As outlined above, the total Co content in tetragonal LLZO scales with the tetrahedral deviation Dtetr from cubic symmetry. Astonishing, the two crystals with largest Dtetr values (extracted close to the LCO|LLZO interface) showed almost no Co incorporation (see Figure , Table ). After full refinement with anisotropic atomic displacement parameters for the framework atoms (La, Zr and O) and isotropic refinement of the Li1, Li2 and Li3 positions, a remaining residual electron density is found in all tetragonal samples, except those with very low Co contents. The residual is located at a position of ~ 0.38, 0, ¼ which is very close to the Li1 12a position in cubic 𝐼4 & 3𝑑 at 3 /8, 0, ¼. This position is in between the Li2 (16f) and Li3 (32g) position of tetragonal crystals and may be seen as a kind of positional disorder in due course of beginning transformation to cubic symmetry. As mentioned, the anisotropic distortion of LLZO towards a tetragonal phase, when the Co decreases, contrasts with our expectations.