Order-disorder and mobility of Li⁺ in the β'- and β-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°C (β'phase, monoclinic P2₁/n, Z = 4; a = 8.81277 (4), b = 8.94520(5), c= 12.37540(6) Å, β = 90.801(1)°) and at 350°C (β phase, orthorhombic Pbna, Z = 4; a = 8.84303(5), b = 8.94120(6), c = 12.41301(8) Å). All Li sites were located by difference Fourier maps in tetrahedral coordination, and both structures were Rietveld-refined to wR(p) = 0.0353(β') and 0.0429 (β). The β' structure is a distortion of β, with a [100] pseudo-twofold axis x, 1/2-y, - z relating all atoms but lithium, which is fully ordered (<Li-O> = 2.02 Å). In the β phase, Li is disordered over four sites, of which two (Li1 and Li2) are symmetry-independent with occupancies 0.34(1) and 0.16(1), respectively, and <Li-O> = 2.17 and 2.16 Å. The disorder fully explains the higher Li⁺ mobility in the β with respect to the β' phase observed from ionic conductivity data. Mechanisms of ion transport are proposed, and the relationship to the NASICON-type α'/α phase of LiZr₂(PO₄)₃ is analyzed in detail. (C) 2000 Academic Press.