A Reinvestigation of Low-Carnegieite by XRD, NMR, and TEM

The crystal structure of low-carnegieite, NaAlSiO4, has been reinvestigated by a combination of X-ray powder diffraction, transmission electron microscopy, solid-state 29Si and 27Al MAS NMR spectroscopy, and electron microprobe analysis. With sufficient homogenization of component oxides and annealing at 1673 K it was possible to obtain a single crystalline phase which could be indexed to an orthorhombic unit cell: Pb 21a, a = 10.261(1), b = 14.030(2), c = 5.1566(6) Å. Electron microprobe analysis confirmed the stoichiometry to be NaAlSiO4. 29Si and 27Al NMR spectroscopy verified the perfect Si:Al ordering. It is now clear from the present work that previous attempts to synthesize low-carnegieite all suffered from incomplete reaction and the presence of other phases. One of these phases has been identified as having a large hexagonal unit cell with a = 13.97(2), c = 15.03(2) Å.