Synthesis and Raman micro-spectroscopy investigation of Li7La3Zr2O12

The Li+ ion conductor Li7La3Zr2O12 was synthesized by solid state reaction as a ceramic with tetragonal and cubic crystal structure. The synthesis parameterswere varied for these polycrystalline samples, e.g. the starting materials for Li and Zr, the cruciblematerials and the thermal treatments. Product formation is sensitive to the ZrO2 starting powder aswell as the crucible material due to reactions between the powdermixture and the cruciblewall. In this study, only the use of an alumina crucible or the addition of alumina resulted in the successful preparation of cubic Li7La3Zr2O12. For single-crystal preparation, flux growth experiments were carried out at 1000 and 1200 °C either in alumina crucibles lined with a gold foil or in magnesia crucibles and using Li2CO3 (Li2O) as flux. The small crystals were separated from the solidified flux by washing larger lumps in water. Irrespective of the additions of alumina, only tetragonal micro-crystals of Li7La3Zr2O12 were obtained. These micro-crystals were studied by micro-Raman scattering spectroscopy. Polarized Raman spectra were recorded either in nearly backscattering or 90° geometry. The majority of Raman modes expected for the tetragonal phase (space group I41/acd, Z=8) were satisfactorily displayed. So far, it has not been possible to make unambiguous symmetry assignments of the observed peaks because of the unknown orientation of the micro-crystals.