Lithium garnet oxides are considered as very promising solid electrolyte candidates for all-solid-state lithium ion batteries (SSLiBs). In this work, we present a cubic garnet-type Li5+2La3Ta2-xSmxO12 (0 <= x <= 0.55) system as a potential electrolyte for SSLiBs. Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) were employed to investigate the structural stability of Li5+2xLa3Ta2-xSmxO12: The results from PXRD and SEM suggested structural and morphological transformation as a function of dopant concentration. In addition to ti -ion transport in Li5+2xLa3Ta2-xSmxO12, the dielectric properties were also investigated in the light of electron energy loss functions, which, showed "some surface energy loss and negligible volume energy loss for the studied garnets. Surface and volume energy loss functions of a mixed conducting LiCoO2 was studied for comparison. The long-term chemical stability of one of members, Li5-3La3Ta1.85Sm0.5O12, was performed on aged sample using PXRD, SEM, and thermogravimetric analysis.