Igor Evangelista

Thermal-expansion mismatch remains a major challenge for the long-term stability of solid oxide fuel-cell and electrolysis-cell materials, particularly for chemically complex perovskite electrodes operating at elevated temperatures. Reliable prediction of thermal expansion in these materials is difficult because anharmonic lattice dynamics, octahedral rotations, chemical disorder, and temperature-driven structural evolution can limit the applicability of conventional phonon-based approaches. Here, we combine quasiharmonic phonon calculations (QHA) and on-the-fly machine-learning force-field molecular dynamics (MLFF-MD) to evaluate finite-temperature structural behavior in LaFeO 3 and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ . LaFeO 3 was used as a benchmark material, and the QHA captures the qualitative expansion trend but overestimates the volumetric thermal expansion coefficient (TEC), giving α V = 4.08 × 10 − 5 K − 1 compared with the high-temperature x-ray diffraction (HT-XRD) value of 3.52 × 10 − 5 K − 1 . In contrast, MLFF-MD gives α V = 3.56 × 10 − 5 K − 1 , in close agreement with the experiment. For La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ , QHA becomes unreliable due to phonon instabilities, while MLFF-MD provides stable finite-temperature trajectories and captures the experimentally observed increase in linear TEC across the rhombohedral-to-cubic transition. Specifically, MLFF-MD predicts an increase in α L from 1.31 × 10 − 5 K − 1 in the low-temperature rhombohedral regime to 1.90 × 10 − 5 K − 1 in the high-temperature cubic regime, consistent with the structural evolution observed experimentally. The remaining quantitative discrepancy with HT-XRD is consistent with the stoichiometric model used here, which isolates the intrinsic thermal expansion while excluding chemical-expansion contributions associated with oxygen nonstoichiometry under experimental conditions. Once trained, the MLFF reduces the cost of an individual molecular-dynamics step by roughly 2 orders of magnitude relative to direct molecular dynamics, enabling substantially longer trajectories and broader sampling of thermally accessible configurations while retaining close agreement with DFT reference energies and forces. These results show that on-the-fly MLFF-MD provides a practical first-principles framework for predicting thermal expansion and phase evolution in disordered solid-oxide perovskites beyond the regime where phonon-based QHA remains reliable.

The durability of solid oxide cells is critically limited by the overall thermochemical expansion of perovskite electrodes, exacerbated by the formation of oxygen vacancies with increasing temperature. This creates a fundamental trade-off, where oxygen vacancies enhance electrochemical functionality at the cost of thermomechanical stability. We independently study thermal and chemical expansion of fifteen perovskite oxides with complex A-site cation mixtures using synchrotron X-ray diffraction, dilatometry, thermogravimetry, and density functional theory. We demonstrate that the Goldschmidt tolerance factor governs both thermal and chemical expansion through octahedral tilting dynamics, which offset bond-stretching vibrations and compensate for vacancy-induced strain. We identify that high-entropy A-site mixing also suppresses chemical expansion via "strain containment", where larger cobalt sites with increased bond covalency locally absorb vacancy-induced strain. This lowers the chemical expansion coefficient by up to 29% relative to the benchmark electrode. Together, these correlations and mechanistic insights map structural and compositional descriptors onto expansion behaviors, providing targeted design principles to tune thermochemical expansion without sacrificing functional defects.

Long-term durability of solid oxide electrolysis cells (SOECs) is strongly constrained by thermomechanical stresses that develop between layered components such as LSCF-based oxygen electrodes, yttrium stabilized zirconia (YSZ) electrolytes, and ceria-based barrier layers. Differences in thermal expansion coefficients (TECs) between these materials generate tensile and compressive stresses during thermal cycling and operation, promoting cracking, delamination, and contact loss at electrode/electrolyte interfaces. Recent modeling and experimental studies underscore that cathode–electrolyte thermal incompatibility is a major driver of performance degradation and mechanical failure in solid oxide cells [1].

Long-term durability of solid oxide electrolysis cells (SOECs) is strongly constrained by thermomechanical stresses that develop between layered components such as LSCF-based oxygen electrodes, yttrium stabilized zirconia (YSZ) electrolytes, and ceria-based barrier layers. Differences in thermal expansion coefficients (TECs) between these materials generate tensile and compressive stresses during thermal cycling and operation, promoting cracking, delamination, and contact loss at electrode/electrolyte interfaces. Recent modeling and experimental studies underscore that cathode–electrolyte thermal incompatibility is a major driver of performance degradation and mechanical failure in solid oxide cells [1].

Developing robust, high-performance anode materials is essential for advancing solid oxide electrolysis cells (O-SOECs) as a key technology for high-efficiency hydrogen production. Among these materials, La _1-x Sr _x Co _y Fe _1-y O _3-δ (LSCF) is widely used as a mixed ionic-electronic conducting anode due to its favorable oxygen transport properties and electrochemical activity under reducing conditions [1]. A key challenge limiting the long-term performance of LSCF is the degradation of oxygen transport kinetics under operating conditions. In particular, the evolution of oxygen vacancies, changes in defect populations, and Sr surface segregation all influence ionic conductivity and ultimately the electrochemical performance and durability of O-SOECs. Understanding oxygen migration mechanisms is therefore central to improving anode functionality, as ionic transport is directly tied to the efficiency of the oxygen evolution reaction (OER) at high operating temperatures. In this work, we employ density functional theory with Hubbard U corrections (DFT+U) to investigate the formation energies of key point defects, including oxygen vacancies and interstitials, and to quantify oxygen diffusion barriers in both rhombohedral and cubic phases (as shown on figure) of LSCF using climbing image nudged elastic band (CI-NEB) calculations. We also explore Sr migration pathways and examine their coupling with oxygen vacancy dynamics to identify how they contribute to long-term degradation phenomena such as SrZrO ₃ formation at the electrolyte interface [2]. We further explore high-entropy oxides (HEOs) with A-site substitutions in perovskite (ABO ₃ ) structures as next-generation anode materials for O-SOECs. These high entropy perovskite oxides (HEPOs)exploit compositional complexity to suppress Sr segregation and improve structural stability and electrochemical performance. Experimental studies have demonstrated that HEPOs, such as (La _0.2 Pr _0.2 Nd _0.2 Sm _0.2 Sr _0.2 )MnO _3-δ , exhibit superior resistance to Sr surface enrichment and phase decomposition compared to conventional LSCF [3]. This is particularly important for mitigating degradation mechanisms such as Cr poisoning, where volatile chromium species, originating from stainless steel interconnects, can react with segregated Sr to form insulating phases at the electrode surface.Compositions like (La _0.2 Sr _0.2 Pr _0.2 Y _0.2 Ba _0.2 )Co _0.2 Fe _0.8 O _3-δ have shown excellent resistance to Cr poisoning, maintaining stable polarization resistance during extended high-temperature operation [4]. By combining first-principles modeling with emerging experimental results, this study establishes a foundational understanding of oxygen migration in both conventional and high-entropy anode materials. These insights directly inform the rational design of next-generation O-SOEC electrodes with improved ionic transport properties and long-term operational stability. Future work will focus on validating predicted migration pathways and degradation mechanisms through electrochemical impedance spectroscopy and high-temperature structural analysis. References: [1] S. Zhang, Mater. Today Commun. 42 , 111280 (2025). [2] D. The, J. Power Sources 275 , 901 (2015). [3] Y. Yang, J. Power Sources 482 , 228959 (2021). [4] Z. Li, ACS Appl. Mater. Interfaces 14 , 24363 (2022). Acknowledgements: This work is funded by the Department of Energy, Office of Energy Efficiency and Renewable Energy, project “Developing High-Entropy Materials as Superior Alternative Electrodes for Long-lasting Oxide-Conducting Oxide Electrolysis Cells (O-SOECs).” Figure 1

The Bi2Se3–In2Se3 layered system has garnered significant attention and extensive research due to its versatile properties, yet its structural properties and phase stability remain elusive. Here, using first-principles calculations with van der Waals interactions, we systematically study the phase stability and transition behavior of (BixIn1−x)2Se3 alloys. Our results reveal a contrasting stability profile between Bi2Se3 and In2Se3, with the former exhibiting a distinct preference for the β phase over the α phase, while the latter shows similar stabilities in both phases, thus partially addressing previously reported ground-state inconsistencies. Exploring composition–structure relationships, we demonstrate that Bi incorporation in low concentrations stabilizes the β phase, consistent with early experimental observations. Further analysis based on the cation orbital properties indicates that the preference of Bi for octahedral sites over tetrahedral ones drives the small critical composition for the α→β phase transition. This work enhances our understanding of phase stability in (BixIn1−x)2Se3 alloys, providing insights for future design of monophasic materials and advanced applications.

Theory of formability and diffusivity of point defects and impurites in Cd(Se,Te)

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO₃ has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO₃ using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO₃ films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO₃ with free electron concentrations, n_3D, from 3.3 × 10¹⁹ cm^-3 to 1.6 × 10²⁰ cm^-3. The films exhibit a maximum room-temperature mobility of 42 cm² V^-1 s^-1 at n_3D = 3.3 × 10¹⁹ cm^-3. Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO₃-based field-effect transistors. The transistors show promise for CaSnO₃'s high-voltage capabilities by exhibiting low off-state leakage below 2 × 10^-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 10⁶. This work serves as a starting point for future studies on the semiconducting properties of CaSnO₃ and many devices that could benefit from CaSnO₃'s exceptionally wide band gap.