Thermal decomposition of oxygen‐containing Ta3N5${\rm {Ta}}_3{\rm {N}}_5$.

Transition metal nitrides, especially tantalum nitrides, are pivotal for applications in extreme environments demanding excellent mechanical properties and thermodynamic stability. Among them, θ$\utheta$‐TaN, a high‐pressure polymorph of tantalum nitride with its exceptional bulk modulus (362 GPa) and hardness (31.7 GPa) promises to have many technological uses. Another nitride, Ta3N5${\rm {Ta}}_3{\rm {N}}_5$, has gained importance as a photocatalyst for water splitting using visible light. The Ta–N phase diagram indicates that the thermal decomposition of pure Ta3N5${\rm {Ta}}_3{\rm {N}}_5$ leads to the formation of ε$\uepsilon$‐TaN. However, Ta3N5${\rm {Ta}}_3{\rm {N}}_5$ usually has some amount of oxygen as an impurity mainly due to its synthesis route. We found that the θ$\utheta$‐TaN phase, which is usually observed at high pressures, is formed during the thermal decomposition of oxygen containing Ta3N5${\rm {Ta}}_3{\rm {N}}_5$. The presence of θ$\utheta$‐TaN is verified using several experimental techniques such as X‐ray diffraction, Raman spectra, high‐angle annular dark field scanning transmission electron microscopy (STEM‐HAADF), and electron energy loss spectroscopy (EELS). Elemental distribution analyzed through energy dispersion X‐ray spectroscopy (XEDS) in STEM reveals about 7 at.% of oxygen in θ$\utheta$‐TaN. First‐principle calculations are performed to examine the thermodynamic stability of oxygen substituted θ$\utheta$‐TaN and pure θ$\utheta$‐TaN via formation enthalpies, elastic constants, and phonon dispersion calculations. The computational studies confirm that oxygen in θ$\utheta$‐TaN enhances its thermodynamic stability. The calculated electron localization functions establish the bonding characteristics between Ta, N, and O, confirming the same. [ABSTRACT FROM AUTHOR]