Ambient-Pressure Polymerization of Carbon Anions in the High

Article pubs.acs.org/IC

Ambient-Pressure Polymerization of Carbon Anions in the HighPressure Phase Mg2C Stevce Stefanoski,*,† Hanyu Liu,‡ Yansun Yao,‡,§ and Timothy A. Strobel*,† †

Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C., 20015, United States Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada § Canadian Light Source, Saskatoon, Saskatchewan S7N 2V3, Canada ‡

ABSTRACT: Experimental and theoretical methods were employed to investigate the ambient-pressure, metastable phase transition pathways for Mg2C, which was recovered after highpressure synthesis. We demonstrate that at temperatures above 600 K isolated C4− anions within the Mg2C structure polymerize into longer-chain carbon polyanions, resulting in the formation of the α-Mg2C3 (Pnnm) structure, which is another local energy minimum for the carbon−magnesium system. Access to the thermodynamic ground state (decomposition into graphite) was achieved at temperatures above ∼1000 K. These results indicate that recoverable high-pressure materials can serve as useful high-energy precursors for ambient-pressure materials synthesis, and they show a novel mechanism for the formation of carbon chains from methanide structures.

■

INTRODUCTION High-pressure synthesis represents a promising tool for the discovery of new materials with exceptional properties. Depending on the strength of interactions between atoms, ambient-pressure recoverability of phases that are thermodynamically stable only at high pressure may be possible.1−3 The recovered materials in these cases are thermodynamically metastable, but they can exhibit mechanical stability and may represent novel high-energy precursors for subsequent ambient-pressure manipulation. This methodology was recently employed for the synthesis of a new allotrope of silicon with exceptional physical properties.2 In this work we demonstrate the applicability of this method to the formation of carbon allylenide chains starting from the recently discovered highpressure carbide Mg2C. Carbides are chemical compounds formed between carbon and less-electronegative elements. Ionic carbides typically include alkali or alkaline-earth metals,4 which, depending on the form of carbon, can be classified into three categories: methanides,5−10 acetylides,11,12 and the least common allylenides.13−17 These families contain methanide anions, C4−, carbon dumbbell pairs (acetylide ions), C22−, and the rare C34− (allylenide ions).4 Interest in carbides stems from the range of exceptional physical properties and unique chemical bonding they exhibit.18−23 The nonionic carbides, for example, include properties such as high-temperature superconductivity,24,25 and relatively high bulk moduli, comparable to that of diamond, in experimentally inaccessible carbon clathrates.26,27 Carbides based on magnesium are particularly interesting not only because compositions with three different carbon anions exist but also for understanding the intriguing Mg−C and C−C bonds important for polar organometallic compounds and © XXXX American Chemical Society

further understanding of the bonding nature of carbon anions.23,28 Magnesium sesquicarbide, or Mg 2 C 3 , exists as two polymorphs: α-Mg2C3 and β-Mg2C3. α-Mg2C3 was synthesized by Novak in 1910,13 and β-Mg2C3 was only recently synthesized by Strobel et al.14 using a high-pressure approach. The α-polymorph crystalizes in the orthorhombic system (space group Pnnm), whereas the β-polymorph is a monoclinic variation of Mg2C3 (space group C2/m).14 α-Mg2C3 is the most stable polymorph at ambient pressure, whereas β-Mg2C3 is the most stable form above 5 GPa, although both structures are thermodynamically metastable at ambient pressure. Structurally, the two polymorphs differ by the mutual organization of the C34− anions: in the orthorhombic form they are canted by ∼70° in an ABAB stacking, whereas all chains are nearly aligned along the c-axis in the monoclinic form.14 Magnesium methanide, or Mg2C, was also synthesized only recently by Kurakevych et al.,5 while initially theoretically predicted by Corkil et al.20 in 1993 using ab initio methods. Mg2C takes on the antifluorite (Li2O) structure (Figure 1) in the cubic crystallographic system (space group Fm3m ̅ ) and becomes stable above 15 GPa (at 0 K) with respect to elemental solids, although recoverable to ambient pressure.5 This compound contains fully ionic methanide C4− anions, which are somewhat analogous to the Si44− anions used in the synthesis of some inorganic clathrates.3 C4− within Mg2C should be distinguished from other interstitial and methanide carbides by the magnitude of the effective charge. Bader charge analysis of Mg2C reveals a net electron transfer of 3.14 e−/C,5 which is Received: August 5, 2015

A

DOI: 10.1021/acs.inorgchem.5b01780 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Crystal structures and phase transformation of Mg2C.

large compared with the more covalent Be2C (3.00 e−/C),29 or the recently discovered Ca2C (2.32 e−/C).30 Metastable phase transitions from silicides and germanides are well-known and can result in the formation of interesting clathrate structures.3 For the case of carbon, no analogous Zintl precursors are known, like Na4Si4 in the case of silicon, although manipulation of certain carbides have resulted in novel forms of so-called carbide-derived carbon.31 In terms of ionic character and valence hybridization, C4− is similar to Si44−. Novel phase transformations in the Si-based systems take place under conditions of elevated temperatures (and typically under vacuum to volatilize the light metal atoms), resulting in the formation of extended Si-based framework structures. Similar manipulations of Mg2C could, in principle, result in the formation of analogous C-based structures, owing to the fully ionic C4− species. Investigating the effect of the partial pressure of Mg on the transition mechanisms in the Mg−C system, for example, by introducing an inert atmosphere instead of vacuum, could be of potential interest for future studies. In this work, experimental and theoretical studies were employed to investigate the metastable transitions from Mg2C, resulting in a new pathway to the α-Mg2C3 (Pnnm) structure.

■

Figure 2. PXRD data of Mg2C at ambient pressures, after heating at selected temperatures. Black tick marks for Mg2C and red for α-Mg2C3 are shown below relevant patterns. Reflections from MgO, Mg and graphite are indicated by *, ▽, and ◇, respectively. (NVT) ensemble for a simulation time of 0.8 ps. The MD trajectories were sampled by a time step of 2.0 fs. The history-dependent potential well was constructed in the simulations using Gaussian energy profiles with a width δs = 15 (kbar·Å3)1/2 and height W = 225 kbar·A3. To reduce the energy barrier for possible phase transitions, the system was pressurized slightly to 5 kbar, in both hydrostatic and nonhydrostatic situations. A recent implementation of the metadynamics method was used to load the uniaxial stresses to the simulation cell.33 The MD components were performed using the Vienna ab initio simulation program34 using projected augmented wave potentials35 employing the Perdew−Burke−Ernzerhof exchange-correlation functional.36 The plane-wave basis set was expanded with an energy cutoff of 520 eV. The Car−Parrinello molecular dynamics (CPMD)37 and linearresponse phonon calculations were performed using the Quantum ESPRESSO package38 with norm-conserving pseudopotentials with an energy cutoff of 60 Rydberg. A fictitious electron mass of the electronic wave function of 50 au and a time step of 4 au were used for the time integration of the ionic motions. For the CPMD calculations, the Brillouin zone (BZ) sampling was restricted to the zone center, and the temperature was controlled by a Nosé thermostat.39 For phonon calculations, a 4 × 4 × 4 q-point mesh was used for BZ sampling, and at each q-point, the dynamical matrix was calculated with an 8 × 8 × 8 k-point mesh.

EXPERIMENTAL AND THEORETICAL METHODS

Synthesis of Mg2C was achieved following the procedure described previously:5 polycrystalline powders of Mg (Sigma-Aldrich, 99.5%) and glassy carbon (Aldrich, 99.95%) were dried in a vacuum oven at 200 °C for ∼12 h and then sealed in Argon. The powders were mixed in Mg/C = 2:1 ratio under an Ar atmosphere and ground and mixed using an alumina mortar and pestle to achieve a homogeneous mixture. The mixture was then pressed within a MgO capsule and enclosed within a Cr-doped MgO octahedral assembly for a multianvil largevolume press. Samples were compressed to a pressure of 15 GPa and then reacted at 1900 K for 30 min, followed by rapid quenching. Powder X-ray diffraction (PXRD) analyses were performed on a Bruker D8 diffractometer with Cu Kα radiation and area detector. Samples were sealed under Ar using a polyimide film before PXRD analysis. Initial syntheses revealed phase-pure Mg2C with traces of MgO that comes from the capsule material (Figure 2). Separate batches of the as-synthesized Mg2C were next introduced into a Swagelok metal fitting inside an Ar-filled glovebox and then placed in a sealed quartz tube and heated under dynamic vacuum of 1 × 10−6 Torr for 2 h in a Mellen tube furnace at temperatures ranging from 500 to 1100 K. The resulting material after each reaction temperature was again analyzed by PXRD. Reaction times under 1 h resulted in low αMg2C3 phase fraction, whereas the α-Mg2C3 phase fraction was found to be relatively consistent and stable when Mg2C was reacted at times as long as 6 h. The behavior of Mg2C under different pressure−temperature conditions was simulated using the density functional theory-based metadynamics method.32 A 2 × 2 × 2 supercell (96 atoms) was employed in the simulations along with a 2 × 2 × 2 k-point mesh for the Brillouin zone sampling. Each metastep consisted of a firstprinciples molecular dynamics (MD) simulation within the canonical

■

RESULTS AND DISCUSSION After the high-pressure/high-temperature Mg2C synthesis reaction, recovered samples consisted of a homogeneous orange-brown colored powder. When opened under the inert Ar atmosphere, it was impossible to completely remove all of the MgO capsule material, but PXRD analysis showed phasepure Mg2C aside from some residual MgO impurity that coated the outside of the sample (Figure 2). Upon heating the recovered material, Mg2C remained thermally stable above 500 K, which is somewhat surprising considering that it is already metastable with respect to Mg + carbon by ∼0.2 electronvolts per atom at atmospheric pressure,5 and interactions are essentially entirely of ionic nature. When Mg2C was heated at 700 K, a phase transformation to α-Mg2C3 was observed to take place following the reaction 3Mg 2C → α‐Mg 2C3 + 4Mg (1) The vapor pressure of Mg at 700 K is most likely insufficient for it to be eliminated from the reaction mixture under the dynamic vacuum conditions, and this is confirmed by the presence of hexagonal close-packed (HCP) Mg PXRD peaks in B

DOI: 10.1021/acs.inorgchem.5b01780 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Figure 2. Earlier studies show that α-Mg2C3 decomposes to Mg and C, that is, α-Mg2C3 → 2Mg + 3C at 970 K.13 We confirm this result from our observations at temperatures above 900 K. When Mg2C was reacted at 1100 K decomposition into graphite-2H was observed (Figure 2), although decomposition into amorphous carbon might, in principle, be possible at lower temperatures. We note that only traces of HCP Mg were observed above 700 K. Above this temperature, elemental Mg was volatilized and evacuated by the vacuum. A reaction of Mg with oxygen is most likely absent under a vacuum of 1 × 10−6 Torr. Also, oxidation of Mg from the atmosphere was ruled out as no traces of Mg(OH)2 were observed. We also note that the lower α-Mg2C3-to-MgO PXRD peak ratios at 900 K as compared to 700 K (Figure 2) confirms the decomposition of α-Mg2C3 into Mg and C. Despite the fact that both Mg2C and α-Mg2C3 are metastable at ambient pressure with respect to their formation elements, the phase transition path was observed to follow reaction pathway (1) at T < ∼900 K. In previous studies,13 thermally treated MgC2 was found to follow the reaction 2MgC2 → α‐Mg 2C3 + C

phonon dispersion relations (Figure 4a,b). The P42/mnm structure has slightly lower static energy than that of the cubic Mg2C structure, by ∼0.03 eV/f.u. At 700 K, the kinetic energies of the Mg and C atoms are sufficient to smear out this energy difference. The low-energy path for this structural fluctuation is characterized by a common Pnnm subgroup.40 The cubic unit cell is converted to the common subgroup via a basic matrix (0, 1, 0), (1/2, 0, −1/2), (−1/2, 0, −1/2) (Figure 3a). The site symmetries for the Mg and C atoms after the conversion are 4g (0.75, 0.5, 0) and 2a (0, 0, 0), respectively. At this temperature, the Mg atoms are mobile and able to diffuse to crystalline voids on the x−y plane, changing their internal coordinates to 4g (0.69, 0.69, 0) (Figure 3b). The C atoms, however, do not change their internal coordinates, but they do translate along with the unit cell deformation. The lengths of the three lattice vectors have substantial changes in the transition, from 5.66, 4.00, and 4.00 Å to 5.06, 5.06, and 3.54 Å (Figure 3c). In the P42/mnm structure, the closet C−C distance is reduced from 3.95 to 3.54 Å, resulting in arrays of linear carbon chains. Further increase in temperature enhances the relative motion of the C atoms and facilitates the formation of the C34− units (Figure 3d). In the metadynamics simulations the C34− units were observed in the structure shortly after the temperature increased to 900 K and above. The C22− units, however, were not formed in the simulations. In this temperature range, the C34− anion most likely has higher thermal stability than the C22− anion. Experimentally, MgC2 is known to decompose into Mg2C3 above 700 K, which provides strong evidence for the stability of the C34− unit. The C34− anion is isoelectronic to CO2, which also has a linear structure. The C−C distances in the C34− units are ∼1.4 Å, much shorter than that in the otherwise equally spaced chains. As such, the formation of C34− would leave voids in the structure, which induces unbalanced forces for the surrounding Mg atoms (Figure 3e). Thus, this procedure is rather dynamic in the simulation; the C34− only settles for a few metasteps, and then the C atoms would move back to their original sites. This observation suggests that excess Mg needs to be evacuated from the system to stabilize the C34− units. The removal of Mg would result in the Mg 2C 3 stoichiometry and a large volume change. A direct simulation of the evacuation of excess Mg is beyond the current capability of MD simulations. Thus, we have manually removed the Mg atoms and obtained an Mg2C3 structure with the P21212 space group. An interesting feature of the P21212 structure is that the C34− unit has an asymmetrical linear structure; the two C−C distances in the C34− unit are 1.41 and 1.27 Å, respectively. The negative charges are distributed nonequally among the two terminal C atoms. The calculated Bader charges revealed that the terminal C atom connected by the longer bond gains approximately one electron from its counterpart at the other end of the unit. Such a charge separation resembles the resonance structure of CO2. As the gas-phase C34− has a symmetric structure, the asymmetrical C34− structure in the P21212 structure reveals significant electrostatic distortions by nearest-neighbor Mg atoms, which causes mechanical instability. The P21212 structure is a transition state and becomes immediately unstable (the imaginary phonon modes appear in entire acoustic branches). To determine the reaction product, we annealed the P21212 structure to 900 K using a CPMD simulation in a NVT ensemble, and then fully relaxed it for 20 ps in an isothermal− isobaric (NPT) ensemble. The structure was found to change with a large reduction of volume. The structure obtained in the

(2) 4−

It is interesting to note that the [CCC] anion (αMg2C3) is observed as an intermediate during the decomposition of both C-rich MgC213 and Mg-rich Mg2C. Thus, it appears that the allylenide anion is the most energetically favorable polycarbon at ∼700 K, irrespective of whether the starting material contains the C4− or [CC]2− anions. This result was also corroborated by metadynamics simulations, which offer further insights into the crystal chemistry of the Mg−C system. Experimentally observed and theoretically predicted Mg−C phases are listed in Table 1. Table 1. Experimentally Synthesized and Theoretically Predicted Binary Mg-C Phases phase Mg2C MgC2 α-Mg2C3 β-Mg2C3 Mg2C Mg2C3 Mg2C3

space group Fm3m ̅ P42/ mnm Pnnm C2/m* P42/ mnm Pnma Pbca

a (Å)

b (Å)

c (Å)

observation

reference

5.45 3.93

5.45 3.93

5.45 5.04

experiment experiment

5 4, 13

6.41 4.83 5.06

5.28 4.70 5.06

3.73 6.05 3.54

experiment experiment theory

12.85 6.23

3.78 5.20

5.32 7.02

theory theory

13 14 present work 14 14

*

β = 126.43°.

First-principles metadynamics simulations were performed to provide insight into the observed reaction and perhaps reveal the transition mechanism. The simulations were performed at six temperatures (100, 300, 500, 700, 900, and 1100 K) starting from a 2 × 2 × 2 supercell of the experimental Mg2C structure. In the simulations the supercells were slightly compressed to 5 kbar, either hydrostatically or nonhydrostatically, to allow for a faster reaction. The first sign of a phase transition appeared at 700 K, where the simulation cell started to fluctuate between the cubic form and an intermediate P42/mnm structure (Figure 3). Both the cubic and the P42/mnm structures of Mg2C are thermodynamically metastable at ambient conditions. The mechanical and dynamical stabilities of these two structures are established by the absence of imaginary frequencies in the C

DOI: 10.1021/acs.inorgchem.5b01780 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. Suggested transition pathway for the formation of α-Mg2C3. (a) Initial cubic Mg2C structure. Mg is shown in orange, and C is in dark brown color. The parent structure is shown by dashed lines. (b) Deformation of the parent structure leading to an intermediate P42/mnm structure. Atomic displacements and cell deformation are indicated by red and blue arrows, respectively. (c) The intermediate P42/mnm structure. (d) Supercell of the P42/mnm structure showing arrays of linear C chains (dashed lines) and the kinetically driven formation of the C34− units (arrows). (e) Permanent transformation of the linear chains to the C34− units. Excessive Mg atoms are to be evacuated (dashed circle), whereas the C34− units are reoriented (arrows). (f) Final α-Mg2C3 structure in the supercell. Black rectangle shows the unit cell.

Figure 4. Calculated phonon dispersion relations for (a) the cubic Mg2C structure, (b) the intermediate P42/mnm structure, and (c) the α-Mg2C3 structure.

atoms, which is a thermally activated process, as well as reorientation of the C34− units (Figure 3f). First-order phase transitions as a result of thermal processing similar to that employed in this study have been observed in other carbide systems, for example, in Na2C2 and Li2C2 comprising the acetylide C22− anions.4 Low- and hightemperature Na2C2 and/or Li2C2 polymorphs exist in these systems, in contrast with the Mg-carbides for which only lowand high-pressure polymorphs are known to date.13,14 It would

NPT simulation was annealed again to 900 K. The ending structure was then fully optimized, which was found to be a distorted α-Mg2C3 structure. Phonon calculations revealed that the α-Mg2C3 structure is dynamically stable (Figure 4c). The low-energy path from the intermediate P21212 structure to the α-Mg2C3 structure is described by a common P21 subgroup;40 this transition results in an energy decrease of ∼0.17 eV/atom. This reaction involves relatively large diffusion of the Mg D

DOI: 10.1021/acs.inorgchem.5b01780 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry be of interest to investigate the possibility of α → β-Mg2C3 phase transformation, as reorientation of the C34− allylenide anions is likely to take place during this reaction. This, in addition to the C4− → [CC]2− transformation, is not only interesting in terms of understanding the crystal chemistry in the Mg−C systems, but also to investigate the possible routes for synthesis of novel materials, such as carbon clathrates. Namely, reconfiguration of the Si44− anions of the Na4Si4 Zintlcomposition into sp3-hybridized Si as a result of a thermal processing results in formation of silicon clathrates.3 Understanding the possible transformations of carbon anions might be a promising route for the synthesis of various carbon-rich materials.

(13) Novak, J. Z. Physik. Chem. 1910, 73, 513. (14) Strobel, T. A.; Kurakevych, O. O.; Kim, D. Y.; Le Godec, Y.; Crichton, W.; Guignard, J.; Guignot, N.; Cody, G. D.; Oganov, A. R. Inorg. Chem. 2014, 53, 7020−7027. (15) Fjellvaag, H.; Karen, P. Inorg. Chem. 1992, 31, 3260−3263. (16) Poettgen, R.; Jeitschko, W. Inorg. Chem. 1991, 30, 427−431. (17) Hoffmann, R.; Meyer, H. J. Z. Anorg. Allg. Chem. 1992, 607, 57− 71. (18) Srepusharawoot, P.; Blomqvist, A.; Araujo, C. M.; Scheicher, R. H.; Ahuja, R. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 125439−125445. (19) Karttunen, A. J.; Fassler, T. F.; Linnolahti, M.; Pakkanen, T. A. Inorg. Chem. 2011, 50, 1733−1742. (20) Corkill, J. L.; Cohen, M. L. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 17138−17144. (21) Yamanaka, S. Dalton. Trans. 2010, 39, 1901−1915. (22) Lambert, C.; von Rague Schleyer, P. Angew. Chem. 1994, 106, 1187−1199. (23) Bickelhaupt, F. M.; van Eikema Hommes, N. J. R.; Fonseca Guerra, C. F.; Baerends, E. J. Organometallics 1996, 15, 2923−2931. (24) Blase, Z.; Bustarret, E.; Chapelier, C.; Klein, T.; Marcenat, C. Nat. Mater. 2009, 8, 375−382. (25) Calandra, M.; Mauri, F. Phys. Rev. Lett. 2008, 101, 016401− 016405. (26) San-Miguel, A.; Kéghélian, P.; Blase, X.; Mélinon, P.; Perez, A.; Itié, J. P.; Polian, A.; Reny, E.; Cros, C.; Pouchard, M. Phys. Rev. Lett. 1999, 83, 5290−5293. (27) Perottoni, C. A.; da Jornada, J. A. H. J. Phys.: Condens. Matter 2001, 13, 5981−5998. (28) Lambert, C.; von Rague Schleyer, P. Angew. Chem., Int. Ed. Engl. 1994, 33, 1129−1140. (29) Herzig, P.; Redinger, J. J. Chem. Phys. 1985, 82, 372−378. (30) Li, Y. − L.; Wang, S. − N.; Oganov, A. R.; Gou, H.; Smith, J. S.; Strobel, T. A. Nat. Commun. 2015, 6, 6974−6983. (31) Presser, V.; Heon, M.; Gogotsi, Y. Adv. Funct. Mater. 2011, 21, 810−833. (32) Martoňaḱ , R.; Donadio, D.; Oganov, A. R.; Parrinello, M. Nat. Mater. 2006, 5, 623−626. (33) Yao, Y.; Klug, D. D. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 214122-1−214122-5. (34) Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (35) Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (37) Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, 2471−2474. (38) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21, 395502−395521. (39) Nosé, S. Mol. Phys. 1984, 52, 255−268. (40) Stokes, H. T.; Hatch, D. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 144114−144126.

■

CONCLUSION We have demonstrated that the recoverable high-pressure composition Mg2C can serve as a high-energy precursor for the ambient-pressure synthesis of the α-Mg2C3 (Pnnm) structure, revealing a novel mechanism for the formation of carbon chains from methanide structures. These experimental results were corroborated by first-principles metadynamics simulations, which elucidate the reaction pathway of the phase transformation at ambient pressure. Understanding the possible transformations of carbon anions is important not only for a better understanding of the crystal chemistry in the Mg-C system but also to investigate potential routes for the synthesis of carbon-rich materials with exceptional physical properties.

■

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by DARPA (ARO Contract No. W31P4Q1310005), Carnegie Canada, and Natural Sciences and Engineering Research Council of Canada (NSERC). XRD studies were supported, in part, by the Washington Diamond Fund. Calculations were performed using computing resources provided by the Univ. of Saskatchewan, WestGrid, and Compute Canada.

■

REFERENCES

(1) Powles, R. C.; Marks, N. A.; Lau, D. W. M.; McCulloch, D. G.; McKenzie, D. R. Carbon 2013, 63, 416−422. (2) Kim, D. Y.; Stefanoski, S.; Kurakevych, O. O.; Strobel, A. T. Nat. Mater. 2015, 14, 169−173. (3) Nolas, G. S. The Physics and Chemistry of Inorganic Clathrates; Springer, 2014. (4) Ruschewitz, U. Coord. Chem. Rev. 2003, 244, 115−136. (5) Kurakevych, O. O.; Strobel, T. A.; Kim, D. Y.; Cody, G. D. Angew. Chem., Int. Ed. 2013, 52, 1−5. (6) Chung, C.; Lagow, R. J. J. Chem. Soc., Chem. Commun. 1972, 1078b−1079. (7) Shimp, L. A.; Lagow, R. J. J. Am. Chem. Soc. 1973, 95, 1343− 1344. (8) Lebeau, M. C. R. Acad. Sci. Paris 1895, 121, 496. (9) Fichter, C; Brunner, E. Z. Anorg. Allg. Chem. 1915, 93, 84−94. (10) von Stackelberg, M.; Quatram, F. Z. Phys. Chem. B 1935, 27, 50. (11) von Stackelberg, M. Z. Phys. Chem. B 1930, 9, 437. (12) Juza, R.; Wehle, V.; Schuster, U. Z. Anorg. Allg. Chem. 1967, 352, 252−257. E

DOI: 10.1021/acs.inorgchem.5b01780 Inorg. Chem. XXXX, XXX, XXX−XXX