THE CRYSTAL-STRUCTURE AND TWINNING LAWS OF THE ORTHORHOMBIC MODIFICATION OF NA2BESIO4

The crystal structure of the solid electrolyte Na2BeSiO4 has been determined and refined both at 295 and 623 K using crystal diffraction data from a sixfold twin collected with MoKα. The structural relations between the twin components have been elucidated. The structure is orthorhombic with the space group symmetry Pca21. The unit cell contains eight formula units of Na2BeSiO4, and the lattice parameters are a = 9.861(2), b = 4.911(1), c = 13.875(3) Å, and V = 671.9 Å3. The refinement yielded Rwiso = 0.068 and Rwan = 0.053 for the data collected at 295 K (2504 reflections with I > 3σI), and Rwiso = 0.059 and Rwan = 0.034 for the 623 K data set (2468 reflections with I > 3σI). The structure can be described as a cristobalite-related framework structure with chains of alternating BeO4 and SiO4 corner-sharing tetrahedra. The Na atoms are found at four different positions in the cavities formed by the tetrahedra. Possible conduction pathways have been deduced. © 1990.