Molecular beam epitaxy synthesis of ternary nitride PrTaN$_2$ and its crystal structure determination
We report the discovery of a novel ternary nitride PrTaN$_2$ 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 YAlO$_3$ 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.