Polytypism and Equilibrium Phase Transitions: In Situ Observation of the Polytypic Phase Transition 2H-12R in PbI2

摘要: Many minerals with two-dimensional structural elements show a specific form of polymorphism that is essentially related to the changing stacking sequences. This effect usually called “polytypism.” Its features have been thoroughly studied in compounds like SiC, CdI2, sulfides, several alloys, and many minerals. The physical reason for formation polytypic phases, however, far from understood. Several models discussed, such as growth (e.g., Trigunayat Chadha, 1971), entropy model Jagodzinski (1954), and, more recently, pseudospin ANNNI (Yeomans Price, 1986). In latter two it tacitly assumed polytypism occurs an equilibrium phenomenon well-defined thermal stability conditions each “phase.” assumption has never systematically justified before on experimental grounds, appears very likely phases some materials, modulated may be result transformation mechanisms Khatchaturyan, 1983) not describable within scheme thermodynamics. Even apparently reversible phase transitions SiC occur via topotactical (Jepps Page, 1984). Moreover, they seem only under drastic changes thermodynamic back reaction, application N2 pressure possibly, slight chemical composition samples. On other hand, there are geological observations which indicate can feature (Christy Putnis, 1987).