Rapid crystal growth of type-II clathrates A₈Na₁₆Si₁₃₆ (A = K, Rb, Cs) by spark plasma sintering

Abstract

Single crystal type-II clathrates A 8 Na 16 Si 136 (A = K, Rb, and Cs) were grown from NaSi and alkali metal halides by spark plasma sintering (SPS). Single crystal structure refinements indicate that K atoms at the 8b crystallographic site are partially occupied IJK 5.8IJ1) Na 16 Si 136 ) while Rb in Rb 8 Na 16 Si 136 and Cs in Cs 8 Na 16 Si 136 are fully occupied. Our results indicate that SPS can be employed in topochemical ion exchange reactions, thus allowing for the rapid synthesis of single crystals of multinary intermetallic phases that cannot be accessible by traditional crystal growth techniques.Inorganic clathrates based on group 14 elements have attracted attention due to their interesting structural and physical properties, for example very low thermal conductivities, 1-3 tunable electrical properties and band gaps with modification of composition, 4,5 and second-order ferromagnetic phase transitions. 6-8 As a result, they are of interest for a variety of potential technological applications, including solid state energy conversion, 1-3,9 photovoltaics, 10,11 magnetocalorics, 12-14 superconductivity, 15-17 and as the anode for Li-ion batteries. [18][19][20] Due to this scientific interest and technological importance an understanding of the fundamental structural and physical properties of inorganic clathrates is essential. In order to achieve this goal various synthetic techniques have been employed to obtain stable or metastable compositions that are not accessible by conventional solid state reaction or single-crystal growth methods. These include soft chemical routes (chemical oxidation), 21-23 kinetically controlled thermal decomposition, 24,25 high-temperature high-pressure synthesis, 26,27 and spark plasma sintering (SPS). 28,29 Among the synthetic approaches described above, SPS is unique in that it applies a pulsed DC current to the specimen through a die assembly that is between two electrodes under uniaxial pressure. SPS processing is well established for the consolidation of ceramics, polymers, semiconductors, and nanocomposites due to the fast heating rates and uniform heating through the specimen inside the die assembly. [30][31][32][33][34][35][36] Recently crystal growth of binary type-I and -II clathrates, Na 8 Si 48 and Na 24 Si 136 , by SPS was achieved. 28,29 In this report we demonstrate that ternary type-II clathrate single crystals can be synthesized via ion-exchange and electrochemical oxidation-reduction (redox) reactions between NaSi and ACl (A = K, Rb, Cs) using SPS. This approach will also be of interest for the synthesis and crystal growth of other intermetallic or metastable ternary, and potentially quaternary, compounds.Single crystals of three type-II clathrates, K 5.8IJ1) Na 16 Si 136 , Rb 8 Na 16 Si 136 , and Cs 8 Na 16 Si 136 , were synthesized by SPS. The NaSi precursor along with each alkali-metal chloride were ground together in a 1 : 1 mass ratio and the resulting mixture was loaded in a graphite die assembly. Tantalum foil was placed between the precursor mixture ...

Structure and Transport Properties of Dense Polycrystalline Clathrate-II (K,Ba)16(Ga,Sn)136 Synthesized by a New Approach Employing SPS

“…The group 14 atoms typically have tetrahedral coordination that form a network encapsulating “guest” species. Most of the research thus far has focused on compositions with the clathrate-I crystal structure, in part due to their interest for thermoelectric applications, although recently there has been a focus on developing new processing techniques for the synthesis of clathrate-II materials [4,5,6,7,8,9,10]. The clathrate-I crystal structure can be represented by the general formula X 2 Y 6 E 46 , and that of the clathrate-II crystal structure by the general formula X 8 Y 16 E 136 , where X and Y are encapsulated guest atoms in the two different interstitial sites, and E represents the group 14 element Si, Ge, or Sn.…”

Precursor routes to quaternary intermetallics: Synthesis, crystal structure, and physical properties of clathrate-II Cs8Na16Al24Si112

“…Site occupancy refinements for both the alkali and framework indicate full occupancy of all crystallographic sites, with good refinement results obtained for Al / Si ratio values of 0.2 / 0.8 for 8a, 0.1 / 0.9 for 32e, and 0.2 / 0.8 for 96g site. The bond distances between framework atoms are very close to those of A 8 Al 8 Si 38 (A = Na, K, Rb, Cs) [27,31] and longer than those of Na 24 Si 136 (2.3578(3) ~ 2.3976(6) Å),[18] Cs 8 Na 16 Si 136 (2.360(1) ~ 2.393(1) Å),[23] and elemental Si (2.368 Å). This is an indication that the larger Al atoms (CN=4 and an effective ionic radius of 0.53 Å, as compared to Si with CN=4 and an effective ionic radius of 0.40 Å)[32] are on the framework.…”

“…[22] Very recently SPS was extended for the synthesis of ternary clathrates employing more than one precursor. [23] In this study we employ KCTD to synthesize the quaternary clathrate-II composition Ca 8 Na 16 Al 24 Si 112 using NaSi and NaAlSi as precursors along with CsCl as the reaction flux.…”

“…For KCTD reactions over 1073 K the product decomposed to α-Si. Phase-pure Cs 8 Na 16 Al 24 Si 112 was obtained at 1073 K. Figure 1 shows the PXRD of Cs 8 Na 16 Al 24 Si 112 together with those of Na 24 Si 136[18] and Cs 8 Na 16 Si 136[23] for comparison.Figure 2shows the Rietveld analysis of Cs 8 Na 16 Al 24 Si 112 , the partial crystal structure is illustrated inFigure 3, and the crystallographic details are given in…”

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization

“…Recently, a thermal decomposition at lower temperatures was employed for the preparation of Na 24- x (Si y Ge 1- y ) 136 and proved to be useful for the synthesis of silicon clathrates [20]. Extensions of the SPS and KCTD approaches have also recently reported, including use of multi-phase precursors or simultaneous ion-exchange and electrochemical reactions to prepare ternary or quaternary clathrates [21,22,23]. …”