🔬 materials science

Controlling nitrogen incorporation in (Eu,Ba,Ca)TiO $_3$ -derived perovskite-type oxynitrides through Fermi Level Engineering

This study demonstrates that Fermi Level Engineering, achieved through A-site cation substitution and the Eu $^{2+/3+}$ redox couple, effectively overcomes intrinsic thermodynamic limitations to significantly enhance nitrogen incorporation in perovskite-type oxynitrides by stabilizing the Fermi level and enabling controlled defect formation.

Original authors: Margarida Barroso, Marie Neumann, Rachele Sciotto, Shama Perween, Magdalena O. Cichocka, Jefferson Bettini, Adrián López Vergara, Patrick Zeller, Gaurav Anand, Verena Streibel, Barbara A. J. Lechner

Published 2026-11-04

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CC BY 4.0

Original authors: Margarida Barroso, Marie Neumann, Rachele Sciotto, Shama Perween, Magdalena O. Cichocka, Jefferson Bettini, Adrián López Vergara, Patrick Zeller, Gaurav Anand, Verena Streibel, Barbara A. J. Lechner, Melanie Appelmann, Oliver Clemens, Ute Kolb, Jan P. Hofmann, Karsten Albe, Andreas Klein, Marc Widenmeyer, Anke Weidenkaff

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Materials scientists often look for ways to make solids that can capture sunlight and turn it into clean fuel, much like a plant does. To do this, they need materials that absorb light well but also conduct electricity efficiently. One promising family of materials is made by taking a standard metal oxide and swapping some of its oxygen atoms for nitrogen. This simple swap changes the material's electronic properties, often making it better at absorbing visible light. However, getting the nitrogen to stay inside the material is notoriously difficult. The process often stops itself before the material can hold enough nitrogen to be truly useful, leaving researchers with a product that is only partially modified and less effective than hoped.

The challenge lies in a hidden electronic tug-of-war that happens inside the crystal structure. When nitrogen enters the material, it brings an extra negative charge. To keep the material electrically balanced, the crystal must create defects or change the state of its metal atoms. In many cases, this balancing act causes the material's internal energy landscape to shift in a way that makes it harder for more nitrogen to enter. It is a self-limiting process where the material essentially locks the door against further modification. Researchers have long suspected that if they could control this internal energy shift, they could force the material to accept more nitrogen, but the precise mechanism remained unclear.

A team of researchers at the Technical University of Darmstadt and other institutions set out to solve this puzzle by studying a specific type of crystal known as a perovskite. They began with a simple version made of barium, titanium, and oxygen. When they subjected this material to a high-temperature reaction with ammonia gas to introduce nitrogen, the process stalled almost immediately. Their measurements showed that the material only accepted a tiny amount of nitrogen, roughly three hundredths of an atom for every formula unit. Through computer simulations, the team discovered why: as the first few nitrogen atoms entered, they caused the material's internal electron energy level to drop significantly. This drop made it energetically unfavorable for any more nitrogen to replace oxygen, effectively shutting down the reaction. The material had reached its limit not because of a lack of nitrogen gas, but because of an electronic barrier it created for itself.

To overcome this barrier, the researchers decided to change the recipe by swapping some of the barium atoms with europium, a rare earth metal, and calcium. They hypothesized that europium could act as a stabilizer. Unlike barium, europium can easily switch between two different electrical states. The team found that when europium was present, it acted as a buffer for the extra negative charge brought by the nitrogen. Instead of letting the internal energy level drop and stop the reaction, the europium atoms changed their own state to balance the charge, keeping the internal energy level high enough to welcome more nitrogen. This strategy worked remarkably well. In samples where they replaced most of the barium with europium and calcium, the material accepted nitrogen up to two-thirds of an atom per formula unit, a massive improvement over the original material.

The success, however, depended heavily on the flexibility of the crystal structure. The researchers discovered that the size of the atoms they used mattered just as much as their electrical properties. When they used barium, which has a large atomic radius, the crystal structure remained rigid and could not stretch enough to accommodate the larger nitrogen atoms without breaking. This structural rigidity limited the nitrogen content, even with the electronic help from europium. In contrast, when they used calcium, which has a smaller radius, the crystal structure became more flexible. It could twist and tilt slightly to make room for the nitrogen atoms without breaking apart. This combination of electronic buffering from europium and structural flexibility from calcium allowed the material to reach its highest nitrogen levels.

The study also revealed how the material behaves on its surface when exposed to water, a critical test for potential fuel production. When the researchers exposed the nitrogen-rich material to water vapor, the surface chemistry responded in a predictable way driven by the same electronic principles. The material's internal energy level, which had been pinned in place by the europium, dictated how electrons moved between the material and the water. This confirmed that the strategy of controlling the internal energy level works not just for building the material, but also for how it functions in real-world conditions.

Ultimately, the work demonstrates that the limits of making these advanced materials are not fixed by the laws of chemistry alone but can be engineered. By understanding and manipulating the internal electron energy levels and the physical flexibility of the crystal lattice, scientists can bypass the self-limiting barriers that have long hindered progress. The researchers showed that by carefully choosing which atoms to swap in, they could guide the material to accept far more nitrogen than previously thought possible, opening a clearer path toward designing better materials for capturing solar energy.

Technical Summary: Controlling Nitrogen Incorporation in (Eu,Ba,Ca)TiO3-Derived Perovskite Oxynitrides via Fermi Level Engineering

Problem Statement
Oxynitride perovskites are promising materials for solar water splitting due to their tunable bandgaps and structural robustness. However, their synthesis via thermal ammonolysis is often hindered by non-uniform nitrogen incorporation and high defect concentrations (e.g., anion vacancies, reduced cations). In pristine barium titanate (BaTiO₃), nitrogen incorporation is self-limiting; as nitrogen (N³⁻) substitutes oxygen (O²⁻), it acts as an acceptor dopant, driving the Fermi level ( $E_F$ ) downward. This shift increases the formation enthalpy of further nitrogen substitution, thermodynamically suppressing additional nitrogen uptake. Consequently, pristine BaTiO₃ typically incorporates less than 0.1 nitrogen atoms per formula unit, far below theoretical limits, limiting its utility as a functional photoabsorber.

Methodology
This study employs a combined approach of Density Functional Theory (DFT) modelling and comprehensive experimental characterization to investigate and manipulate defect thermodynamics in perovskite oxynitrides.

Theoretical Modelling: DFT calculations were performed on rhombohedral BaTiO₃ to map defect formation energies for oxygen vacancies ( $V_O$ ) and nitrogen substitutions ( $N_O$ ) as a function of the Fermi level. Charge transition levels (CTLs) and charge neutrality conditions were solved self-consistently to predict the feedback mechanism between nitrogen concentration and $E_F$ .
Synthesis: Pristine BaTiO₃ and A-site substituted variants ( $Eu_{1-x}Ba_xTiO_3$ and $Eu_{1-x}Ca_xTiO_3$ with $x=0.2, 0.8$ ) were synthesized via direct thermal ammonolysis.
Characterization: The study utilized a suite of advanced techniques, including:
- Structural Analysis: Powder Neutron Diffraction (PND), High-Resolution Synchrotron XRD, and 3D Electron Diffraction (3DED) to determine crystal symmetry, lattice parameters, and anion ordering.
- Composition: Hot Gas Extraction (HGE) for precise quantification of oxygen and nitrogen content.
- Electronic Structure: Synchrotron-based X-ray Photoelectron Spectroscopy (XPS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) to analyze valence states (Eu²⁺/Eu³⁺, Ti⁴⁺/Ti³⁺), core-level shifts, and Fermi level positioning.
- In Situ Treatments: Redox cycling (O₂/UHV) and Near-Ambient Pressure XPS (NAP-XPS) with water vapor exposure to probe surface electronic stability and charge transfer mechanisms.

Key Results

Self-Limiting Mechanism in Pristine BaTiO₃: DFT and experimental data confirm that nitrogen incorporation in pristine BaTiO₃ is electronically self-limiting. As $N_O$ forms, $E_F$ shifts downward, eventually crossing the charge transition level where oxygen vacancy formation becomes energetically more favorable than further nitrogen substitution. This results in a maximum nitrogen content of only ~0.033 per formula unit.
Fermi Level Pinning via Eu²⁺/Eu³⁺ Redox: Introducing Europium (Eu) on the A-site provides a charge compensation mechanism distinct from vacancy formation. The Eu²⁺/Eu³⁺ redox couple acts as an electronic buffer. Upon ammonolysis, Eu²⁺ oxidizes to Eu³⁺ to balance the negative charge of incorporated N³⁻. This process pins the Fermi level at the Eu charge transition level (~2.9 eV above the Valence Band Maximum), keeping $E_F$ high enough to maintain favorable formation enthalpies for nitrogen substitution.
Structural Flexibility Dictates Maximum Uptake: While Eu-substitution enables higher nitrogen uptake, the crystal structure's ability to accommodate the resulting strain determines the final limit:
- Ba-rich systems: The large ionic radius of Ba²⁺ enforces a rigid tetragonal ($P4mm$) or distorted orthorhombic structure that cannot easily relax. This leads to significant internal bond-angle variance in the Ti(O,N)₆ octahedra, limiting nitrogen content to ~0.14 for the Eu-poor composition ( $x=0.8$ ) and ~0.44 for the Eu-rich composition ( $x=0.2$ ).
- Ca-rich systems: The smaller Ca²⁺ ion allows for a flexible orthorhombic ($Pnma$) structure with octahedral tilting ( $a^+b^-b^-$ ). This flexibility accommodates the lattice strain induced by N³⁻ incorporation and Eu oxidation more effectively. Consequently, the Eu-rich Ca-substituted sample ( $Eu_{0.8}Ca_{0.2}Ti(O,N)_3$ ) achieves a nitrogen content of 0.65 per formula unit, approaching near-stoichiometric limits without phase segregation.
Surface Electronic Stability: In situ NAP-XPS revealed that the Fermi level, pinned at the Eu²⁺/Eu³⁺ CTL, governs surface charge exchange. Unlike BiVO₄, which undergoes reduction upon water exposure, these Eu-oxynitrides exhibit surface oxidation (Eu²⁺ to Eu³⁺) in water vapor, indicating the CTL remains electrochemically active and stable under relevant conditions.

Significance and Claims
The paper claims to demonstrate that Fermi Level Engineering (FLE) is a predictive and rational framework for overcoming intrinsic thermodynamic limitations in oxynitride synthesis. By utilizing the Fermi level as a descriptor, the authors show that:

The self-limiting nature of nitridation in pristine oxides can be bypassed by introducing redox-active cations (Eu) that pin the Fermi level at a favorable energy.
Successful high-nitrogen incorporation requires a dual strategy: electronic engineering (via the Eu²⁺/Eu³⁺ couple) to maintain a high Fermi level, and structural engineering (via Ca substitution) to provide the lattice flexibility necessary to accommodate the resulting strain.
This approach shifts the limiting constraints of nitridation from purely energetic barriers to structural tolerances, offering a pathway to design functional oxynitrides with controlled defect chemistry and higher nitrogen content for applications such as solar water splitting.

The work establishes a direct correlation between defect chemistry, crystal symmetry, and electronic structure, providing a unified concept to guide the synthesis of complex perovskite oxynitrides.

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