AFe 1/2 B 1/2 O3(A-Ba, Sr, Ca; B-Nb, Ta, Sb) ceramics were synthesized and temperature dependencies of the dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of the dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. The analyses of these data establish a Maxwell-Wagner mechanism as a main source for the phenomenon observed.

A metastable perovskite BiFe 0.5 Sc 0.5 O 3 synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions was obtained in two different polymorphs, antipolar P nma and polar I ma2, through an irreversible behavior under a heating/cooling thermal cycling. The I ma2 phase represents an original type of a canted ferroelectric structure where Bi 3+ cations exhibit both polar and antipolar displacements along the orthogonal [110] p and [110] p pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the P nma and the I ma2 structural modifications exhibit a long-range antiferromagnetic ordering with a weak ferromagnetic component below T N ∼ 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 I ma2 phase is described by a fifth-degree free-energy invariant and is expected to be small.

Raman spectra of sodium niobate (NaNbO 3 ) were obtained in all phases and revealed a significant disorder in the high-temperature U, T2 and T1 phases and a complicated folding of the Brillouin zone at the transitions into modulated S, R, P and N phases associated with the competitive zone-boundary soft modes (in-phase and out-of phase octahedral tilts) along the M-T-R line. An extensive Raman study combined with x-ray diffraction (XRD) and dielectric measurements confirmed the presence of the incommensurate (INC) phase in sodium niobate. XRD experiments revealed the invar effect in the temperature interval 410-460 K corresponding to the INC phase associated with rotations of the NbO 6 octahedra modulated along the b-direction. Our experiments suggest that the phase P consists of three phases: monoclinic (P m ) between 250 and 410 K, INC between 410 and 460 K, and orthorhombic (P o ) between 460 and 633 K. At the low-temperature transition to the ferroelectric rhombohedral N phase all folded modes originating from the M-and T-points of the Brillouin zone abruptly disappear, Raman spectra in the N phase become much simpler and all peaks were assigned.

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature T(c)=403  K. Above the Burn's temperature T(d)≈580  K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below T(d), a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near T(d) is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation.