Lithium stuffed garnet oxides such as Li₇La₃Zr₂O₁₂ have attracted considerable attention as promising candidate for solid electrolytes for All Solid State Lithium Ion Batteries. Their high Li-ion conductivity of ca. 1x10^-3 Scm^-1, wide temperature stability window, stability against metallic lithium and compatibility with high voltage new electrode materials implies new perspectives for large-scale applications for elevated temperatures as well new cell designs for thin film chip-integrated micro-batteries. The key to a successful application of Li-garnets is to ensure high Li-ion conductivity and electrochemical stability with other battery components.

Despite the great interest on this class of materials, there is still uncertainty on the role of La- and Zr-sites co-doping on the structural stability and phase as a function of ionic radii of the dopant and Li-transport properties. Here we present extensive study on structural properties and ionic transport of the Li_6.75La_3-xM_xZr_1.75Ta_0.25O₁₂ system, where M=Er³⁺, Y³⁺, Nd³⁺, Gd³⁺, Sr²⁺, Ba²⁺, Ca²⁺. Utilizing X-Ray diffraction, Raman spectroscopy and electron microscopy we discuss crystal structure properties and stability of the materials with respect to the ionic transport properties analysed by impedance spectroscopy in wide temperature window for future electrolyte applications in all solid state batteries.