Phase transitions of hairy black holes in massive gravity and thermodynamic behavior of charged AdS black holes in an extended phase space

We study the thermodynamic behavior of static and spherically symmetric hairy black holes in massive gravity. In this case, the black hole is surrounded in a spherical cavity with a fixed temperature on the surface. It is observed that these black holes have a phase transition similar to the liquid-gas phase transition of a Van der Waals fluid. Also, by treating the cosmological constant Λ as a thermodynamic pressure P, we study the thermodynamic behavior of charged anti--de Sitter black holes in an ensemble with a pressure of P and an electric potential Φ as the natural variables. A second order phase transition is observed to take place for all the values of the electric potential Φ.