title: Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: a play in the octahedra to tetrahedra ratio in oxygen-deficient perovskites.
abstract: Three different perovskite-related phases were isolated in the SrGa(1-x)Sc(x)O(2.5) system: Sr(2)GaScO(5), Sr(10)Ga(6)Sc(4)O(25), and SrGa(0.75)Sc(0.25)O(2.5). Sr(2)GaScO(5) (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 Sc(3+) and Ga(3+) over octahedral and tetrahedral positions, respectively. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) (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) 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 Sr(10)Ga(6)Sc(4)O(25) 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 Sr(2)GaScO(5), this structure features a complete ordering of the Sc(3+) and Ga(3+) cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr(10)Ga(6)Sc(4)O(25) is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc(3+) by Ga(3+) leads to the formation of the cubic perovskite phase SrGa(0.75)Sc(0.25)O(2.5) (x = 0.25) with a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa(0.75)Sc(0.25)O(2.5)·xH(2)O hydrate, which exhibits a proton conductivity of ∼2.0 × 10(-6) S/cm at 673 K.
authors.affiliation: Institute of Problems of Chemical Physics, RAS, Acad. Semenov avenue 1, 142432 Chernogolovka, Moscow Region, Russia.
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Chernov
authors.forename: Yuri A
authors.initials: YA
authors.lastname: Dobrovolsky
authors.forename: Sergey Ya
authors.initials: SY
authors.lastname: Istomin
authors.forename: Evgeny V
authors.initials: EV
authors.lastname: Antipov
authors.forename: Jekabs
authors.initials: J
authors.lastname: Grins
authors.forename: Gunnar
authors.initials: G
authors.lastname: Svensson
authors.forename: Nadezhda V
authors.initials: NV
authors.lastname: Tarakina
authors.forename: Artem M
authors.initials: AM
authors.lastname: Abakumov
authors.forename: Gustaaf
authors.initials: G
authors.lastname: Van Tendeloo
authors.forename: Sten G
authors.initials: SG
authors.lastname: Eriksson
authors.forename: Seikh M H
authors.initials: SM
authors.lastname: Rahman