Study on the structural and electrical properties of the double perovskite oxide SrMn0.5Nb0.5O3−δ

Abstract

The new non-stoichiometric mixed perovskite SrMn_0.5Nb_0.5O_3−δ has a cubic double perovskite structure similar to that of Sr₂CrNbO₆ with space group Fmm (225), a = 7.9338(3) Å, V = 499.39(6) ų according to X-ray diffraction. The material is redox stable and maintains its structure in a reducing atmosphere. After reducing in 5% H₂ at 900 °C for 6 hours, SrMn_0.5Nb_0.5O_3−δ still exhibits a cubic structure with space group Pmm (221), a = 4.0022(5) Å, V = 64.10(8) ų. A lattice volume expansion of 2.7% was observed during the reduction. TGA analysis indicates SrMn_0.5Nb_0.5O_3−δ loses 0.125 oxygen per formula unit from 500 to 950 °C in 5% H₂. This weight change is consistent with a reduction from SrMn_0.5Nb_0.5O₃ to SrMn_0.5Nb_0.5O_2.875. 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% H₂. The conductivities of this material in air, humidified 5% H₂ and 5% H₂ were 1.23, 6.4 × 10⁻² and 3.1 × 10⁻² S cm⁻¹ respectively at 900 °C. The decrease of d.c. conductivity of SrMn_0.5Nb_0.5O_3−δ at p_O2 below 10⁻¹² atm indicates p-type electronic conduction. The higher apparent conduction activation energy and lower conductivity in H₂ than in air may be due to the contribution of lattice expansion which results in poorer overlap of both σ and π bonds, which makes the hopping of electron holes more difficult. The d.c. conductivity of SrMn_0.5Nb_0.5O_3−δ at low p_O2 exhibits a p_O2^1/6 dependence that is interpreted by a simple defect chemistry model.