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Topic: Silicon allotropes  (Read 1083 times)

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Offline GSRush

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Silicon allotropes
« on: October 17, 2019, 09:23:49 PM »
Dear All!

I want to synthesize silicon allotrope. Not just cubic silicon, but orthorhombic.
But in the first step, I need to synthesize Li7Si12.
Is it possible or not?
Any ideas about how to do it?

The main idea can be found at this reference.

Any ideas?

Offline AWK

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Re: Silicon allotropes
« Reply #1 on: October 18, 2019, 02:37:57 AM »
View the Li-Si phase diagram
AWK

Offline Enthalpy

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Re: Silicon allotropes
« Reply #2 on: October 18, 2019, 08:02:31 AM »
Wow, direct-gap silicon would be fantastic!
https://www.chemistryworld.com/news/new-silicon-allotrope-could-revolutionise-solar-cells/7977.article
Not just for solar cells, which demand a very low cost. Optical communications within supercomputers or between PC accept much smaller components with an easier cost.

If you achieve tiny zones of direct-gap silicon on a standard wafer, that will be perfect for optical communications. Implant Na heavily there, recrystallise, evaporate? I fear Na pollutes the whole wafer. Diffusion barrier of W, Pt?

If the mean composition Li7Si12 segregates upon cooling, you could try ultra-fast cooling by spraying material droplets on a rotating substrate, as is done for vitreous magnetic materials, which are subsequently sintered. But you didn't tell: I fear your Li7Si12 shall be single-crystal?

The other attempt I saw evaporated Na from an Si-Na crystal. Li is less volatile. The necessary temperature risks to recrystallise Si. Or?

Offline GSRush

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Re: Silicon allotropes
« Reply #3 on: October 18, 2019, 11:36:36 AM »
View the Li-Si phase diagram
Thanks a lot!  ;D

Offline GSRush

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Re: Silicon allotropes
« Reply #4 on: October 18, 2019, 11:42:38 AM »
Wow, direct-gap silicon would be fantastic!
https://www.chemistryworld.com/news/new-silicon-allotrope-could-revolutionise-solar-cells/7977.article
Not just for solar cells, which demand a very low cost. Optical communications within supercomputers or between PC accept much smaller components with an easier cost.

If you achieve tiny zones of direct-gap silicon on a standard wafer, that will be perfect for optical communications. Implant Na heavily there, recrystallise, evaporate? I fear Na pollutes the whole wafer. Diffusion barrier of W, Pt?

If the mean composition Li7Si12 segregates upon cooling, you could try ultra-fast cooling by spraying material droplets on a rotating substrate, as is done for vitreous magnetic materials, which are subsequently sintered. But you didn't tell: I fear your Li7Si12 shall be single-crystal?

The other attempt I saw evaporated Na from an Si-Na crystal. Li is less volatile. The necessary temperature risks to recrystallise Si. Or?

Thanks for your answer, but I am a physicist and not a chemist.
I thought that it will be easy, for example, to mix 7 Li and 12 Si and heat up this mixture.

Offline Enthalpy

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Re: Silicon allotropes
« Reply #5 on: October 18, 2019, 05:46:50 PM »
7 moles Li for 12 Si, or in mass proportion 0.1260 Li for 0.8740 Si.

Put in a pot, melt everything, stir, and with little luck you get a liquid alloy with the right proportion. The pot will pollute the alloy, but you know that from usual silicon already.

But as you cool the liquid alloy and expect it to crystallize, Li and Si will combine according to favourable proportions, and reject the excess Li or Si in the melt. These proportions correspond to more stable crystal shapes and compositions, not necessarily to the one you prefer, and they depend also on the liquid's composition, so the formed crystals can evolve over time as the proportion of the rejected element increases in the liquid over time.

For instance enough C in Fe makes carbide crystals in a matrix made mostly of Fe.

In the phase diagrams for Li-Si, I don't even see a mention of the Li7Si12 phase you seek. Maybe you first get Si-rich crystals, more and more alloyed with Li over time, then approximately Li4Si3, and at the end LiSi. Or Li12Si7 at the end. It would be nice if someone could confirm.
http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Li-Si.jpg&dir=FTlite
http://jes.ecsdl.org/content/160/8/A1232/F3.large.jpg

One known answer is ultrafast cooling. When crystals don't have time to form, you obtain a glass whose composition is frozen and homogeneous. Subsequent treatments, well below any melting point, can then act on the proportion and size of crystals.

If you want single-crystal Li7Si12, I have no idea how to obtain it. Maybe pressure forces this particular composition. Or just a seed of that crystal lets obtain a boule by Czochralski or Bridgman. Is there literature?

Offline GSRush

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Re: Silicon allotropes
« Reply #6 on: October 18, 2019, 11:40:34 PM »
7 moles Li for 12 Si, or in mass proportion 0.1260 Li for 0.8740 Si.

Put in a pot, melt everything, stir, and with little luck you get a liquid alloy with the right proportion. The pot will pollute the alloy, but you know that from usual silicon already.

But as you cool the liquid alloy and expect it to crystallize, Li and Si will combine according to favourable proportions, and reject the excess Li or Si in the melt. These proportions correspond to more stable crystal shapes and compositions, not necessarily to the one you prefer, and they depend also on the liquid's composition, so the formed crystals can evolve over time as the proportion of the rejected element increases in the liquid over time.

For instance enough C in Fe makes carbide crystals in a matrix made mostly of Fe.

In the phase diagrams for Li-Si, I don't even see a mention of the Li7Si12 phase you seek. Maybe you first get Si-rich crystals, more and more alloyed with Li over time, then approximately Li4Si3, and at the end LiSi. Or Li12Si7 at the end. It would be nice if someone could confirm.
http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Li-Si.jpg&dir=FTlite
http://jes.ecsdl.org/content/160/8/A1232/F3.large.jpg

One known answer is ultrafast cooling. When crystals don't have time to form, you obtain a glass whose composition is frozen and homogeneous. Subsequent treatments, well below any melting point, can then act on the proportion and size of crystals.

If you want single-crystal Li7Si12, I have no idea how to obtain it. Maybe pressure forces this particular composition. Or just a seed of that crystal lets obtain a boule by Czochralski or Bridgman. Is there literature?

Thanks a lot for your reply!
Especially about ultrafast cooling.

The Li7Si12 is not synthesized yet, but there are some articles about
Li7Ge12:

https://doi.org/10.1016/j.solidstatesciences.2010.12.038




Offline AWK

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Re: Silicon allotropes
« Reply #7 on: October 19, 2019, 02:46:23 AM »
If you look at the phase diagram - you need a temperature of almost 1200°C to melt your mixture and silicon will crystallize when cooling, and the Li12Si7 phase will solidify at 648°C regardless of the cooling speed (unless the phase diagram for atmospheric pressure is not true). Based on this phase diagram, it is difficult to predict what will happen under HTHP conditions. Zeolite o-Si24 was probably obtained from Na-Si alloy.
AWK

Offline GSRush

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Re: Silicon allotropes
« Reply #8 on: October 19, 2019, 07:49:34 PM »
If you look at the phase diagram - you need a temperature of almost 1200°C to melt your mixture and silicon will crystallize when cooling, and the Li12Si7 phase will solidify at 648°C regardless of the cooling speed (unless the phase diagram for atmospheric pressure is not true). Based on this phase diagram, it is difficult to predict what will happen under HTHP conditions. Zeolite o-Si24 was probably obtained from Na-Si alloy.

Yes, zeolite Si24 was produced from Na-Si alloy.
But I am looking for Li7Si12.

Offline Enthalpy

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Re: Silicon allotropes
« Reply #9 on: October 21, 2019, 03:34:20 PM »
GSRush, could you confirm you seek Li7Si12, not the Li12Si7 indicated everywhere? Just to be sure.

[...] The Li12Si7 phase will solidify at 648°C regardless of the cooling speed [...]

Ultrafast cooling achieves unexpected things. I used an Al alloy containing >10% Zn (for >700MPa yield strength, wow!) which couldn't have existed from casting nor extrusion
http://www.rsp-technology.com/site-media/user-uploads/rsp_alloys_overview_2018lr.pdf

But "ultrafast" isn't the usual water-cooling tinkering. It's like 1MK/s, faster than microscopic precipitates form. The usual process sprays droplets of the liquid alloy on a highly conductive drum that spins quickly and is cooled actively. The droplets freeze by contact before precipitates form.

[Later edit: I've probably understood it wrongly. Precipitates seem to form, only much finer and uniform than usual. There
http://www.rsp-technology.com/technology/microstructure.html
"The various intermetallic compounds as well as the components with a low solubility are distributed finely and homogeneously over the metal matrix, thereby forming a very homogeneous microstructure."]

Then, the obtained flakes are sintered together, hence with convincing pressure, at a temperature low enough that precipitates can't form - but the flakes must be somewhat malleable, which Li7Si12 could be or not. Optionally, the obtained material undergoes some heat treatment.

A semiconductor hypothetically obtained that way would be amorphous or polycrystalline, hence my question, whether a single-crystal is sought. Recrystallisation would probably lead to the wrong compounds and allotropes, unless a seed or pressure suffices to favour the desired one.

==========

I wonder if atomic layer deposition (ALD) can produce the Li7Si12. For that, some orientation of Li7Si12 must show alternating planes of pure Si and pure Li. Ideally, standard Si would match two lattice constants of Li7Si12 if Si is cut at the proper and accurate angle, to serve as a seed.
« Last Edit: October 21, 2019, 06:30:16 PM by Enthalpy »

Offline Enthalpy

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Re: Silicon allotropes
« Reply #10 on: October 22, 2019, 01:40:03 PM »
Li7Si12 is not synthesized yet
So obviously, you seek Li7Si12, not the already known Li12Si7. My bad.

Offline Enthalpy

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Re: Silicon allotropes
« Reply #11 on: October 27, 2019, 12:14:15 PM »
An other and possibly better seed or substrate to grow Li7Si12 could be GaxAl1-xInyAszP1-y-z and other variants. The proportions adjust their lattice constant continuously and accurately, so it would match the (expected) constants of Li7Si12 to make its growth selective and easier.

I don't remember if these crystals have a zincblende structure. If Li7Si12 has two identical lattice constants, it might grow along the third direction.

This applies to thin epitaxy, especially Atomic Layer Deposition (ALD), and to the growth of more bulky crystals, for instance by Czochralski, where the adjusted crystal would be a seed.

Marc Schaefer, aka Enthalpy

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