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

The effect of A-site substitution by alkaline earth metals (A=Ba, Sr, Ca) on the crystal structure and properties of the Nd(1-x)A(x)MNn(0.5)Fe(0.5)O(3-delta) (x = 0, 0.25) perovskite-like oxides were studied in the temperature range of 25-1000 degrees C in air. The Nd(1-x)A(x)Mn(0.5)Fe(0.5)O(3-delta) 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-delta and Nd0.75Ba0.25Mn0.5Fe0.5O3-delta (sp. gr. Pnma); and monoclinic unit cell for Nd0.75Ca0.25Mn0.5Fe0.5O3-(delta) and Nd0.75Sr0.25Mn0.5Fe0.5O3-delta (sp. gr. P2(1)/n). The refinement results for the single-phase samples revealed that the unit cell volume decreased while decreasing the size of A-dopant in Nd(1-x)A(x)Mn(0.5)Fe(0.5)O(3-delta). Oxygen content and oxidation state of Mn in Nd(1-x)A(x)Mn(0.5)Fe(0.5)O(3-delta) 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 Nd(1-x)A(x)Mn(0.5)Fe(0.5)O(3-delta) samples was almost independent of temperature even at high temperatures except for NdMn0.5Fe0.5O3-delta. The thermal expansion coefficient (TEC) for Nd0.75Ba0.25Mn0.5Fe0.5O3-delta was found to be 10.1 x 10(-6) K-1 in the range of 25-800 degrees 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-delta at 1000 degrees 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.