True Structure of Trigonal Bipyramidal SiO₄F^- Species in Siliceous Zeolites

This paper describes the true geometry of the pentacoordinated SiO 4 F - units found in several all-silica zeolite systems, as determined by ab initio density functional calculations. The SiO 4 F - units in the two zeolites (sodalite and ferrierite) studied contain Si−F bonds of 1.71 and 1.76 Å in length, and all the Si−O bonds in the units are elongated with respect to the Si−O bonds in the tetrahedral SiO 4 unit typically found in a zeolite. All the bond angles in the SiO 4 F - unit agree with those expected for a regular trigonal bipyramidal XY 5 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.