Instability of the Ionic Conductor Li₆BaLa₂B₂O₁₂ (B = Nb, Ta): Barium Exsolution from the Garnet Network Leading to...

The instability of the two garnets Li₆BaLa₂B₂O₁₂ (B = Nb, Ta) has been studied on samples prepared in powder form by solid-state reaction. For this study, we coupled different techniques: powder X-ray diffraction, IR spectrometry, thermal analysis, transmission electron microscopy, and complex impedance spectroscopy. We showed that in ambient air and at low temperature (<150 °C), a spontaneous Li⁺/H⁺ exchange occurs. At higher temperature (500-700 °C), a progressive exsolution of the barium from the garnet framework is observed, leading to the formation of a second garnet, BaCO₃, and a 3D cubic perovskite. To conclude this work, we studied the impact of barium exsolution on the ionic conductivity measured by complex impedance spectroscopy. We observed a significant decrease in the starting bulk conductivity (60%) when the pellet is heated at 500 °C for 5 h.