The Tetragonal-Cubic Phase Transition of Lithium Garnet Oxide Li7La3Zr2O12
Authors: Wang Yuxing , Lai Wei 
Published: February 2020

Lithium-stuffed garnets have become one of the most studied lithium ionic conductors in the oxide system owing to the high ionic conductivity and good stability [1, 2]. The compound Li7La3Zr2O12 (LLZ) was reported to crystallize in either the cubic or tetragonal symmetries in different studies [3, 4]. The tetragonal phase transformation was accompanied by a complete ordering of Li. It has been proved that cation doping could stabilize the cubic phase.[5] Alternatively, H2O and CO2 were shown to be responsible for phase transition in aged LLZ samples.[6] In this study, we examine the effect of Ta doping and H2O/CO2 exposure on the phase transition of LLZ materials using diffraction techniques and in-situ impedance measurement. It’s important to note that the strict control over synthesis condition is a necessity for carrying out such study and we took cautions to ensure purity.
We have shown computationally that the Li-ordering may be driven by the increasing first neighbor Li-Li repulsion as Li content increases.[7] Therefore, any cation doping strategy that results in a lower Li content has the potential of disrupting Li-ordering thus transforming the phase. As shown from the X-ray diffraction results on the series of carefully prepared Ta-doped LLZ (Li7-xLa3Zr2-xTaxO12, x=0-0.6, Al-free, minimal air exposure), the intermediate compositions (x=0.1-0.5) consist of coexisting cubic phase and tetragonal phase possibly with different chemical makeup, contradicting results in previous studies.[8] It is likely that the high conductivity phase Li6.75La3Zr1.75Ta0.25O12 (LLZT0.25) is not thermodynamically stable. Synchrotron experiments (APS proposal accepted) are expected to help us determine the phase fraction and exact composition. Two possibilities could explain the cubic phase of high Li content compositions in previous study: 1) unintentional doping of Al from firing medium which further lowered Li content; 2) phase transition induced by extended exposure to the air. To evaluate the effects of moisture and CO2 on phase transition and conductivity of LLZ, we measured the impedance change of impurity-free LLZ pellet in-situ under H2O or CO2 gas flow at different temperatures and characterize the phase ex-situ. In the control group, conductivities of LLZ sample up to 750 °C under argon were measured and the phase transition at around 630 °C was clearly evident from the Arrhenius plot. On the other hand, tetragonal LLZ transformed to cubic phase upon exposure to moisture at 250 °C or CO2 at 120 °C. Impedance measurement indicated that despite having cubic symmetry, the transformed LLZ had a lower conductivity than the initial phase at room temperature in both cases. It should be noted that the phase transition of LLZ under argon and under H2O or CO2 are different in nature: the former is first order phase transition due to increasing entropic contribution; the latter may be due to compositional changes, for instance, removal of Li ions and/or incorporation of protons in garnets.

Figure 1. (a) Arrhenius plot of LLZ under argon flow; (b) X-ray diffraction patterns showing the tetragonal-to-cubic phase transition induced by H2O and CO2; (c) X-ray diffraction patterns of the series Li7-xLa3Zr2-xTaxO12, x=0-0.6.
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