Investigation of the phase transitions in cesium by the average atom model

Using the average atom method (quasizone model) we show that the cesium cold curve has two minimums — the first one is at relatively small densities 0.3 g/cm$^3$ and the second one is at higher densities 6.2 g/cm$^3$. The first one leads to the usual critical point characterizing the liquid-vapor phase transition, which is confirmed by experimental data. The reason for the second minimum is connected with s-d transition of valence electron. This leads to a second phase transition with the critical point at temperature 6400 K, density 5.3 g/cm$^3$ and pressure approximately 61000 atm. This phase transition takes place in the cesium plasma state. In this degenerate nonideal plasma with mean ion charge 1.5 and density 3 g/cm$^3$ jumps in a highly ionized state with mean charge of 3.5 and density 6.5 g/cm$^3$.