True structure of trigonal bipyramidal SiO4F- species in siliceous zeolites

This paper describes the true geometry of the pentacoordinated SiO4F- units found in several all-silica zeolite systems, as determined by ab initio density functional calculations. The SiO4F- units in the two zeolites (sodalite and ferrierite) studied contain Si-F bonds of 1.71 and 1.76 Angstrom in length, and all the Si-O bonds in the units are elongated with respect to the Si-O bonds in the tetrahedral SiO4 unit typically found in a zeolite. All the bond angles in the SiO4F- unit agree with those expected for a regular trigonal bipyramidal XY5 unit. The calculated structure is shown to reproduce accurately an averaged structural model determined by standard crystallographic methods. The approach we have developed in this work has general implications for the solution of complex local structure problems in solids.