Crystal structure and magnetic properties of Ba-ordered manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd)

The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd) with a cation ratio Ln3+/Ba2+ ≫ 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln0.70Ba0.30MnO3 synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size <D> of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O2] = 10−4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn2O5 phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln2O3 (P $$\bar 3$$ m1, Z = 1) and (3) MnO (Fm $$\bar 3$$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size <D> = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn2O5 exhibit ferrimagnetic properties with Néel temperatures T N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn2O6, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln0.90Ba0.10MnO3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.