Gold boils at 2750K. It is relatively inert. It has a speed of sound of 500m/s at 3000K, 645m/s at 5000K. We can design some interesting gold-based phase change turbines. Liquid gold passes through the heat exchanger, to make the most of its thermal conductivity. It then boils and the gas is used to drive a turbine between the boiling and condensation temperature, which helps achieve a high pressure ratio.
The turbine blades would have to be actively cooled TaHC.
Efficiency would be roughly 36%.
And it would be the solar system's BLINGiest power generating system, ever.
Gold is not especially common and very massive, but a boiling gold turbine would be hilariously blingy indeed.
Gold doesn't strongly bond to Carbon, so that's one less safety aspect to be concerned about. Some forms of active cooling may have catastrophic failure mode due to gold vapor deposition, but the very low melt temperature means that we can probably be safe against aggregation via centrifugal action. Just don't use a microscopic pore/channel system to attempt to form a pressure wall between gold/turbine blade.
Does gold like to alloy to Ta or Hf? It'll probably work decently as long as it doesn't like anything present in the chamber.
I can't say I fully understand the properties of saturated gold vapor against different materials, but a not unreasonable analogue is mercury (another relatively inert metal, though one we are familiar with being a liquid). There is a fairly short list of metals compatible with it on the periodic table (notably iron). And, troublesomely, it tends to weaken/destroy things it amalgamates with.
If you can use amorphous carbon blades, you should. In CoaDE, that's what our high-temperature turbopumps are made from, and I don't see a reason why it wouldn't work IRL. TaHfC has much less strength and much more mass, so it can't spin as fast.
Another candidate ceramic I'd consider is zirconium carbide, ZrC, due to having very similar strength and melting point to amorphous carbon, just increased mass (though still not nearly as heavy as TaHfC). I don't know about its creep and brittleness though, sorry.