[txnf]

Most of these bizarre properties can be traced to the mixed valent character of plutonium - that is the electrons in the partially filled 5f shell of the plutonium atom hybridize in the solid phase with the valence electrons, and thus for some properties they are valence like and others they are core like. This is quite similar to some lanthanide (4f) elements like cerium, which also show strange variety of allotropes. The mixed valent character causes plutonium to have a variety of exotic physics effects, completely unrelated to it's most notorious use in nuclear weapons. Examples of these include heavy fermion metals, novel superconductivity, etc..

[khuey]

It would be extremely funny if the room-temperature superconductor we're searching for is some plutonium-based compound or alloy.

[PaulHoule]

See https://en.wikipedia.org/wiki/Gas_mantle for a an unusual property of Thorium which must have to with having so many electrons.

[perihelions]

Is it specifically the emissivity that's so unusual and makes it the preferred material? I don't know that "low emissivity in infrared + high emissivity in visible" is a rare property—I recall reading about space satellite thermal engineering and seeing long lists of common materials sorted by visible/infrared emissivity ratios [0]. And there are a lot of them, in every category. I suspect the key thing is ThO2 is a super-refractory with a melting point of (approximately—these are hard to measure!) 3,350° C [1]. (About the same as the tungsten filaments in the old-school type of lightbulbs—something with pretty similar considerations). I suspect "things that maintian structural integrity in hot gas flame" is really the key discriminator here, the rare property that prunes out most candidates.

I'm not any sort of expert on this, to make very clear! Just a curious geek.

[0] (That's basically a proxy for the radiative equilibrium temperature in space: visible emissivity measuring absorption of sunlight, infrared emissivity measuring emission of waste heat. (To those unfamiliar, absorption and emission are exactly the same, at a specified wavelength: the physics is reversible). ThO2 for example, you'd expect would get extremely hot in space).

[1] https://en.wikipedia.org/wiki/Thorium_dioxide