Dyonic Taub-NUT-AdS: Unconstrained Thermodynamics and Phase Structure

Here we extend the approach developed in [1] to study the thermodynamics of Taub-NUT-AdS and dyonic Taub-NUT-AdS solutions. Furthermore, we investigate in details the possible phase structures of the dyonic Taub-NUT-AdS solution. We show that the first law, Gibbs-Duhem and Smarr's relations are all satisfied for both solutions. Our study of phase structures shows some intriguing features, which were not reported before, among which the existence of two distinguished critical points with a region of continuous phase transitions in between, and the possibility of merging them into one point. To analyze these phases we consider both canonical and mixed ensembles. The two distinguished critical points occur for the canonical case as well as the mixed cases with $1/2\ensuremath{\le}{\ensuremath{\phi}}_{e}<1$. Another interesting case is the mixed ensemble with ${\ensuremath{\phi}}_{e}\ensuremath{\ge}1$, where we have one critical point but the continuous phase-transition region in the $P\ensuremath{-}T$ diagram is close to the origin, in contrast with what happens in Reissner-Nordstrom-AdS solutions and Van der Waals fluids, i.e., the continuous phase transition happens only for low-enough pressures and temperatures.