Ion transport and phase transition in Li7-xLa3(Zr2-xMx)O12 (M = Ta5+, Nb5+, x=0, 0.25)

Due to their favourable combination of high ionic conductivity and stability versus elemental lithium, garnet-related lithium ion conductors Li7La3Zr2O12 have raised strong interest for both all-solid-state batteries and as protective layers for anode materials. Here we study the correlation between structure and ion mobility in Li7-xLa3(Zr2-xMx)O-12 (x = 0, 0.25; M = Ta5+, Nb5+) combining Molecular Dynamics (MD) simulations, bond valence (BV) studies and experimental characterisation. In situ XRD demonstrates a tetragonal-to-cubic phase transition above 450 K for LixLa3Zr2O12. MD simulations using our new BV-based Morse-type force field reproduce static (lattice constants, thermal expansion, phase transition) and dynamic characteristics of this material. Simulations and structure refinements for the tetragonal phase accordingly yield an ordered Li distribution. The majority of Li fully occupies the 16f and 32g octahedral sites. Out of the two tetrahedral sites only the 8a site is fully occupied leaving the 16e tetrahedral sites with slightly higher site energy due to the tetragonal distortion vacant. For the cubic phase recent structural studies either suggest a major Li+ redistribution to nearly fully occupied tetrahedral sites and distorted octahedral sites with a low occupancy (which leads to unphysically short Li-Li distances) or suggest the existence of additional Li sites. MD simulations however show that the lithium distribution just above the phase transition closely resembles that in the tetragonal phase with only slightly more than 1/3 of the now equivalent tetrahedral 24d sites and almost half of the distorted octahedral 96h sites occupied, so that overly short Li-Li distances are avoided. Pentavalent doping enhances ionic conductivity by increasing the vacancy concentration and by reducing local Li ordering. At higher temperatures Li is gradually redistributed to the tetrahedral sites that can be occupied up to a site occupancy factor of 0.56. BV pathway analysis and closely harmonizing Li trajectories demonstrate that the two partially occupied Li sites of similar site energy form a 3D network suitable for fast ion conduction. The simulated diffusion coefficient and its activation energy closely match the experimental conductivities. The degree of correlation of the vacancy-type Li+ ion migration is analyzed in terms of the van Hove correlation function.