In this study we investigate different types of interfaces in a multiferroic BiFeO₃. By means of atomistic modelling we show that some configurations of interfaces can greatly enhance the permittivity by hosting terahertz-range collective polar fluctuations. The key result is that the permittivity is modified in the THz range. This fundamental finding and its understanding can be used in a design of new materials [ J. Hlinka, M. Pasciak, S. Körbel, and P. Márton, Phys. Rev. Lett. 119, 057604 (2017) ].

A central mode excitation in the THz range, due to anharmonic dynamics of cations, shows critical behaviour towards T~400 K. At lower frequencies, around 10 GHz, another excitation (ν_DW) appears below T_m~330K, which is related to the development of ferroelectric microdomains.
In addition, several relaxations below the phonon frequencies, play an essential role in the dielectric response of the crystal. The main contribution to the permittivity comes from a strong relaxation (ν₀₁) present in the GHz range at high temperatures which slows down on cooling following the Arrhenius law. A second relaxation with lower frequency (ν₀₂) slows down, as well, contributing to the permittivity mainly near T_m. Both these relaxations can be assigned to polar fluctuations, probably flipping and breathing of polar nanodomains.

Altogether, the four mechanisms explain, above the kHz range, the ferroelectric transition in SBN-81 as well as its relaxor character, which differs from the behaviour displayed by SBN-61 and lead-based relaxors.

We have synthesized and characterized mesomorphic behaviour of a new type of bent-core dimers in which bent-core molecules are connected core-to-core via the alkoxy chain. Interestingly, despite short linkage connecting the mesogenic cores, which should stiffen the molecular structure, the liquid crystalline properties characteristic for bent-core mesogens have been observed. Dimers are more stable in comparison with analogous monomers and the type of mesophases strictly depends on the length of the terminal chains. The shortest studied homologues formed the intercalated smectic A phase, For the dimers with intermediate terminal chains, the columnar phase appears, built of small layer fragments arranged into body centred 2-dimensional crystallographic lattice. The longest homologues exhibit the SmCP phase with antiferroelectric type of switching in the applied electric field [ M. Horčic, J. Svoboda, V. Novotná, D. Pociecha, and E. Gorecka, Chem. Commun. 53, 2721 (2017)].

The lowest-frequency phonon of the E symmetry polarized perpendicular to the local dipole moments undergoes softening towards T* ≈ 400 K, which gives evidence about a local structural phase transition. This softening is also responsible for previously observed high temperature dependence of permittivity, which follows the Curie-Weiss law with the same critical temperature even without any anomaly in the low-frequency permittivity [D. Nuzhnyy et al, Phys. Rev. B 96, 174113 (2017)].