  We investigate the thermodynamic properties of 3+1 dimensional black holes in
asymptotically de Sitter spacetimes, conformally coupled to a real scalar
field. We use a Euclidean action approach, where boundary value data is
specified at a finite radius `cavity' outside the black hole, working in the
extended phase space where the cosmological constant is treated as a
thermodynamic pressure. We examine the phase structure of these black holes
through their free energy. For the MTZ subclass of solutions, we find
Hawking-Page-like phase transitions from a black hole spacetime to thermal de
Sitter with a scalar field. In the more general case, Hawking-Page-like phase
transitions are also present, whose existence depends further on a particular
cosmic censorship bound.
