Evidence of non‐adiabatic small polaron hopping conduction in Bi0.1A0.9MnO3 (A = Ca, Sr, Pb)

Abstract

AbstractA detailed study of the electrical transport properties of semiconducting perovskites Bi0.1A0.9MnO3 (A = Ca, Sr, Pb of increasing ionic radius ri) have been made over a wide range of temperature 80–375 K. XRD studies indicate a change of structural differences with change of ionic radius ri of A. The dc conductivity of the Sr‐ and Pb‐doped samples with larger ri is much lower than that of the Ca‐doped sample of smaller ri. Unlike the Sr‐ and Pb‐doped samples, little negative magnetoresistance is also observed in the Ca‐doped sample (below 115 K at 1.5 T field). The resistivity data below θD/2 (θD is the Debye temperature) are fitted well with the Mott's variable range hopping (VRH) model, whereas the nearest‐neighbour small polaron hopping mechanism satisfactorily fitted the high‐temperature (above θD/2 ≈ 200 K) conductivity data. The condition of the non‐adiabatic small polaron hopping mechanism is valid for all the samples. The estimated electron–phonon interaction constant γp (= 2WH/hνph) of the highly resistive Sr‐ and Pb‐doped samples are much higher (≫4) than those of the low‐resistive Ca‐doped oxide sample (≈4). Large values of γp destroy spin ordering in the Sr‐ and Pb‐doped samples, while for the lower value of γp in the Ca‐doped sample spin ordering favours high conductivity and show a negative magnetoresistance (only in the low‐temperature region). Low‐temperature (T < θD/2) frequency‐dependent ac conductivity data indicate that the conduction mechanism in the highly resistive Sr‐ and Pb‐doped samples is primarily due to hopping of Anderson‐localized charge carriers.