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

Abstract

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.

Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications

“…This has proven important since both phase selectivity and large scale synthesis are the well known challenges with pure silicon clathrate synthesis. 34 Most commonly, synthesis of Si, Ge and alloyed N which has historically received ertain factors into consideration inked to the relatively high vapor , thermal decomposition of the acuum or inert atmosphere in w temperatures (350-450°C, nt), resulting in microcrystalline with respect to decomposition) n closed (i.e., sealed) vessels and Since the clathrate products are (which is a line compound), a 8 Si 46 and Na 24 Si 136 will likely shift toward the Si-rich composilace. In light of these considerasure of Na could be maintained ed upon crystal nucleation as the ntinuously changed, growth of slowly removed from Na 4 Si 4 phase with spatially separated lume under uniaxial pressure.…”

“…Phase selection in Si clathrates formed via thermal decomposition of the NaSi can be achieved by controlling the Na vapor pressure. 33,34 Fully filled type I clathrates prefer high local Na vapor pressure (e.g. Ar atmosphere) during thermal decomposition of NaSi.…”

“…Recently, a HF-HNO 3 etch technique was demonstrated to more rapidly reduce Na concentration in type II silicon clathrates (Na x Si 136 ). 34 This etch is commonly used as a dopant selective etch for d-Si. Though the mechanism of Na removal is under investigation, this technique offers a faster route (on the order of minutes rather than hours/days) to achieving semiconducting clathrates.…”

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.…”

Predicted Siliconoids by Bridging Si9 Clusters through sp3-Si Linkers

“…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].…”