Synthesis, Phases, Structures, and Electrical Properties of Melilite-Type La1+xSr1−xGa3S6O1+0.5x Materials

Deformation flexibility of two-dimensionally connected tetrahedral network and Ga3+ ion with variable coordination are reported to be the key factor for oxide ion migration in La1+xSr1−xGa3O7+0.5x materials, which are well-known oxide ion conductors with remarkable conductivity at the temperature range 600–900 °C. In this work, La1+xSr1−xGa3S6O1+0.5x materials which also adapt melilite structure were prepared at high temperature in a quartz tube after being vacuumed. The prepared samples were then studied by complementary techniques, including X-ray diffraction (XRD), alternative current (AC) impedance spectroscopy, and density functional theory (DFT). The XRD data together with Rietveld refinement results showed that La atoms failed to replace Sr atoms in La1+xSr1−xGa3S6O1+0.5x materials, leading to two phases in the products. This was consistent with the high defect formation energy (higher than 2.76 eV) of La atoms substituting for Sr atoms. In addition, alternative current impedance spectroscopy measurements demonstrated no ionic conduction responses in these melilite La1+xSr1−xGa3S6O1+0.5x materials, agreeing well with the fact that the La atoms were not able to substitute for Sr atoms and no interstitial oxide ions were introduced in any phases of the products.