In situ high-temperature neutron diffraction study of A-site deficient perovskites with transition metals on the B-sublattice and structure- conductivity correlation

Neutron powder diffraction has been used for the first time to explore crystal structure and phase transformations in the Mn and Sr doped A-site deficient La_0.95Ni_0.6Fe_0.4O₃ (LSNFM series) as a function of temperature and upon different doping regimes. The range of A-site deficiency in the simple perovskite structure with Ni, Fe, and Mn cations on the B-sublattice is very low, and all the A-site deficient nominal compositions explored can be represented as a mixture of the cation stoichiometric perovskite phase with rhombohedral structure and NiO as a secondary phase in the temperature range of 45-460 °C. The a lattice parameter and the unit cell volume of the perovskite phase in the LSNFM compositions increase upon Sr and Mn doping. Exsolution of NiO from the perovskite phase in the Mn doped La_0.95Ni_0.6Fe _0.4O₃ with the rise in the temperature was stronger compared to that in the LSNFM compositions with a low level of Sr doping (0.004 ≥ x ≥ 0.042). The local distortions of the [NiO₆] octahedra, existing most probably due to the mismatch in the Ni³⁺-O and Ni ²⁺-O bond lengths, seems to be compensated through the incorporation of Sr cations with a larger ionic radius onto the A-sublattice of the perovskite phase. Reversible high-temperature phase transition from rhombohedral (R3̄c) to cubic (Pm3̄m) symmetry occurs above 530-600 °C within the cation stoichiometric perovskite phase in all the A-site deficient compositions. The increase in the degree of Sr substitution in the LSNFM series diminishes the high temperature phase transition within the cation stoichiometric perovskite phase. The change in the conductivity of the Mn and Sr substituted compositions at room temperature can be explained through the increase in the lattice parameters of the perovskite phase. At high temperatures there is no correlation between the evolution of the phase equilibria and the transport properties of the compositions in the LSNFM series. Mechanism of the band formation and variation of its components with the temperature have been discussed.