The project will study the high frequency ferroelectric response (mainly above 1 MHz) and lattice dynamcs of several relaxor and ferroelectric materials, in order to understand the important microwave range (1-100 GHz), where relaxation processes at the mesoscale takes place. This is important to understand the conection between macroscopic properties, like permittivity, and the microscopic atomic structure. We will focus in iniaxial relaxor materials like SrBaNb₂O₆ and similar, but also in ferroelectrics based on BaTiO₃ and PbTiO₃, because all of them show responses in the 10-100 GHz range, that were not properly studied and understood up to now. The experiences and techniques of our department and the partner organization will complement to cover the broad band multiexperimental approach needed.

Domain walls in ferroelectrics are naturally formed 2D solitons with a defined, nm-thick polarization profile stable over macroscopic lateral dimensions. Strong coupling of the polarization gradient with strain drastically changes the material properties within the domain wall thickness. Increasing attention is paid to these mobile interfaces because the characterization tools have recently reached the desired nanoscale resolution, needed to uncover the rich spectrum of new phenomena expected there. We are convinced that some ferroelectrics can also host 1D analogues of domain walls, i.e. spontaneously formed ferroelectric line solitons, similar to the recently experimentally confirmed Bogdanov-Yablonskitype magnetic skyrmion lines. We wish to extend the explorations also to these interesting topological objects and to pave a path to the experimental discovery of the ferroelectric skyrmion phases, analogous to the vortex states in superconductors and skyrmion phases of chiral magnets.

Caloric effects of ferroelectric and ferromagnetic materials have recently received significant attention in solid-state cooling. The electrocaloric effect in bulk ferroelectrics promises easy operation, however it is quite small. The magnetocaloric effect is much stronger but requires application of high magnetic fields and cooling agents. Theoretical models predict that the caloric effect can be larger in multiferroic materials which accommodate multiple ferroic states and which can display various cross-coupling effects arising from coupled mechanical, magnetic, and electric order parameters. The existence of such coupling offers a unique opportunity to explore combined caloric effects (called multicaloric effects). However, to date the experimental evidence for such multicaloric effects in multiferroic materials is very limited. This project aims to explore synergy of several order parameters in novel multiferroic single-phase and composite materials, which could lead to a design of solid state cooling systems based on a multicaloric effect.

Relaxor behaviour occurs in chemically-substituted ferroelectrics due to the disruption of the long-range correlation of cation displacements. Substitution can be performed either with homovalent or heterovalent cations. Then either non-polar regions are created in the polar lattice matrix, or polar defect complexes are formed, respectively. These non-polar or polar Polarization Decorrelation Regions (POLDERs) have different correlation length and can respond differently to the applied electric fields. In heterovalent-substituted (non chargecompensated) Ba-based perovskites, relaxor behaviour appears for much lower substituent contents than in homovalent-substituted ones (e.g. at 7 wt.% substituent content against 35 wt.%, respectively). It is vital to study the formation, distribution and interaction of polar and nonpolar POLDERs on the nano- to mesoscale in order to tune macroscopic dielectric properties in these relaxors. This project for the first time will study the role of POLDERs in disrupting longrange ferroelectricity.

This work aims to obtain and characterize physico-chemically Phase Change Materials (PCMs) with improved chemical and thermal stability for thermal energy storage in buildings with nearly zero energy consumption. Once obtained, the PCMs will be tested in the laboratory modules with various geometry as a preliminary step for implementing them in the thermally energy storage systems for buildings.

The recently discovered coexistence of both ferroelectricity and non-conventional skyrmionic spin textures in the lacunar spinel GaV₄S₈ (GVS) promises outstanding magnetoelectric effects to happen in these compounds. Notably, GVS is the first and only multiferroic material known to date that potentially might find its way into top-modern applications such as skyrmionic memories. In order to significantly advance the fundamental understanding, we propose in this bilateral project to shed light onto the fundamental static and dynamic behavior of GVS and its related compounds, through the concerted nanoscale approach between theory and experiment. More precisely, we uniquely combine the local-scale experimental inspection (by using various scanning probe techniques and optical spectroscopy) with multi-scale modeling strategies (i.e. ab-initio, phase field modelling, etc.). The two participating teams in Prague/Czech Republic and Dresden/Germany form the ideal basis in order to conduct this research in the proposed bilateral project.