Crystal Growth of Intermetallics from the Joint Flux: Exploratory

Jan 7, 2019 - Synopsis. In this study, the joint flux technique is introduced, providing a tool to tune the valence electron concentration through the...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Crystal Growth of Intermetallics from the Joint Flux: Exploratory Synthesis through the Control of Valence Electron Count Valeriy Yu. Verchenko,*,†,‡ Andrei V. Mironov,† Zheng Wei,§ Alexander A. Tsirlin,∥ Evgeny V. Dikarev,§ and Andrei V. Shevelkov† †

Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia § Department of Chemistry, University at Albany, SUNY, Albany, New York 12222, United States ∥ Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany

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‡

S Supporting Information *

ABSTRACT: In this study, we modify the flux-growth method for the purpose of exploratory synthesis of ternary intermetallic compounds. Our concept is based on the assumption that valence electron count plays a crucial role in the stability of polar intermetallic compounds of different structure types. Control of the valence electron count parameter is made possible through the use of an excess of two metals having a different number of valence electrons. By gradually changing the ratio between these metals in the joint flux, we scan the gross number of valence electrons and explore the crystallization of new compounds. In the ternary system Re−Ga−Zn, we detect compounds belonging to three structure types, ReGa5, PtHg4, and V8Ga41, while gradually increasing the content of Zn metal in the flux. Two new compounds, ReGa3Zn and Re8Ga41−xZnx with x = 21.2(5), are obtained in the form of high-quality single crystals, and the former compound shows the narrow-gap semiconducting behavior favorable for high thermoelectric performance.

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INTRODUCTION

The use of liquid metals as high-temperature solvents and reaction media has proved to be instrumental in the synthesis and crystal growth of intermetallic compounds. Compared to conventional solid-state techniques, the flux-growth method enables synthesis of intermetallics in the form of high-quality crystals that fosters their detailed structural and physical characterization. Anisotropy of physical properties can be studied on sufficiently large single crystals, but their growth is often a challenging task that requires careful optimization of the synthetic conditions. The flux-growth method has been considerably developed over the last decades. Early reviews cover the use of p-metals: Sn and Pb, Al, Ga, and In, Sb and Bi, as well as metals of s-, d- and f-blocks: Li and Na, Mg, Zn, Cu, Co, and Ce, as metallic fluxes for crystal growth of binary, ternary, and quaternary intermetallics of more than 50 different structure types.3,4 In a recent review,5 emphasis is placed on the crystal growth of intermetallics with strong electronic correlations, highlighting that the method can be adapted to certain types of functional materials. In this paper, we propose that the flux-growth method can be modified for the purpose

The demand for new ideas of exploratory synthesis of intermetallic compounds is evident nowadays. Featuring complex compositions and crystal structures, peculiar chemical bonding, and useful functional properties, intermetallic compounds create a lot of puzzling questions, and the new approaches in exploratory synthesis based on chemical principles that govern structural stability help in understanding this complex chemistry. In the recent study, the idea of finetuning the valence electron count in Au-based electron-poor intermetallics revealed how different structure types are stabilized on the border between quasicrystals and polar intermetallic compounds featuring interesting bonding patterns for Au, such as columns, layers, and tetrahedral frameworks.1 In another study, the idea of exploratory synthesis employing machine-learning approaches enabled systematization of the data for many binary and ternary systems, shedding light on the stability of intermetallics with simple crystal structures.2 Chemical and mathematical concepts when they are combined with exploratory synthesis not only lead to new compounds but also help in understanding the complex chemistry of intermetallics in general. © XXXX American Chemical Society

Received: November 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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Changing the VEC parameter may trigger crystallization of such intermetallic compounds, thus enabling facile and easy-inimplementation exploratory synthesis within the chosen ternary system. By employing high annealing temperatures and slow cooling rates, we expect thermodynamically stable compounds, however, the appearance of metastable phases can not be ruled out. In this paper, we focus on the Re−Ga−Zn system and report two new compounds, which were isolated in the form of single crystals from the melts with different Ga/Zn ratios. One of those new compounds, namely ReGa3Zn, features the 18-electron configuration and exhibits the narrowgap semiconducting behavior, thus perfectly matching our goal of finding new semiconducting intermetallics.

of exploratory synthesis of new intermetallic compounds, which show proximity to the semiconducting ground state and, thus, may be potential thermoelectrics. Besides the intermetallic narrow-gap semiconductors, there are many metallic systems for which a band gap or pseudogap may exist, but the Fermi level is located in the conduction band. Such intermetallics can also be obtained by the developed joint flux technique. Intermetallic compounds with the narrow-gap semiconducting behavior are rare. Examples include the marcasite-type antimonides FeSb26 and CrSb2,7 IrIn3-type compounds FeGa3,8 RuGa3,8 and RuIn3,9 Nowotny chimney ladder phases RuGa2,10 RuAl2,11 and Fe2Ge3,12 half-Heusler compounds TiNiSn,13 ZrCoSb,14 and ZrCoBi,14 and some other variants. Semiconducting intermetallics may exhibit interesting thermoelectric properties in different temperature ranges. At low temperatures, huge values of thermopower were reported for FeSb2,15 CrSb2,7 and FeGa3,16 which can be explained by the unconventional phonon-drag effect.17 The phonon-drag mechanism involves the metallic in-gap states observed by nuclear quadrupole resonance technique in FeGa3 and FeSb2.18 Recent high-precision structural studies show that the in-gap states appear owing to an extremely small amount of interstitial Fe atoms in the real structure of FeGa3.19 On the other hand, at high temperatures, high values of thermoelectric figure-ofmerit ZT were observed for RuIn3−xAx (A = Sn, Zn),20 NiTiSn-based,21 and ZrCoBi-based22 materials. With these findings, semiconducting intermetallic compounds form a promising field for new highly efficient thermoelectric materials. A characteristic feature of the semiconducting intermetallic compounds is that they follow electron counting rules. Indeed, TiNiSn, ZrCoSb, and ZrCoBi obey the 18-electron rule known for half-Heusler compounds,23 while FeSb2, IrIn3-type intermetallics, and Nowotny chimney ladder phases follow the 18 − n rule formulated for polar intermetallic compounds in general.24 The valence electron count (VEC) is a key factor that determines stability of different intermetallic structure types. Most of the intermetallic structure types are characterized by well-defined ranges of the VEC, and in some structure types, the “stable” value of the VEC corresponds to the semiconducting ground state. Scanning the VEC parameter in the chosen system may be used to transit between different structure types22 and to search for new intermetallic compounds, in particular, for new narrowgap semiconductors that are interesting for thermoelectric applications. Given the special role of the valence electron count and manifest advantages of the flux-growth technique, we propose to combine them and use two flux metals with different numbers of valence electrons for tuning the gross valence electron count in a ternary system. Considering, for example, crystal growth from the excess of indium and tin added to a dmetal, it is experimentally easy to gradually change composition of the flux from the pure indium-based system to the tin-based case, and all intermediate compositions would possess intermediate numbers of the gross valence electron count. Our approach is significantly different from the traditional exploratory synthesis in a ternary metallic system. By using the excess of two metals with low melting points, which usually form the eutectic-type combination, we consider the transition-metal-poor region of the ternary phase diagram searching for compounds enriched in the nontransition metals.

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EXPERIMENTAL SECTION

Crystal Growth. Crystal growth was performed from the joint flux using the excess of Ga and Zn metals. Re powder (Sigma-Aldrich, 99.995%), Ga pieces (Sigma-Aldrich, 99.9995%), and Zn pieces (Sigma-Aldrich, 99.99%) were used as starting materials. The Re[Ga(100−y)/100Zny/100]50 compositions were prepared with y varying between 0 and 100. The values of y = 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, and 90 were used in syntheses. The weighed starting materials were placed inside quartz ampules, which were evacuated to the residual pressure of 2σ(I)] R1/wR2 (all) goodness-of-fit residual electron density (e̅ Å−3)

Re8Ga41−xZnx

Re1.02(6)Ga3.01(2)Zn0.97(4)

Re8.0(4)Ga19.8(3)Zn21.2(5)

125 × 150 × 140

90 × 107 × 125

Enraf Nonius CAD4 Ag Kα, 0.56083

Bruker D8 Venture Mo Kα, 0.71073

295

100

cubic Im3̅m a = 5.7611(6)

trigonal R3̅ a = 13.8723(9), c = 14.7887(10) 2464.7(3)

191.21(3)

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RESULTS To perform crystal growth from the joint flux, we prepared samples with excess of Ga and Zn metals with respect to Re. In these samples, we used different Ga/Zn ratios and scanned through different intermediate compositions between the binary systems Re−Ga and Re−Zn. Further in the text, composition of the flux is referred to the Zn content y in mol % assuming that the sum of Ga and Zn is equal to 100%. In the range of 0 ≤ y ≤ 60, crystal growth resulted in well-formed single crystals, while for y > 60, only unreacted rhenium powder was obtained. Phase composition of the products strongly depends on the Zn content in the flux. For 0 ≤ y ≤ 5, the ReGa5-based solid solution was obtained as a single-phase product. This compound will not be considered further in the text. For 15 ≤ y ≤ 45, a new compound with the composition ReGa3Zn was isolated, while for 50 ≤ y ≤ 60, another new compound Re8Ga41−xZnx was obtained. Typical single crystals of ReGa3Zn and Re8Ga41−xZnx are shown in Figure 1. The

8.00 8.60 31.92 40.31 0 ≤ h ≤ 10, 0 ≤ k ≤ 10, −25 ≤ h ≤ 25, −25 ≤ k ≤ −10 ≤ l ≤ 2 25, −26 ≤ l ≤ 26 420 62855 89 3462 78 3268 0.049 Ψ-scan

0.052 numerical

0.768/1.000 5

0.676/1.000 78

0.0208/0.0252

0.0278/0.0545

0.0243/0.0268 1.02 1.19/−1.49

0.0308/0.0551 1.42 2.27/−2.24

Information. Atomic coordinates for the crystal structures of ReGa3Zn and Re8Ga41−xZnx are given in Tables S2 and S3, respectively, and the selected interatomic distances are given in Table S4 of the Supporting Information. Electron density was extracted by the maximum-entropy method (MEM) using the Dysnomia program29 and visualized using the VESTA software.30 Electronic Structure Calculations. Electronic structure calculations were performed within the framework of density functional theory (DFT) using full-potential local-orbital minimum basis bandstructure FPLO code31 (version 14.00−47). The local-density (LDA) exchange−correlation functional32 was used in the scalar-relativistic approximation. Integrations in the k-space were performed by an improved tetrahedron method33 on a grid of 16 × 16 × 16 k-points. Virtual crystal approximation (VCA) was used to account for the mixing of Ga and Zn atoms. For both compounds, the experimental crystal structure parameters obtained by single-crystal X-ray diffraction were used for calculations. Crystal structures were reduced to the respective Niggli unit cells using the VESTA software.30 Further, the VCA atoms were introduced in the positions of Ga/Zn atoms with the values of Z = 30.75 for ReGa3Zn and Z = 30.48 for Re8Ga41−xZnx with x = 21.2. Additionally, for ReGa3Zn, the calculations were performed in the full-relativistic regime to account

Figure 1. Optical images of single crystals of ReGa3Zn (a) and Re8Ga41−xZnx (b).

ReGa3Zn and Re8Ga41−xZnx compounds were investigated in detail with respect to their composition, crystal and electronic structures, and thermodynamic and transport properties. ReGa3Zn: Composition and Crystal Structure. The ReGa3Zn compound can be obtained by slow cooling of the Re−Ga−Zn melt with the Zn content of 15 ≤ y ≤ 45. As revealed by EDX spectroscopy, elemental mapping shows uniform distribution of Re, Ga, and Zn across the crystal surface (Figure 2). According to quantitative analysis, crystals isolated from the flux with y = 20, 25, and 30 have similar compositions, Re1.03(6)Ga3.03(2)Zn0.94(4), Re1.02(6)Ga3.01(2)Zn0.97(4), and Re1.02(7)Ga3.03(2)Zn0.95(7), respectively, which are equal within their standard deviations. Also, C

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (Top) EDX mapping of the Re, Ga, and Zn species for a crystal obtained from the Re−Ga−Zn flux with y = 25. (Bottom) Experimental (black points) and calculated (red line) PXRD patterns of the same sample. Peak positions are given by black ticks, and the difference plot is shown as a black line in the bottom part. Peaks of the impurity phase are marked with asterisks The inset shows PXRD patterns of the y = 20 (black line) and y = 30 (red line) samples at high 2θ angles highlighting the absence of shift of peak positions.

Figure 3. (Top) Polyhedral view of the ReGa3Zn crystal structure. Atoms are shown as displacement ellipsoids on the probability level of 95%. Re atoms are shown in red and Ga and Zn in gray. The unit cell is shown by the red lines. (Bottom) Electron density in the unit cell obtained by MEM. The interstitial sites are shown in black.

PXRD patterns of these samples show the absence of systematic shift of peak positions evidencing that lattice parameters do not vary between the studied samples (Figure 2). Thus, we assume that ReGa3Zn has no noticeable homogeneity range. The selected single crystals were examined by single-crystal X-ray diffraction. ReGa3Zn crystallizes in the Im3̅m (no. 229) space group with a = 5.7611(6) Å and belongs to the PtHg4 structure type. Rietveld refinement against the PXRD data clearly shows that this structural model is representative for the entire sample (Figure 2). On the PXRD pattern, there are several weak diffraction reflections with the maximum relative intensity of 4.6% that belong to an unidentified impurity phase. For these reflections, tentative indexing is possible in the primitive hexagonal unit cell with a ∼ 9.02 Å and c ∼ 3.28 Å. In the crystal structure of ReGa3Zn, statistically mixed Ga and Zn atoms form cubic arrangement, in which one-fourth of the cubes are centered by Re atoms in the ordered way (Figure 3). The Re−Ga/Zn interatomic distances of 2.4946(3) Å are observed. Analysis of the single-crystal X-ray diffraction data by the MEM method reveals the residual electron density in the centers of the hollow (Ga/Zn)8 cubes [(0.5; 0; 0) crystallographic position], indicating the presence of interstitial defects. This residual electron density corresponds to ∼2 e̅ per unit

cell. From the single-crystal XRD data alone, it is not obvious whether the interstitial defects are caused by the extremely small amount of extra Re atoms or Ga and Zn atoms in the real structure of ReGa3Zn. ReGa3Zn: Electronic Structure and Physical Properties. Electronic structure of ReGa3Zn was calculated using VCA to model the mixing of Ga and Zn atoms. In the VCA scheme, an effective atom Ga/Zn having the nuclear charge of 30.75 is added in the position, where Ga and Zn atoms are located. For this atom, both the 4s and 4p valence orbitals are involved in the hybridization and can be populated by valence and conduction electrons. Figure 4 shows the calculated electronic structure. The bottom of the valence band is primarily composed of the Ga/Zn 4s contribution, while at the top of the valence band and in the bottom of the conduction band, 4p states of Ga/Zn atoms and 5d states of Re atoms are present. Strong hybridization between the Re 5d states and Ga/Zn 4p states opens a direct band gap of 0.42 eV at the Γ point of the first Brillouin zone. The Fermi level is located in D

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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K−4, and δ = 2.34(4) μJ mol−1 K−6. We assume that the small value of the Sommerfield coefficient is due to deviations from the ideal structure and the presence of a residual electron density. However, electrical transport is obscured by the Ga/ Zn disorder, and only a semiconducting behavior with a tiny band gap is observed. Re8Ga41−xZnx: Composition and Crystal Structure. Crystals of the Re8Ga41−xZnx compound were obtained from the Re−Ga−Zn flux with y = 50 and 60. According to the EDX analysis, crystals possess the following compositions: Re8.0(6)Ga20.2(3)Zn20.8(6) for y = 50 and Re8.0(4)Ga19.8(3)Zn21.2(5) for y = 60, which are basically indistinguishable. EDX mapping collected on the y = 60 crystal reveals uniform distribution of Re, Ga, and Zn across the crystal surface, indicating that there is no phase separation at least above the level of 1 μm2, which is the spatial resolution of the used EDX setup (Figure 6). According to PXRD, the obtained samples are single-phase. PXRD pattern of the y = 60 sample can be indexed in the trigonal R-centered unit cell with the parameters a = 13.895(1) Å and c = 14.801(1) Å. The absence of peak splitting on the PXRD pattern also rules out the presence of two isostructural phases with similar unit cell parameters, as might be expected for the compound with mixed Ga and Zn elements. Crystals grown from the flux with y = 60 were studied by single-crystal X-ray diffraction technique. The Re8Ga41−xZnx compound [x = 21.2(5) according to EDX] crystallizes in the V8Ga41 structure type [R3̅ space group (no. 148), a = 13.8723(9) Å, c = 14.7887(10) Å]. In the crystal structure, Re atoms occupy two crystallographic positions, while Ga and Zn atoms statistically mix in nine other positions. Both the Re1 and Re2 positions possess the same coordination polyhedra Re(Ga/Zn)10 known as centaur polyhedra. Each polyhedron consists of one-half of a Re(Ga/Zn)8/2 cube and one-half of a Re(Ga/Zn)12/2 icosahedron. The Re−Ga/Zn interatomic distances vary between 2.47 and 2.84 Å. In addition to the Re(Ga/Zn)10 polyhedra, there are (Ga/Zn)(Ga/Zn)12 cuboctahedra centered by Ga and Zn atoms located in the special 3a crystallographic position (0; 0; 0). Condensation of the Re(Ga/Zn)10 polyhedra and (Ga/Zn)(Ga/Zn)12 cuboctahedra through the common triangular faces completes the threedimensional crystal structure of Re8Ga41−xZnx.

Figure 4. Electronic structure of ReGa3Zn with Re 5d (red points), and Ga/Zn 4s (green points) and 4p (blue points) contributions.

the band gap, resulting in a narrow-gap semiconducting behavior. Resistivity of ReGa3Zn decreases with increasing temperature as expected for a nonmetallic system. Two temperature regimes are visible in the resistivity curve. At T < 230 K, resistivity roughly follows the activation behavior that can be attributed to the impurity in-gap states. However, the value of a band gap for the in-gap states is too small, which impedes its exact determination. At T > 280 K, resistivity of ReGa3Zn follows the activation behavior with the narrow band gap of ∼0.03 eV. Obviously, electronic structure calculations overestimate the band gap value due to the crudeness of VCA scheme. Including the spin−orbit coupling yields the narrowing of the band gap to the value of 0.17 eV, which is, again, larger that the experimental one. In addition, the real structure of ReGa3Zn contains the residual electron density as revealed by the MEM analysis. Such defects may not only produce in-gap states but also lead to the overall narrowing of the band gap. To obtain further insight into the ground state of ReGa3Zn, heat capacity was measured at low temperatures in zero magnetic field. The low-temperature heat capacity was approximated using the equation cp = γT + βT 3 + δT 5 (Figure 5) yielding γ = 4.0(1) mJ mol−1 K−2, β = 0.129(5) mJ mol−1

Figure 5. (Left) Resistivity of ReGa3Zn as a function of temperature in zero magnetic field. (Right, top) ln ρ vs 1/T plot of the resistivity data. (Right, bottom) Low-temperature heat capacity of ReGa3Zn in zero magnetic field plotted as cp/T vs T2. Solid red lines are least-squares fits of the data, as described in the text. E

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (Top, left) Experimental (black points) and calculated (red line) PXRD patterns of the y = 60 sample. Peak positions are given by black ticks, the difference plot is shown as a black line in the bottom part. (Top, right) EDX composition maps of the Re, Ga, and Zn species for the y = 60 crystal. (Bottom) Polyhedral view of the Re8Ga41−xZnx crystal structure. Atoms are shown as displacement ellipsoids on the probability level of 95%. Re(Ga/Zn)10 polyhedra are shown in gray and (Ga/Zn)(Ga/Zn)12 cuboctahedra in green. Re atoms are shown in red and Ga and Zn in gray. The unit cell is shown by the red lines.

x = 21.2. The calculated density of states at the Fermi level yields bare Sommerfield coefficient of γbare = 11.6 mJ mol−1 K−2. Resistivity of Re8Ga41−xZnx [x = 21.2(5)] increases with increasing temperature as expected for a metallic system. The relatively small residual resistance ratio (RRR) can be explained by the fact that the measurements were performed on a polycrystalline sample. Moreover, the small RRR value may originate from the tendency of resistivity toward saturation, as observed in similar systems based on 4d transition metals and p-metals.35 Low-temperature heat capacity of the same sample measured in zero magnetic field is shown in the inset of Figure 7. The data were fitted using the equation cp = γT + βT 3 + δT 5 yielding γ = 31.8(6) mJ mol−1 K−2, β = 1.38(2) mJ mol−1 K−4, and δ = 16.8(1) μJ mol−1 K−6. Large value of the Sommerfield coefficient indicates a noticeable contribution of the conduction electrons to the total heat capacity in agreement with the metallic behavior deduced from the electronic structure calculations and from the resistivity measurements. The enhancement of the Sommerfield coefficient with respect to the calculated value γbare may indicate large effective masses of charge carriers and possible electronic correlations in Re8Ga41−xZnx.

Although Ga and Zn atoms are not distinguishable in the Xray diffraction experiments, analysis of interatomic distances in the crystal structure of Re8Ga41−xZnx may provide an insight into their distribution. The distances between Re atoms and Ga1−Ga5 atoms are in the range of 2.47−2.58 Å, whereas longer contacts are realized with Ga6, Ga7, and Ga8 atoms: These distances vary between 2.71 and 2.84 Å. Notably, the Ga7 and Ga8 atoms form the (Ga/Zn)(Ga/Zn)12 cuboctahedron around the Ga9 position. Therefore, Re−Ga/Zn interatomic distances are systematically larger for the Ga/Zn atoms from the cuboctahedron unit. This situation is reminiscent of that observed in the isomorphous V8Ga41−xZnx, Cr8Ga41−xZnx, and Mn8Ga41−xZnx compounds where the formation of Zn13 cuboctahedron clusters was deduced from the neutron powder diffraction experiments. 3 4 In Re8Ga41−xZnx, which contains as much as 21.2(5) Zn atoms per formula unit, the same situation may be anticipated with Zn atoms preferentially occupying the Ga7, Ga8, and Ga9 crystallographic positions of the crystal structure. Re8Ga41−xZnx: Electronic Structure and Physical Properties. Electronic structure of Re8Ga41−xZnx calculated for x = 21.2 is shown in Figure 7. Similar to ReGa3Zn, the top of the valence band and the bottom of the conduction band are formed by the narrow Re 5d bands and more dispersed Ga/Zn 4p bands. Hybridization between these states opens a pseudogap located at the relative energy of E − EF ∼ −0.7 eV at the Γ point. The Fermi level is located in the conduction band, indicating the metallic ground state of Re8Ga41−xZnx for

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DISCUSSION Rhenium forms a limited number of intermetallic compounds. In the binary system Re−Zn, no compounds were reported to F

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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in the form of single crystals from the joint flux with the tunable Ga/Zn ratio. This way, each compound could be synthesized individually. ReGa3Zn and Re8Ga41−xZnx (x = 21.2) possess 18 and 19.725 valence electrons per Re atom, respectively, assuming that Zn contributes 2 electrons from the 4s2 shell. Accordingly, different ranges of the VEC are observed for the corresponding structure types, PtHg4 in the case of ReGa3Zn and V8Ga41 in the case of Re8Ga41−xZnx. Whereas the PtHg4-type intermetallics are known for VEC = 18 and 19,38 compounds of the V8Ga41 structure type have VEC in the range between 19.86 and 21.375.34,39,40 Gradual change in the Ga/Zn ratio of the flux leads to an optimization of the VEC parameter within the possible structure types. In the Re−Ga− Zn system, we observe not only the optimization of the VEC that results in two new compounds but also the switching between three structure types in the series ReGa5 (VEC = 22) − ReGa3Zn (VEC = 18) − Re8Ga41−xZnx (VEC = 19.725), when the content of Zn metal is gradually increased in the flux. The minimum of VEC is realized for the ReGa3Zn compound. Alternatively, the calculation of the VEC parameter per one atom in the formula unit gives monotonous dependence of the VEC versus the Ga/Zn ratio: ReGa5 (VEC = 3.67 e̅ per atom) − ReGa3Zn (3.6) − Re8Ga41−xZnx (3.22). Interestingly, there is a clear connection between the ReGa3Zn and Re8Ga41−xZnx crystal structures (Figure 8). In

Figure 7. (Top) Electronic structure of Re8Ga41−xZnx for x = 21.2. Re 5d and (Ga/Zn) 4p contributions are shown by red and blue points, respectively. (Middle) Density of states of Re8Ga41−xZnx for x = 21.2. Re 5d, Ga/Zn 4s, and Ga/Zn 4p contributions are shown in red, green, and blue, respectively. Position of the Fermi level is indicated by the dashed line. (Bottom) Resistivity of the y = 60 sample as a function of temperature in zero magnetic field. The inset shows the low-temperature heat capacity plotted as cp/T vs T 2. The solid red line is the least-squares fit of the data.

Figure 8. (Top, left) Fragment of the ReGa3Zn crystal structure. (Top, right) Re8(Ga/Zn)41 supercube in the crystal structure of Re8Ga41−xZnx. (Bottom) Connection of two Re8(Ga/Zn)41 supercubes.

the latter, eight Re(Ga/Zn)10 polyhedra share their triangular faces with a (Ga/Zn)(Ga/Zn)12 cuboctahedron forming a distorted supercube Re8(Ga/Zn)41. This construction, which is the main building unit of the Re8Ga41−xZnx crystal structure, can be directly compared to the unit cell of ReGa3Zn, in which eight Re(Ga/Zn)8 cubes surround another one located in the center of the unit cell. The replacement of this central Re(Ga/ Zn)8 cube by a (Ga/Zn)(Ga/Zn)12 cuboctahedron accompanied by the deformation of the entire crystal structure yields the Re8Ga41−xZnx building unit. Re8(Ga/Zn)41 supercubes are

date, and two compounds were discovered in the Re−Ga system. The ReGa3 compound belonging to the IrIn3 structure type was synthesized under high-pressure conditions.36 The ambient-pressure compound, ReGa5, crystallizes in its own structure type and belongs to the family of gallium cluster superconductors.37 By performing syntheses in the ternary Re−Ga−Zn system, we were able to discover two new compounds, ReGa3Zn and Re8Ga41−xZnx. They were obtained G

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

new members of this family may lead to new superconductors with unusual microscopic properties of the superconducting state. The joint flux technique developed in this study produced a new endohedral cluster compound, Re8Ga41−xZnx, as well as a narrow-gap semiconductor, ReGa3Zn. Control of the valence electron count through the use of joint flux has significant potential as a method for exploratory synthesis. Some indications of this can be found in the previous literature. For instance, the use of Cr−Ga−Zn and Mn−Ga− Zn ternary fluxes leads to the new compounds Cr8Ga41−xZnx and Mn8Ga41−xZnx of the V8Ga41 type.34 Compounds of this structure type do not form in the respective binary systems. Another example shows how the diversity of ternary compounds can be explored when the excess of Ga and Cu metals is applied to transition metals of groups 5 and 6. Ternary compounds V2Cu3Ga8, Mo2Cu3Ga8, and W2Cu3Ga8 with the crystal structures reminiscent of the PtHg4 structure type are formed.44 Moreover, the superstructure compounds V11Cu9Ga4645 and Ta7Cu10Ga3446 were obtained. Recent results show that the Ga/Cu combination can also be applied to a rare-earth metal Yb yielding complex ternary intermetallics Yb6(CuGa)50 and Yb6(CuGa)51.47 Finally, our exploratory syntheses in the systems Mo−Ga−Zn, Mo−Ga−Sn, and Re− Ga−Sn resulted in new compounds as well, which will be presented in the upcoming publications.

interconnected by the corners of Re(Ga/Zn)10 polyhedra forming the crystal structure of Re8Ga41−xZnx. The similarity between ReGa3Zn and Re8Ga41−xZnx is also evident when comparing their electronic structures. Although the electronic structure of Re8Ga41−xZnx contains a larger number of bands in the vicinity of the Fermi energy owing to the complexity of its crystal structure, both compounds show the special feature at the Γ point. For ReGa3Zn, the opening of a direct band gap is observed, whereas for Re8Ga41−xZnx, a pseudogap is formed separating valence and conduction bands. ReGa3Zn is an electron-precise compound with the value of VEC = 18. Thus, the Fermi level is located in the band gap in accordance with the 18 − n rule24 leading to the semiconducting behavior, which is directly seen in the transport and thermodynamic measurements. On the other hand, Re8Ga41−xZnx is an electron-efficient compound with VEC = 19.725 for the experimental value of x = 21.2(5). In this case, the Fermi level is located in the conduction band corresponding to the metallic behavior, which is also confirmed experimentally. The discovered Re−Ga−Zn compounds can be compared to Mo−Ga intermetallics that recently attracted interest as endohedral gallium cluster superconductors with relatively high transition temperatures.37 Mo8Ga41, which is isomorphous to Re8Ga41−xZnx, is a superconductor with the critical temperature of Tc = 9.8 K in zero magnetic field.41 The related Mo6Ga31 compound also exhibits superconducting properties below Tc = 8 K.42 Also in these series, the hypothetical MoGa4 compound was considered in light of systematic studies of chemical bonding in the PtHg4 family of intermetallics.38 The MoGa4 − Mo8Ga41 − Mo6Ga31 series is an interesting example of the evolution of crystal and electronic structures and physical properties upon increase of the valence electron count. The hypothetical MoGa 4 compound, which is isomorphous to ReGa3Zn, is electron precise possessing a VEC = 18. Electronic structure calculations show the formation of pseudogap at the Fermi level for MoGa4 predicting the bad metallic behavior. The Mo8Ga41 and Mo6Ga31 compounds, which crystal structures contain Ga13 cuboctahedra along with the MoGa10 polyhedra, are enriched in Ga with respect to MoGa4 and possess higher values of VEC. As a consequence, they exhibit metallic properties in the normal state due to the excess of valence electrons that fill the conduction band. Moreover, the superconductivity is found in both compounds, and the transition temperature seems to correlate with the VEC parameter.37 The critical temperature decreases from Tc ∼ 10 K in Mo8Ga41 with the VEC = 21.375 to ∼8 K in Mo6Ga31 with the VEC = 21.5. The interrelations found in the MoGa4 − Mo8Ga41 − Mo6Ga31 series once again underline the significance of the VEC parameter in stability and physical properties of intermetallics. The newly discovered narrow-gap semiconductor ReGa3Zn with the experimental band gap of 30 meV may be interesting as a potential thermoelectric material. In the upcoming studies, we intend to address its thermoelectric properties in detail. On the other hand, the Re8Ga41−xZnx compound is isostructural with Mo8Ga41, which is a superconductor below Tc = 9.8 K in zero magnetic field showing strong electron−phonon coupling in the superconducting state.41 Moreover, a nontrivial two-gap behavior is proposed for Mo8Ga41 based on the muon spin rotation/relaxation measurements.43 Although Re8Ga41−xZnx is metallic in the studied temperature range, it clearly represents the endohedral cluster family of compounds, and the search for

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CONCLUSIONS

In this study, we showed that the modification of flux-growth method through the use of joint flux containing two low-melt aliovalent metals enables fine-tuning and adjustment of the valence electron count parameter in the chosen system. Crystal growth from the joint flux with excess of Ga and Zn metals resulted in two new intermetallic compounds, which were obtained individually in the form of high-quality single crystals. The ReGa3Zn compound was isolated from the flux with the relative content of Zn of 15 ≤ y ≤ 45, while Re8Ga41−xZnx was synthesized in the range of 50 ≤ y ≤ 60. ReGa3Zn crystallizes in the PtHg4 structure type and shows no noticeable homogeneity range. Possessing 18 valence electrons per Re atom, it exhibits narrow-gap semiconducting behavior with the experimental band gap of 30 meV. Electronic structure calculations within the VCA scheme confirm the opening of a band gap and corroborate the semiconducting behavior deduced from the resistivity measurements. The Re8Ga41−xZnx compound with x = 21.2(5) has no significant homogeneity range, too. Re8Ga41−xZnx features the crystal structure of the V8Ga41 type, which is closely related to the PtHg4 structure type. This similarity is reflected in the fact that in the calculated electronic structures of ReGa3Zn and Re8Ga41−xZnx, a band gap and a pseudogap are opened at the Γ point, respectively. However, in contrast to ReGa3Zn, Re8Ga41−xZnx possesses the larger value of VEC = 19.725 for x = 21.2. Accordingly, the Fermi level is located in the conduction band leading to the metallic behavior, which is confirmed by the resistivity and heat-capacity measurements. With the narrow band gap of 30 meV, the electron-precise semiconducting compound ReGa3Zn attracts interest as a promising thermoelectric material. Future studies of thermoelectric properties of this compound are highly desirable. H

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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narrow-gap semiconductor CrSb2. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 235136. (8) Häussermann, U.; Boströ m, M.; Viklund, P.; Rapp, Ö .; Björnängen, T. FeGa3 and RuGa3: Semiconducting Intermetallic Compounds. J. Solid State Chem. 2002, 165, 94−99. (9) Bogdanov, D.; Winzer, K.; Nekrasov, I. A.; Pruschke, T. Electronic properties of the semiconductor RuIn3. J. Phys.: Condens. Matter 2007, 19, 232202. (10) Evers, J.; Oehlinger, G.; Meyer, H. Semiconducting behaviour of RuGa2. Mater. Res. Bull. 1984, 19, 1177−1180. (11) Mandrus, D.; Keppens, V.; Sales, B. C.; Sarrao, J. L. Unusual transport and large diamagnetism in the intermetallic semiconductor RuAl2. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 3712. (12) Verchenko, V.Yu.; Wei, Z.; Tsirlin, A. A.; Callaert, C.; Jesche, A.; Hadermann, J.; Dikarev, E. V.; Shevelkov, A. V. Crystal Growth of the Nowotny Chimney Ladder Phase Fe2Ge3: Exploring New FeBased Narrow-Gap Semiconductor with Promising Thermoelectric Performance. Chem. Mater. 2017, 29, 9954−9963. (13) Aliev, F. G.; Kozyrkov, V. V.; Moshchalkov, V. V.; Scolozdra, R. V.; Durczewski, K. Narrow band in the intermetallic compounds MNiSn (M = Ti, Zr, Hf). Z. Phys. B: Condens. Matter 1990, 80, 353− 357. (14) Evers, C. B. H.; Richter, C. G.; Hartjes, K.; Jeitschko, W. Ternary transition metal antimonides and bismuthides with MgAgAstype and filled NiAs-type structure. J. Alloys Compd. 1997, 252, 93− 97. (15) Bentien, A.; Johnsen, S.; Madsen, G. K. H.; Iversen, B. B.; Steglich, F. Colossal Seebeck coefficient in strongly correlated semiconductor FeSb2. Europhys. Lett. 2007, 80, 17008. (16) Wagner-Reetz, M.; Kasinathan, D.; Schnelle, W.; Cardoso-Gil, R.; Rosner, H.; Grin, Yu.; Gille, P. Phonon-drag effect in FeGa3. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 195206. (17) Tomczak, J. M. Thermoelectricity in correlated narrow-gap semiconductors. J. Phys.: Condens. Matter 2018, 30, 183001. (18) Gippius, A. A.; Verchenko, V.Yu.; Tkachev, A. V.; Gervits, N. E.; Lue, C. S.; Tsirlin, A. A.; Büttgen, N.; Krätschmer, W.; Baenitz, M.; Shatruk, M.; Shevelkov, A. V. Interplay between localized and itinerant magnetism in Co-substituted FeGa3. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 104426. (19) Wagner, F. R.; Cardoso-Gil, R.; Boucher, B.; Wagner-Reetz, M.; Sichelschmidt, J.; Gille, P.; Baenitz, M.; Grin, Yu. On Fe-Fe Dumbbells in the Ideal and Real Structures of FeGa3. Inorg. Chem. 2018, 57, 12908. (20) Kasinathan, D.; Wagner, M.; Koepernik, K.; Cardoso-Gil, R.; Grin, Yu.; Rosner, H. Electronic and thermoelectric properties of RuIn3‑xAx (A = Sn, Zn). Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 035207. (21) Liu, Y.; Sahoo, P.; Makongo, J. P.; Zhou, X.; Kim, S. J.; Chi, H.; Uher, C.; Pan, X.; Poudeu, P. F. Large enhancements of thermopower and carrier mobility in quantum dot engineered bulk semiconductors. J. Am. Chem. Soc. 2013, 135, 7486−7495. (22) Zhu, H.; He, R.; Mao, J.; Zhu, Q.; Li, C.; Sun, J.; Ren, W.; Wang, Y.; Liu, Z.; Tang, Z.; Sotnikov, A.; Wang, Z.; Broido, D.; Singh, D. J.; Chen, G.; Nielsch, K.; Ren, Z. Discovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency. Nat. Commun. 2018, 9, 2497. (23) Graf, T.; Felser, C.; Parkin, S. S. P. Simple rules for the understanding of Heusler compounds. Prog. Solid State Chem. 2011, 39, 1−50. (24) Yannello, V. J.; Fredrickson, D. C. Generality of the 18-n Rule: Intermetallic Structural Chemistry Explained through Isolobal Analogies to Transition Metal Complexes. Inorg. Chem. 2015, 54, 11385−11398. (25) Petříček, V.; Dušek, M.; Palatinus, L. Crystallographic computing system JANA2006: General features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345. (26) Palatinus, L.; Chapuis, G. Superflip - a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J. Appl. Crystallogr. 2007, 40, 786−790.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03083. Tables with crystallographic data, X-ray crystallographic files in CIF format, and density of states plot calculated for ReGa3Zn (PDF) Accession Codes

CCDC 1877143 and 1877157 contain 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Valeriy Yu. Verchenko: 0000-0002-8000-425X Andrei V. Shevelkov: 0000-0002-8316-3280 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Sergey Kazakov for his help with the PXRD experiments. We acknowledge the use of the Bruker D8 Advance powder X-ray diffractometer purchased under the Lomonosov MSU program of development. The work is supported by the Russian Science Foundation, grant no. 17-1301033. V.Yu.V. appreciates the financial support from the Mobilitas program of the European Science Foundation, project no. MOBJD449. A.A.T. is grateful for the financial support by the Federal Ministry for Education and Research under the Sofja Kovalevskaya Award of the Alexander von Humboldt Foundation. E.V.D. thanks the National Science Foundation, grant no. CHE-1337594.

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REFERENCES

(1) Lin, Q.; Miller, G. J. Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization. Acc. Chem. Res. 2018, 51, 49− 58. (2) Oliynyk, A. O.; Mar, A. Discovery of Intermetallic Compounds from Traditional to Machine-Learning Approaches. Acc. Chem. Res. 2018, 51, 59−68. (3) Canfield, P. C.; Fisk, Z. Growth of single crystals from metallic fluxes. Philos. Mag. B 1992, 65, 1117−1123. (4) Kanatzidis, M. G.; Pöttgen, R.; Jeitschko, W. The Metal Flux: A Preparative Tool for the Exploration of Intermetallic Compounds. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (5) Phelan, W. A.; Menard, M. C.; Kangas, M. J.; McCandless, G. T.; Drake, B. L.; Chan, J. Y. Adventures in Crystal Growth: Synthesis and Characterization of Single Crystals of Complex Intermetallic Compounds. Chem. Mater. 2012, 24, 409−420. (6) Petrovic, C.; Kim, J. W.; Bud’ko, S. L.; Goldman, A. I.; Canfield, P. C.; Choe, W.; Miller, G. J. Anisotropy and large magnetoresistance in the narrow-gap semiconductor FeSb2. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 155205. (7) Sales, B. C.; May, A. F.; McGuire, M. A.; Stone, M. B.; Singh, D. J.; Mandrus, D. Transport, thermal, and magnetic properties of the I

DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (27) Petříček, V.; Dušek, M. JANA2000; Institute of Physics, Czech Academy of Sciences: Prague, Czech Republic, 2000. (28) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (29) Momma, K.; Ikeda, T.; Belik, A. A.; Izumi, F. Dysnomia, a computer program for maximum-entropy method (MEM) analysis and its performance in the MEM-based pattern fitting. Powder Diffr. 2013, 28, 184−193. (30) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272. (31) Koepernik, K.; Eschrig, H. Full-Potential Nonorthogonal LocalOrbital Minimum-Basis Band-Structure Scheme. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1743. (32) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244. (33) Blö chl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin-Zone Integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16223. (34) Viklund, P.; Svensson, C.; Hull, S.; Simak, S. I.; Berastegui, P.; Häussermann, U. From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: A Remarkable Onset of Zn-Cluster Formation in an Intermetallic Framework. Chem. - Eur. J. 2001, 7, 5143−5152. (35) Gunnarsson, O.; Calandra, M.; Han, J. E. Colloquium: Saturation of electrical resistivity. Rev. Mod. Phys. 2003, 75, 1085. (36) Popova, S. V.; Fomicheva, L. N. New phases in Re-Ga and OsGa systems, obtained at high pressure. Inorg. Mater. 1982, 18, 205− 208. (37) Xie, W.; Luo, H.; Phelan, B. F.; Klimczuk, T.; Cevallos, F. A.; Cava, R. J. Endohedral gallide cluster superconductors and superconductivity in ReGa5. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, No. E7048. (38) Häussermann, U.; Viklund, P.; Boström, M.; Norrestam, R.; Simak, S. I. Bonding and physical properties of Hume-Rothery compounds with the PtHg4 structure. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 125118. (39) Bezinge, A.; Yvon, K.; Decroux, M.; Muller, J. On the existence of binary Mo8Ga41 and its properties. J. Less-Common Met. 1984, 99, L27−L31. (40) Koffi, A.; Ade, M.; Hillebrecht, H. Single crystal studies of binary compounds Ta/Ga - A system with experimental and crystallographic peculiarities. J. Solid State Chem. 2016, 238, 88−102. (41) Verchenko, V.Yu.; Tsirlin, A. A.; Zubtsovskiy, A. O.; Shevelkov, A. V. Strong electron-phonon coupling in the intermetallic superconductor Mo8Ga41. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 064501. (42) Fisher, O. Properties of high field superconductors, containing localized magnetic moments. Helv. Phys. Acta 1972, 45, 331. (43) Verchenko, V.Yu.; Khasanov, R.; Guguchia, Z.; Tsirlin, A. A.; Shevelkov, A. V. Two-gap superconductivity in Mo8Ga41 and its evolution upon vanadium substitution. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 134504. (44) Kuntze, V.; Lux, R.; Hillebrecht, H. V2Cu3Ga8, Mo2Cu3Ga8 and W2Cu3Ga8 - New compounds with a novel order variant of a bcc packing and motifs of self-similarity. J. Solid State Chem. 2007, 180, 198−206. (45) Lux, R.; Kuntze, V.; Hillebrecht, H. Synthesis and crystal structure of cubic V11Cu9Ga46 - A 512-fold super structure of a simple bcc packing. Solid State Sci. 2012, 14, 1445−1453. (46) Koffi, A.; Ade, M.; Hillebrecht, H. Synthesis, Single Crystal Growth and Crystal Structure of Ta7Cu10Ga34 - a 8 × 4 × 2 Super Structure of CsCl. Z. Anorg. Allg. Chem. 2016, 642, 350−354. (47) Mishra, V.; Oliynyk, A. O.; Subbarao, U.; Sarma, S. C.; Mumbaraddi, D.; Roy, S.; Peter, S. C. Complex Crystal Chemistry of Yb6(CuGa)50 and Yb6(CuGa)51 Grown at Different Synthetic Conditions. Cryst. Growth Des. 2018, 18, 6091−6099.

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DOI: 10.1021/acs.inorgchem.8b03083 Inorg. Chem. XXXX, XXX, XXX−XXX