La doped effects on structure and thermoelectric properties of Sr ₂ MnMoO ₆ double-perovskite oxides

The thermoelectric properties of the double-perovskite-type oxide Sr ₂ MnMoO ₆ were investigated in terms of La doping at the A-site of the oxide. The Bragg reflection of Sr _2-x La _x MnMoO ₆ was indexed with a monoclinic unit cell and the structure was determined in the monoclinic space group P2 ₁ /n, due to the good agreement between the observed and calculated patterns after Rietveld refinement. The electrical conductivity of the oxides, σ, showed a semiconductor characteristic, increasing monotonically from 0.1 to 0.5S/cm at room temperature to 5-50S/cm at 1250K. The Seebeck coefficient, S, indicated the oxides to be of n-type. The absolute values of S increased with increasing temperature. These absolute values increased at just x=0.1, and then drastically decreased with increasing x ranging from 0.3 to 1.0. The power factor of the oxides, S ² σ, increased with increasing temperature; Sr _1.9 La _0.1 MnMoO ₆ showed approximately 0.12mW/mK ² at 1250K, which is the largest value among all the samples examined in this study. The thermal conductivities of the oxides showed a temperature independent behavior, ranging approximately 0.6-1.0W/mK, and the values were significantly low. The dimensionless figure of merit, ZT=S ² σT/κ, increased with the increasing temperature. The highest ZT value of 0.13 was observed for Sr _1.9 La _0.1 MnMoO ₆ at 1100K.