Order-disorder and mobility of Li+ in the beta ' and beta-LiZr2(PO4)(3) ionic conductors: A neutron diffraction study

Neutron diffraction profiles at high resolution (HRPD, ISIS Facility, U.K.) were collected on powder samples of LiZr2(PO4)(3) at 20 degrees C (beta' p hase, monoclinic P2(l)/n, Z = 4; a = 8.81277 (4), b = 8.94520 (5), 12.37540 (6) Angstrom, beta = 90.801 (1)degrees) and at 350 degrees C (beta phase, orthorhombic Pbna, Z = 4; a = 8.84303 (5), b = 8.94120 (6), c = 12.41301 (8 ) Angstrom). All Li sites were located by difference Fourier maps in tetrah edral coordination, and both structures were Rietveld-refined to wR(p) = 0. 0353 (beta') and 0.0429 (beta), The beta' structure is a distortion of beta , with a [100] pseudo-twofold axis x, 1/2 - y, - z relating all atoms but l ithium, which is fully ordered (< Li-O > = 2.02 Angstrom). In the beta phas e, Li is disordered over four sites, of which two (Li1 and Li2) are symmetr y-independent with occupancies 0.34 (1) and 0.16 (1), respectively, and < L i-O > = 2.17 and 2,16 Angstrom, The disorder fully explains the higher Limobility in the beta with respect to the beta' phase observed from ionic co nductivity data. Mechanisms of ion transport are proposed, and the relation ship to the NASICON-type alpha'/alpha phase of LiZr2(PO4)(3) is analyzed in detail. (C) 2000 Academic Press.