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4hv.org :: Forums :: Chemistry
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Impossible compounds?

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Conundrum
Mon Oct 03 2016, 08:25AM Print View
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: Republic of Guernsey
Posts: 3803
Hi.
I read somewhere that lead carbide (PbC2) is not possible due to a very high thermodynamic barrier that encourages less energetic compounds to form.

Interestingly anecdotal reports of its accidental production by melting of BiPb alloy (eutectic) in a helium atmosphere in a SiC/C reinforced crucible at over 1000C suggest that it might be feasible to produce some this way.

So far one suggestion that did come up is to use cleaned and treated pyrolytic carbon and bismuth/lead alloy in close proximity then focus several dozen W of blue laser light under pure helium onto the underside of the carbon in a strong electric field over tens of hours with a refractory tantalum counter-electrode hoping to form the desired PbC2:Bi intermediate alloy by forced electromigration at 980C

Intriguingly if PbC2:Bi was stable it could be a candidate for a high temperature superconductor as the anomalous electron configuration might encourage the formation of d waves with the Pb dampening down the troublesome charge-density waves that normally prevent superconductivity, in this case the electrons and holes might balance exactly.
This could also happen at a very narrow range of temperatures, possibly less than 0.05K although well above room temperature.

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Conundrum
Tue Jan 31 2017, 05:18AM
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: Republic of Guernsey
Posts: 3803
Add "metallic hydrogen" to the list of previously theoretical compounds. Apologies for double post, this is newsworthy as it could possibly net Ashcroft and others a well deserved Nobel.

I also suggested a few years back that PdH:S might be a RTSC candidate in light of the discovery of 212K HTSC in H3S.
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Enceladus
Thu Feb 09 2017, 12:26AM
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Posts: 46
Conundrum wrote ...

I also suggested a few years back that PdH:S might be a RTSC candidate in light of the discovery of 212K HTSC in H3S.


*H2S

WRT your comments regarding PbC2, I assume you are saying that this particular cabide of lead ie: [Pb2+][C2 2-], lead acetylide, appears not to be possible. What little information there seems to be available seems to agree with you, but my thinking is that intermetallic compounds of lead and carbon may, in fact exist, even if they lack a well defined stoichiometry or indeed, a Pb-C bond. Even if lead acetylide did exist it would probably be ionic in nature without any covalent bond between lead and carbon.

Maybe you could synthesize calcium lead carbide by "doping" the CaC2 with a lead salt.. for whatever good that would do you.

Personally, a yet undiscovered compound/allotrope I am looking forward to seeing evidence of is carbyne. For those unfamiliar with the stuff, it is a hypothetical allotrope of carbon consisting of strings of carbon atoms with alternating triple and single bonds. It's tensile strength and electrical conductivity as a material is expected to far exceed that of even carbon nanotubes. The trouble with graphene and CNT's and perhaps eventually carbyne is having to constantly hear about how marvelous they are without ever getting to see and feel them in reality. I don't know about you guys but I'm becoming very impatient waiting for the future to get here.
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