Crystal Growth through Field-Assisted Electrochemical Redox and Ion

Sep 16, 2015 - Crystal Growth through Field-Assisted Electrochemical Redox and ... that K4.2Na3.8Si46 is metallic and that the Fermi level lies within...
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Crystal Growth through Field-Assisted Electrochemical Redox and Ion-Exchange Reactions: A Case Study of K4.2Na3.8Si46 Clathrate‑I Yongkwan Dong and George S. Nolas* Department of Physics, University of South Florida, Tampa, Florida 33620, United States S Supporting Information *

ABSTRACT: Single crystals of the ternary clathrate-I compound K4.2Na3.8Si46 were grown by a unique ion-exchange process employing spark plasma sintering (SPS). Single crystal structure refinements of clathrateI K4.2Na3.8Si46 at 100 and 300 K indicate that the 6d crystallographic site contains both Na and K. Extended Hückel tight binding calculations suggest that K4.2Na3.8Si46 is metallic and that the Fermi level lies within bands having Si−Si antibonding character. The selective synthesis of clathrate-I and -II compositions was possible by changing the reaction temperature. Our results indicate that SPS can be employed in electrochemical redox and ion-exchange reactions simultaneously, thus allowing for the rapid synthesis of single crystals of multinary inorganic clathrate phases that cannot be accessible by traditional crystal growth techniques. acting as a reactive flux for clathrate synthesis.17 With this in mind, we have successfully synthesized ternary inorganic clathrate-I and -II compositions by SPS, from a precursor made of the mixture of NaSi and KCl, using electrochemical redox and ion-exchange reactions simultaneously. In addition to the electrochemical redox reaction of the binary,14 Na+ ions also react with KCl to form NaCl thus allowing K to be available for encapsulation in the clathrate framework. K4.2Na3.8Si46 is the first example of a clathrate-I compound containing both K and Na, and we report its structural and compositional characterization as well as electronic structure. The selective synthesis of ternary clathrate-I and -II compositions by varying the SPS processing temperatures is also discussed. K4.2Na3.8Si46 single crystals were synthesized by SPS from a Na4Si4 and KCl mixture. Each binary starting material was ground together in a 1:2 mass ratio, and the resulting mixture was loaded in a graphite die assembly inside an N2 filled glovebox due to the air-sensitivity of Na4Si4 and hygroscopicity of KCl. In order to provide a tight fit of the die assembly and to prevent direct reaction with the die or punches, tantalum foil was placed between the precursor mixture and the graphite die and punches. After mounting this assembly into the SPS reaction chamber, the chamber was evacuated and flushed three times with high-purity N2. The SPS process was performed under a vacuum of 10 mTorr, a uniaxial pressure of 100 MPa, and a pulsed DC current with a pulse-on and -off time of 36 and 2 ms, respectively. A temperature ramp rate of 25 K/min was used to 773 K before holding at this temperature for 3 h. After the reaction, the current and pressure were released and

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ver since the introduction of inorganic clathrates by Cros and co-workers,1 ternary group 14 inorganic clathrates have been extensively investigated because of their interesting structural and physical properties. The group 14 inorganic clathrates-I and -II structures, chemical formulas A6B2X46 and A16B8X136, respectively, are composed of a three-dimensional open framework of face sharing X20 dodecahedra and X24 tetrakaidekahedra for clathrate-I and X20 dodecahedra and X28 hexacaidekahedra for clathrate-II.2 Guest atoms A and B can reside inside these polyhedra and are typically alkali, alkaline earth, or rare earth metals. Framework substitution by triel elements (group 13) is also possible. Clathrates continue to be of interest for various technical applications including photovoltaics, thermoelectrics, magnetocalorics, and superconductivity.2−13 Although inorganic clathrates are very important scientifically and technologically, relatively few compositions have been synthesized due to the synthetic challenge in the preparation of certain types of compositions. To overcome serendipitous syntheses by traditional methods and obtain targeted thermodynamically metastable compositions, various synthetic techniques have been employed.2 Among the newly introduced synthetic approaches, spark plasma sintering (SPS) is unique in that it applies a pulsed DC current to the specimen enclosed in a punch and die assembly under uniaxial pressure. SPS processing is relatively well-known for consolidation of ceramics, polymers, semiconductors, and nanocomposites;7 however, the synthesis of new single-crystals by SPS is rare.14,15 Beekman et al.14 demonstrated the crystal growth of clathrate-II Na24Si136 by SPS from a NaSi precursor by an electrochemical redox reaction. The selective synthesis of binary Na−Si clathrate-I and -II compositions was also possible by changing the applied SPS temperature and pressure.15 In addition, it is well-known that elemental K can be obtained by ion-exchange of KCl with Na at high temperatures,16 KCl also © XXXX American Chemical Society

Received: March 10, 2015 Revised: August 23, 2015

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DOI: 10.1021/acs.cgd.5b00329 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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group Pm3n̅ (#223) with two positions for the alkali metals on the crystallographic 2a and 6d sites and three for Si atoms on 6c, 16i, and 24k sites. No additional symmetry was found using the ADDSYM28,29 algorithm in the PLATON30 program of packages. Initial refinement with only K on the 6d site showed that wR2 and R1 are 0.0850 and 0.0298, respectively, with Ueq = 0.0388 Å2, and the minimum residual hole (−1.11 e/Å3) is very close to the 6d site (with a distance of 0.37 Å). These large Ueq and residual hole values indicate that the 6d site likely contains both K and Na. The final refinement results indicated wR2 and R1 to be 0.0532 and 0.0222, respectively, and the minimum and maximum residual peaks being away from the 6d site with Ueq = 0.0231 Å2. All Si positions were fully occupied and had spherical displacement parameters indicative of no structural disorder on the Si framework. The refinement input from the 300 K data was used for the 100 K data in order to refine the crystal structure,27 which resulted in the composition K4.2(1)Na3.8(1)Si46. Detailed refinement results, atomic coordinates, atomic displacement parameters, and selected bond lengths and angles are given in the Supporting Information. The temperature dependence of the ADPs can be considered an indication of the degree of dynamic disorder. For all the atoms in K4.2Na3.8Si46 the ADPs are larger at 300 K than 100 K, much larger for the cations than for Si in the framework (Figure 2), and show a very similar temperature dependence as that for

the graphite tooling was cooled to room temperature under vacuum. The product of reaction was separated from any unreacted Na4Si4 and halides by washing with ethanol and distilled water14,18 and then sonicated for 30 min to remove any potential surface contaminants from the crystals. EDX analyses of the single crystals were accomplished with an Oxford INCA X-Sight 7582 M equipped SEM (JEOL JSM6390LV) before sonication. Powder XRD data were collected with a Bruker AXS D8 Focus powder diffractometer with a Cu anode (Kα, λ = 1.5406 Å) in Bragg−Brentano geometry configuration. Single crystal XRD data were collected using a Bruker AXS SMART APEX II CCD diffractometer using Cu Kα radiation (λ = 1.54178 Å) for the 300 K data and a Bruker D8 Venture PHOTON 100 CMOS system equipped with a Cu Kα INCOATEC Imus microfocus source (λ = 1.54178 Å) for the 100 K data. The initial cubic cell constants and orientation matrix were obtained by using APEX2 (difference vectors method).19 Data integration and reduction were performed using SaintPlus 6.01.20 An empirical absorption correction was performed by a multiscan method implemented in SADABS.21 The initial input files for solving the crystal structure were prepared by XPREP implemented in APEX2.19 The structure was solved using SHELXS-97 (direct methods)22 and refined using SHELXL-97 (full-matrix least-squares techniques)22 contained in the WinGX program package.23 Thermal gravimetry (TG) and differential thermal analyses (DTA) measurements were carried out with a SDT Q600 (TA Instruments) under N2 flow with the specimen heated from 293 to 1023 K at a rate of 10 K/min. Electronic band structure calculations were carried out by means of the extended Hückel tight binding (EHTB)24,25 for the stoichiometric compound K6Na 2Si46 at 216k-point set in the irreducible wedge corresponding to the one-eighth 3D Brillouin zone of the cubic system. The CAESAR2.0 program package26 was utilized for calculations. The atomic orbital parameters for the EHTB calculation used default values. Figure 1 shows a scanning electron microscope (SEM) image of a K4.2Na3.8Si46 single crystal cluster after SPS processing.

Figure 2. Atomic displacement parameters at 100 and 300 K for K4.2Na3.8Si46. Also shown are the Si20 dodecahedron (right, Na) and Si24 tetrakaidekahedron (left, K/Na) with thermal ellipsoids for all atoms corresponding to 99% probability.

other Si clathrates.31 The dynamic disorder of the alkali metals in the Si20 dodecahedra are isotropic while anisotropic in the Si24 tetrakaidecahedra. The ADP values can be used to estimate Einstein temperatures providing an indication of the local disorder of the cations within the Si framework. Using this approach32 we estimate Einstein temperatures of 170 K for Na on the 2a crystallographic site (Si20) and 139 K for K (and Na) on the 6d crystallographic site (Si24). The Einstein temperature for Na in Si20 is very similar to that for Na8Si46,33 while the value for K in Si24 is slightly higher than that for the case of K7.5Si46,34 presumably due to the fact that Na is also present in Si24 for K4.2Na3.8Si46. Electronic structure calculations for stoichiometric K6Na2Si46 were carried out assuming complete occupancy of the crystallographic 6d site with K atom. We note that the electronic structures of K6Na2Si46 and K4.2Na3.8Si46 are very similar because contributions from K and Na to the conduction and the valence bands are insignificant. The density of states and band structure are given in Figures S2 and S3, respectively, of the Supporting Information. The valence and conduction

Figure 1. (a) SEM image of a K4.2Na3.8Si46 single crystal cluster after SPS and (b) the result of elemental analysis.

Cube-like clathrate-I bluish single crystals typically stick together and are ∼50 μm in size. After sonication, several crystals were analyzed by energy dispersive X-ray analysis (EDX) from multiple spots resulting in an average composition (K4.1(2)Na4.1(7)Si46.0(6)) that is consistent with the refined composition. From differential thermal analysis (DTA), shown in Figure 1S, a large exothermic peak was observed at 1023 K. This exotherm can be assigned to the decomposition of the clathrate phase, from powder X-ray diffraction (PXRD) that indicates α-Si after thermal analysis. Several crystals were selected to check for crystal uniformity and to determine the unit cell parameters at two different temperatures.27 The composition crystallizes in the cubic space B

DOI: 10.1021/acs.cgd.5b00329 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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bands are dominated by the s and p states of Si, while contribution from K and Na are negligibly small, as expected, and the K and Na states are barely hybridized with the Si46 conduction bands due to the parametrization of the extended Hückel method. A total of 192 valence electrons per cell (Z = 1) fill the states up to a Fermi level in energy of −2.54 eV. This energy level is just above the edge of the conduction band composed of Si−Si antibonding character, as indicated in Figure 3. This is in agreement with our results and the ab initio calculations for Na8Si46.35 This presumably implies a high electrical conductivity as in the case of Na8Si46.33 Figure 4. Powder XRD patterns from crushed single crystals demonstrating the selective synthesis of single crystal K4.2Na3.8Si46 and K5.8(1)Na16Si136 clathrates at 100 MPa at different SPS processing temperatures.

multinary intermetallic compositions, in addition to multinary clathrates, that are not accessible by traditional synthetic methods.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00329. Detailed single crystal refinements, bonds and angles, thermal analyses, and electronic structures (PDF) Crystallographic cif file for K4.2Na3.8Si46 at 100 K (CIF) Crystallographic cif file for K4.2Na3.8Si46 at 300 K (CIF)

Figure 3. Si−Si COOP curves for K6Na2Si46 obtained from EHTB calculations.

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Previously it was proposed that an electrochemical redox reaction caused by the applied electrical current passing through the specimen during SPS is the main driving mechanism for clathrate crystal growth by SPS,2,14 with the Na4Si4 precursor acting like a solid electrolyte that is capable of sodium transport toward the cathode. Our results suggest that an electrochemical redox reaction occurs simultaneously with an ion-exchange reaction between KCl and Na. Tetrahedral Si44− units from the Na4Si4 precursor start to oxidize to form Si46 framework at the anode, while elemental Na is formed by reduction of Na+ ions at the cathode through an electrochemical redox reaction. During this process Na+ also reacts with KCl to form NaCl allowing K+, together with the remaining Na+ cations, to be available for encapsulation in the resulting polyhedra of the clathrate framework. A further point of interest to note is that the selective synthesis of clathrate-I and -II compositions was achieved by changing the reaction temperature, as shown in Figure 4, with clathrate-I K4.2Na3.8Si46 being obtained at 773 K and clathrate-II K5.8(1)Na16Si136 being obtained at 873 K.36 At intermediate temperatures, a mixture of the two phases was obtained. Single crystals of a new ternary clathrate-I compound K4.2Na3.8Si46 were synthesized by using a mixture of Na4Si4 and KCl as the precursor for ion-exchange/electrochemical redox reactions by SPS. The K and Na atoms at the crystallographic 6d site are in a 7:3 ratio and are located at the center of the Si24 tetrakaidekahedron with minimal static disorder. Thermal analysis indicates that K4.2Na3.8Si46 decomposes at 1023 K and forms α-Si. Electronic structure calculations indicate that K4.2Na3.8Si46 is metallic and that the Fermi level lies within the conduction band mainly composed of Si46 antibonding character. Our SPS synthetic approach can reproduce these crystals easily and rapidly. This approach may also be applied to the synthesis and crystal growth of other

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the U.S. Department of Energy, Basic Energy Sciences, Division of Materials Science and Engineering, under Award No. DE-FG02-04ER46145.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00329 Cryst. Growth Des. XXXX, XXX, XXX−XXX