Egor Koemets

Interfaces in oxide heterostructures act as critical regions to induce emergent and novel types of functionality in thin films. Herein, we demonstrate the creation of bulk ferroelectric heterostructures (BFHs) in monolithic ferroelectrics by nanoscale elemental partitioning, unlocking functionalities beyond the reach of conventional ferroelectrics and thin-film devices. The unique percolating, compositionally modulated networks act as built-in heterointerfaces that generate coupled ferroelectric-electrostatic-elastic fields, which stabilize charged domain walls and imprint predefined domain configurations throughout the bulk. In exemplar BiFeO ₃ -BaTiO ₃ ceramics, the BFH approach raises the Curie temperature to 824°C (increment Δ T _C > 350°C) and yields superior high-temperature piezoelectric responses ( d ₃₃ > 400 picocoulombs per newton, k ₃₃ > 0.4), surpassing those of existing high-temperature piezoelectrics. Domain-engineered BFHs further unlock programmable control of domain switching, enabling the strongest yet tunable internal bias fields (>8 megavolts per meter) in bulk materials and record-high reversible shear electrostrains (up to 0.98%) at supercoercive field levels. The BFH design concept provides a transformative platform for next-generation ferroelectric and electromechanical transduction devices with extended temperature, high-power, and high-stress capability. It also establishes a universal framework for programmable control across ferroic families, including ferroelastic, ferromagnetic, and multiferroic materials in polycrystalline, single-crystal, and thin-film forms.

The crystallographic structure of iron under extreme conditions is a key benchmark for state-of-the-art experimental and computational methods. Moreover, it plays a crucial role in our understanding of planetary cores, as it strongly influences the interpretation of observational data and, consequently, our knowledge of their internal structure and dynamics. However, even the crystal structure of pure solid iron under Earth's core conditions remains uncertain, with the commonly expected hexagonal close-packed phase being energetically competitive with several cubic structures. In this study, iron was compressed in a diamond anvil cell to pressures exceeding 200 GPa and dynamically probed near its melting point using MHz-frequency x-ray pulses from the European X-ray Free-Electron Laser. In the pressure range between 120 and 160 GPa, the high temporal resolution of the structural snapshots at elevated temperatures revealed the face-centered cubic phase, extending its observed stability to higher pressures than previously reported. Brief appearances of intermediate or disordered phases, including body-centered cubic (bcc) and stacking-faulted structures, were also observed. Above 230 GPa, new diffraction reflections emerged at high temperatures and are interpreted as evidence for the nucleation of a previously unreported phase, which subsequently evolved into a powderlike diffraction pattern characteristic of the bcc structure. The unprecedented time resolution of the structural data acquired during rapid heating and cooling cycles provides insights into the kinetics and mechanisms of phase transformations in iron at these extreme pressures.

Static compression experiments using diamond anvil cells (DAC), combined with x-ray diffraction (XRD) are widely used to study material properties at extreme pressures. Recent advances enable time-resolved XRD during dynamic compression along tailored loading paths. However, the influence of compression rate on derived properties remains poorly understood, and the use of tailored compression paths—particularly pressure cycling—requires further investigation. Here, we employed time-resolved XRD in a dynamic DAC to compress MgO in a Ne pressure medium along a tailored compression path consisting of a ramp, plateau, and cyclic loading. Pressure-volume relations derived from ramp compressions at average compression rates of 0.22 and 0.46 GPa s − 1 agree closely with previous static experiments, with minor but detectable systematic deviations in the faster compression run. Stress analysis suggests that this deviation might be related to an increased flow strength of Ne under fast compression. During the postramp plateaus, MgO pressures continued to increase, highlighting the effect of compression rate on the time required for the coupled mechanical DAC system to reach equilibrium. Under cyclic loading, we observe phase shifts between oscillating differential stresses and volume strains in Ne, possibly indicating that viscoelastic/-plastic behavior of either Ne or the gasket material contributed to the overall response. We found this effect more pronounced for lower frequencies and larger cycling amplitudes. The findings presented in this paper improve our understanding of material behavior during dynamic loading in the DAC and provide reference data and methodology for studies of complex materials and loading paths.

The development of pulsed intense x-ray sources, such as free electron laser, offers new avenues for high pressure experiments. Here, we study the feasibility and metrology of x-ray heating in diamond anvil cells at the European x-ray free electron laser. This method enables one to volumetrically heat the sample while inhibiting chemical migration and probing the crystallographic structure of the sample throughout the heating with a high repetition rate. We focus our study on iron, whose phase diagram is well established up to 100 GPa, to explore the possibilities and limitations of this technique. We volumetrically heat iron samples at starting pressures ranging from 10 to 138 GPa, using the x-ray beam pulsed at 4.5 MHz in a serial pump-and-probe experimental design. Experimental challenges arise from temperature gradients within the sample, changes in temperature at the 100 ns timescale, the difficulty of direct temperature estimates, the effect of thermal pressure, and the presence of metastable crystallites due to rapid cycles of heating and cooling. Hence, we develop a multi-crystal-like data processing method that allows us to account for sample heterogeneity in probed conditions. We then calibrate our measurements using known physical properties of iron under pressure. Thermal pressure in our experiments increases from 4% of the isochoric prediction at 10 GPa to 23% at 138 GPa, and we show that our data are in agreement with most previous observations of iron in this pressure range. The method can now be implemented at higher pressures and temperatures and on materials with unknown phase diagrams.

The iron spin crossover in (Mg_1‐xFe_x)O ferropericlase causes changes to its physical properties that are expected to affect seismic velocities in Earth's lower mantle. We present new time‐resolved pressure‐volume measurements of iron‐rich ferropericlase (x_Fe = 0.40, 0.59) and combine the results with literature data with x_Fe = 0.04–0.6 to investigate the dependence of ferropericlase elastic properties on iron content. We infer the relationship between unit‐cell volume, pressure and iron content directly from the data by training Mixture Density Networks and derive bulk modulus, density and bulk sound velocity from the outputs. This allows us to constrain the effect of the spin crossover on these properties and estimate their uncertainties for different iron contents. Our findings indicate that the spin crossover may significantly alter the physical properties of ferropericlase in iron‐enriched regions in the lowermost mantle, with implications for the interpretation of seismic heterogeneities observed near the core‐mantle boundary.

The reduction of Co-based oxides doped with Al³⁺ ions has been studied using in situ XRD and TPR techniques. Al³⁺-modified Co₃O₄ oxides with the Al mole fraction Al/(Co + Al) = 1/6; 1/7.5 were prepared via coprecipitation, with further calcination at 500 and 850 °C. Using XRD and HAADF-STEM combined with EDS element mapping, the Al³⁺ cations were dissolved in the Co₃O₄ lattice; however, the cation distribution differed and depended on the calcination temperature. Heating at 500 °C led to the formation of an inhomogeneous (Co,Al)₃O₄ solid solution; further treatment at 850 °C provoked the partial decomposition of mixed Co-Al oxides and the formation of particles with an Al-depleted interior and Al-enriched surface. It has been shown that the reduction of cobalt oxide by hydrogen occurs via the following transformations: (Co,Al)₃O₄ → (Co,Al)O → Co. Depending on the Al distribution, the course of reduction changes. In the case of the inhomogeneous (Co,Al)₃O₄ solid solution, Al stabilizes intermediate Co(II)-Al(III) oxides during reduction. When Al³⁺ ions are predominantly on the surface of the Co₃O₄ particles, the intermediate compound consists of Al-depleted and Al-enriched Co(II)-Al(III) oxides, which are reduced independently. Different distributions of elemental Co and Al in mixed oxides simulate different types of the interaction phase in Co₃O₄/γ-Al₂O₃-supported catalysts. These changes in the reduction properties can significantly affect the state of an active component of the Co-based catalysts.

Goethite, α‐FeOOH, is a major phase among oxidized iron species, commonly called rust. We studied the behavior of iron (III) oxyhydroxide up to 81 GPa and 2100 K using in situ synchrotron single‐crystal X‐ray diffraction. At high pressure‐temperature conditions FeOOH decomposes forming oxygen‐rich fluid and different mixed valence iron oxides (previously known phases of Fe₂O₃, Fe₃O₄, Fe₅O₇, and novel Fe₇O₁₀ and Fe_6.32O₉). Rust is known to form as a byproduct of anoxygenic prokaryote metabolism that took place massively from about 3.8 billion years (Ga) ago until the Great Oxidation Event (GOE) ∼2.2 Ga ago. Rust was buried on the ocean floor and was transported into the mantle as a consequence of plate tectonics (started ∼2.8 Ga ago). Our results suggest that recycling of rust in Earth's mantle contributes to redox conditions of the early Earth and formation of oxygen‐rich atmosphere.