Disclosed is a solid state electrolyte comprising a compound of Formula 1 Li7−a*α−(b−4)*β−xMaαLa3Hf2−βMbβO12−x−δXx  (1) wherein Ma is a cationic element having a valence of a+; Mb is a cationic element having a valence of b+; and X is an anion having a valence of −1, wherein, when Ma includes H, 0≤α≤5, otherwise 0≤α≤0.75, and wherein 0≤β≤1.5, 0≤x≤1.5, and (a*α+(b−4)β+x)>0, 0≤δ≤1.