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

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.