Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: a Play in the Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites

Three different perovskite-related phases were isolated in the SrGa1–xScxO2.5 system: Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5. Sr2GaScO5 (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å] with complete ordering of Sc3+ and Ga3+ 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. I41/a, a = 17.517(1) Å, c = 32.830(3) Å]. 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)­O1.6–SrO0.8–(Sc/Ga)­O1.8–SrO0.8−]2.... Similar to Sr2GaScO5, this structure features a complete ordering of the Sc3+ and Ga3+ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr10Ga6Sc4O25 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 SrGa0.75Sc0.25O2.5 (x = 0.25) with a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa0.75Sc0.25O2.5·xH2O hydrate, which exhibits a proton conductivity of ∼2.0 × 10–6 S/cm at 673 K.