Introduction

Abstract

To consider phase states and phase transitions, we must look first to thermodynamic concepts. According to classical thermodynamics, the phase or aggregate state of an ensemble of interacting atoms or molecules is a uniform spatial distribution of atoms or molecules that is restricted by boundaries. A transition between two phases of a macroscopic system has a stepwise character and results from variation of thermodynamic parameters, typically (but not necessarily) the temperature. Most commonly, the variable controlling the phase and phase change is an intensive variable. A thermodynamic description of phase transitions has advantages and disadvantages. The advantage of this description is its universal character; it is suitable for many kinds of systems with different interactions between atoms or molecules. But for this reason, a thermodynamic description of aggregate states and phase transitions is formal and does not allow one to exhibit the nature of phenomena under consideration at a microscopic level.