A-site substitution effect on crystal structure and properties of Nd1−xAxMn0.5Fe0.5O3−δ (ACa, Sr, Ba; x = 0, 0.25)

The effect of A-site substitution by alkaline earth metals (ABa, Sr, Ca) on the crystal structure and properties of the Nd1−xAxMn0.5Fe0.5O3−δ (x = 0, 0.25) perovskite-like oxides were studied in the temperature range of 25-1000 °C in air. The Nd1−xAxMn0.5Fe0.5O3−δ samples were synthesized by the citrate-nitrate combustion technique. X-ray powder diffraction (XRPD) patterns were refined by the Rietveld method using orthorhombic unit cell for NdMn0.5Fe0.5O3−δ and Nd0.75Ba0.25Mn0.5Fe0.5O3−δ (sp. gr. Pnma); and monoclinic unit cell for Nd0.75Ca0.25Mn0.5Fe0.5O3−δ and Nd0.75Sr0.25Mn0.5Fe0.5O3−δ (sp. gr. P21/n). The refinement results for the single-phase samples revealed that the unit cell volume decreased while decreasing the size of A-dopant in Nd1−xAxMn0.5Fe0.5O3−δ. Oxygen content and oxidation state of Mn in Nd1−xAxMn0.5Fe0.5O3−δ increased in the row: Nd0.75Ca0.25Mn0.5Fe0.5O2.81, Nd0.75Sr0.25Mn0.5Fe0.5O2.85, Nd0.75Ba0.25Mn0.5Fe0.5O2.93 at all temperature studied. Oxygen non-stoichiometry of the Nd1−xAxMn0.5Fe0.5O3−δ samples was almost independent of temperature even at high temperatures except for NdMn0.5Fe0.5O3−δ. The thermal expansion coefficient (TEC) for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ was found to be 10.1 × 10−6 K−1 in the range of 25-800 °C. Temperature dependencies of total conductivity possessed semiconductor-type behavior and were interpreted within the small polaron hopping mechanism. The maximum value of conductivity (25 S/cm) was obtained for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ at 1000 °C in air. The Seebeck coefficient values decreased with temperature from positive to negative values indicating that both electrons and electron holes may contribute to the electric conductivity of the oxides.