Crystal chemistry of high-pressure BaSi4O9 in the trigonal (P3) barium tetragermanate structure

Crystals of BaSi4O9 synthesized at 4 GPa and 1000 degrees C were determined to be isostructural with barium tetragermanate [trigonal, space group P3, a =11.2469(11) and c = 4.4851(6) Angstrom, V = 491.3(1) Angstrom(3)]. The s tructure (R = 2.4%) features a corner-linked framework of three-member sili cate tetrahedral rings, which are cross-linked by isolated silicate octahed ra. Ba cat-ions occupy tenfold-coordinated sites in channels defined by the silicate framework. This structure is 4.2% denser than the topologically s imilar benitoite form of high-pressure BaSi4O9, which was produced by grind ing the barium tetragermanate-type crystals described in this report.