All samples were stored in an argon glove box to prevent proton-Li exchange in the garnet. Phase analysis was performed by X-ray diffraction (XRD) using a Bruker D8 diffractometer with Cu source from 15 -80 2θ with a step size of 0.018°. Variable temperature measurements were conducted in a similar manner on a Bruker D8 instrument equipped with an Anton Parr heating stage from 50°C up to a maximum of 1000°C.