The Superstructure of Lead Tetragonal Tungsten Bronze

The structures of the oxyorthogermanate La2(GeO4)O and the apatite-structured La9.33 (GeO4)6O2 have been refined from powder neutron diffraction data. La2(GeO4)O crystallizes in a monoclinic unit cell (P21/c) and is cation stoichiometric in contrast to previous reports. La9.33(GeO4)6O2 crystallizes in a hexagonal unit cell (P63/m) and the powder diffraction data show anisotropic peak broadening that is observed in electron diffraction patterns as incommensurate diffuse spots at hkq reciprocal planes (with q=1.6-1.7) and can be attributed to a correlated disorder in the "apatite channels". This compound was doped up to a nominal composition close to M2La8(GeO4)6O2 with M=Ca, Sr, Ba. The dopant ions preferentially occupy the 4f sites as the number of La vacancies decreases. The measured ionic conductivity of La9.33(GeO4)6O2 is about 3 orders of magnitude larger than for La2 (GeO4)O at high temperatures and decreases with increasing dopant content from the highest value of about 0.16 S cm-1 at 1160 K.