Sr2GaScO5, Sr10Ga6Sc 4O25, and SrGa0.75Sc0.25O 2.5: A play in the octahedra to tetrahedra ratio in oxygen-deficient perovskites

Article abstract of DOI:10.1021/ic202236h

Three different perovskite-related phases were isolated in the SrGa _1-xSc_xO_2.5 system: Sr₂GaScO ₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5. Sr₂GaScO ₅ (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) A, b = 15.1594(1) A, and c = 5.70926(4) A] with complete ordering of Sc³⁺ and Ga ³⁺ over octahedral and tetrahedral positions, respectively. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. I4 ₁/a, a = 17.517(1) A, c = 32.830(3) A]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ can be described as a stacking of eight perovskite layers along the c axis...[-(Sc/Ga)O_1.6-SrO_0.8-(Sc/Ga)O_1.8- SrO_0.8-]₂.... Similar to Sr₂GaScO₅, this structure features a complete ordering of the Sc³⁺ and Ga ³⁺ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr ₁₀Ga₆Sc₄O₂₅ is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc³⁺ by Ga³⁺ leads to the formation of the cubic perovskite phase SrGa_0.75Sc_0.25O _2.5 (x = 0.25) with a = 3.9817(4) A. This compound incorporates water molecules in the structure forming SrGa_0.75Sc _0.25O_2.5·xH₂O hydrate, which exhibits a proton conductivity of ～2.0 × 10^-6 S/cm at 673 K.