raman-active. The calculated Raman spectrum of the H-Li2Mo4O13 crystal is presented in Figure 2 and compared with the experimental one. The comparison shows a satisfactory agreement between the calculated and experimental frequencies and an overall agreement between the intensities. It is worth noting that the calculated intensities of some modes in the high-frequency region are underestimated. Similar results were also found in the calculated Raman spectra of K2MonO3n+1 (n = 1, 2, and 3) crystals;16 we thus believe that the underestimation is due to the inherent shortcoming of the calculated method. The calculation demonstrates that the H-Li2Mo4O13 crystal has 84Ag Raman-active modes and provides the atomic displacements of each mode. The two lowest-frequency Raman modes (2Ag), located at 69 and 73 cm−1 (calculated values, which are the same below), are primarily due to the librational vibrations of the molybdate groups. The other seven Raman external modes (7Ag), located at 234, 369, 376, 379, 431, 452, and 460 cm−1, are assigned to the translational vibrations of the lithium ions. The remaining Raman modes (75Ag) are attributed to the internal modes of the molybdate groups, and their corresponding vibrational bands can be divided into three regions/types (see Figure 2): (1) The bands below 460 cm−1 arise from the wagging/twisting vibrations of the molybdate groups, the translations of the lithium ions, or their combination. (2) The bands in the 420−746 cm−1 range arise from the bending vibrations of the Mo−O−Mo bonds. (3) The bands in the 845−991 cm−1 range arise from the stretching vibrations of the MoO bonds. Atomic displacements for five typical Raman bands are illustrated in Figure 3. The Raman spectra of the Li2Mo4O13 polycrystalline powder recorded below its melting point show an expected broadening of bands and an expected decrease in frequency with increasing
Figure 4. Raman spectra of the H-Li2Mo4O13 polycrystalline powder recorded at (a) room temperature and (b) 550 °C. Raman spectra of the Li2Mo4O13 melt recorded at (c) 570 and (d) 585 °C.
of chemical bonds in the H-Li2Mo4O13 crystal structure.31 This result implies that the H-Li2Mo4O13 crystal structure is stable below its melting point. According to the Li2MoO4−MoO3 phase diagrams reported by Van Der Wielen et al.,32,33 Li2Mo4O13 melts incongruently, which indicates that a melt with a composition different from that of Li2Mo4O13 will be produced upon melting the HLi2Mo4O13 polycrystalline powder. In view of this, we collected the Raman spectra of the melts when H-Li2Mo4O13 melted partially (Figure 4c) and completely (Figure 4d). Interestingly, the two spectra are exactly the same, indicating that the melt composition was fixed at Li2Mo4O13 throughout the melting process, and thus H-Li2Mo4O13 must be a congruently melting compound. The conclusion is consistent with the experimental results found by Gatehouse and Miskin.25 In their experiment, a metastable L-Li2Mo4O13 crystal was directly produced by slowly cooling a Li2Mo4O13 melt. The congruently melting behavior of Li2Mo4O13 suggests that the Li2Mo4O13 melt and H-Li2Mo4O13 crystal, as well as the L-Li2Mo4O13 crystal, are probably made 14131
DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
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Inorganic Chemistry
optimization, the unit cell parameters and atomic positions were relaxed in order to obtain the most stable conformation. The result is shown in Figure 6. The optimized Mo−O bond
up of different molybdate groups with the same chemical composition. Figure 4 shows that the Raman bands of low and middle frequency change drastically in the crystal−melt transition. According to the above-calculated results, the low- and middlefrequency bands are mainly related to the wagging/twisting vibrations of the molybdate groups or the bending vibrations of the Mo−O−Mo bonds (see Figures 2 and 3). The spectral change proves that the molybdate groups in the melt are different from those in the crystal. On the other hand, the highfrequency bands of the crystal corresponding to the stretching vibrations of the MoO bonds (see Figure 2 and 3) persist in the melt spectrum, suggesting that the MoO bonds still exist in the melt. The H-Li2Mo4O13 crystal structure is built up of Li+ ions and Mo−O ribbons that are formed by edge-sharing Mo−O octahedra or corner-sharing Mo−O pyramids.23 The crystal structure includes three types of Mo−O bonds: MoO double bonds with lengths shorter than 180 pm, Mo−O single bonds with lengths from 180 to 210 pm, and Mo−O weak bonds with lengths larger than 210 pm. According to the chemical bonding theory for describing a crystal growth/melting process,34,35 we consider that the melting of H-Li2Mo4O13 results from breakage of the weak Mo−O bonds. Along with the melting, the complicated Mo−O ribbons in the crystal structure are destroyed and then transform to Mo4O132− chains, each of which comprises four corner-sharing MoO 3 Ø/MoO 2 Ø 2 tetrahedra, as shown in Figure 5. Therefore, the Mo4O132−
Figure 6. Structural model of the Li2Mo4O13 melt (top) and its experimental and calculated Raman spectra (bottom): (a) experimental spectrum; calculated spectra broadened by a Lorentzian lineshape function with (b) experimental fwhm’s and (c) a fixed fwhm of 1 cm−1.
lengths and O−Mo−O angles coincide well with the reported values (see Figure S3 and Tables S4 and S5 for more details). The optimized structure consists of two types of Mo−O bonds: the first one is the MoO double bond, which is pertinent to the terminal oxygen atoms, ranging from 171.2 to 175.8 pm in length; the second one is the Mo−O single bond with a length of about 190 pm, which is pertinent to the bridging oxygen atoms. The Raman spectra of the Li2Mo4O13 melt were calculated on the basis of the optimized structural model and are presented in Figure 6b,c along with the experimental spectrum (Figure 6a). Considering that different Raman bands show different degrees of broadening at high temperatures but the degrees are difficult to predict, experimental full widths at halfmaximum (fwhm’s) were used here to broaden the calculated Raman bands (100, 80, and 50 cm−1 are used in the frequency ranges lower than 500 cm−1, from 500 to 900 cm−1, and larger than 900 cm−1, respectively). The final result is shown in Figure 6b. The spectrum broadened with a fixed fwhm of 1 cm−1 is also presented as Figure 6c in order to provide more detailed spectral information. Both of the calculated frequencies and peak profiles are in agreement with the experimental results, indicating that the optimized model can be used to describe the Li2Mo4O13 melt. The calculated intensities of high-frequency bands are found to be lower than the experimental ones, which is similar to the calculated spectrum of the Li2Mo4O13 crystal and also due to the inherent shortcoming of the calculated method. The Li2Mo4O13 melt model comprises two Li+ ions and one Mo4O132− chain that is formed by four corner-sharing MoO3Ø/ MoO2Ø2 tetrahedra. The model contains 19 atoms and thus has 57 vibrational modes (57A). With the exception of three acoustic modes (3A), the rest of the 54 modes (54A) are optical modes. DFT calculation demonstrates that the 54
Figure 5. (a) H-Li2Mo4O13 crystal structure viewed along the c axis. (b and c) Mo4O132− chains viewed along directions slightly deviating away from the b axis. The Li+ ions are omitted in the structure.
chain is the probable anion motif of the Li2Mo4O13 melt. As mentioned above, the L-Li2Mo4O13 crystal structure can be formed by cooling a Li2Mo4O13 melt. Upon breakage of the weak Mo−O bonds in a manner similar to that used for the HLi2Mo4O13 crystal, the molybdate groups in the L-Li2Mo4O13 crystal structure can also be regarded as an assembly of Mo4O132− chains,25 which further supports the theory that the Mo4O132− chain is the anion motif of the Li2Mo4O13 melt. On the basis of the above structural analyses, a Li2Mo4O13 melt structural model was constructed. One Mo4O132− chain, together with two Li+ ions, was placed in a monoclinic unit cell. Then the geometry of the model was optimized. During 14132
DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
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Inorganic Chemistry
942 and 950 cm−1 and those of the MoO3Ø tetrahedra at 888 and 898 cm−1; the MoO symmetric stretching vibrations of the MoO2Ø2 tetrahedra are located at 980 and 981 cm−1 and those of the MoO3Ø tetrahedra at 952 and 960 cm−1. The result reveals that the MO bonds of the MoO2Ø2 tetrahedra are stronger than those of the MoO3Ø tetrahedra.
optical modes are all Raman-active. Their frequencies and mode assignments are shown in Figure 7 (see Table S6 for
4. CONCLUSIONS Factor group analysis and the DFT method have been used to analyze the Raman spectrum of the H-Li2Mo4O13 crystal. The crystal has 171 vibrational modes (84Ag + 87Au), including three acoustic modes (3Au), six librational modes (2Ag + 4Au), 21 translational modes (7Ag + 14Au), and 141 internal modes (75Ag + 66Au). All of the 84Ag modes are Raman-active. The Raman bands below 460 cm−1 are related to the wagging/ twisting vibrations of the molybdate groups, the translations of the Li+ ions, or their mixture; the Raman bands in the range of 420−746 cm−1 arise from the bending vibrations of the Mo− O−Mo bonds; the Raman bands in the range of 845−991 cm−1 are attributed to the stretching vibrations of the MoO bonds. The Li2Mo4O13 crystal melts congruently. In the melting process, two types of weak Mo−O bonds in the crystal break. As a result, the Mo−O octahedra/pyramids in the Li2Mo4O13 crystal structure transform to Mo−O tetrahedra in the Li2Mo4O13 melt structure. The anion motif in the melt was demonstrated to be the Mo4O132− chain that is formed by four corner-sharing MoO3Ø/MoO2Ø2 tetrahedra. The Raman bands in the range of 160−440 cm−1 are mainly attributed to the wagging/twisting vibrations of the Mo4O132− chain; the Raman bands in the range of 460−570 cm−1 mainly arise from the motions of the Li+ ions; the Raman bands with frequencies of 747, 869, and 876 cm−1 can be assigned to the bending vibrations of the Mo−O−Mo bonds; the Raman bands with frequencies larger than 880 cm−1 belong to the stretching vibrations of the MoO bonds. The Mo−O−Mo bonds connecting two Mo−O tetrahedra are stronger than those connecting two Mo−O octahedra/pyramids; the MoO bonds of the MoO2Ø2 tetrahedra are stronger than those of the MoO3Ø tetrahedra. The work provides a new method to study the Raman spectra and structures of alkali molybdate melts. The results can be used to unravel the subtle spectral characteristics of various alkali molybdate melts rich in MoO3 and further to determine their composition−structure relationship.
Figure 7. Calculated Raman bands of the Li2Mo4O13 melt and their mode assignments (ν, stretching vibration; δ, bending vibration; ω, wagging vibration; τ, twisting vibration).
more details). The bands in the range of 160−440 cm−1 are attributed to the wagging and twisting vibrations of the Mo4O132− chain. The bands in the range of 460−570 cm−1 mainly arise from the motions of the Li+ ions. The bands with frequencies of 747, 869, and 876 cm−1 can be assigned to the bending vibrations of the Mo−O−Mo bonds. The bands with frequencies larger than 880 cm−1 are related to the stretching vibrations of the MoO bonds. Five typical vibrational modes are depicted in Figure 8.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 8. Five typical Raman modes of the Li2Mo4O13 melt and their frequencies.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02263. Convergence tests for the Li2Mo4O13 crystal and melt total energies with respect to the Γ-centered k-point grids and cutoff energies, calculated Li2Mo4O13 crystal vibrational modes and their frequencies, the initial Li2Mo4O13 melt structural model and its geometrical parameters, and calculated Li2Mo4O13 melt vibrational modes and their frequencies (PDF)
A comparison between Figures 2 and 7 shows that the frequencies of the Mo−O−Mo bending vibrations of the Li2Mo4O13 melt are higher than those of the Li2Mo4O13 crystal, which implies that the Mo−O−Mo bonds connecting two Mo−O tetrahedra are stronger than those connecting two Mo−O octahedra/pyramids. The Mo4O132− chain includes two types of MoO bonds: one belongs to the terminal MoO3Ø tetrahedra and the other to the middle MoO2Ø2 tetrahedra. Our calculated results demonstrate the deduction made by Voronko et al. that the MoO stretching vibrations of the MoO2Ø2 tetrahedra have higher frequencies than those of the MoO3Ø tetrahedra.10 For example, the MoO antisymmetric stretching vibrations of the MoO2Ø2 tetrahedra are located at
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S.M.W.). Fax: 86 0551 65591054. 14133
DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
Article
Inorganic Chemistry *E-mail: jlyou@staff.shu.edu.cn (J.L.Y.). Fax: 86 021 56331482.
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ORCID
Songming Wan: 0000-0002-8923-7914 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The work is financially supported by the National Natural Science Foundations of China (Grants 51372246 and 21773152) and Open Project of State Key Laboratory of Advanced Special Steel, Shanghai University. The calculations were partially performed at the Center for Computational Science, CASHIPS.
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DOI: 10.1021/acs.inorgchem.7b02263 Inorg. Chem. 2017, 56, 14129−14134
