A-Site Order-Disorder Evolution in Double Manganites RBaMn2O6 (R = Pr, Nd).

Structural, magnetic and optical properties of double manganites PrBaMn₂O₆ and NdBaMn₂O₆ are studied depending on the degree of ordering in the A-position. It is shown that, annealing induced disordering goes in an initial stage throw formation of a two-phase state: a phase with a high A-site ordering with T_C ∼ 280–300 K and a phase with a low ordering of ~30% with significantly lower T_C coexist. In partially ordered PrBaMn₂O₆ samples there are metal-insulator transitions near T_C of the each coexisting phases. In partially ordered NdBaMn₂O₆ in a highly A-site ordered high-temperature phase, there is no metal-insulator transition due to the destruction of the channels for metallic conductivity formed by the ordering of the x²‒y² orbitals. If a metal-insulator transition takes place, the application of a magnetic field leads to the effect of magnetotransmission near T_C, which is associated with a change in the concentration of delocalized charge carriers. [ABSTRACT FROM AUTHOR]