Based on our findings, we hypothesize that the Co incorporation in LLZO leads to Ga dissolution (although both have the same oxidation state and similar ionic radii) and lattice distortion that introduces high residual stresses due to lattice constraints towards the bulk. The build-up of elastic energy gets finally released by fracture. Since Co is not thermodynamically stable in LLZO structure, we believe that Co will segregate at the fracture surfaces by stressmediated diffusion. Since the Ga originally present to stabilize the cubic phase has been removed in favor of Co, LLZO undergoes a phase change to tetragonal LLZO. Moreover, the fracture of the LLZO crystal leads to a slight tilting between the new formed LLZO domains explaining the complex twinning behavior observed.