Ceria based solid electrolytes

The present invention relates to compositions of matter represented by the general formula LnLn'O wherein Ln is selected from the group consisting of Sm, Gd, Y, and mixtures thereof; Ln' is selected from the group consisting of La, Pr, Nd, Pm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, A is selected from the group consisting of Mg, Ca, Sr and Ba, 0.05 ≤ x ≤ 0.25, 0 ≤ x' ≤ 0.25, 0 ≤ y ≤ 0.03, 0.001 ≤ z ≤ 0.03, 0.05 ≤ x + x' ≤ 0.25, 0.001 ≤ y + z ≤ 0.03, wherein δ is a number which renders the composition of matter charge neutral. The compositions can be formed into sintered bodies suitable for use as solid electrolytes in devices including solid-state oxygen generators. Such sintered bodies have greater than 95% theoretical density at temperatures at or below 1600°C, and can be produced by a solid-state method.