Origin of the structural phase transition in Li7La3Zr2O12

TL;DR

This study uncovers the atomic-level mechanisms behind the phase transition in LLZO, revealing how Li ion ordering, structural strain, and doping influence the transition from low to high conductivity phases, guiding improved synthesis.

Contribution

It provides a detailed atomic-scale understanding of the phase transition in LLZO, highlighting the roles of Li ordering, structural distortion, and doping effects, which was previously not well understood.

Findings

Li ion ordering stabilizes the tetragonal phase.

Doping introduces vacancies that favor the cubic phase.

Transition involves Li ion hopping and is affected by dopant levels.

Abstract

Garnet-type Li7La3Zr2O12 (LLZO) is a solid electrolyte material with a low-conductivity tetragonal and a high-conductivity cubic phase. Using density-functional theory and variable cell shape molecular dynamics simulations, we show that the tetragonal phase stability is dependent on a simultaneous ordering of the Li ions on the Li sublattice and a volume-preserving tetragonal distortion that relieves internal structural strain. Supervalent doping introduces vacancies into the Li sublattice, increasing the overall entropy and reducing the free energy gain from ordering, eventually stabilizing the cubic phase. We show that the critical temperature for cubic phase stability is lowered as Li vacancy concentration (dopant level) is raised and that an activated hop of Li ions from one crystallographic site to another always accompanies the transition. By identifying the relevant mechanism and critical concentrations for achieving the high conductivity phase, this work shows how targeted synthesis could be used to improve electrolytic performance.