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Inspired by this question, what is the substance or compound with the highest melting point? At standard atmospheric pressure and ever-increasing temperature which substance a) eventually becomes liquid b) does so at the highest temperature?

I don't know much about glasses or related semisolids, so perhaps a reference viscosity? Glass transition temperatures are kind of like melting points, but some materials have a glass transition temperature and a melting point.

Some initial casting about on the internet finds that rhenium and tungsten have melting points at about 3400 C, and Hafnium Carbide claims to be 'one of the highest' at 4100 Kelvin. Does it actually melt or just decompose and then sublimate? 4100 Kelvin is its 'refractory temperature', but I'm not certain how that's defined.

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there are reports, that crust of neutron star is solid, composed of iron nuclei and degenerate electrons. The surface temperature of such star is about $10^6$ Kelvin. –  permeakra Aug 12 at 7:14

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up vote 4 down vote accepted

Well, according to this Wikipedia article, it might be an alloy of tantalum carbide and hafnium carbide, specifically $\ce{Ta_4HfC_5}$. A melting point of 4,215 °C is listed. But who really knows the answer to your question with so many new, proprietary alloys with complex compositions being fabricated these days.

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It seems like 4-5 thousand kelvin is where the high-performance ceramics melt. I wonder if there would be interest in a melting-point competition, with teams sponsored by Dow, Du Pont, etc. Like Darpa's self-driving car thing they do in the desert. –  Jeremy Kemball Aug 12 at 21:01
    
It's human nature to compete. Set up a competition and people will be there. –  ron Aug 12 at 21:27

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