Pressure dependence of Tc from chemical equilibrium

Chemical equilibrium theory analogous with dissociation of molecules or ionization of gas of atoms is applied to high-Tc superconductors. The starting point are performed pairs, which exist in the normal state and can be treated as Coulomb bosons with charge 2e. Above Tc the pairs (B++) decay into fermions (h+) according to the equilibrium reaction B++ ↔ 2h+. Using an approximate chemical equilibrium constant, proportional to pressure the authors derive a universal two-parameter expression for the pressurized optimum transition temperature TcMAX(P). The formula shows that TcMAX(P) develops a maximum and beyond the maximum it starts to come down. The authors also show that in interesting pressure range P > 10 GPa, the expansion in powers of pressure diverges