Cubic phases of garnet-type Li7La3Zr2O12: the role of hydration

Abstract

We address the controversial issue of the structural stability of Li₇La₃Zr₂O₁₂ garnets, focusing on the mechanisms that result in the transformation from tetragonal to cubic symmetry. We show that undoped tetragonal Li₇La₃Zr₂O₁₂ not exposed to humidity at any moment undergoes a reversible phase transition to cubic symmetry at T_c ≃ 645 °C that we ascribe to lithium dynamic effects. On the other hand, a close correlation has been found between the appearance of a cubic phase between 100 and 200 °C in X-ray diffractograms and the presence of water, either in the atmosphere in which experiments are performed or already in the starting material. The natures of the high and low-temperature cubic garnets are totally different: the one found above the phase transition does not involve any change in the stoichiometry, whereas the cubic phase formed at low temperature is a hydrated, lithium defective phase, due to the combined effect of water insertion into the garnet structure and the H⁺/Li⁺ exchange mechanism. Differences in the actual compositions of the samples depending on their thermal history are corroborated by TG-MS experiments. Chemical reactions and phases formed along the thermal evolution are elucidated with the help of Raman spectroscopy.