Critical point of melting and its change at the decreases of a nanocrystal size

Abstract

It is shown that the structures of the crystal and the liquid become close at high temperatures and pressures. This should lead to appearance of a melting critical point. The appearance and disappearance of the S-loop of the phase transition (PT) of the first order on the isotherm of the simple matter state equation in the crystal-liquid (C-L) PT region is studied based on a three-phase model. Calculations performed for argon have shown that the S-loop of the C-L PT on the isotherm of the state equation arises due to a sharp decrease and subsequent increase in pressure associated with the appearance of delocalized atoms at an isothermal increase in the specific volume. With an increase in temperature on the isotherm, the pressure associated with the delocalization of atoms passes from the negative region (where it compresses the system) to the positive region (where it stretches the system). Such behavior of this function leads both to the appearance of the S-loop of C-L PT on the isotherm of the state equation, and the disappearance of the S-loop of isotherm at high temperatures with the formation of the critical point of C-L PT. It is shown that during the transition to the nanocrystal, the critical temperature and pressure decrease, and the critical molar volume increases. Calculations in the framework of the three-phase model of a simple matter showed that the structure at the critical point of C-L PT is close to an amorphous packing. It is shown that for a nanocrystal with a Gibbs free surface shape different from the energetically optimal shape, the size dependences of the parameters of the critical point of C-L PT are more noticeable. If one face of a nanosystem with a given surface shape lies on the substrate, then the size dependences of the parameters of the critical point of C-L PT become weaker.