Isolating the effect of doping in the structure and conductivity of (Ln_1-xM_x)FeO_3-δ perovskites

A series of iron perovskites with the general formula AFeO_3-δ (A = Ln_1-xM_x; Ln = La, Nd and/or Pr; M = Sr and/or Ca) has been prepared by conventional solid state reaction. In order to isolate the effect of divalent dopant concentration from the A cation steric effects, the whole group has a fixed mean A cation radius <r_A> ≈ 1.22 Å and cation size disorder σ²(r_A) ≈ 0.003 Å² but variable doping x. The structure changes with x from orthorhombic (0.2 ≤ x ≤ 0.4) through rhombohedral (0.5 ≤ x ≤ 0.7) to a mixture of rhombohedral and cubic for x = 0.8. SEM images show that the average grain size increases with alkaline-earth content. All samples show a systematic dependence of conductivity upon doping, which can be directly related to the concentration of charge carriers. Conductivity fits indicate a p-type semiconducting, small-polaron hopping mechanism, in which the activation energies remain similar for all doping levels, x, throughout the series.