Isolating the effect of doping in the structure and conductivity of (Ln1−xMx)FeO3−δ perovskites

A series of iron perovskites with the general formula AFeO3−δ (A = Ln1−xMx; 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 <rA> ≈ 1.22 Å and cation size disorder σ2(rA) ≈ 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.