The concentration range of the stability of polar (R3c) and antipolar phases in the Bi 1Àx RE x FeO 3 (RE-La -Dy) solid solutions has been determined by X-ray study of the polycrystalline samples. Both polar and antipolar phases become less stable with a decrease of the rare earth ionic radii. It is stimulated by a reduction of the rare-earth ions polarizability with a decrease in ionic radii. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar one exhibits dominantly antiferromagnetic behavior near the polar-antipolar morphotropic boundary. The local piezoelectric response decreases with increase in antipolar phase content in the mixed polarantipolar structural state. It is suggested that the piezoelectric activity is associated with polar (R3c) phase.

Crystal structures of Bi1–x Lnx FeO3 systems (Ln = La, Nd, Eu) are studied using X‐ray and neutron diffraction, magnetization measurements as well as electron microscopy. It was shown that the unit cell of the Bi1–x Lax FeO3 system is described with the R3c space group in the concentration interval 0 < x < 0.15, with the space group Imma (0.4 < x < 0.5) and with the Pnma at x > 0.5. The mixed‐phase state was observed in the range 0.15 < x < 0.4 independent of synthesis conditions. A similar sequence of the phase transformations was observed for Bi1–x Ndx FeO3 system. In the case of Bi1–x Eux FeO3 series the unit cell is rhombohedral (R3c) at x < 0.08 and orthorhombic (Pnma) at x > 0.20, whereas no evidence for Imma phase was found. The magnetization study has revealed the spontaneous magnetization to be associated with Pnma and Imma phases that do not allow conventional ferroelectricity. The ferroelectric R3c phase exhibits metamagnetic behavior due to transition from an incommensurately modulated antiferromagnetic state to a weak ferromagnetic state. Substitution of Bi3+ with Ln3+ leads to a strong decrease of external magnetic field inducing a metamagnetic transition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of treatment conditions on the magnetic and magnetotransport properties of A-site ordered PrBaMn2O6−δ manganites is examined. The parent oxygen-stoichiometric A-site ordered PrBaMn2O6 samples were prepared from oxygen-stoichiometric A-site disordered Pr0.50Ba0.50MnO3 ones by using a ‘two-step’ synthesis method. The most significant structural feature of the A-site ordered manganites is that the MnO2 sublattice is sandwiched by two kinds of rock-salt layers, PrOx and BaO. The oxygen-stoichiometric A-site ordered PrBaMn2O6 demonstrates a ferromagnetic metal to paramagnetic insulator transition with the Curie point at about 320 K. These compounds are stable in air up to 1300 °C. The A-site ordered PrBaMn2O6 samples were next reduced in flowing argon as well as being treated under high pressure conditions. The reduction of the A-site ordered PrBaMn2O6 samples leads to the appearance of an antiferromagnetic state with a Néel point of about 140 K, while the A-site order remains. Upon high pressure treatment the degree of A-site order decreases, which reduces TC to 180 K. The magnetotransport properties of A-site ordered manganites treated under different conditions are discussed in terms of the manganese valence, oxygen content and degree of A-site order.

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