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Correlation Between Dopant Atom Evaporation Field and Measured Site Preference in Atom Probe Tomography

Using Zr-doped ErMnO3 as a model system, this study demonstrates that preferential retention of atoms during atom probe tomography, driven by evaporation field differences and analysis temperature, can significantly distort measured dopant site preferences and lead to incorrect interpretations of atomic positions.

Original authors: Kasper A. Hunnestad, Constantinos Hatzoglou, Frida H. Danmo, Zewu Yan, Edith Bourret, Francois Vurpillot, Antonius T. J. van Helvoort, Sverre Selbach, Dennis Meier

Published 2026-09-29

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CC BY 4.0

Original authors: Kasper A. Hunnestad, Constantinos Hatzoglou, Frida H. Danmo, Zewu Yan, Edith Bourret, Francois Vurpillot, Antonius T. J. van Helvoort, Sverre Selbach, Dennis Meier

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

To understand how modern electronics work, scientists often look at the tiny impurities added to materials, known as dopants. These are trace amounts of foreign atoms mixed into a solid crystal to change its electrical or magnetic properties. The challenge is that these atoms are incredibly small and sparse, sometimes appearing only once in a million host atoms. To see them, researchers use a powerful imaging technique called atom probe tomography. This method works by taking a needle-shaped piece of material, making it extremely sharp, and then using high voltage or laser pulses to strip atoms off the surface one by one. As each atom flies off, a detector identifies what it is and records where it came from. By collecting millions of these events, scientists can build a three-dimensional map of the material, showing exactly where every single atom sits. This is crucial for designing better devices, but it relies on a perfect understanding of how the atoms leave the surface. If the atoms do not fly off in the order they are expected to, the final map can be distorted, placing atoms in the wrong spots.

A team of researchers set out to investigate this very issue using a specific type of crystal called hexagonal erbium manganite. They added a tiny amount of zirconium to this crystal, a dopant that is theoretically expected to replace the larger erbium atoms within the structure. Based on the known sizes of these atoms and standard computer models, the zirconium should sit comfortably in the spots reserved for erbium. However, when the researchers used atom probe tomography to look at the sample, the data told a different story. The map showed the zirconium atoms sitting in the spots meant for the much smaller manganese atoms. This was a clear contradiction. The researchers realized that the imaging technique itself was creating an illusion. The zirconium atoms were not actually in the wrong place; rather, they were stubbornly refusing to leave the surface at the right time. Because zirconium requires a stronger electric field to evaporate than the surrounding atoms, it stayed behind while its neighbors flew off. By the time the zirconium finally broke free, the computer reconstruction had already moved the next layer of atoms into position, causing the zirconium to be plotted in the wrong location.

To prove that this "stubbornness" was the cause, the team ran a series of experiments and computer simulations. They tested samples with different amounts of zirconium and changed the temperature and energy of the laser pulses used to strip the atoms. When they increased the concentration of zirconium, the effect became more pronounced, confirming that the presence of these high-field atoms was altering the local environment. When they lowered the laser energy, the distortion decreased, and the zirconium atoms appeared closer to their true positions. The computer simulations, which tracked the exact order in which atoms should leave a perfect crystal, matched these findings perfectly. They showed that when a dopant atom has a higher evaporation field than its neighbors, it lags behind, creating a gap that the reconstruction software fills incorrectly. This lag can be so severe that the atom is not just placed in the wrong layer, but can even delay the evaporation of the atoms beneath it, warping the entire local structure.

The study concludes that the apparent position of a dopant atom in these images is not just a matter of where it sits in the crystal, but also a result of how easily it evaporates compared to its neighbors. The researchers demonstrated that without accounting for these differences in evaporation strength, scientists can easily misinterpret where an atom belongs, potentially leading to incorrect conclusions about how a material works. By understanding that some atoms hold on tighter than others, the team provided a way to correct these maps. This insight is vital for anyone trying to see the atomic details of complex materials, ensuring that the tiny impurities controlling the behavior of future technologies are seen exactly where they truly are.

Technical Summary: Correlation Between Dopant Atom Evaporation Field and Measured Site Preference in Atom Probe Tomography

Problem Statement
Atom Probe Tomography (APT) is a powerful tool for analyzing dopants and impurities in complex oxides at the atomic scale. However, reliable extraction of atomic-level information, such as site preference (which lattice site a dopant occupies), is compromised when dopant and matrix atoms possess different field evaporation behaviors. In standard APT reconstruction protocols, a single evaporation field is often assumed for the entire system. When a dopant has a significantly different evaporation field than the matrix, "preferential retention" occurs: atoms with higher evaporation fields evaporate later than expected. This delay causes them to be incorrectly positioned in the reconstructed volume, potentially leading to erroneous conclusions about their crystallographic site. While this effect is known in layered systems, it is subtler and harder to identify in lightly doped materials where dopants constitute only a small fraction of the total atoms.

Methodology
The authors utilized Zr-doped hexagonal erbium manganite ( $ErMnO_3$ ) as a model system to investigate the relationship between dopant evaporation fields and measured site preferences.

Sample Preparation: Single crystals of $Er_{0.998}Zr_{0.002}MnO_3$ (0.04 at.% nominal Zr) and polycrystalline samples with higher doping (2 at.% Zr) were prepared. Specimens were oriented such that the hexagonal $c$ -axis aligned with the needle axis to maximize spatial resolution at the (001) pole.
APT Experiments: Measurements were performed using a laser-pulsed LEAP 5000XS. The study varied intrinsic parameters (dopant concentration: 0.04 at.% vs. 2 at.%) and extrinsic parameters (analysis temperature/laser pulse energy: 5 pJ vs. 0.5 pJ).
Data Analysis: Spatial Distribution Maps (SDMs) were calculated by summing coordinate differences between ionic species along the crystallographic $c$ -axis. This allowed the authors to determine the average distance of Zr and Er atoms relative to Mn atomic planes.
Simulations: Field-evaporation simulations were conducted using a Monte-Carlo algorithm on a simplified cubic lattice of alternating Er and Mn planes. Zr dopants were introduced on Er sites with varying relative evaporation fields ( $F_{Zr}$ ) ranging from 0.5 to 1.8 times the evaporation field of Er ( $F_{Er}$ ).

Key Results

Contradiction with Theory: Based on Shannon ionic radii and Density Functional Theory (DFT), $Zr^{4+}$ is expected to substitute $Er^{3+}$ on the A-site. However, initial APT reconstructions of the low-doped sample placed Zr atoms on the Mn B-site (coinciding with Mn planes in the SDM), contradicting theoretical expectations.
Identification of Preferential Retention: The discrepancy was attributed to the high evaporation field of Zr relative to Er and Mn. The Zr atoms were retained on the surface after the Er planes evaporated, causing them to be reconstructed as if they belonged to the subsequent Mn planes.
Concentration and Temperature Dependence:
- Concentration: Increasing the Zr concentration to 2 at.% increased the average evaporation field of the Er/Zr planes. While preferential retention of Er planes became more pronounced, the Zr peak in the SDM shifted away from the Mn peak and closer to the Er peak, indicating a partial correction of the artifact.
- Temperature: Reducing the laser pulse energy (lowering the analysis temperature) reduced the evaporation field differences between species. This suppression of retention effects further shifted the Zr peak away from the Mn position, aligning better with the expected Er-site preference.
Simulation Insights: Simulations confirmed that high-field dopants ( $F_{Zr} > F_{Er}$ ) exhibit delayed evaporation, resulting in negative depth displacements in reconstruction. The effect is asymmetrical; while low-field dopants saturate at small positive displacements, high-field dopants can be retained beyond the subsequent atomic plane. Furthermore, retained dopants were found to perturb the evaporation sequence of neighboring matrix atoms, causing local distortions in the reconstruction.

Significance and Claims
The paper demonstrates that the apparent site preference of dopants in APT is not solely a function of chemical substitution but is heavily influenced by field evaporation dynamics. The authors claim that without accounting for dopant-specific evaporation fields, APT analysis of lightly doped oxides can lead to incorrect interpretations of atomic positions.

The work establishes that:

Preferential retention is a primary source of reconstruction artifacts in doped complex oxides.
The magnitude of the positional error is governed by the relative evaporation field of the dopant, its concentration, and the experimental analysis temperature.
Systematic variation of experimental conditions (concentration and temperature) combined with field-evaporation simulations is necessary to distinguish between true site preference and evaporation-induced artifacts.

The authors conclude that adequate control experiments and a deeper understanding of field evaporation dynamics are essential for the reliable APT-based analysis of individual dopant atoms in solid systems.

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