On the critical melting point of a simple matter

The problem of appearance and disappearance of the S-loop of the first-order phase transition (PT) in an isotherm of the equation of state near the crystal–liquid (CL) PT was studied using the three-phase model of a simple matter. The calculations carrier out for argon is shown that the S-loop of CL PT in an isotherm of the equation of state appears because of a sharp decrease and subsequent increase in the pressure related to the formation of delocalized atoms as the specific volume increases isothermally. As temperature increases, the pressure in an isotherm related to the delocalization of atoms transits from the negative region (where it compresses the system) to the positive region (where is stretches the system). Such a behavior of this function leads to the formation of the S-loop of CL PT in an isotherm of the equation of state and also to the disappearance of the S-loop of CL PT at high temperatures with the formation of the critical point of CL PT. The change in the parameters of the CL PT critical point is studied as the number of atoms in the nanosystem decreases. It was shown the critical temperature and pressure decrease in going to a nanosystem, and the critical molar volume increases. The calculations in terms of the three-phase model of a simple matter show that the structure in the CL PT critical point is close to the amorphous packing. The parameters of this amorphous structure in the CL PT critical point are changed slightly as the number of atoms in the nanosystem decreases.