A closer look into close packing: pentacoordinated silicon in a high-pressure polymorph of danburite.

Anna Pakhomova,Konstantin Glazyrin,Maxim Bykov,Liudmila Gorelova,Leonid Dubrovinsky,Biliana Gasharova,Hanns-Peter Liermann,Sergey Krivovichev,Elena Bykova,Mohamed Mezouar

doi:10.1107/s2052252517010612

Abstract

Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB2Si2O8, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si2O7 groups of danburite first transform into chains of vertice-sharing SiO5 trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO6 octahedra (danburite-III). It is suggested that the unusual formation of an SiO5 configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II.

Highlights

Silicates are widely used in different chemical processes ranging from catalysis to cement production
Pentacoordinated silicon is extremely rare in inorganic compounds, and the reason why SiO4 tetrahedra transform directly into SiO6 octahedra without transitional SiO5 coordination is not clear
Ionic oxide compounds are known for their tendency of forming a close-packed arrangement of oxygen atoms, with cations filling up the voids between the anions

Summary

Introduction

Silicates are widely used in different chemical processes ranging from catalysis to cement production. Due to the high geological relevance of silicates, numerous X-ray diffraction studies have been performed in order to understand their crystal structures as a function of chemical composition, temperature and pressure. While Pauling (1929) gave rules to describe the principles of structural organization of ionic compounds under ambient conditions, several ‘rules of thumb’ were outlined by Prewitt & Downs (1998) to address the high-pressure evolution of silicates. The crystal chemistry of rock-forming silicates with four- and six-coordinated silicon is well established, whereas only a very few examples of structures with five-coordinated Si are known. These examples are limited to cases where SiO5 groups coexist with SiO4 tetrahedra and/or SiO6 octahedra.

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Conclusion