Sergey Ya. Istomin

Target-aimed synthesis of anion-deficient perovskites.

The brownmillerite-type A 2B 2O 5 structure is considered as a parent one, giving rise to different derivatives: layered double perovskites, A-site and anion-vacancy-ordered perovskites, and the perovskite-like compounds with crystallographic shear planes. The structural relationships and synthesis pathways for these classes of materials are discussed with particular attention to the ordering at the A or B sublattices, anion vacancy ordering, and their mutual interaction.

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 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.

Synthesis and characterisation of reduced niobates [Sr1−xLnxNb2O6, Ln = La, Nd]

Sr[sub 1-x]Ln[sub x]Nb[sub 2]O[sub 6], Ln = La (x [le] 0.4), Nd (x [le] 0.3) solid solutions with a CaTa[sub 2]O[sub 6]-type structure have been studied. No superconductivity was found in the prepared samples down to 5K. The CaTa[sub 2]O[sub 6] type crystal structure of Sr[sub 0.6]La[sub 0.4]Nb[sub 2]O[sub 6] was refined from X-ray powder diffraction data: R[sub i] = 0.054, R[sub p] = 0.117. The oxidation process of the lanthanum-containing compounds was studied. A drastic decrease of the lanthanum content, accompanied by the formation of the cation vacancies at the Sr/La site occurs during oxidation. Electrical and magnetic properties were studied.

Sr0.75Y0.25Co0.5Mn0.5O3 − y Perovskite Cathode for Solid Oxide Fuel Cells

The Sr 0.75 Y 0 . 25 Co 0.5 Mn 0.5 O 3-y , (SYCM) oxide with a cubic perovskite structure was examined as a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity, thermal expansion coefficient (TEC), and reactivity with gadolinia-doped ceria (GDC) or yttria-stabilized zirconia (YSZ) were studied. Reflections from SrZrO 3 (~6 wt %) after heat-treatment of the SYCM:YSZ mixture at 900°C for 48 h and no reaction after heat-treatment with GDC were observed. In the low and intermediate temperature region (473-873 K), the TEC value is 13.3 ppm K -1 and most closely matches the common IT-SOFC electrolyte material GDC, but the TEC value increases to 19.6 ppm K -1 at 873-1073 K. The investigations of the electrochemical properties of the model SOFCs at 900°C with the SYCM cathode show that substituting the SYCM cathode for the standard (La,Sr)Mn03 cathode improves the cell performance.

A neutron powder diffraction study of Na1−xSrxTaO3 (x=0.2 and 0.3)

Black powder samples of the reduced oxotantalates Na1−x Srx TaO3 x = 0.2 and 0.3 were prepared via solid state reactions from appropriate amounts of NaTaO3 ,S r 5Ta4O15 and Ta (99.99%) in sealed tantalum ampoules at 1500 ◦ C. They crystallize in variants of the perovskite type structure (aper). According to structural refinements from neutron powder diffraction data Na 0.8Sr0.2TaO3 crystallizes orthorhombic in the GdFeO3 type structure with unit cell parameters a ≈ √ 2 × aper ,b ≈ 2 × aper, c ≈ √ 2 × aper and space group Pnma. The structure of the more reduced phase Na0.7Sr0.3TaO3 is tetragonal with unit cell parameters a ≈ √ 2 × aper ,c ≈ 2 × aper and space group