  We examine the thermodynamic behaviour of four-dimensional charged and
uncharged de Sitter black holes enclosed in an isothermal cavity, in the
extended phase space where the cosmological constant is treated as a
thermodynamic pressure. We demonstrate the presence of a novel
pressure-dependent phase transition in a compact region of phase space that
does not appear in asymptotically anti-de Sitter black holes, and find a highly
non-linear equation of state that does not lead to the usual interpretation of
a van der Waals fluid.
