Precursor Routes to Complex Ternary Intermetallics: Single-Crystal and Microcrystalline Preparation of Clathrate-I Na8Al8Si38 from NaSi + NaAlSi (original) (raw)
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Inorganic Chemistry, 2015
Single crystals of the ternary clathrate-I Na 8 Al 8 Si 38 were synthesized by kinetically controlled thermal decomposition (KCTD), and microcrystalline Na 8 Al 8 Si 38 was synthesized by spark plasma sintering (SPS) using a NaSi + NaAlSi mixture as the precursor. Na 8 Al x Si 46−x compositions with x ≤ 8 were also synthesized by SPS from precursor mixtures of different ratios. The crystal structure of Na 8 Al 8 Si 38 was investigated using both Rietveld and single-crystal refinements. Temperature-dependent transport and UV/vis measurements were employed in the characterization of Na 8 Al 8 Si 38 , with diffuse-reflectance measurement indicating an indirect optical gap of 0.64 eV. Our results indicate that, when more than one precursor is used, both SPS and KCTD are effective methods for the synthesis of multinary inorganic phases that are not easily accessible by traditional solid-state synthesis or crystal growth techniques.
Journal of Solid State Chemistry, 2016
A new quaternary clathrate-II composition, Cs 8 Na 16 Al 24 Si 112 , was synthesized by kinetically controlled thermal decomposition (KCTD) employing both NaSi and NaAlSi as the precursors and CsCl as a reactive flux. The crystal structure and composition of Cs 8 Na 16 Al 24 Si 112 were investigated using both Rietveld refinement and elemental analysis, and the temperature dependent transport properties were investigated. Our results indicate that KCTD with multiple precursors is an effective method for the synthesis of multinary inorganic phases that are not easily accessible by traditional solid-state synthesis or crystal growth techniques.
Simple Approach for Selective Crystal Growth of Intermetallic Clathrates
Chemistry of Materials, 2011
We report on a new method for the synthesis of single-crystal intermetallic clathrates. Alkali-metal is slowly removed from an alkali-silicide precursor by reaction of the vapor phase with spatially separated graphite, in a closed volume under uniaxial pressure, to form single-crystals of the binary intermetallic clathrates Na 8 Si 46 and Na 24 Si 136. Single-crystal structure refinement for Na 8 Si 46 is reported for the first time. For both Na 8 Si 46 and Na 24 Si 136 , full occupation of all Si framework sites as well as full Na occupancy in both polyhedra was observed. In addition to comprising a simple method for selective, phase-pure crystal growth of clathrates such as Na 8 Si 46 or Na 24 Si 136 which was previously challenging to achieve, this or similar approaches are applicable in the preparation of new compositions from different alkali-metal precursors.
Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization
Materials, 2016
The binary intermetallic clathrates K 8-x Si 46 (x = 0.4; 1.2), Rb 6.2 Si 46 , Rb 11.5 Si 136 and Cs 7.8 Si 136 were prepared from M 4 Si 4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb 11.5 Si 136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si 20 dodecahedra, was found in all compounds. SPS preparation of Na 24 Si 136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K 7.6 Si 46 and K 6.8 Si 46 in the temperature range 4-700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.
Synthesis, Crystal Structure, and Transport Properties of Na22Si136
Journal of Electronic Materials, 2009
The type II clathrate Na 22 Si 136 is prepared by the thermal decomposition of NaSi. Thermal analysis indicates this phase is metastable yet has a relatively high decomposition temperature. Rietveld analysis indicates that Na in the larger Si 28 cage is shifted off-center, analogous to observations in some type I clathrates. Temperature-dependent electrical and thermal transport properties are reported for Na 22 Si 136 , for which the spark plasma sintering technique was found to be effective in achieving intergrain sintering in the consolidated specimen. The potential that type II clathrate materials possess for thermoelectric applications is discussed.
Synthesis of the intermetallic clathrate Na 2 Ba 6 Si 46 by oxidation of Na 2 BaSi 4 with HCl
Science and Technology of Advanced Materials, 2007
A new preparation route to the intermetallic clathrate-I compound Na 2 Ba 6 Si 46 is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na 2 BaSi 4 is oxidized with gaseous HCl at 673 K to Na 2 Ba 6 Si 46 , NaCl and BaCl 2. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na 2 Ba 6 Si 46 (Na 1.94(1) Ba 6.06(1) Si 46 , space group Pm3n, a ¼ 10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na 2 Ba 6 Si 46 is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na 2 Ba 6 Si 46 was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na 2 Ba 6 Si 46 obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe.
Synthesis and Properties of Single-Crystalline Na4Si24
Crystal Growth & Design, 2018
Na4Si24 is the precursor to Si24, a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si24 has potential to transform silicon-based optoelectronics including solar energy conversion. However, the lack of large, pure crystals has prevented the characterization of intrinsic properties and has delayed deposition-based metastable growth efforts. Here, we report an optimized synthesis methodology for single-crystalline Na4Si24 with crystals approaching the millimeter-size scale with conditions near 9 GPa and 1123 K. Single-crystal diffraction was used to confirm the open-framework structure, and Na atoms remain highly mobile within the framework channels, as determined by electrical conductivity and electron energy loss spectroscopy (EELS) measurements. An epitaxial relationship between Na4Si24 and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy (HRTEM), is proposed to facilitate the growth of high-quality Na4Si24 crystals from DC-Si wafers mixed with metallic Na, and could provide a viable path forward for scaling efforts of Na4Si24 and Si24.
Crystal Growth & Design, 2015
Single crystals of the ternary clathrate-I compound K 4.2 Na 3.8 Si 46 were grown by a unique ion-exchange process employing spark plasma sintering (SPS). Single crystal structure refinements of clathrate-I K 4.2 Na 3.8 Si 46 at 100 and 300 K indicate that the 6d crystallographic site contains both Na and K. Extended Huckel tight binding calculations suggest that K 4.2 Na 3.8 Si 46 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.
Crystal Growth & Design, 2013
Three different sodium-silicon clathrate compounds-Na 8 Si 46 (sI), Na 24 Si 136 (sII) and a new structure, NaSi 6-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 NaSi 6 , which crystallizes in the orthorhombic Eu 4 Ga 8 Ge 16 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 NaSi 6), 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