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

Abstract

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