Rietveld refinement of Na1.8Be0.9Si1.1O4; an ionic conductor with a cristobalite-related structure

The structure of the ionic conductor Na1.8Be0.9Si1.1O4 has been refined with the Rietveld Profile Refinement technique, based on X-ray powder diffraction data. The unit cell parameters were found to be a=4.920, b=9.876 and c=13.922 Å. With space group symmetry Pcba assumed, the refinement yielded RF=0.049 and Rwp=0.088. The cristobalite-related structure is built up of corner-shared [MeO4]-tetrahedra, with Me=Si and Be. The Si4+ and Be2+ ions are statistically distributed and the Na+ ions, located in the cavities formed by the tetrahedra, are found to be five-and four-coordinated. The findings are discussed in relation to the structure of Na2BeSiO4 and migration pathways for the Na+ ions.