High-yield synthesis of semiconductive type-II Si clathrates with low Na content

Ohashi, Fumitaka; Hattori, Minoru; Ogura, Takuya; Koketsu, Yuzo; Himeno, Roto; Kume, Tetsuji; Ban, Takayuki; Iida, Tamio; Habuchi, Hitoe; Natsuhara, H.; Nonomura, Shuichi

doi:10.1016/j.jnoncrysol.2011.12.058

Cited by 11 publications

(8 citation statements)

References 11 publications

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“…Local vapor pressure of Na is exceptionally difficult to measure directly; all discussion of vapor pressure herein is inferred from observation of Na deposition and reactor geometry. In all reactions, 0.5 g of NaSi powder was decomposed at 370 °C for 24 h. Most of the high-yield type II Si clathrate syntheses report extensive annealing of NaSi (~12-60 h) before thermal decomposition; 33,37 no such precursor annealing was required for the type II clathrate synthesis described herein.…”

Section: Clathrate Phase Selectionmentioning

confidence: 99%

“…In comparison to prior hot wall reactor approaches, the synthesis time to reach low Na is quite rapid. 3,32 Additionally, these low Na levels are achieved with minimal annealing of the NaSi precursor, unlike prior efforts which required annealing between 12-60 h. 33,37 Further, this synthesis technique did not require intermittent washing with HCl to remove accumulated material on the surface of the powder. No significant phase change from type II to I was observed during Na removal.…”

Section: Na Removal Using Cold Plate Approachmentioning

confidence: 99%

“…NaSi) under vacuum. 3,23,25,29,[32][33][34][35][36][37][38] During thermal decomposition, it is hypothesized that cation sublimation leads to charge imbalance in the mono-silicide, which causes subsequent rearrangement and bonding of silicon atoms into the four-coordinate cage structures of the clathrate. During this process, the remaining cations are expected to template cage formation.…”

Section: Introductionmentioning

confidence: 99%

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Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

et al. 2014

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Phase selective synthesis of type II silicon clathrates from thermal decomposition of NaSi has previously been limited to small quantities due to the simultaneous formation of competing phases. In this work we show that the local sodium vapor pressure during the NaSi precursor decomposition is a critical parameter for controlling phase selection. We demonstrate synthesis techniques that allow us to tune the local Na vapor pressure, yielding type I or II clathrate products that are ≥90 wt.% phase pure. The "cold plate" reactor design discussed in this work maintains low Na vapor pressure during thermal decomposition of NaSi, thus yielding large scale, phase selective synthesis of type II silicon clathrates. The low Na vapor pressure maintained in this reactor is also shown to efficiently produce low Na (x ~1; Na x Si 136 ) Si clathrate through Na sublimation. To further reduce sodium occupancy (x < 1), we demonstrate etching of Na x Si 136 in HF/HNO 3 solutions, which rapidly yields a clathrate product with reduced x. 29 Si NMR and electron spin resonance (ESR) characterization validate the low Na occupancy of Si clathrate synthesized.The acid etch also selectively dissolves the type I silicon clathrate impurity phase, thereby enabling the synthesis of large quantities of phase pure type II silicon clathrate with low Na content.

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Section: Clathrate Phase Selectionmentioning

confidence: 99%

Section: Na Removal Using Cold Plate Approachmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

et al. 2014

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“…Regarding these two types of Si clathrates, the extraction of Na from Na 8 Si 46 exhibiting a type‐I structure has never been reported, whereas the type‐II Si clathrates exhibit a non‐stoichiometric feature of Na + . [ 8–17 ] Here, type‐II Na 24 Si 136 , which is a promising candidate for synthesizing guest‐free Si clathrates as a precursor, was focused on.…”

Section: Introductionmentioning

confidence: 99%

A Novel Technique for Controlling Anisotropic Ion Diffusion: Bulk Single‐Crystalline Metallic Silicon Clathrate

et al. 2022

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Na‐free Si clathrates consisting only of Si cages are an allotrope of diamond‐structured Si. This material is promising for various device applications, such as next‐generation photovoltaics. The probable technique for synthesizing Na‐free Si clathrates is to extract Na+ from the Si cages of Na24Si136. Vacuum annealing is presently a well‐known conventional and effective approach for extracting Na. However, this study demonstrates that Na+ cannot be extracted from the surface of a single‐crystalline type‐II metallic Si clathrate (Na24Si136) in areas deeper than 150 µm. Therefore, a novel method is developed to control anisotropic ion diffusion: this is effective for various compounds with a large difference in the bonding strength between their constituent elements, such as Na24Si136 composed of covalent Si cages and weakly trapped Na+. By skillfully exploiting the difference in the chemical potentials as a driving force, Na+ is homogeneously extracted regardless of the size of the single crystal while maintaining high crystallinity. Additionally, the proposed point defect model is evaluated via density functional theory, and the migration of Na+ between the Si cages is explained. It is expected that the developed experimental and computational techniques would significantly advance material design for synthesizing thermodynamically metastable materials.

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“…Computationally, many novel well-ordered Si allotropes of various dimensionality have been proposed [9,10], but only a few of those were experimentally verified, such as the low-density Si allotropes [11] with clathrate-type structures [12][13][14][15][16][17][18][19]. The focus of the research lies on the description, understanding, and discovery of well-performing photovoltaic materials as well as models for bulk silicon surfaces [20,21].…”

Section: Introductionmentioning

confidence: 99%

Predicted Siliconoids by Bridging Si9 Clusters through sp3-Si Linkers

Jantke

Fässler

2018

Inorganics

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Charged and neutral silicon clusters comprising Si atoms that are exclusively connected to atoms of the same type serve as models for bulk silicon surfaces. The experimentally known nido-[Si 9 ] 4− Zintl cluster is investigated as a building block and allows for a theoretical prediction of novel silicon-rich oligomers and polymers by interconnection of such building units to larger aggregates. The stability and electronic properties of the polymers { 12 ) n }, as well as of related oligomers are presented.

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High-yield synthesis of semiconductive type-II Si clathrates with low Na content

Cited by 11 publications

References 11 publications

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

A Novel Technique for Controlling Anisotropic Ion Diffusion: Bulk Single‐Crystalline Metallic Silicon Clathrate

Predicted Siliconoids by Bridging Si9 Clusters through sp3-Si Linkers

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