Crystal structures and crystal chemistry in the system Na1+xZr2SixP3−xO12

As part of a search for skeleton structures for fast alkali-ion transport, the system Na1+xZr2SixP3−xO12 has been prepared, analyzed structurally and ion exchanged reversibly with Li+, Ag+, and K+ ions. Single-crystal x-ray analysis was used to identify the composition NaZr2P3O12 and to refine its structure, which has rhombohedral space group R3̄c with cell parameters ar = 8.815(1)Å and cr = 22.746(7)Å. A small distortion to monoclinic symmetry occurs in the interval 1.8 ≤x≤ 2.2. The structure for Na3Zr2Si2PO12, proposed from powder data, has space group C2c with am = 15.586(9)Å, bm = 9.029(4)Å, cm = 9.205(5)Å, and β = 123.70(5)° Both structures contain a rigid, three-dimensional network of PO4 or (SiO4) tetrahedra sharing corners with ZrO6 octahedra and a three-dimensionally linked interstitial space. Of the two distinguishable alkali-ion sites in the rhombohedral structure, one is completely occupied in both end members, the occupancy of the other varies across the system from 0 to 100 percent. Several properties are compared with the fast Na+-ion conductor β-alumina.