HIGH PRESSURE CRYSTAL CHEMISTRY: 'STUFFED' FRAMEWORK STRUCTURES AT HIGH-PRESSURE

摘要

This chapter explores the role of alkali and alkaline earth atoms in complex framework structures synthesized at high pressure, as well as their role in these compounds at high pressure using the procrystal model. This model approximates the electron density distribution of a crystal by superimposing spherically-averaged electron density distributions of static, ground state neutral atoms at the observed atomic sites in the crystal. We assume that a bonded interaction occurs between a pair of atoms if a topological feature, a (3,-1) critical point, exists in the electron density between the atoms. The bonded interactions of non-framework cations in tetrahedral frameworks (low albite and microcline), novel high-pressure frameworks with corner-linked SiO_4 and SiO_6 polyhedra (high-pressure Na-silicates, titanites, KTiOPO_4) and octahedral frameworks (perovskites) are described. The role of the non-framework cations in these compounds at high pressure is also investigated, including phase transitions.