Revisiting the Si–Te System: SiTe2 Finally Found ... - ACS Publications

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Revisiting the Si−Te System: SiTe2 Finally Found by Means of Experimental and Quantum-Chemical Techniques Kai C. Göbgen,† Simon Steinberg,† and Richard Dronskowski*,†,‡ †

Institute of Inorganic Chemistry and ‡Jülich-Aachen Research Alliance (JARA-FIT and -HPC), RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany S Supporting Information *

ABSTRACT: Through explorations of the silicon−tellurium system we identified the extremely air-sensitive, red Si1.67(4)Te3Si1.11(3)Te2 that is a silicon-deficient relative of the previously reported Si2Te3. The crystal structure features hexagonal closest packed layers of tellurium atoms with disordered [Si2] dumbbells residing in about 50% of the octahedra of every second layer enclosed by the tellurium atoms. In addition to the determination of the crystal structure for this silicon telluride, we probed the opportunity of the existence of a SiTe2 adopting the Si2Te3-structure by means of quantum chemical techniques. The investigations of the electronic structures and a subsequent chemical bonding analysis based on the projected Crystal Orbital Hamilton Population (pCOHP) technique for two “SiTe2” models revealed a tendency to align the [Si2] dumbbells parallel to the c axis to maximize Si−Te bonding. However, the disorder of the [Si2] dumbbells appears to be a consequence of non-equilibrium condensation into the solid state.

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INTRODUCTION The binary tetrel-tellurides GeTe, SnTe, and PbTe have attracted immense interest due to their applications as materials for data storage or thermoelectric energy conversion.1−3 Because silicon features a low commodity price as the second highest abundant element on earth,4 it is clear that a SiTe solidstate material showing the same properties as GeTe, SnTe, or PbTe would cause a drastic decrease in manufacturing costs. Hence, there have been several approaches to project synthesis of a solid state SiTe;5,6 yet, the electronic structures and the thermodynamic properties have to date been determined solely for gaseous SiTe molecules from explorations on SiTe in the vapor phase.7−10 While the determination of the SiTe crystal structure is still left open, there has also been a lot of controversy regarding the composition and the actual crystal structure of a known silicon telluride.11 Previous examinations identified a material with the stoichiometry Si2Te3 for the Si−Te system,7,10,12−14 whereas other research detected another composition, namely, SiTe2.15−17 Although there has been conclusive evidence for the presence of a Si2Te3 rather than a SiTe2 crystallizing with the CdI2-type,6,10,13 yet, the existence of a CdI2-type SiTe2 is still reported in textbooks and databases.18,19 More recent research demonstrated the capability of Si2Te3 nanoribbons and -plates to serve in near-infrared photodetection or LEDs and the potential of bulk Si2Te3 to be used as a state-of-the-art thermoelectric material;20,21 yet, the widths of the band gaps reported for this silicon telluride, which is expected to be a ptype semiconductor, vary widely.12,13,16,22,23 Inspections of the electronic structures for Si2Te3 revealed that the orientations of the Si dumbbells affect the width of the band gap.6,24 © 2017 American Chemical Society

In conjunction with the growth of high-quality Si2Te3 crystals, which are suited for a recently proposed high-pressure synthesis of a feasible SiTe from Si and Si2Te3, we critically revisited the controversy between Si2Te3 and SiTe2 by means of both experimental and quantum chemical techniques. In doing so, we determined the crystal structure of a silicon telluride with the overall composition Si1.67(4)Te3.

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EXPERIMENTAL TECHNIQUES

Syntheses. Si1.67(4)Te3 was prepared from reactions of the pure elements tellurium (Merck, > 99%) and silicon (North Chemical, > 99.8%). Mixtures of ∼250 mg total weight with the initial stoichiometry of Si2Te3 were ground, loaded into fused-silica tubes, and then flame-sealed under a vacuum of at least 2 × 10−3 mbar. The samples were then placed in computer-controlled furnaces, heated to 1273 K at a rate of 80 K/h, left at this temperature for 48 h, cooled to 473 K in 670 h, and finally cooled to room temperature at a rate of 100 K/h. The product appeared as gray-red powder containing red, plateshaped hexagonal crystals that are extremely sensitive to air and moisture in agreement with previous reports on the red silicon telluride.15,23 Due to the extreme sensitivity of the product to air and moisture, all sample preparations had to be completed under a dry argon atmosphere in a glovebox (MBraun, Garching, Germany). X-ray Diffraction Studies. The purities of the samples were checked based on phase analyses of powder X-ray diffraction data sets, which were collected on a STOE StadiP diffractometer at room temperature (Stoe & Cie GmbH, Darmstadt, Germany; area detector; Cu Kα radiation; λ = 1.54059 Å). The samples were first pestled and, then, loaded in capillaries that were closed with a resistance wire in a Received: July 20, 2017 Published: August 28, 2017 11398

DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405

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Inorganic Chemistry glovebox. The program WINXPOW25 was used for the data as well as phase analyses. Single-crystals were selected from the bulk and fixed in capillaries, which were sealed in the glovebox. The single crystals were subsequently transferred to a Bruker APEX CCD diffractometer (Bruker Inc., Madison, USA; Mo Kα; λ = 0.71073 Å) that was used for initial determinations of the quality of the selected crystals and the collection of the sets of single-crystal X-ray intensity data at room temperature. The integration of the raw intensity data sets and a multiscan absorption correction were carried out with the programs SAINT+ and SADABS, respectively.26 The structure was solved in the initial space group P3̅ (no. 147) utilizing direct methods (SHELXS-97) and refined in least-squares refinements on F2 including anisotropic atomic displacement parameters (SHELXL).27,28 A topological analysis of the initial structure model pointed to the super space group P3̅1c as the highest possible space group for Si1.67(4)Te3, in which the structure was finally solved and refined. The Platon code29 was used for transformation of the initial structure model into that corresponding to the space group P3̅1c. Close inspections of the Fourier map for Si1.67(4)Te3 revealed the presence of high residual electron densities in the vicinity of the Si2 site (Wyckoff position 12i). Further examinations of the distances and the geometry between the centers of the residual electron densities and the surrounding tellurium as well as silicon atoms corroborated the occurrence of two different Wyckoff sites for these disordered [Si2] dumbbells. Accordingly, the aforementioned residual electron densities were assigned to the Si3 atoms (Wyckoff site 4e) and with these settings the residual electron density converged to 2.11 e−/Å3. Analog analysis of the residual electron density in the near of the Si1 sites did not provide any hints to a similar type of disorder within these octahedrons. Another inspection of the diffraction patterns and an additional topological examination of the refined structure model did not reveal any higher symmetry for Si1.67(4)Te3, while the feasibility of stacking faults for the layered crystal structure is reviewed in the Results and Discussion section. Details of the data collection and the refinement parameters may be obtained from Table 1, while the atomic positions are listed in Table 2. Computational Details. Structural optimizations and electronic structure computations on “SiTe2” models were carried out with the projector augmented wave method30 (PAW) as implanted in the Vienna Ab Initio Simulation Package (VASP).31−35 Correlation and exchange were depicted by the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA−PBE).36 Because this functional

type fails to fully describe the van der Waals interactions between the Te layers, which do not encompass any silicon atoms, correction terms (GGA+D)37,38 were added to all calculations to take the dispersive nature of the interactions between these layers into account. Sets of 10 × 10 × 5 k-points were employed to sample the first Brillouin zones and the energy cutoff of the plane wave basis set was 500 eV. All computations converged until the energy difference between two iterative steps fell below 10−8 (and 10−6) eV/cell for the electronic (and the ionic) relaxation. A (chemical) bonding analysis for the “SiTe2” models was completed utilizing the projected Crystal Orbital Hamilton Population (pCOHP) technique,39 a variant of the COHP method, in which the bonding, the nonbonding, and the antibonding interactions are revealed from the off-site projected DOS weighted by the corresponding Hamilton matrix elements.40 Since the COHP method requires the use of crystal orbitals constructed from local basis sets, the Hamilton-weighted populations had to be projected from plane-wave sets using the Local Orbital Basis Suite Toward Electronic-Structure Reconstruction code (LOBSTER39−42). The DOS and −pCOHP curves were visualized with the aid of the wxDragon software package.43 The vibrational properties of the “SiTe2” model 2 that corresponds to the structure model with the lowest total energy were examined based on the phonon band structures and density-of-states, which were computed with the finite-displacement method44 as implemented in the PHONOPY program.45 In this approach, the phonon frequencies are evaluated from force constant matrices, for which the interatomic forces within supercells were calculated utilizing the projector augmented wave (PAW) method30 in the Γ-point approximation an approach that has also been employed elsewhere.6,46−48 The computations of the interatomic forces were conducted for supercells corresponding to 4 × 4 × 2 expansions of the original unit cell, while a reciprocal-space mesh of 10 × 10 × 5 used for the determination of the vibrational eigenvalues resulted in well-converged phonon DOS (PhDOS).

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RESULTS AND DISCUSSION Previous research on the Si−Te phase diagram detected a red air-sensitive silicon telluride for this system; however, the actual composition and crystal structure of this telluride has remained controversial: certain examinations revealed a Si2Te3 composition,7,12−14 while other studies identified a CdI 2-type SiTe2.15−17,22,49 In particular, both structure models feature hexagonal closest packed layers of tellurium atoms, but they differ in the distributions and amounts of Si atoms. In Si2Te3, the Si atoms establish disordered [Si2] dumbbells residing in 2/3 of all octahedral voids of every second layer enclosed by Te atoms, whereas Si atoms reside in all octahedral voids of every second layer encased by Te atoms in SiTe2. Further research on the Si-richer regions of the Si−Te system bared a clathrate-type material, i.e., Si20−x Te7+x (x ∼ 2.5), which was obtained under high-temperature high-pressure conditions.50,51 On the other hand, explorations on the Te-richer part of the Si−Te system revealed the crystallization kinetics of two silicon telluride glasses, that is, Si15Te85 and Si20Te80.52 In the context of the growth of high-quality Si2Te3 single crystals that are suited for a recently reported high-pressure synthesis of a solid-state SiTe,6 we detected a silicon telluride with the overall composition Si1.67(4)Te3, whose crystal structure and electronic structure will be discussed in the following. A phase analysis based on comparing the measured PXRD patterns to those of Si1.67(4)Te3 as well as potential side-products revealed the presence of the target compound besides the reagent tellurium (Figure S1). Indeed, previous research22 on the compositions in the Si−Te system already indicated that the differences between the weight percentages of the constituents for SiTe2 and Si2Te3 are

Table 1. Details of the Crystal Structure Investigation and Refinements for Si1.67(4)Te3Si1.11(3)Te2 Si1.67(4)Te3 fw space group a, Å c, Å Vol., Å3 Z density (calcd), g/cm3 μ, mm−1 F (000) θ range index ranges no. of reflns collected no. of independent reflns/Rint no. of reflns with I > 2σ (I)/Rσ refinement method data/restraints/parameter goodness-of-fit on F2 final R indices [F2 > 2σ(F2)] R indices (all data) largest diff. peak and hole, e−/Å3

429.64 P3̅1c (no. 163) 7.426(3) 13.479(5) 643.7(4) 4 4.43 13.7 717 3.0−26.3 −9 ≤ h ≤ 6, −6 ≤ k ≤ 9, −12 ≤ l ≤ 16 2475 447/0.050 357/0.041 full matrix least-squares on F2 447/0/33 1.18 R1 = 0.068; wR2 = 0.204 R1 = 0.072; wR2 = 0.211 2.11 and −3.03 11399

DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405

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Inorganic Chemistry Table 2. Atomic Positions and Equivalent Isotropic Displacement Parameters for Si1.67(4)Te3Si1.11(3)Te2 atom

position

x

y

z

Uiso/Ueq, Å2

Occ. (1000 °C), so the disorder of the [Si2] dumbbells could be a consequence of a non-equilibrium condensation into the solid state. Why, then, is an alignment of all [Si2] dumbbells parallel to the c axis not achieved?21 And yet, the “SiTe2” model 3 is 26.83 kJ/mol higher in energy than the “SiTe2” model 2 with the lowest total energy, probably because the Fermi level falls into a DOS maximum of the “SiTe2” model 3, which typically indicates an electronic instability (Figure S6).62 Accordingly, the alignments of all [Si2] dumbbells parallel to the c axis is energetically disadvantageous due to an electronically unfavorable situation. To comprehend the origin of the diverse disorders of the [Si2] dumbbells in the experimentally determined Si 1.67(4) Te 3 structure model at the atomic scale, we followed up with an examination of the electronic properties for the “SiTe2” models 1 and 2. An examination of the densities-of-states (DOS) curves for the two “SiTe2” models, whose structure models are generated based on that of Si1.67(4)Te3Si1.11(3)Te2, reveals that the states around the Fermi levels, EF, stem mostly from the Te-p as well as Si-p atomic orbitals (AOs; Figures 3, S3 and S4). The Fermi levels fall into band gaps for the “SiTe2” models 1 and 2, which typically signifies a semiconductor. Hence, this outcome agrees well with the red color observed for the silicon telluride and is in stark contrast to the results of the band structure computations for a CdI2-type SiTe2, for which EF falls at a peak in the DOS.6 Furthermore, electronically favorable situations are attained for the “SiTe2” model 1 and 2, while EF in the CdI2-type SiTe2 falls at a maximum of the DOS pointing to an electronically unfavorable situation for this structure.6 In addition, we also examined the vibrational properties of the “SiTe2” model 2, which corresponds to the lowest total energy. Because (negative) imaginary modes are not obvious in the phonon band structure of the “SiTe2” model 2 (Figure S5), there are no indicators pointing to a dynamic instability for the inspected structure.

Figure 2. Representations of the two different types of disorders for the [Si2] dumbbells established by the Si2 and Si3 atoms (blue circles, left bottom) and the Si1 atoms (red circles, right bottom) within the tellurium octahedra (90% thermal ellipsoids). Top: overview of the locations of the respective octahedra in the crystal structure of Si1.67(4)Te3.

among materials with layered crystal structures.49 More recent analyses on the growth of Si2Te3 nanoribbons and nanoplates were based on high-resolution transmission electron microscopy imaging and demonstrated that a verifiable detection of disorders is set hurdles due to the extreme sensitivity of the silicon telluride to air and moisture.20 Taking previous research on the stoichiometry and the crystal structure of this silicon telluride into consideration,14 the present results imply that Si1.67+xTe3 does not only display a considerable disorder of the [Si2] dumbbells, but also features a moderate composition range of 0.00 ≤ x ≤ 0.33, while maintaining the same overall structural characteristics. At this point, one may wonder if a SiTe adopting the Si2Te3 type of structure could be obtained; yet, recent quantum chemical examinations6 bared the presence of negative (imaginary) frequencies in the phonon band structures of such a SiTe, which should be dynamically unstable.57 To assess if a Si1.5Te3SiTe2 composition with the aforementioned structure model is electronically stable and also agrees with the observed physical properties,16,23 we followed up with a gedankenexperiment, in which we examined the electronic structure for a projected “SiTe2” with such structure model. Furthermore, we modified the orientations of the [Si2] dumbbells within those tellurium octahedra which are supposed to enclose the Si2 and Si3 sites in Si1.67(4)Te3 to gain insight into the origin of the disorder of the [Si2] dumbbells. Electronic Structure. More recent research on the electronic and the vibrational properties of the CdI2-type SiTe2 revealed a maximum at the Fermi level and the presence of negative (imaginary) modes in the phonon band structure, which point to both electronic and dynamic instabilities for this structure model.6 Additional examinations of the DOS curves and the phonon band structures for diverse models approximating the actual crystal structure of Si2Te3 bared not 11401

DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405

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Table 3. Average −IpCOHP/Bond Values (eV) of the Si−Si and the Si−Te Contacts in the Diverse “Si2Te3” and “SiTe2” Modelsa model

Si−Si

Si−Te

“Si2Te3”-I “Si2Te3”-II “Si2Te3”-III “SiTe2”-I “SiTe2”-II

4.6498 4.6538 4.6052 4.4456b 4.4536

4.1449 4.0322 4.1077 4.0461 4.1800

a Computational details regarding the diverse “Si2Te3” models and all −IpCOHP/bond values of selected interactions in the “Si2Te3” models have been reported elsewhere.6 b−IpCOHP values corresponding to Si−Si contacts ≥2.5 Å were not included.

Based on the hypothesis of a complete electron transfer from silicon to tellurium, a formal electron distribution according to the formula ([Si2]6+)(Te2−)3 could be achieved for Si2Te3; however, a similar treatment to a SiTe2(Si4+)(Te2−) whose crystal structure is derived from that of Si2Te3 cannot account for the presence of strong Si−Si bonds. On the basis of Pearson′s electronegativities,63 in which Si (4.77) is slightly less electronegative than tellurium (5.49), there is no clear tendency of Te to fully withdraw the valence electrons from silicon and, hence, the aforementioned complete distributions of electrons from silicon to tellurium should be treated with care. A closer inspection of the Si−Te −pCOHP curves (Figure 3) reveals that these interactions change from bonding to antibonding states at −1.21 eV for the “SiTe2” model 1 and −1.39 eV for the “SiTe2” model 2. Notably, such transitions from bonding to antibonding states below the Fermi levels are also evident in the Si−Te −pCOHP curves of the diverse inspected “Si2Te3” models.6 Under consideration of more recent research on the electronic structures of phase-change materials, in which vacancies are introduced to minimize the occupations of antibonding states,64,65 it may be inferred that the structure alleviates this electronically unfavorable situation by reducing the amount of silicon from Si2Te3 to SiTe2. Previous examinations14 on the structural features of the parental Si2Te3 showed that significant differences in the local atomic environments of the silicon atoms are evident solely for the Si−Te contacts. More specifically, the Si−Te distances between the tellurium atoms and the [Si2] dumbbells parallel to the c axis were observed to exhibit a smaller variance than those separations between the tellurium atoms and the [Si2] dumbbells that are tilted relative to the c axis.14 In that connection, it is worth noticing that more recent quantum chemical examinations on the chemical bonding in Si2Te3 revealed that the vast majority of the bonding population resides between the Si−Te contacts due to the high bond frequencies of these heteroatomic contacts.6 A comparison of the average Si−Te −IpCOHP/bond values for the “SiTe2” models (Table 3) reveals that the −IpCOHP values of these interactions are slightly larger in the “SiTe2” model 2 than in the “SiTe2” model 1. To understand this outcome in more detail, we followed up with a geometrical examination of the local atomic environments in the “SiTe2” model 2, in which each possible orientation of the [Si2] dumbbells is implemented. An inspection of the Si−Te bond lengths indicates that the average Si−Te distances between tellurium atoms and [Si2] dumbbells aligned parallel to the c axis are shorter than those between Te atoms and [Si2] dumbbells that are tilted with respect to the c axis (Table 4). Because the −IpCOHP values

Figure 3. Densities-of-states (DOS) and projected Crystal Orbital Hamilton Populations (−pCOHP) curves of the “SiTe2”models 1 (a, top) and 2 (b, bottom); the Fermi levels, EF, are represented by the black horizontal lines.

A (chemical) bonding analysis based on the projected Crystal Orbital Hamilton Populations (pCOHP) and their respective integrated values (IpCOHP) for both “SiTe2” models (Figure 3, Tables 3, S1 and S2) demonstrates that the vast majority of the bonding populations resides between the Si−Te contacts as well as the Si−Si dumbbells enclosed by the tellurium octahedrons. The Te−Te −IpCOHP values (Tables S1 and S2) are much smaller than to those of the Si−Te and Si−Si separations and are indicative of a weakly bonding character for these homoatomic contacts. As the −IpCOHP values tend to scale similarly to bond strength,62 the magnitudes of the Te− Te −IpCOHP values in relation to those of the Si−Te and Si− Si contacts imply the presence of (weaker) dispersion interactions for the Te−Te separations enclosing the empty octahedral voids between the Te layers. Accordingly, it can be inferred that strong bonding interactions are evident solely between the Si−Te and the Si−Si contacts. 11402

DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405

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crystal structures of the diverse “SiTe2” models; DOS curves of the “SiTe2” model 3 (PDF)

Table 4. Average Si−Te Distances and −IpCOHP/Bond Values in the “SiTe2” Model 2 Corresponding to the Lowest Total Energya interaction

d ⟨Si−Te⟩ (Å)

−IpCOHP/Bond (Si−Te)

Si1−Te Si2−Te Si3−Te

2.552 2.550 2.529

4.0771 4.2027 4.2601

Accession Codes

CCDC 1562977 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

a

Si atoms residing on the Si3 sites assemble [Si2] dumbbells parallel to the c axis, whereas the [Si2] dumbbells containing Si1 and Si2 atoms are tilted relative to the c axis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

have a tendency to scale alike to bond strength, typically the magnitude of the −IpCOHP values increases as the bond length decreases.62 In fact, the average Si−Te −IpCOHP/bond values corresponding to the Si−Te separations with [Si2] dumbbells aligned parallel to the c axis are larger than those of the Si−Te contacts with [Si2] dumbbells that are tipped relative to the c axis (Table 4). Accordingly, it can be inferred that the tendency to align the [Si2] dumbbells parallel to the c axis originates from the attempt to maximize the Si−Te −IpCOHP/bond values and, accordingly, overall bonding. Interestingly, there is a more evident reduction of the silicon amount on the Si1 and Si2 sites (∼20%) than on the Si3 positions (∼8%) from Si2Te314 to Si1.67(4)Te3, which may emphasize the tendency to maximize Si−Te bonding in this telluride.

ORCID

Richard Dronskowski: 0000-0002-1925-9624 Author Contributions

K. C. G. conducted the synthesis of Si1.67(4)Te3, prepared the samples for the SCXRD as well as PXRD experiments and carried out the crystal structure determination as well as phase analysis, S. St. determined the crystal structure, computed the electronic structures and prepared the manuscript, R. D. prepared the manuscript and supervised the project. Funding

This work was supported in part by the Deutsche Forschungsgemeinschaft (K. C. G. and R. D.) and the Verband der chemischen Industrie e. V. (FCI), Frankfurt a. M. (Liebigstipend to S. St.).

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Notes

CONCLUSION In summary, we report on the synthesis and the structure determination of Si1.67(4)Te3, which exhibits the same structural features as Si2Te3.14 The outcome of the present investigation implies that Si1.67(4)Te3 and Si2Te3 are both components of a series Si1.67+xTe3 with a composition range of 0.00 ≤ x ≤ 0.33 and, furthermore, provides a plausible explanation for the varying reports regarding the actual composition of this telluride. Additional examinations with respect to a plausible existence of a SiTe2 adopting the Si1.67+xTe3 structure model were accomplished based on “SiTe2” models and demonstrate the opportunity of the presence of such a SiTe2 that should be electronically as well as dynamically stable. From subsequent chemical bonding analyses for these “SiTe2” models, it is clear that the tendency to orientate the [Si2] dumbbells parallel to the c axis stems from the attempt to maximize Si−Te bonding. However, a comparison of the total energies between these “SiTe2” models implies that the difference between the total energies is located in the range of possible reaction temperatures such that the disorder of the [Si2] dumbbells is a consequence of a non-equilibrium condensation into the solid state. A full orientation of all [Si2] dumbbells parallel to the c axis is not present due to an electronically unfavorable situation.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Tobias Storp, Dr. Paul Müller, and Björn Faßbänder for the collection of the sets of SXCRD and PXRD data and the IT Center at RWTH Aachen University for the allocation of the computer time (JARA-HPC project jara0144).

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REFERENCES

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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.7b01847. Measured and simulated PXRD patterns of Si1.67(4)Te3; site- and orbital-projected DOS curves as well as −IpCOHP/bond values for selected interactions in the “SiTe2” models 1 and 2; phonon band structures and PhDOS of the “SiTe2” model 2; representations of the 11403

DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405

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DOI: 10.1021/acs.inorgchem.7b01847 Inorg. Chem. 2017, 56, 11398−11405