Phase conversions in calcium manganites with changing Ca/Mn ratios and their influence on the electrical transport properties

Abstract

The phase-interconversions between the spinel-, brownmillerite-, defect rocksalt and perovskite-type structures have been investigated by way of (i) introducing deficiency in A-sites in Ca_xMn_2−xO₃ (0.05 ≤ × ≤ 1) i.e., by varying Ca/Mn ratio from 0.025 to 1 and (ii) nonstoichiometric CaMnO_3−δ (CMO) with 0.02 ≤ δ ≤ 1. The temperature dependence of resistivity (ρ–T) have been investigated on nonstoichiometric CaMnO_3−δ (undoped) as well as the CMO substituted with donor impurities such as La³⁺, Y³⁺, Bi³⁺ or acceptor such as Na¹⁺ ion at the Ca-site. The ρ–T characteristics of nonstoichiometric CaMnO_3−δ is strongly influenced by oxygen deficiency, which controls the concentration of Mn³⁺ ions and, in turn, affects the resistivity, ρ. The results indicated that the substitution of aliovalent impurities at Ca-site in CaMnO₃ has similar effects as of CaMnO_3−δ (undoped) annealed in atmospheres of varying partial pressures whereby electron or hole concentration can be altered, yet the doped samples can be processed in air or atmospheres of higher $P_{{{\text{O}}_{ 2} }}$. The charge transport mechanisms of nonstoichiometric CaMnO_3−δ as against the donor or acceptor doped CaMnO₃ (sintered in air, $P_{{{\text{O}}_{ 2} }}$ ~ 0.2 atm) have been predicted. The ρ (T) curves of both donor doped CaMnO₃ as well as non-stoichiometric CaMnO_3−δ, is predictable by the small polaron hopping (SPH) model, which changes to the variable range hopping (VRH) at low temperatures whereas the acceptor doped CaMnO₃ exhibited an activated semiconducting hopping (ASH) throughout the measured range of temperature (10–500 K).