A new ternary nitride was not pulled from a bulk material—it was grown as a thin film under highly nitriding conditions, then identified from limited diffraction data. The result is PrTaN2, an unexplored crystal phase. Ternary complex nitrides with lanthanides are attracting attention because of their rich physical properties.

These materials are expected to show superconductivity, ferromagnetism, and ferroelectricity. However, despite many theoretical predictions of stable ternary nitrides, the number of experimentally realized compounds remains limited.

One major challenge is controlling group V to VII five-d transition metals during synthesis, whose melting points exceed three thousand degrees Celsius. Another challenge is achieving sufficiently strong and pure nitridation conditions; previously reported ternary nitrides require high pressure of about eight gigapascals or strong nitriding agents such as sodium azide.

The work reports the discovery of a novel ternary nitride, PrTaN2, achieved by thin-film growth using molecular beam epitaxy, or MBE. Compounds composed of lanthanides and tantalum have already been investigated in bulk form. However, this particular phase had not been reported to date, suggesting that thin-film growth can provide access to phases not readily obtained in bulk synthesis.

The MBE system offers advantages for the synthesis challenges: e-beam evaporators can handle high-melting-point elements. A radical source using pure nitrogen provides favorable nitridation conditions, and reactive radical species can produce a higher effective nitrogen chemical potential than bulk growth using ammonia or azide.

This highly nitriding environment enabled the discovery of the novel ternary nitride PrTaN2. To confirm that the observed crystalline phase corresponds to a new material, the study used detailed X-ray diffraction measurements and scanning transmission electron microscopy.

Because multiple candidate phases exist in the Pr-Ta-N system, detailed crystallographic analysis was used to rule out those possibilities. The space group and atomic coordinates were determined with a fitting procedure based on the structure factor.

PrTaN2 thin films were grown on YAlO3, or YAO, zero-zero-one-c substrates in a custom-designed MBE system equipped with e-beam evaporators for generating Pr and Ta fluxes. Controlling the flux is challenging because tantalum has a melting point of three thousand seventeen degrees Celsius, but electron impact emission spectroscopy precisely controlled the flux.

The growth rate was maintained at zero point zero two five nanometers per second, and twenty minutes of growth resulted in a film thickness of thirty nanometers. For nitridation, pure nitrogen was introduced into a radio-frequency radical source operated at thirteen point fifty-six megahertz with a radio-frequency power of four hundred watts.

The nitrogen flow rate was two point zero standard cubic centimeters per minute, corresponding to a background nitrogen pressure of one times ten to the minus five torr, while the growth temperature was eight hundred degrees Celsius. For structural determination, X-ray diffraction measurements were performed at room temperature using both the SAGA Light Source and a Bruker D8.

The SAGA Light Source had a photon flux at least one hundred times higher than the copper source, which was useful for detecting weak diffraction peaks from small crystalline volumes. The copper source provided constant X-ray intensity, whereas the SAGA Light Source intensity decreased over time.

Therefore, the copper source was used for fitting the measured X-ray diffraction peaks, and the reported lattice parameters came from Bruker D8 data. The study synthesized and identified the novel ternary nitride PrTaN2. The film resistivity was higher than ten to the third ohm meters at room temperature, and paramagnetic behavior was observed.

X-ray diffraction revealed an orthorhombic crystal system with lattice constants of three point ninety-seven plus or minus zero point zero eight angstroms, four point zero zero plus or minus zero point zero eight angstroms, and four point zero two plus or minus zero point zero one angstroms.

The PrTaN2 thin film grew along the c direction on a YAO zero-zero-one-c substrate. PrTaN2 is directly grown on the substrate rather than obtained through a reduction process. The formation of perovskite PrTaN3 was not observed within the accessible growth conditions.

In PrTaN3, the valences of Pr and Ta are expected to be plus four and plus five, whereas both Pr and Ta are considered to be in the plus three state in PrTaN2. The ionization energy of Pr three plus is thirty-eight point nine electron volts lower than that of Pr four plus, which favors the formation of the trivalent state.

High-angle annular dark-field scanning transmission electron microscopy images were obtained along the one-zero-zero and one-one-zero directions of PrTaN2. Clear atomic ordering was observed in both crystallographic directions.

Because high-angle annular dark-field scanning transmission electron microscopy intensity is related to atomic number, comparison of line profiles allows identification of the atomic columns corresponding to Pr and Ta. The red curve had higher intensity than the blue curve, indicating Ta and Pr columns, respectively.

In the line profile for the [100] direction, the red and blue peaks alternate laterally, whereas in the [110] profile they remain aligned across successive oscillations. The FFT panels reinforce this: each pair shares a dominant peak, with additional peaks visible at the second and third harmonics in the [100] case and at the second harmonic in the [110] case.

Detailed X-ray diffraction measurements are key to confirming that the obtained phase corresponds to PrTaN2. The PrTaN2 zero-zero-one peak and higher-order diffractions up to zero-zero-six were observed, indicating high crystalline coherence along this direction.

Ta three N five zero-one-two and zero-two-three peaks, together with their higher-order diffraction peaks, were also observed as secondary phases. Ta three N five is metastable in the Ta-N system compared with TaN and Ta two N.

Its presence as a secondary phase suggests that growth occurred under a high effective nitrogen chemical potential. Under these conditions, enhanced nitrogen incorporation may facilitate nitrogen-rich phases such as PrTaN2 that are not readily obtained under bulk equilibrium synthesis conditions.

For all measured crystallographic planes, the experimental d values agreed with the calculated values within errors. The phi scans showed consistent phi angles between neighboring peaks for the two-zero-four and three-one-two planes. Together, these results are consistent with an orthorhombic crystal system, and the estimated lattice constants agree with the experimental data.

The ratios q sub z over q sub x for the one-zero-three and one-one-three reflections were nearly identical for the PrTaN2 film and the YAO substrate, indicating that the film is essentially strain-free. The lattice constants were therefore not significantly affected by substrate-induced strain.

The in-plane crystal orientation was not completely arbitrary but reflected the four-fold symmetry of the YAO substrate. This behavior suggests preferentially oriented growth influenced by the substrate rather than fully coherent epitaxy. Figure four shows high-resolution reciprocal-space maps around the YAO (103)c and (113)c substrate reflections.

In both panels, distinct PrTaN₂ film peaks appear alongside the substrate peak, with intensity plotted on a logarithmic color scale. The aligned reciprocal-space positions indicate a relaxed film sharing the substrate’s crystallographic axis, while the observed (113) film reflection also helps distinguish the structure from candidate phases forbidden by their extinction rules.

The choice of unit cell based solely on X-ray diffraction data is not necessarily unique because there remains a degree of freedom in assigning h-k-l indices in terms of integer multiples. To constrain this ambiguity and determine a specific chemical formula and crystal structure, diffraction peaks from multiple crystallographic planes must be analyzed together with scanning transmission electron microscopy observations.

The study systematically ruled out alternative candidate phases using combined X-ray diffraction and scanning transmission electron microscopy analyses. Pr two Ta N three with orthorhombic symmetry is another possible candidate, and a similar compound, Ce two Ta N three, has been reported.

However, this possibility can be ruled out based on the high-angle annular dark-field scanning transmission electron microscopy measurements. STEM image simulations for Pr two Ta N three were performed using ReciPro.

In the one-zero-one direction, the experimental data show an atomic sequence of Pr, Ta, Pr, Ta. The fitting procedure determines the crystal structure even from thin-film X-ray diffraction data by converting observed peak intensities into observed structure factors and selecting candidate space groups from extinction rules.

Allowed atomic positions are then assigned according to Wyckoff positions. An optimization problem minimizes the root-mean-square residual between experimental and calculated structure-factor values for the observed h-k-l reflections.

Finally, root-mean-square values are compared among space groups, followed by screening based on chemical bond length. The procedure assumes the smallest translational periodicity compatible with the observed diffraction pattern and adopts a z equals one orthorhombic average structure.

On the left, the flowchart visibly loops back when not all space groups have been explored or when the RMS condition is not yet minimized, then proceeds to compare the RMS values. On the right, the reproduced structure shows a central Pr atom surrounded by Ta and two differently labeled nitrogen sites, N1 and N2, within the orthorhombic axes.

Figure two shows a symmetric X-ray diffraction scan along the YAO [001] direction, measured at SAGA-LS using a wavelength of one point zero zero angstroms. The labeled PrTaN2 reflections extend from zero zero one through zero zero six, supporting crystalline coherence along this direction.

Peaks from Ta3N5, including the zero one two and zero two three reflections and higher-order peaks, also appear, indicating secondary-phase formation and a nitrogen-rich growth environment. Figure three presents X-ray diffraction scans targeting seven PrTaN₂ crystallographic planes through different tilt angles, along with an in-plane scan at chi equal to zero point five degrees.

The labeled PrTaN₂ peaks appear alongside YAlO₃ substrate reflections, providing multiple diffraction positions for testing the proposed orthorhombic structure and estimating d-spacings, which are summarized in Table one. The Pr and Ta positions were taken from the scanning transmission electron microscopy images, further reducing the number of free variables and making the statistical errors in the fitting results sufficiently small.

The cation positions determined from STEM placed Pr at zero point fifty-one plus or minus zero point zero four, zero point fifty-three plus or minus zero point zero four, and zero point forty-nine plus or minus zero point zero one relative to Ta at zero, zero, zero. Within the error bars, the only position satisfying the PrTaN2 stoichiometry and the allowed Wyckoff-position constraints was zero point five, zero point five, zero point five.

The actual fitting therefore used Ta at zero, zero, zero and Pr at zero point five, zero point five, zero point five. The root-mean-square values alone could not uniquely determine a single structural model, even among the best candidates. For the top five structurally distinguishable candidates, root-mean-square values ranged only from zero point one seven three five to zero point one seven five zero, so the differences were insufficient for definitive structure determination.

The worst root-mean-square value was forty-four point five for Pmmm. Additional screening was therefore performed using Pr-N bond distances. For cases one through three, the resulting Pr-N distances were about two point zero angstroms, and no known compound was reported to exhibit such a short Pr-N bond distance.

The simple sum of the ionic radii of Pr and N is approximately two point five to two point eight angstroms, while the Pr-N distance in rock-salt PrN is two point fifty-eight angstroms; for these reasons, only category four was considered physically plausible. In category four, the Pr-N distance is approximately two point eight angstroms, and the estimated space group is P222 with a best root-mean-square value of zero point one seven four.

The atomic coordinates and bond lengths are shown in Tables three and four, while Figure five-b shows the crystal structure of PrTaN2 using the estimated atomic coordinates. One possible way to reconcile the discrepancy is to introduce excess nitrogen at interstitial sites.

However, there are too many possible interstitial sites to establish a unique structural model. Determining the positions of excess nitrogen atoms will require further structural studies, such as single-crystal neutron diffraction measurements on PrTaN2.

A novel PrTaN2 thin film was grown on a YAO substrate using a custom-designed molecular beam epitaxy system. Detailed STEM and X-ray diffraction measurements revealed an orthorhombic crystal system, and the estimated lattice constants were consistent with the X-ray diffraction data.

Reciprocal-space-map measurements indicated that the film is essentially strain-free. For substrate selection, not only lattice mismatch but also surface stability under nitridation conditions should be considered. The work shows that molecular beam epitaxy can stabilize previously unexplored ternary nitrides and that structure-factor fitting, constrained by microscopy and chemistry, can determine their crystal structure from thin-film data.