10.1007/s11446-008-3004-5

Abstract

In this study, we consider properties of structural phase transitions proceeding in halide crystals under extreme conditions of high pressures and analyze the role of vacancies in the discovered dimensional effect of the polymorphic B1-B2 transformation. We also describe the observed anomalous behavior of surface characteristics of certain alkali-halide crystals.Structural phase transitions including those of the B1-B2 type (i.e., transitions between NaCl-type and CsCl-type structures) have been the subject of numerous experimental and theoretical studies [1][2][3][4]. We should emphasize the importance of investigating properties of polymorphic transformations in small crystalline particles, which is associated with both the rapid development of the physics of ultradisperse systems [5,6] and the wide propagation of these materials in technology. Dimensional effects in ultradisperse systems have attracted great attention since they lead to new properties unusual for homogeneous macroscopic bodies, which is important for application in practice. For example, reduction of the melting temperature in these systems [7] is used to produce solders; the application of ultradisperse powders as dopes in technological processes of powder metallurgy abruptly reduces the activation energy (in particular, the agglomeration energy) and increases the material's strength [7].The statistical theory of structural phase transitions in faceted crystals is of clear interest since constructing the theory of finite-size solids is associated with correctly setting the boundary conditions. In this case, the energy contribution of crystal-lattice defects, in particular, vacancies, noticeably affects many thermodynamic and physicochemical properties of a substance. The investigation of the concentration dependence of these defects in ultradisperse systems on the particle's size turns out to be quite important [8]. It is reasonable to perform rigorous theoretical analysis of phase transformations in small particles in accordance with the thermodynamics of small systems (the Hill method) [9,10]. This is associated with the fact that the thermodynamics of macroscopic Gibbs systems [11] does not allow us to take into account correctly the contribution of the surface energy and the energy of defects into the system energy.This study is devoted to investigating properties of structural phase transitions in small-size alkali-halide crystals in extreme conditions of high pressures. Specific attention is paid to calculation of the B1-B2-transition pressure as a function of the crystal size. The results obtained in the framework of the Gibbs [11] and Hill [9] approaches with allowance for the surface contribution to the system's thermodynamic potential are compared.Numerous experimental data (see, e.g., [12]) show that the pressures of polymorphic transformations in small crystalline particles in massive samples are different. These features are associated with enhancing the role of the surface energy with the reduction in the particle size [6].In [1...