A structural investigation of La-2(GeO4)O and alkaline-Earth-doped La-9.33(GeO4)(6)O-2

摘要

The structures of the oxyorthogermanate La-2(GeO4)O and the apatite-structured La-9.33(GeO4)602 have been refined from powder neutron diffraction data. La2(GeO4)O crystallizes in a monoclinic unit cell (P2(1)/c) and is cation stoichiometric in contrast to previous reports. La9.33(GeO4)(6)O-2 crystallizes in a hexagonal unit cell (P6(3)/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)(6)O-2 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 La-9.33(GeO4)(6)O-2 is about 3 orders of magnitude larger than for La-2(GeO4)O at high temperatures and decreases with increasing dopant content from the highest value of about 0.16 Scm(-1) at 1160 K. (C) 2002 Elsevier Science (USA). [References: 31]