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Synthesis and Characterization of Compositionally Complex (Gd/Ho/Er/Dy)2Zr2O7 Thin Film Combinatorial Library

This study employs high-throughput combinatorial thin-film synthesis and multimodal characterization to map the structure-property relationships of (Gd/Ho/Er/Dy)2Zr2O7 ceramics, revealing that the largest experimentally observed lattice parameter, rather than cation size disorder or equiatomic composition, correlates with a pronounced minimum in thermal conductivity.

Original authors: Dalton A. Pearl, Jade Holliman Jr, Reece Emory, Joshua Safin, Aditya Raghavan, Kamyar Barakati, Andrew H. Jones, Ethan A. Scott, Jack C. Lasseter, Adam Corrao, Daniel Olds, Bruce Ravel, Sergei K. Kali

Published 2026-09-04

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CC BY 4.0

Original authors: Dalton A. Pearl, Jade Holliman Jr, Reece Emory, Joshua Safin, Aditya Raghavan, Kamyar Barakati, Andrew H. Jones, Ethan A. Scott, Jack C. Lasseter, Adam Corrao, Daniel Olds, Bruce Ravel, Sergei K. Kalinin, Patrick E. Hopkins, Katharine Page, Philip D. Rack

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

Imagine a world where the engines that power our airplanes and electricity generators could run hotter, faster, and more efficiently. The key to unlocking this potential lies in a special kind of protective skin: a ceramic coating that shields metal parts from extreme heat. For decades, scientists have relied on a specific material to do this job, but it has a limit. When temperatures climb too high, this material becomes unstable and eventually fails. To build the next generation of engines, researchers need a new kind of ceramic that can withstand these punishing conditions while keeping heat from passing through. The challenge is finding a material that is not just a single chemical compound, but a complex mixture of several different elements, all mixed together in a way that creates a stable, heat-blocking structure.

A team of researchers set out to solve this puzzle by creating a vast library of these complex ceramic mixtures on a single, thin sheet of glass. Instead of making one sample at a time, they used a technique called combinatorial sputtering, which is like spraying paint from four different cans simultaneously onto a moving canvas. By carefully controlling the spray, they created a continuous gradient where the amount of each of the four rare-earth ingredients changed smoothly from one side of the sheet to the other. This allowed them to test thousands of different chemical recipes in a single experiment, searching for the perfect balance that would stop heat from flowing.

The scientists were looking for a specific type of crystal structure known as a defect fluorite, which is a disordered arrangement of atoms that is excellent at scattering heat. They suspected that mixing many different-sized atoms together would create enough chaos in the crystal lattice to block heat effectively. However, their results revealed a surprising twist. The spot on their glass sheet that blocked heat the best was not where the ingredients were mixed in equal parts, nor was it where the difference in atomic sizes was the greatest, as many previous theories had predicted. Instead, the best insulator was found in a specific region where the atoms had arranged themselves into a slightly larger, more relaxed crystal structure.

To understand why this happened, the team peered deep into the material using powerful X-ray beams. They checked the chemical makeup and the local environment of every atom, confirming that the atoms were in the right state and that the basic building blocks of the crystal were consistent across the entire sheet. They also measured the size of the tiny grains that make up the material and found that the best insulator did not have the smallest grains, ruling out the idea that tiny boundaries between grains were the cause of the heat blocking. The data showed that the material with the lowest heat flow had the largest crystal spacing and the least amount of internal strain in a specific direction.

This discovery suggests that the secret to stopping heat in these complex ceramics is not just about throwing different-sized atoms together to create chaos. It is about how the crystal lattice itself stretches and relaxes to accommodate that complexity. The researchers found that when the lattice expands in a particular way, it becomes a more effective barrier against heat flow, even if the chemical mixture is not the one most people would guess. By mapping out these relationships, the team has provided a new roadmap for designing better thermal barrier coatings. Their work shows that by looking at how the crystal structure relaxes and expands, rather than just focusing on the ingredients, scientists can find new materials that allow our engines to run hotter and more efficiently than ever before.

Technical Summary: Synthesis and Characterization of Compositionally Complex (Gd/Ho/Er/Dy)2Zr2O7 Thin Film Combinatorial Library

Problem Statement
Next-generation thermal barrier coatings (TBCs) for advanced combustion engines require ceramic materials with lower thermal conductivities and superior high-temperature phase stability compared to the current industry standard, yttria-stabilized zirconia (YSZ). While rare-earth zirconates (RE2Zr2O7) and high-entropy oxides (HEOs) offer promising properties, the vast compositional space of compositionally complex rare-earth zirconates (CCREZrOx) makes the discovery of optimal non-equiatomic formulations difficult using traditional bulk synthesis methods. Existing research often focuses on equiatomic compositions or assumes that cation size disorder alone dictates thermal transport, potentially overlooking other structural descriptors.

Methodology
The authors employed a high-throughput combinatorial approach to synthesize and characterize a thin-film library of (GdDyHoEr)2Zr2O7.

Synthesis: The library was created using combinatorial reactive magnetron sputtering (CRMS) with four rare-earth/zirconium alloy targets (Gd/Zr, Dy/Zr, Ho/Zr, Er/Zr) arranged in a co-sputtering geometry. This generated a continuous gradient of cation concentrations across a 100 mm substrate.
Structural and Chemical Characterization:
- Composition: Energy-dispersive X-ray spectroscopy (EDS) and Knudsen-surface source modeling were used to map elemental gradients.
- Crystal Structure: Laboratory and synchrotron X-ray diffraction (XRD) were utilized for phase identification and whole-pattern fitting (WPF) to extract lattice parameters and microstrain.
- Local Environment: X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were performed at multiple edges (Gd, Dy, Ho, Er, Zr) to probe valence states, coordination environments, and bond distances.
- Morphology: Automated atomic force microscopy (A-AFM) combined with multi-objective Bayesian optimization (MOBO) was used to map grain size and structural correlation length.
Thermal Property Analysis: Steady-state thermoreflectance (SSTR) was used to map thermal conductivity across the library with high spatial resolution.

Key Results

Phase Stability: XRD confirmed the formation of a single-phase defect fluorite structure (space group Fm3̅m) across the entire composition space, despite the varying ionic radii of the rare-earth cations.
Local Structure: XANES and EXAFS revealed minimal variation in cation valence states (3+ for RE, 4+ for Zr), local coordination environments, and nearest-neighbor bond distances across the library. The local environment suggests a defect-fluorite structure with some local orthorhombic-like distortion, consistent across different compositions.
Thermal Conductivity: A pronounced minimum in thermal conductivity (0.4–0.6 W m⁻¹ K⁻¹) was identified in the Dy/Gd-rich quadrant.
- This minimum did not coincide with the equiatomic composition (where configurational entropy is maximized).
- It did not coincide with the region of maximum calculated cation size disorder.
- It did not correlate with grain size or microstrain, as the region of lowest conductivity corresponded to the largest grain sizes and lowest microstrain.
Structural Correlation: The region of lowest thermal conductivity coincided with the largest experimentally measured lattice parameters and the minimum in the reflection-dependent displacement of the (200) peak ( $\Delta d_{200}$ ). This region deviated from Vegard's law predictions based on average ionic radius.

Key Contributions

Workflow Demonstration: The paper establishes a high-throughput workflow combining combinatorial thin-film synthesis with multimodal characterization (XRD, XAFS, AFM, SSTR) to rapidly identify structure-property relationships in complex ceramics.
Discovery of Non-Equiatomic Optima: The study demonstrates that the optimal thermal barrier composition in this system is non-equiatomic, challenging the common focus on equiatomic high-entropy formulations.
New Structural Descriptor: The authors propose that the way the fluorite lattice accommodates compositional complexity—specifically through structural relaxation mechanisms that expand the average lattice parameter and minimize reflection-dependent distortions ( $\Delta d_{200}$ )—is a more informative descriptor for thermal transport than cation size disorder alone.

Significance and Claims
The paper claims that while cation size disorder is a driving force, it is not the sole determinant of low thermal conductivity in these materials. Instead, the specific mechanism by which the fluorite lattice relaxes to accommodate size mismatch (resulting in anomalous lattice expansion and reduced directional distortion) appears to be the critical factor. The authors conclude that this high-throughput approach enables the identification of previously inaccessible structure-property relationships, offering a pathway to accelerate the discovery of next-generation thermal barrier materials and other complex functional ceramics. The work suggests that future optimization should focus on tuning these structural relaxation mechanisms through composition and processing rather than solely maximizing configurational entropy or size mismatch.

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