Order–Disorder and Mobility of Li+ in the β′- and β-LiZr2(PO4)3 Ionic Conductors: A Neutron Diffraction Study

Abstract

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 wRp=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.