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arXiv 1209.0690cond-mat.mtrl-sci
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Li7La3Zr2O12结构相变的起源

Origin of the structural phase transition in Li7La3Zr2O12

N. Bernstein, M. D. Johannes, Khang Hoang

摘要:

石榴石型Li7La3Zr2O12(LLZO)是一种具有低电导率四方相和高电导率立方相的固体电解质材料。使用密度泛函理论和可变细胞形状的分子动力学模拟,我们表明,四方相稳定性是依赖于同时订购的锂离子的锂亚晶格和体积保持四方扭曲,减轻内部结构应变。超价掺杂将空位引入到Li亚晶格中,增加整体熵并降低来自有序化的自由能增益,最终稳定立方相。我们表明,立方相稳定性的临界温度降低,作为锂空位浓度(掺杂剂水平)的提高,并激活跳跃的锂离子从一个晶体位置到另一个总是伴随着过渡。通过确定实现高电导率相的相关机制和临界浓度,这项工作显示了如何使用靶向合成来改善电解性能。

英文摘要:

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.

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