Materials Data on Na8Si7O18 by Materials Project

Abstract

Na8Si7O18 crystallizes in the trigonal R-3m space group. The structure is three-dimensional. there are three inequivalent Na1+ sites. In the first Na1+ site, Na1+ is bonded to six O2- atoms to form distorted NaO6 octahedra that share corners with six equivalent SiO4 tetrahedra, faces with three equivalent NaO8 hexagonal bipyramids, and a faceface with one SiO6 octahedra. There are three shorter (2.23 Å) and three longer (2.46 Å) Na–O bond lengths. In the second Na1+ site, Na1+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are four shorter (2.55 Å) and four longer (2.63 Å) Na–O bond lengths. In the third Na1+ site, Na1+ is bonded to eight O2- atoms to form distorted NaO8 hexagonal bipyramids that share corners with four equivalent NaO8 hexagonal bipyramids, corners with two equivalent SiO4 tetrahedra, edges with two equivalent SiO6 octahedra, edges with four equivalent SiO4 tetrahedra, and faces with two equivalent NaO6 octahedra. There are a spread of Na–O bond distances ranging from 2.28–2.72 Å. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to six equivalent O2- atoms to form SiO6 octahedra that share corners with six equivalent SiO4 tetrahedra, edges with six equivalent NaO8 hexagonal bipyramids, and faces with two equivalent NaO6 octahedra. All Si–O bond lengths are 1.83 Å. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share a cornercorner with one NaO8 hexagonal bipyramid, a cornercorner with one SiO6 octahedra, corners with two equivalent NaO6 octahedra, corners with two equivalent SiO4 tetrahedra, and edges with two equivalent NaO8 hexagonal bipyramids. The corner-sharing octahedra tilt angles range from 30–58°. There are a spread of Si–O bond distances ranging from 1.59–1.68 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to three Na1+ and two equivalent Si4+ atoms to form a mixture of distorted edge and corner-sharing ONa3Si2 trigonal bipyramids. In the second O2- site, O2- is bonded in a 2-coordinate geometry to three Na1+ and two Si4+ atoms. In the third O2- site, O2- is bonded in a 5-coordinate geometry to four Na1+ and one Si4+ atom.