Synthesis, Crystal Structure, and Magnetic Properties of R2Mg3SiPn6

Article pubs.acs.org/IC

Synthesis, Crystal Structure, and Magnetic Properties of R2Mg3SiPn6 (R = La, Ce; Pn = P, As) Jian Wang, Joshua T. Greenfield, and Kirill Kovnir* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States S Supporting Information *

ABSTRACT: Four new quaternary pnictides, R2Mg3SiPn6 (R = La, Ce; Pn = P, As), were synthesized via high-temperature solid-state reactions and gas-phase transport reactions with iodine. Their crystal structures were determined by single crystal X-ray diffraction. All four compounds are isostructural and crystallize in a new structure type in the orthorhombic space group Pnma (No. 62, Z = 4), Pearson symbol oP48. The crystal structures of R2Mg3SiPn6 are composed of twodimensional puckered MgP3 layers, which are connected in a three-dimensional framework by P−P dimers and MgSiP4 double-tetrahedral chains. Rare-earth cations are encapsulated inside the channels of the framework running along [010]. Quantum-chemical calculations predict that La2Mg3SiP6 is an indirect narrow bandgap semiconductor. The Mg−P bonding in MgP4 tetrahedra and MgP6 octahedra was analyzed by means of crystal orbital Hamilton population (COHP) analysis. Magnetic characterization of Ce-containing compounds confirmed the trivalent nature of cerium atoms and revealed complex ferrimagnetic ordering at low temperatures.

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INTRODUCTION Transition and rare-earth metal pnictides have attracted the attention of solid-state chemists and materials scientists for their structural diversity and interesting physical properties.1−9 Recent discoveries of superconductivity in transition metal arsenides and phosphides, such as RFeAsO10,11 (R = rare-earth metal), AFe2As2 (A = K, Rb, Cs, Sr, Ba),12−14 AFeAs (A = Li, Na),15,16 BaNi2P2,17 LaMPn (M = Ir and Rh; Pn = P and As),18 and A2Cr3As3 (A = K, Rb, Cs),19,20 reinvigorate the interest in this field. In our search for potential thermoelectrics based on metal−pnictide framework compounds capable of encapsulating alkali-earth and rare-earth compounds, we explored Cu− P21−27 and Zn−P frameworks.28,29 Zn−P frameworks are capable of encapsulating not only isolated rare-earth cations but also one-dimensional chains of halogen-centered Cl@La4 and Br@La4 tetrahedra.28 This is possible due to the high structural flexibility of Zn which exhibits 3-fold and 4-fold coordination by P atoms. Increasing the Zn coordination number to six, such as in the crystal structure of La4Zn7P10 where Zn is located inside an octahedron of P atoms, resulted in the displacement of Zn atom from the octahedron center to adopt a 4 + 2 coordination.29 Such a displacement can be eliminated by partial substitution of Mg for Zn,29 since MgP6 octahedra are common building blocks in the structures of ternary and quaternary Mg-containing phosphides. In the course of synthesizing the hypothetical compound “La4Mg7P10”, where all Zn is replaced with Mg, we came across a new quaternary compound La2Mg3SiP6, where the source of Si was the reduction of the silica ampule by Mg. After establishing the © 2017 American Chemical Society

structure and composition of this phase we synthesized singlephase samples and explored the tunability of this structure by performing La/Ce and P/As replacements. In this work we report the synthesis, crystal structure, chemical bonding, and magnetic properties of La2Mg3SiP6 and cerium- and arseniccontaining analogues, La 2 Mg 3 SiAs 6 , Ce 2 Mg 3 SiP 6 , and Ce2Mg3SiAs6. According to our magnetic characterization, despite the presence of only one crystallographically independent Ce position, the Ce-containing compounds exhibit complex ferrimagnetic behavior.

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EXPERIMENTAL SECTION

Synthesis. All preparation and handling of samples were performed in an argon-filled glovebox with the O2 level below 1 ppm. All starting materials are commercial grade and were used as received: La (Alfa Aesar, 99.9%), Ce (Ames Laboratory, 99.996%), Mg (Alfa Aesar, 99.98%), Si (Alfa Aesar, 99.99%), red P (Alfa Aesar, 99%), and I2 (Alfa Aesar, resublimed crystals, 99.9985%). Single crystals of La2Mg3SiP6 were first found in a transport reaction upon loading La:Mg:P:I2 in a 3:4:6:0.15 ratio, using I2 as a transport agent. This mixture was placed in silica ampule, evacuated, and sealed. The ampule was heated from room temperature to 1073 K at a rate of 40 K/h, and then annealed at this temperature for 144 h. Then the ampule was cooled down to room temperature over 48 h. Many tiny black needle-like crystals were found in the reaction products together with amorphous powder and binary La and Zn phosphides. Single crystal X-ray diffraction on selected crystals revealed the La2Mg3SiP6 composition. Due to the highly air-sensitive nature of this compound, Received: April 24, 2017 Published: June 24, 2017 8348

DOI: 10.1021/acs.inorgchem.7b01015 Inorg. Chem. 2017, 56, 8348−8354

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Inorganic Chemistry Table 1. Selected Crystal Data and Structure Refinement Parameters for R2Mg3SiPn6 (R = La, Ce; Pn = P, As) fw CSD number T radiation, wavelength cryst syst space group, Z unit cell dimensions a, Å b, Å c, Å V, Å3 density (calcd), g/cm3 μ, mm−1 data/params GOF final R indicesa [I > 2σ(I)] rinal R indicesa [all data]

La2Mg3SiP6

La2Mg3SiAs6

Ce2Mg3SiP6

Ce2Mg3SiAs6

564.66 g/mol 432964 90(2) K Mo Kα, 0.71073 Å orthorhombic Pnma (No. 62), 4




11.401(1) 8.213(1) 10.677(1) 1001.5(2) 3.745 9.61 1338/65 1.06 R1 = 0.021 wR2 = 0.035 R1 = 0.038 wR2 = 0.040

11.784(1) 8.484(1) 10.915(1) 1091.2(2) 5.042 25.99 2042/65 1.04 R1 = 0.023 wR2 = 0.040 R1 = 0.035 wR2 = 0.044

11.356(2) 8.188(1) 10.564(2) 982.3(2) 3.835 10.37 1769/65 1.04 R1 = 0.021 wR2 = 0.033 R1 = 0.031 wR2 = 0.036

11.722(1) 8.449(1) 10.819(1) 1071.5(2) 5.150 27.00 1734/65 1.05 R1 = 0.022 wR2 = 0.040 R1 = 0.031 wR2 = 0.043

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2, and w = 1/[σ2Fo2 + (AP)2 + BP], P = (Fo2 + 2Fc2)/3; A and B are weight coefficients.

a

as ω-scans with a 0.4° step width and integrated with the Bruker SAINT software package.30 Multiscan absorption corrections were applied.30 The solution and refinement of the crystal structures were carried out using the SHELX-2014 suite of programs.31 In the final stage of the refinement the occupancies of Si and P atomic sites were allowed to vary freely in the crystal structure of La2Mg3SiP6. The deviation from the full occupancy for each atomic site was smaller than 1 esd. When the Si site was set to be P, the refined occupancy, 96.0(4)%, was distinctly smaller than 100% by 10 esd. When P sites were set to be Si, the refined occupancies varied in the range 105− 110% with esd of less than 1%. These occupancy tests allowed for an unambiguous assignment of the P and Si atomic positions. The final refinements were performed using anisotropic atomic displacement parameters for all atoms. A summary of pertinent information relating to unit cell parameters, data collection, and refinements is provided in Table 1, and the atomic parameters and interatomic distances are provided in Tables S2 and S3. Further details of the crystal structure determination may be obtained from Fachinformationszentrum Karlsruhe, Germany, by quoting the depository numbers CSD432964−432967. Electronic Structure Calculations. Electronic structure calculations and crystal orbital Hamilton population bonding analyses (COHP) were carried out using the tight binding-linear muffin tin orbitals-atomic sphere approximation (TB-LMTO-ASA) program.32 The Barth−Hedin exchange potential was employed for the LDA calculations.33 The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. The basis set used contained La(6s,5d,4f), Mg(3s,3p), Si(3s,3p), and P(3s,3p) orbitals with downfolded La(6p), Mg(3d), Si(3d), and P(3d) functions. The density of states and band structures were calculated after converging the total energy on a dense k-mesh of 24 × 24 × 24 points with 2197 irreducible k-points. Magnetic Measurements. Magnetic measurements were performed on polycrystalline samples placed in gelatin capsules. Samples were transferred to the magnetometer under inert atmosphere, and exposure to ambient atmosphere was kept as short as possible. Magnetic susceptibility was measured in the temperature range 2−300 K in an applied field of 10 mT, and isothermal magnetization measurements were performed in an applied field of 0−7 T using a Quantum Design MPMS-XL SQUID magnetometer.

the composition determined by energy-dispersive X-ray (EDX) microanalysis can only verify that the metal ratios in the sample are La/Mg/Si = 1.8/3.0/1.1 when normalized to 3 Mg atoms (Table S1). No incorporation of iodine was found in the crystals. The source of Si in the sample was assumed to be from the silica ampule. A new reaction was performed with the starting materials La:Mg:Si:P:I2 in a 2:3:1:6:0.15 ratio and using the aforementioned temperature profile. Similar black needle-like crystals were found in the reaction with a significantly larger yield, and were characterized as exactly the same La2Mg3SiP6 phase. Crystals of the other three title compounds, La2Mg3SiAs6, Ce2Mg3SiP6, and Ce2Mg3SiAs6, were also synthesized through the same temperature profile by loading the appropriate elements in a R:Mg:Si:Pn:I2 = 2:3:1:6:0.15 (R = La, Ce; Pn = P, As) ratio. All of the title compounds are extremely air-sensitive and decompose into amorphous powders within a few minutes of exposure to ambient atmosphere. The polycrystalline samples of Ce2Mg3SiP6 and Ce2Mg3SiAs6 for magnetic properties characterization were obtained via solid-state reaction of elements without iodine. The elements were loaded in the stoichiometric 2:3:1:6 ratio into carbonized silica ampules, which were subsequently evacuated and flame-sealed. The ampules were heated from room temperature to 1073 K over 17 h and then annealed at this temperature for 144 h. After the furnace was turned off, the samples were ground and reloaded into new ampules in the glovebox, resealed, and reheated using the same temperature profile as the first annealing. The same procedure was repeated for a third time, and after the third cycle the fine black polycrystalline powders of Ce2Mg3SiP6 and Ce2Mg3SiAs6 were proved to be single phase by means of powder Xray diffraction (Figures S1 and S2). Elemental Analysis. Elemental analysis of selected crystals was carried out on a Hitachi S4100T scanning electron microscope (SEM) with energy-dispersive X-ray (EDX) microanalysis (Oxford INCA Energy) to check for possible incorporation of iodine into the samples. Due to the partial sample decomposition upon transferring from the glovebox into the SEM, a reliable determination of P content was not possible. EDX analyses confirmed the presence of silicon and the absence of iodine in the selected samples, as well as the metal ratios La/Mg/Si = 1.8(1)/3.0/1.1(1), which agrees well with the composition determined from single crystal diffraction (Table S1). Single Crystal X-ray Diffraction. Single crystal diffraction experiments were collected at 90 K using a Bruker Apex II diffractometer with Mo Kα radiation. The data sets were recorded 8349


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Å).40 The bond interactions within MgP4 tetrahedra and MgP6 octahedra were further elucidated by COHP analysis (vide inf ra). The Si−P interatomic distances, 2.262(2)−2.308(2) Å, are typical for covalent Si−P bonds in SiP (2.27 Å),41 Cs2SiP2 (2.28 Å),42 and NiSi2P3 (2.24 Å to 2.27 Å).43 The P−P distance in La2Mg3SiP6, 2.242(2) Å, is typical for homoatomic bond distances which are also observed in BaP3 (2.19−2.23 Å),44 SrP2 (2.19−2.23 Å),45 Ba8Au16P30 (2.12−2.30 Å),46 BaAu2P4 (2.21 and 2.23 Å),47 and Ba4Mg2.8Cu11.2P10 (2.22 Å).22 The La3+ atoms are surrounded by eight P atoms at distances of 2.931(1)−3.122(1) Å, which are comparable to many compounds such as LaP2 (2.93−3.18 Å),48 LaP5 (2.99−3.12 Å),49 La4Mg1.5Zn8.5P10 (2.96−3.15 Å),29 LaCu1+xP2 (2.92−3.12 Å),24 and La3Zn4P6Cl (2.97−3.09 Å).28 La2Mg3SiAs6 is isostructural to La2Mg3SiP6 and has larger unit cell volume due to the larger covalent radius of As compared to P. In turn, the unit cell volume for Ce2Mg3SiP6 is smaller than the volume for La2Mg3SiP6 due to the smaller ionic radius of Ce3+ compared to that of La3+ (Table 1). The composition of La2 Mg 3 SiP 6 can be written as (La3+)2(Mg2+)3(Si4+)(P3−)4(P24−) assuming formal oxidation states of −3 for isolated P atoms, and −2 for atoms forming P2 dumbbells. The electron-balanced nature of La2Mg3SiP6 is further verified by band structure calculations. The electron count in Ce2Mg3SiP6 should be identical, (Ce3+)2(Mg2+)3(Si4+)(P3−)4(P24−), suggesting a + 3 oxidation state for Ce, which was confirmed by magnetic measurements. Electronic Structure. The band structure and density of states (DOS) for La2Mg3SiP6 are shown in Figure 2.

RESULTS AND DISCUSSION Crystal Structure. All the title compounds are isostructural, and for clarity, only the crystal structure of La2Mg3SiP6 is discussed (Figure 1). La2Mg3SiP6 crystallizes in the ortho-

Figure 1. (Left) Polyhedral representation of the crystal structure of La2Mg3SiP6 together with (right) its two main constituent fragments, a [MgP3] layer and a [MgSiP4] chain (right): La, gray; Mg, red; Si, blue; P, orange; MgP6 octahedra, green; SiP4 tetrahedra, blue; MgP4 tetrahedra, red. The unit cell is shown as a black line.

rhombic space group Pnma (No. 62), Pearson symbol oP48, Wyckoff sequence d3c6. While compounds with this Wyckoff sequence have been reported,34,35 we found no similar bonding connectivity in the polyanion and cation arrangements and consider this compound to be a new structure type. There are one La, two Mg, one Si, and five P symmetry-independent atomic positions in the unit cell of La2Mg3SiP6, which are all fully occupied. The crystal structure of La2Mg3SiP6 is composed of a three-dimensional anionic framework of [Mg3SiP6]6− encapsulating La3+ cations (Figure 1). (Left) Polyhedral representation of the crystal structure of The three-dimensional [Mg3SiP6]6− framework is built from two main fragments: puckered [MgP3] layers and onedimensional [MgSiP4] chains. The [MgP3] layers are composed of MgP6 octahedra (green in Figure 1) which share edges along the [010] direction and vertices along the [001] direction. Such layers are linked together in the three-dimensional framework by homoatomic P−P bonds between P atoms from different [MgP3] layers. Additionally, such layers are connected via onedimensional [MgSiP 4 ] chains which are composed of alternating MgP4 and SiP4 tetrahedra, which are, respectively, red and blue in Figure 1. Within the [MgSiP4] chain, each MgP4 tetrahedron shares one edge and two vertices with three neighboring SiP4 tetrahedra, and vice versa. We have recently reported an isostructural but not isoelectronic one-dimensional fragment [Zn2P4] in the crystal structure of the compound La4Zn7P10.29 Each tetrahedron in the [MgSiP4] chains shares two edges with surrounding [MgP3] layers (Figure S5). The distances between Mg and P in the distorted [MgP6] octahedra fall in the range 2.619(2)−2.809(2) Å, which is comparable to many binary and ternary Mg phosphides such as MgP4 (2.61−2.86 Å),36 Mg8Ir23P8 (2.71−2.80 Å),37 and BaMg2P2 (2.63−2.635 Å).38 The Mg−P interatomic distances in the [MgSiP4] chains, 2.523(1)−2.591(3) Å, are shorter than those in the MgP6 octahedra, which is expected for a lower coordination number. Similar short Mg−P distances were reported for compounds where Mg is tetrahedrally coordinated by P atoms, such as MgSiP2 (2.60 Å)39 and Mg3P2 (2.56−2.61

Figure 2. (Top) Band structure and (bottom) density of states of La2Mg3SiP6.

La2Mg3SiP6 was predicted to be an indirect bandgap semiconductor with a 0.1 eV separation between the Γ and Z points.50 The direct bandgaps of 0.35 and 0.36 eV are observed for Γ and U points, respectively. The top of the valence band exhibits similar dispersion of orbitals along the main crystallographic directions, Γ → X, Γ → Y, and Γ → Z, which is 8350


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Inorganic Chemistry expected for a three-dimensional framework compound. In La2Mg3SiP6, the main contribution to the top of the valence band is from P3p orbitals with smaller contributions of La and Mg orbitals. The bottom of the conduction band exhibits small dispersion along Γ → X and Γ → Z directions, but strong dispersion along Γ → Y and other directions parallel to [010]. The bottom of the conduction band is mainly composed of La5d orbitals and P3p orbitals while the Mg contribution is minimal. Location of the La cations in the framework channels running along the [010] direction explains the observed dispersions of the conduction band orbitals. The La4f orbitals mainly contributed to the states above 2 eV (Figure S3). Si orbitals make small steady contributions to both the valence and conduction bands. To further elucidate the Mg−P bonding within MgP4 tetrahedra and MgP6 octahedra, COHP calculations were performed on La2Mg3SiP6. The COHP analysis indicates strong covalent bonding for the Si−P and P−P interactions of 2.26 and 2.24 Å, respectively. The values of integrated −ICOHP are 3.11 (Si−P) and 3.38 (P−P) eV/bond. In Figure 3 the COHP

Ba4Mg2.8Cu11.2P10.22 The presence of both MgP4 tetrahedra and MgP6 octahedra in the same crystal structure of La2Mg3SiP6 with variable bond distances demonstrates the flexibility of Mg−P bonding. Magnetic Properties. The temperature dependences of the magnetic susceptibilities for Ce2Mg3SiP6 and Ce2Mg3SiAs6 are shown in Figure 4. The temperature dependences of the

Figure 3. −COHP plot for the selected interactions between Mg and P in La2Mg3SiP6. Color code and integrated −ICOHP values are given in the legend.

Figure 4. Temperature-dependent magnetic susceptibilities for Ce2Mg3SiP6 (blue △) and Ce2Mg3SiAs6 (orange □) measured in an applied field of 0.01 T. Insets: High-temperature part of magnetic susceptibility together with a modified Curie−Weiss fit ().

curves for the selected Mg−P distances are shown: Mg1−P1 of 2.81 Å and Mg1−P4 of 2.62 Å in the MgP6 octahedron, and Mg2−P1 of 2.52 Å and Mg2−P4 of 2.59 Å in the MgP4 tetrahedron. COHP analysis shows that Mg−P bonding is welloptimized with all bonding Mg−P interactions below the Fermi level, and the essentially nonbonding interactions just above the Fermi level, 0−1.5 eV. Strong antibonding character is observed for interactions at higher energies. The only exception is the Mg1−P4 bond which has a small bonding character at the bottom of the conduction band at 0.2 eV. The strongest Mg−P interaction is found for the shortest distance, Mg2−P1 of 2.52 Å, with an integrated −ICOHP value of 1.70 eV/bond. The weakest interaction is found for the longest distance, Mg1−P1 of 2.81 Å, with an integrated −ICOHP value of 0.91 eV/bond, which still indicates a significant degree of bonding. For comparison, the Zn−P interactions within ZnP6 octahedra are 2.75 and 3.35 Å with integrated −ICOHP values of 0.97 and 0.03 eV/bond, respectively.29 In turn, similar distances in the tetrahedron and octahedron, Mg1−P4 and Mg2−P4, 2.62, and 2.59 Å, have comparable values of the −ICOHP, 1.35 and 1.48 eV/bond, respectively. In complex phosphides, the MgP4 and MgP6 fragments can be found in separate compounds, while 5coordinated MgP5 fragments are rare; one example is the MgP5 square pyramidal coordination that was reported in

magnetic susceptibilities exhibit an abrupt upturn at 23 K (P) and 11 K (As) indicating ferro- or ferrimagnetic ordering of the Ce magnetic moments. The high-temperature parts of the susceptibility dependences were fit with the modified Curie− Weiss law taking into account the temperature-independent contribution, χ0, which was similar for both samples (insets in Figure 4); without χ0, unreasonably high magnetic moments were observed for Ce (>3 μB). The modified Curie−Weiss fit resulted in similar effective magnetic moments for Ce for both compounds, which are also close to the expected value of 2.54 μB for Ce3+.51 The most striking difference was observed for the asymptotic Curie temperature, θ. For the P-containing compound a value of +20(1) K was calculated, while a value of −20(4) K was calculated for the As-containing phase. This indicates that in Ce2Mg3SiP6 the nearest-neighbor interactions between Ce atoms are ferromagnetic, while in Ce2Mg3SiAs6 such interactions are antiferromagnetic, suggesting ferrimagnetic behavior. The ferrimagnetic nature of both phases was confirmed by isothermal magnetization studies in the range 0.01−7 T (Figure 5). At the applied field of 0.01 T the Ce magnetic moments in Ce2Mg3SiP6 are already partially saturated at the moment of 0.17 μB/Ce (Figure S4). With the increase of the field there is a broad metamagnetic transition at 3.5 T. The maximum 8351


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Figure 6. Top: An arrangement of the Ce atoms in the crystal structures of Ce2Mg3SiP6 and Ce2Mg3SiAs6. One-dimensional bands running along [010] are clearly visible. The shortest Ce−Ce interband distances are 5.89 Å (P) and 5.78 Å (As). Bottom: The intraband distances are shown separately for the isolated Ce bands for both compounds.

Figure 5. Isothermal (T = 2 K) field dependences of magnetization for Ce2Mg3SiP6 (blue △) and Ce2Mg3SiAs6 (orange □). The low-field region is shown in the inset for the As-containing compound. The lines are drawn to guide the eyes.

by P are present in the reported compounds. The two ceriumcontaining compounds, Ce2Mg3SiP6 and Ce2Mg3SiAs6, have been purified by means of solid-state reaction and are demonstrated to have complex ferrimagnetic interactions and magnetic ordering of Ce moments at low temperatures. Electronic structure calculations indicate that La2Mg3SiP6 is an electron-balanced indirect bandgap semiconductor with a predicted bandgap value of 0.1 eV.

observed nonsaturated moment at 7 T is 0.83 μB/Ce, which is only 40% of the expected gJ value of 2.14 μB/Ce. The results of the field- and temperature-dependent studies indicate that the nature of the magnetic interactions in Ce2Mg3SiP6 is complex. In accordance with the Curie−Weiss fit results, the isothermal behavior of Ce2Mg3SiAs6 is different from that of the P-containing analogue. Small magnetic moments, typical for antiferromagnets, are observed at low fields, 5.7 Å (Figure 6). Ferroand antiferromagnetic interactions may occur both within and between the bands. Comprehensive neutron diffraction investigations are necessary to further clarify the magnetic structure of the reported compounds.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01015. Calculated and experimental powder XRD patterns; tables with crystallographic information; EDS results; and additional structural, magnetic and DOS figures (PDF) Accession Codes

CCDC 1547185−1547188 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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CONCLUSIONS Four new quaternary pnictides, R2Mg3SiPn6 (R = La, Ce; Pn = P, As), have been synthesized and structurally characterized. All the compounds are isostructural and crystallize in a new structure type in the orthorhombic space group Pnma. The crystal structure of these new compounds is composed of a three-dimensional Mg−Si−Pn framework encapsulating R cations. Both octahedral and tetrahedral coordinations of Mg

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian Wang: 0000-0003-1326-4470 Kirill Kovnir: 0000-0003-1152-1912 Notes

The authors declare no competing financial interest. 8352


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ACKNOWLEDGMENTS We thank Peter Klavins for the help with the SQUID measurements. This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0008931.

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