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If I squeeze hydrogen superhard with 400-500GPa of pressure, it becomes metal so is that solid or liquid? I know gallium melts on my hand and it's metal.

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    $\begingroup$ At room temperature? $\endgroup$ Commented Feb 3 at 6:07
  • $\begingroup$ @Chemomechanics: I only know it takes high pressure and temperature to create metallic hydrogen but I don't know if it stays that way once the condition changes back to room temperature and 1 atm😅 $\endgroup$
    – user6760
    Commented Feb 3 at 6:18
  • $\begingroup$ The wiki seems to indicate that's a current question. $\endgroup$
    – Triatticus
    Commented Feb 3 at 6:48
  • $\begingroup$ At 1 atm and room temperature hydrogen is gaseous. If you have metallic hydrogen and immediately change the ambient to 1atm and room temperature it will probably sublimate or survive in a metastable state, see physics.stackexchange.com/q/308290/226902 and physics.stackexchange.com/a/255090/226902 $\endgroup$
    – Quillo
    Commented Feb 3 at 10:43

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One-component systems have two independent thermodynamic degrees of freedom. Then, fixing the pressure is not enough to determine the thermodynamic state uniquely and, consequently, the equilibrium phase. By increasing the temperature at constant pressure, every known solid undergoes a solid-liquid transition. Liquid metals are well-known systems demonstrating that metallicity is a physical property independent of the phase of matter.

A recent review of the available information about the Hydrogen phase diagram (Goncharov, A. (2020). Phase diagram of hydrogen at extreme pressures and temperatures; updated through 2019. Low Temperature Physics, 46(2), 97-103) reports a figure from which I can read that at $600$ GPa the solid-liquid transition temperature is between $100$ and $200$ K.

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