Synthesis and characterization of perovskite-type SrxY1 xFeO3 d (0.63rxo1.0) and Sr 0.75 Y 0.25 Fe 1 y M y O 3 d (M¼Cr, Mn, Ni), (y ¼0.2, 0.33, 0.5)

Oxygen-deficient ferrates with the cubic perovskite structure SrxY1 � xFeO3� d were prepared in air (0.71r xr 0.91) as well as in N2 (x ¼0.75 and 0.79) at 1573 K. The oxygen content of the compounds prepared in air increases with increasing strontium content from 3 � d¼ 2.79(2) for x ¼0.75 to 3 � d¼ 2.83(2) for x¼ 0.91. Refinement of the crystal structure of Sr0.75Y0.25FeO2.79 using TOF neutron powder diffraction (NPD) data shows high anisotropic atomic displacement parameter (ADP) for the oxygen atom resulting from a substantial cation and anion disorder. Electron diffraction (ED) and highresolution electron microscopy (HREM) studies of Sr0.75Y0.25FeO2.79 reveal a modulation along / 100 Sp with G7 � 0.4/ 100 Sp indicating a local ordering of oxygen vacancies. Magnetic susceptibility measurements at 5–390 K show spin-glass behaviour with dominating antiferromagnetic coupling between the magnetic moments of Fe cations. Among the studied compositions, Sr0.75Y0.25FeO2.79 shows the lowest thermal expansion coefficient (TEC) of 10.5 ppm/K in air at 298–673 K. At 773– 1173 K TEC increases up to 17.2 ppm/K due to substantial reduction of oxygen content. The latter also results in a dramatic decrease of the electrical conductivity in air above 673 K. Partial substitution of Fe by Cr, Mn and Ni according to the formula Sr0.75Y0.25Fe1� yMyO3� d (y¼ 0.2, 0.33, 0.5) leads to cubic perovskites for all substituents with y ¼0.2. Their TECs are higher in comparison with un-doped Sr0.75Y0.25FeO2.79. Only M ¼Ni has increased electrical conductivity compared to un-doped Sr0.75Y0.25