Ehrenfest scheme for complex thermodynamic systems in full phase space
  For a thermodynamic system with multiple pairs of intensive/extensive variables and the thermodynamical coefficients attain finite or infinite values on the phase boundary, we obtain the two classes of Ehrenfest equations in the full phase space, and find that the rank of the matrix for these equations can tell us the dimensions of the phase boundary. We also apply this treatment to the RN-AdS black hole. 
Zixu Zhao and Jiliang Jing
gr-qc hep-th