Study on the structural and electrical properties of the perovskite oxide SrMn_0.5Nb_0.5O_{3- d}

The new non-stoichiometric mixed perovskite SrMn0.5Nb0.5O3-delta has a cubic double perovskite structure similar to that of Sr2CrNbO6 with space group Fm (3) over barm (225), a = 7.9338(3) Angstrom, V = 499.39(6) Angstrom(3) according to X-ray diffraction. The material is redox stable and maintains its structure in a reducing atmosphere. After reducing in 5% H-2 at 900 degreesC for 6 hours, SrMn0.5Nb0.5O3-delta still exhibits a cubic structure with space group Pm (3) over barm (221), a = 4.0022(5) Angstrom, V = 64.10(8) Angstrom(3). A lattice volume expansion of 2.7% was observed during the reduction. TGA analysis indicates SrMn0.5Nb0.5O3-delta loses 0.125 oxygen per formula unit from 500 to 950 degreesC in 5% H,. This weight change is consistent with a reduction from SrMn0.5Nb0.5O3 to SrMn0.5Nb0.5O2.875. The morphology of this material does not significantly change on reduction according to SEM observation. A.c. impedance measurements indicate that electronic conduction is probably dominant both in air and 5% H-2.(.) The conductivities of this material in air, humidified 5% H-2 and 5%, H-2 were 1.23, 6.4 x 10(-2) and 3.1 x 10(-2) S cm(-1) respectively at 900 degreesC. The decrease of d.c. conductivity of SrMn0.5Nb0.5O3-delta at p(o2), below 10(-12) atm indicates p-type electronic conduction. The higher apparent conduction activation energy and lower conductivity in H-2 than in air may be due to the contribution of lattice expansion which results in poorer overlap of both sigma and Tu bonds, which makes the hopping of electron holes more difficult. The d.c. conductivity of SrMn0.5Nb0.5O3-delta at low p(o2) exhibits a p(o2)(1/6) dependence that is interpreted by a simple defect chemistry model.