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Molecular Beam Epitaxy Synthesis of Ternary Nitride PrTaN2 and its Crystal Structure Determination

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Kosuke Takiguchi, Yoshiharu Krockenberger, Eisuke Magome, Yoji Kunihashi, Hideki Yamamoto

Some materials may exist only when they are built as extremely thin layers. This study creates one such compound and works out its hidden atomic arrangement from limited evidence.

Abstract

We report the discovery of a novel ternary nitride PrTaN2 synthesized as a thin film using molecular beam epitaxy. The combination of e-beam evaporation for refractory elements and a radio-frequency nitrogen radical source enables growth under a highly nitriding environment, providing access to phases not readily obtained in bulk synthesis. Structural characterization by X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy reveals that the compound crystallizes in an orthorhombic structure and grows with a well-defined orientation on YAlO3 substrates, while remaining essentially strain-free. To determine the crystal structure from limited thin-film diffraction data, we developed a fitting procedure based on structure factors. By combining extinction rules with constraints from Wyckoff positions, the number of fitting parameters is significantly reduced, enabling reliable structure determination. Systematic exclusion of alternative candidate phases in the Pr-Ta-N system, together with structure factor fitting, identifies the space group as P222 and determines the atomic coordinates. The present results demonstrate that thin-film growth with molecular beam epitaxy can stabilize previously unexplored ternary nitrides, and establish a practical approach for structural determination in such systems. This work provides a pathway for the exploration of new complex nitride materials.

Transcript

Some materials may exist only when they are built as extremely thin layers. This study creates one such compound and works out its hidden atomic arrangement from limited evidence. These complex nitrides are expected to show superconductivity, ferromagnetism, and ferroelectricity, but the number of experimentally realized compounds remains limited.

One major challenge is controlling the very high-melting-point metals during synthesis. Another is achieving sufficiently strong and pure conditions for adding nitrogen. Previously reported ternary nitrides require high pressure or strong nitrogen-providing chemicals, making the search difficult.

This particular phase had not been reported in bulk form, suggesting that thin-film growth can provide access to phases that are not readily obtained in bulk synthesis. The growth system can handle high-melting-point elements and supply nitrogen in a more reactive form than ammonia or azide.

That creates a highly nitriding environment for film growth, and these advantages enabled the discovery of the new ternary nitride. To confirm that the observed crystalline phase was a new material, detailed X-ray measurements and electron microscopy were performed. Because several other candidate phases could exist in the same chemical system, detailed crystal analysis was used to rule out those alternatives.

The crystal’s overall symmetry and atomic positions were determined through detailed crystallographic analysis, using a fitting procedure based on its structure factor. This atomic map confirms an ordered structure: the two kinds of atoms alternate along one direction, but line up in the other.

That directional shift is the key structural clue behind the material’s unusual behavior. The new ternary nitride was synthesized and identified as a thin film. At room temperature, the film has high electrical resistance and shows paramagnetic behavior.

The crystal has an orthorhombic structure, and the thin film grows along one direction on the substrate. Detailed X-ray measurements are key to confirming that the obtained phase corresponds to the proposed compound. The main crystal peak and its higher-order reflections are observed, indicating strong crystal coherence along the growth direction.

Additional peaks show that other material phases are also present as secondary phases. The fit quality alone could not uniquely determine a single structural model, even among the best candidates. The best candidates had only small differences in fit quality, so those differences were insufficient for definitive structure determination.

Additional screening was therefore performed using the distances between praseodymium and nitrogen atoms. The thin film was grown on a substrate using a custom-designed growth system, and detailed electron microscopy and X-ray measurements revealed an orthorhombic crystal system.

The estimated dimensions of the crystal matched the X-ray measurements, and the film was essentially free of strain. For future growth, the substrate’s surface stability under nitrogen-rich conditions must be considered, not just how well its dimensions match the film.

A carefully controlled thin film can produce a nitride that bulk methods had not revealed, while its structure can still be identified from incomplete measurements. That widens the search for materials with useful electrical and magnetic behavior.

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