Topological Modeling of Reconstructive Phase Transitions through Periodic Surfaces: Tin Dioxide I - A Calcium Chloride to Palladium Fluoride Type Transition

First order phase transitions in solids are under thermodynamic as well as kinetic control. Different from molecular modelling we focus here on tracing cooperative rearrangements of groups of atoms by help of periodic equisurfaces (PES) derived from selected structure factors, which are common to the structures under consideration. Such drastic Fourier filtering is very helpful in deriving the cooperative pictures of first order phase transitions. This report analyses the transition of SnO2 from the calcium chloride modification to the cubic high pressure form called Pa3 phase. An outline is given how to define a common unit cell for the two limiting structures based on the assumption of smallest net diffusion, i.e. conserving most structural features. Our approach garanties a minimum of integral displacement but has only indirect relation to energy considerations, such as small unit cells, short pathways and preservation of dominat structural features.