Na-Si Clathrates Are High-Pressure Phases - American Chemical

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Na-Si clathrates are high-pressure phases: A meltbased route to control stoichiometry and properties Oleksandr O Kurakevych, Timothy Alan Strobel, Duck Young Kim, Takaki Muramatsu, and Viktor V Struzhkin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg3017084 • Publication Date (Web): 03 Dec 2012 Downloaded from http://pubs.acs.org on December 10, 2012

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Crystal Growth & Design

Na-Si clathrates are high-pressure phases: A meltbased route to control stoichiometry and properties Oleksandr O. Kurakevych*, Timothy A. Strobel, Duck Young Kim, Takaki Muramatsu and Viktor V. Struzhkin Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA KEYWORDS: Clathrates, Silicon, Sodium, High Pressure, Phase Stability, Crystal growth ABSTRACT Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24Si136 (sII) and a new structure, NaSi6–were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900-1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. Highpressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous

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reports of their synthesis may be viewed as non-equilibrium, precursor-based routes to highpressure phases at low-pressure conditions. Introduction. Silicon clathrates1 (or simply clathrates throughout this communication) are analogues of the water-based clathrate hydrates,2 often crystallizing as high-symmetry cubic structures. Si atoms within these compounds take on a sp3 hybridization state to form a covalent network of polyhedral cavities that host appropriately sized “guest” atoms. The alkali and alkaline earth metals can be intercalated into these cages that extend in three dimensions. Cubic structure I (sI, Pm3 n , a ~ 10.2 Å) clathrate contains two small 512 cages (twelve pentagonal faces) and six

larger 51262 cages (twelve pentagonal faces and two hexagonal faces) with 46 silicon atoms per unit cell, whereas a unit cell of cubic structure II (sII, Fd3 m , a ~ 14.7 Å) is comprised of sixteen 512 cages and eight large 51264 cages with 136 Si atoms. The rigid covalent 3-D network and intercalation nature of these compounds give rise to a combination of interesting properties including remarkable incompressibility;3,4 glass-like thermal conductivity;5 metallic, semi-6 and superconductivity;7, 8 photovoltaic properties;6 and potential for hydrogen storage materials.9 Most known silicon clathrates have been synthesized at ambient pressure or under deep vacuum using Zintl compounds as precursors or through different types of chemical reactions.1, 6 Up to now, chemical decomposition (arc-remelting or heating in vacuum/Ar atmosphere) remains the principal methods for Na-Si clathrate synthesis.6, 10, 11 Later, chemical oxidation was proposed as an alternative route,12 as well as spark plasma treatment of the Na4Si4 precursor.10 Although single crystals of some Si clathrates can be grown by techniques just mentioned,10, 13 the equilibrium synthesis from a melt remains a challenging task for well-controlled crystal growth.


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The knowledge of thermodynamic stability domains in the terms of composition, temperature and pressure is, therefore, of primary importance. The generalized thermodynamic stabilities of silicon clathrate phases are not clear. Guest-free Si clathrates have been predicted to be stable only at negative pressures below -3 GPa14, 15 (a direct analogy may be drawn between the negative pressure stability of empty silicon and water-based clathrates). While some Ba- and halogen-based silicon clathrates can be synthesized at high pressure,16, 17 ab initio results on the high-pressure stability of Na-Si clathrates suggest that these phase are thermodynamically unstable with respect to decomposition at high-pressure conditions.10 Thus, the p-T domains for thermodynamic stability and the possibility for equilibrium crystal growth need clarification. At present, the phase equilibria for the Na-Si system remain the least understood as compared with other clathrate systems (e.g., Ba-Si). Sodium is the lightest metal that is known to intercalate the Si cages alone and its understanding may lead, for example, to the synthesis of Li-, Mg- and Be-bearing clathrates. The experimentally established binary phase diagram at ambient pressure contains only one congruently melting compound of Na and Si, sodium silicide (Na4Si4), and clathrate formation has not been observed under (quasi-) equilibrium conditions.18 In situ study of Na-Si clathrate growth under high-temperature, high-vacuum conditions allows one to conclude that its formation is principally determined by mutual structural relationships of phases and dynamic phenomena (e.g., sodium volatility), rather than thermodynamic stability of corresponding phases.19 Contrary to Ba-based clathrates,20 high-pressure synthesis has not been explored so far for the Na-Si system. The remarkable negative values of atomic ∆V of clathrate formations (~1.5-2.5


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cm3 / mol, Tab. S1), compared with the elemental constituents, allows one to suggest that highpressure should facilitate their formation. HP synthesis is the most reliable method for elaboration and properties control of high-pressure phases such as diamond,21, 22 cubic boron nitride23, 24 and boron-rich compounds.25 The quasi-equilibrium growth of HP phases from solvents allows production of high-purity single crystals of a given habit and high-quality powders. In this paper we report experimental and theoretical results that unambiguously indicate that Na clathrates are thermodynamically stable high-pressure phases. Stoichiometric sodium clathrates of structure I (Na8Si46) and a new metallic clathrate compound NaSi6 were synthesized directly from the elements, allowing for new opportunities for melt-based growth under equilibrium conditions. Over the range of conditions studied, the Na24Si136 clathrate (sII) only forms as an intermediate compound prior to crystallization of the sI phase. Experimental. Sodium clathrates were prepared from elemental Na/Si mixtures (20 mol% of Na, i.e., ~5% excess as compared with stoichiometric sI and sII clathrates). The initial mixture was ground for one hour in a porcelain mortar contained within a glovebox under a high-purity Ar atmosphere. The mixtures were loaded into double-walled capsules, the inner one from Ta and the outer one from Au (Fig. S1a). Such simple design of the capsule allows one to keep Na-rich liquid under high-pressure and high-temperature conditions for a long time (at least up to 12 hours) without hermetic sealing of the initial capsule. The capsules were then introduced into high-pressure cells: (1) octahedral-shaped assemblies for multi-anvil (18/11 type, pressures from 3 to 8 GPa, Fig. 1a)26 and (2) standard half-inch assemblies for piston-cylinder (pressures from 1 to 3 GPa),


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and compressed to the desired pressure. The time-pressure-temperature conditions of experiments are given in Tab. S2. In some cases, the heating was performed in two steps, i.e., 30 min at 675 K, and the remaining time (1.5 – 24 h) at the final temperature. The main purpose of this pre-heating was to avoid the blow-out of overheated liquid sodium from the cell at the initial stages of synthesis, i.e., before Na reacts with Si. After the accomplishment of this process, the temperature was decreased either by switching off the power, or by slowly decreasing power over a 10-60 minute duration. The latter allowed us to study crystallization in (quasi-) equilibrium conditions.

a

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Figure 1. (a) Double-stage multi-anvil assemblies (left) and high-pressure cell (right) used for synthesis. (b) Images of metallic Na8Si46 (left) and NaSi6 (right) synthesized at high pressure.

The recovered samples were easily removed from the Ta/Au capsules (Fig. S1a). No reaction between the Na/Si mixture and capsule was detected to 1275 K at 6 GPa and to 1000 K at 3 GPa (Fig. S1b). Recovered samples were washed in water, rinsed with ethanol, and dried in air.


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Fig. 1b shows a typical picture of recovered clathrate particles with metallic brilliance, similar to previous reports for stoichiometric clathrates.10 Results and Discussion Powder X-ray diffraction (PXRD) from samples recovered between 2-6 GPa revealed the formation of sI clathrate. The presence of a small amount of silicon in the recovered samples likely indicates the peritectic character of the Na-Si phase diagram under pressure, with the peritectic reaction LNa/Si + Si = Na8Si46. 2-D XRD indicated the presence of both primary grown clathrate crystals and fine-grained powder formed during eutectic crystallisation (Fig. S6 and related comments in Supporting Information). Individual polycrystaline pieces of pure sI clathrate were easily isolated (up to 500 µm in length and even larger, Fig. 1b), and Fig. 2a shows the PXRD pattern of sI clathrate formed under quasi-equilibrium conditions at 6 GPa with a = 10.208(1) Å. Correspondence between our experimental lattice constants and literature values

(a = 10.198 Å),27 in addition to Rietveld refinements of site occupancies (Tab. S3), indicate that the recovered phase is approximately stoichiometric, i.e., Na8Si46.The formation of sI clathrate directly from the elements indicates a region of high-pressure thermodynamic stability on the NaSi binary phase diagram. While sI was always observed between 2-6 GPa under quasi-equilibrium conditions of slow cooling (~2-10 Kmin-1), the rapid quench of the system to ambient temperature (by abruptly switching off the heating power) led to the recovery of both sI and sII phases. Fast and/or lower temperature treatment of the reacting mixture may also occasionally lead to a combination of sII and diamond-Si, without remarkable traces of sI clathrate (Fig. S2). According to X-ray diffraction data, the lattice parameter of this sII clathrate (a = 14.76(2) Å) is expanded by ~0.3%


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as compared to stoichiometric sII obtained by thermal decomposition under vacuum (a = 14.72 Å).28 Evidently, sII is not thermodynamically stable between 2-6 GPa over the temperature range studied, but its formation may be induced under transient recrystallization conditions. This result might be explained in terms of Ostwald’s rule of stages (i.e., easier nucleation of a less stable phase in terms of chemical potential, prior to the stable phase formation), due to structural similarity and close Gibbs energies. A similar HP-effect was recently observed in other cage-based crystal structures of elemental boron.29

Na8Si46

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Figure 2. Rietveld refinements of PXRD patterns for Na8Si46 (a) and NaSi6 (b) crystallized under slow cooling at 6 and 8 GPa, respectively. Corresponding structures are shown to the right with host silicon (blue) and guest sodium (yellow) atoms.


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When pressure was increased to 8 GPa, the formation of a novel Na-Si compound, NaSi6 was observed. It has the Eu4Ga8Ge1630 structural type (Fig. 2b), never reported thus far for an alkali metal. The structure is composed of sp3-bonded Si atoms, which form tunnels, intercalating Na atoms along the a-axis. The Na-Na distance of 4.106(1) Å is the shortest metal-metal distance for this structural family (e.g. Ba-Ba distance in isostructural BaSi6 is 4.479 Å), but quite reasonable given the small diameter for Na. The Si-Si distances vary from 2.368 to 2.380 Å, very close to those of sI clathrate Na8Si46 (2.333-2.413 Å), and shorter than in BaSi6 (2.400-2.469 Å). The SiSi-Si angles vary in a wide range between 99.2 and 134.0° similarly to BaSi6 (97.8-136.4°) and are remarkably less uniform than in cubic sI Na8Si46 (106.1-124.1°). Each sodium atom has 14 nearest Si atoms (3.197-3.436 Å, Fig. 5Sa) and 2 Na atoms along a-axis. Contrary to sI and sII clathrate structures, where Si atoms form cages produced by only slightly distorted pentagonal and hexagonal rings, the Si matrix of NaSi6 is produced by hexagonal rings with wellpronounced boat and chair configurations (Fig. 5Sb). As compared to BaSi6,31 SrSi6,32 and CaSi6,33 the corresponding sodium compound forms at substantially lower pressure: 8 GPa as compared to 11.5 GPa and 10 GPa for BaSi6 and CaSi6, respectively. The pressure for NaSi6 formation allows consideration of this phase for a largevolume production, e.g., in the toroid-type high-pressure systems,34 contrary to the alkali-earth metal analogues. The electrical resistivity of a polycrystalline piece of ~500x500 µm2 (for microstructural details see Fig. S6 and related comments in Supporting Information) of NaSi6 was measured as a function of temperature by the standard four-electrode technique, using a Physical Property Measurement System (PPMS: Quantum Design, Inc.). Overall, NaSi6 exhibits metallic behavior, with the electrical resistivity decreasing with decreasing temperature (Fig. 3). Below 75 K, the


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electrical resistivity begins to increase with further temperature decrease, a feature never observed so far for related compounds within this structural family. The residual resistivity ratio value (RRR, defined here as ratio between ρ300K and a minimal value of ρ, ρmin ) is between 13 and 100 depending on the estimate of ρmin, which can be taken at 75 K (for the lower estimate of RRR) or taken at 0 K by extrapolating a least-squares fit of the high-temperature part of T-ρ curve (Fig. 3). Contrary to NaSi6, the resistivity of sI obtained at 3 GPa shows monotonic behavior with temperature (Fig. 3), but the RRR value is also very high ~24. Typical RRR values for clathrates obtained without pressure are between 1 and 6, even for single crystals35 (the best value of 36 was recently achieved36 for a single crystal of Na8Si46 obtained by the so-called "slow controlled removal of Na" method).37 In fact, high pressure synthesis provides samples with remarkably improved electrical properties, even in the case of bulk material. To present time such improvement was unachievable by non-equilibrium crystal growth methods. The observed minimum at about 75 K on the resistivity curve for NaSi6 might be caused by the particularities of the electronic band structure and by an unknown scattering process. The detailed study of this phenomenon is beyond the scope of this paper, but our results suggest that the resistivity increase in NaSi6 at very low temperatures (formal value of ρ300K/ ρ2K for NaSi6 is ~6) is not due to structural defects, inter-grain boundaries or an experimental artifact (the lowtemperature resistivity curve of sI does not show such behavior, Fig. 3).


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2 1 0 -1 -2 -3

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Figure 3. Electrical resistivity of NaSi6 (blue triangles) and Na8Si46 crystallized at 3 GPa (dark yellow circles) as a function of temperature. Red line shows the quadratic fit of NaSi6 resistivity data (above 75 K). Inset shows the calculated band structure of NaSi6 at 1 MPa.

While the formation of sI clathrate and NaSi6 directly from the elements indicates high-pressure thermodynamic stability of these phases, we performed density functional theory (DFT) calculations38 in order to further elucidate the nature of their stability. Geometry optimizations were performed within the framework of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization39 for the exchange-correlation functional implemented in Quantum Espresso. For Brillouin zone integration, we used the Monkhorst-Pack scheme40 and checked convergence of ground state calculations with uniformly increasing kpoint meshes for each structure. We used a plane-wave basis-set cutoff of 60 Ry and generated a 8x8x8 k-point grid meshed for a Brillouin-zone integration.41 Calculations of phonons of NaSi6 were performed with density functional perturbation theory with a uniform 4x4x4 mesh.


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Our DFT calculations reveal that Na4Si4, sI, sII and NaSi6 are all stable against decomposition into the elements at high-pressure conditions (Fig. S4). Fig. 4 shows the calculated enthalpy differences, ∆H, between the clathrate structures and a mixture of d-Si and Na4Si4 Zintl phase, which are the experimentally observed thermodynamically stable phases at ambient conditions.18 In order to compare the thermodynamic stability, the total composition has been fixed to 15 at%, i.e. we compared the systems NaSi5.667 for structure II, (NaSi5.75 - 0.083 Si) for structure I, and (NaSi6 – 0.333 Si) for NaSi6. These results (Fig. 4) indicate that both sI and NaSi6 are indeed thermodynamically stabilized by high-pressure conditions: sI and NaSi6 become the most enthalpically favorable structures at approximately 4 and 9.5 GPa, respectively. From our experimental HPHT data, the corresponding values of pressure are 2.5(5) GPa for sI and 7(1) GPa for NaSi6 at 900-1100 K, demonstrating very good qualitative agreement between predicted and experimental domains of stability.

Isopleth 15 at% Na ab initio, 0 K

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Figure 4. Calculated enthalpies of clathrate phases relative to Na4Si4-Si system (15 at% of Na) as a function of pressure at 0 K (lines). Shaded boxes at bottom show the experimental pressures of


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d-Si, sI and NaSi6 crystal formation at high temperatures from Na-Si melt (see text and Supporting Information for details). Calculations suggest that sII clathrate (dashed curve) is the stable ground state from 0 to 4 GPa (0 K), however, our experimental data prove that, although enthalpically close to sI (see text), sII is not thermodynamically stable in this pressure range over the range of temperatures studied.

Our DFT calculations suggest that sII clathrate is the stable ground state from 0 to 4 GPa, yet experiments performed below 2 GPa resulted in the recrystallization of Si. At ambient pressure we calculate sII to be more stable than Na4Si4+Si by 2.5 meV/atom, whereas sII is more stable than “sI – Si” by 5.2 meV/atom. At 4.2 GPa we calculate sI to become more stable than sII. The absence of sII in quasi-equilibrium experiments and the lack of this clathrate phase on the ambient-pressure binary Na-Si phase diagram are in apparent disagreement with our calculations; however, enthalpy differences between these compounds are small. Finite temperature effects and kinetic barriers across the various phases may contribute to the differences observed. Nevertheless, the observation of sII in rapid-quench experiments certainly verifies the seemingly isoenthalpic nature of these phases. Conclusion The results reported in the present communication open new perspectives for high-pressure synthesis and properties control of new advanced materials. The high-pressure thermodynamic stability of Na-Si clathrate phases allows for a melt-based synthesis approach, which could be very useful for compositional control in mixed phases (e.g., Na+Ba etc.), high-quality single crystals, and for precise tuning of the occupancy ratios. All phases formed in this pressure


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domain allow for larger-volume scaling of materials (from 40 cm3 for cubic sI at ~3 GPa to 1 cm3 for the orthorhombic Eu4Ga8Ge16 structure at ~8 GPa). The consistency of experimental results with ab initio calculations may justify the future application of this approach to prediction of new covalent sp3 intercalation compounds (e.g, carbon-rich compounds). Finally, our results reveal the existence of multiple chemical mechanisms that allow for synthesis of high-pressure phases “without pressure”. Since the Na-Si clathrates are stable only under high-pressure conditions (> 2 GPa), previous reports of their synthesis may be viewed as non-equilibrium, precursor-based routes to high-pressure phases at low-pressure conditions. The understanding of such intrinsic interrelationships between thermodynamics and kinetics is thus the next step to explore that could open potential for other precursor-based synthesis of high-pressure phases. ASSOCIATED CONTENT Supporting Information. Experimental information, stabilities and structural details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by DARPA under ARO contract number W911NF-11-1-0300. The PPMS-related work was supported by Energy Frontier Research in Extreme Environments Center (EFree) under award number DE-SG0001057. ABBREVIATIONS HP, high pressure; HT, high temperature; DFT, density functional theory; GGA, generalized gradient approximation; PPMS, Physical Property Measurement System; RRR, Residualresistance ratio; PXRD, powder x-ray diffraction; sI and sII, crystal structures of clathrate Na8Si46 and Na24Si136 respectively. REFERENCES 1.

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For Table of Contents Use Only. SYNOPSIS We report that high pressure is a promising tool to synthesize and control the properties of sodium-silicon clathrate compounds (Na8Si46, Na24Si136 and NaSi6), which can also be obtained without pressure by chemical routes. Our experimental and theoretical results unambiguously indicate that Na-doped clathrates are thermodynamically stable high-pressure phases that form directly from the elements.


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Na-Si Clathrates Are High-Pressure Phases - American Chemical

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