A metastable perovskite ${\text{BiFe}}_{0.5}$ ${\text{Sc}}_{0.5}$ ${\text{O}}_3$ synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions was obtained in two different polymorphs, antipolar $Pnma$ and polar $Ima2$, through an irreversible behavior under a heating/cooling thermal cycling. The $Ima2$ phase represents an original type of a canted ferroelectric structure where Bi${}^{3+}$ cations exhibit both polar and antipolar displacements along the orthogonal ${\left[110\right]}_p$ and ${\left[1\overline{1}0\right]}_p$ pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the $Pnma$ and the $Ima2$ structural modifications exhibit a long-range antiferromagnetic ordering with a weak ferromagnetic component below $T_N\sim 220$ K. Analysis of the coupling between the dipole, magnetic, and elastic order parameters based on a general phenomenological approach revealed that the weak ferromagnetism in both phases is mainly caused by the presence of the antiphase octahedral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar $Ima2$ phase is described by a fifth-degree free-energy invariant and is expected to be small.

Ceramic samples of a new perovskite multiferroic PbFe_0.5Sb_0.5O₃ (PFS) with a differing degree S of long-range compositional ordering of the Fe³⁺ and Sb⁵⁺ cations were fabricated by a variation of the high-pressure-synthesis conditions. The S values estimated from the XRD data varied from 0.93 down to 0.17. The unit cell parameter was found to decrease with increasing S. In line with these results the fraction of the singlet component in the Mössbauer spectra was found to decrease with decreasing S. However, the temperatures and diffusion of dielectric permittivity maxima as well as the temperature of magnetic phase transition determined from Mössbauer data do not change substantially with S. These results imply that PFS remains highly ordered at the nanoscale.

Perovskite (1-x)PbFe_0.5Nb_0.5O₃ - xPbFe_0.5Sb_0.5O₃ solid solutions have been obtained for the first time using a high-pressure synthesis method. X-ray diffraction studies revealed a gradual increase of long-range compositional order degree S in the arrangement of Fe³⁺ and Sb⁵⁺(Nb⁵⁺) cations beginning from x ≈ 0.6. Room-temperature Mössbauer spectra of compositions with S > 0.6 besides doublet include also a singlet component; its fraction increases with S and reaches ≈0.8 for pure PbFe_0.5Sb_0.5O₃. Temperature dependences of the relative intensity of doublet (singlet) component of Mössbauer spectra exhibit two anomalies which are supposed to correspond to the antiferromagnetic phase transition temperatures in the regions with differing S values. Magnetic phase transition in PbFe_0.5Sb_0.5O₃ takes place at ≈ 25 K.

Magnetic susceptibility of two types of perovskite compounds A(Fe_0.5M_0.5)O₃ A=Ba,Sr with M=Nb and Sb differing in the degree of chemical ordering was studied between 2 and 380 K. In the low temperature region, these compounds exhibit antiferromagnetic and spin-glass transition at T = 16−30 K. For the ordered compounds (M = Sb) the inverse susceptibility depends on T linearly up to the highest temperatures used. This enables the determination of the Weiss temperature θ and effective moment of the Fe³⁺ ion expressed by a number of Bohr magnetons, p_eff. In contrast to it, the compounds with M = Nb are characterized by a higher degree of the glassines and by a concave character of the inverse susceptibility vs T dependence in the high temperature region. In these cases, the values of p_eff and θ could be estimated only very roughly by extrapolation to higher temperatures. The observed behavior was qualitatively explained from the Curie-Weiss law taking into account a distribution of the values θ, which reflects the presence of different exchange interactions between the Fe³⁺ ions. The disordered samples (M = Nb) exhibit a roughly two times larger $|\theta |/T_N$ ratio values as compared to the ordered (M = Sb) ones.