Abstract
Silicon clathrate, an important allotrope of silicon, has attractive opto-electronic properties for energy applications. However, it remains an experimental challenge to synthesize electrically undoped, intrinsic clathrate. Here we show, through high-throughput computer modeling, that unconventional silicon phases spontaneously nucleate from liquid silicon in the presence of noble gases under high pressure and high temperature. In particular, our results show that a medium-sized noble gas, for example, argon, can trigger the nucleation and growth of inert-gas silicon clathrate, whereas a small noble gas such as helium is able to induce the formation of an unconventional, inclusion-type compound Si2He. The formation of both silicon phases can be attributed to the same thermodynamic and kinetic rationale that explains the crystallization of clathrate hydrate, an isostructural analog. Our findings, along with the gained molecular insights, thus strongly suggest a viable experimental synthesis route for these silicon phases using noble gases at high pressure.
Highlights
Silicon clathrate, an important allotrope of silicon, has attractive opto-electronic properties for energy applications
While only six-membered rings are present in the diamond cubic (DC) structure, open-frameworks can contain a variety of ring structures, for example, five-membered rings
Despite the novel properties of open-framework structures and the remarkable theoretical predictions for new allotropes, the synthesis of these metastable structures represents a major challenge. This is because metastable allotropes are local minima on free energy landscape, they are usually separated by large kinetic barriers which are difficult to overcome through conventional synthesis
Summary
An important allotrope of silicon, has attractive opto-electronic properties for energy applications. Despite the novel properties of open-framework structures and the remarkable theoretical predictions for new allotropes, the synthesis of these metastable structures represents a major challenge This is because metastable allotropes are local minima on free energy landscape, they are usually separated by large kinetic barriers which are difficult to overcome through conventional synthesis. This certainly represents a major experimental challenge because empty clathrate was predicted to be thermodynamically stable only under negative pressure[10,11,12] Another possibility is to use electrically inert guests that fit geometrically within cavities and weakly interact with the host atoms, for example, noble gas elements. Despite the high promise and predicted high stability, experimental synthesis of these inert gas inorganic clathrate has not been reported[17]
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