Abstract
Despite the wide applicability of oxynitrides from photocatalysis to refractory coatings, our understanding of the materials has been limited in terms of their thermodynamics. The configurational entropy via randomly mixed O/N or via cation vacancies are known to stabilize oxynitrides, despite the positive formation enthalpies. Here, using tin oxynitrides as a model system, we show by ab initio computations that oxynitrides in seemingly charge-unbalanced composition stabilize by forming pernitrides among metal-(O,N)6 octahedra. The nitrogen pernitride dimer, =(N-N)=, results in the effective charge of −4, facilitating the formation of nitrogen-rich oxynitrides. We report that the dimer forms only in structures with corner-sharing octahedra, since the N-N bond formation requires sufficient rotational degrees of freedom among the octahedra. X-ray photoemission spectra of the synthesized tin oxynitride films reveal two distinct nitrogen bonding environments, confirming the computation results. This work opens the search space for a novel kind of oxynitrides stabilized by N dimer formation, with specific structural selection rules.
Highlights
Doping or mixing different species of anions in ceramics’ lattice may extend the known horizon of materials properties
To study whether specific O-N ordering affects the thermodynamics of oxynitrides, we study 30 symmetrically distinct O-N configurations for each structure and composition
The formation energies of cubic fluorite phase (Pa3) show a large spread between −0.31 eV/f.u. and 0.29 eV/f.u. among different O-N configurations, while those of the other crystal structures remain nearly fixed at 0.26 eV/f.u
Summary
Doping or mixing different species of anions in ceramics’ lattice may extend the known horizon of materials properties. Sputtering rutile SnO2 under ammonia/nitrogen environment results in nitrogen-rich tin oxynitrides in fluorite-type (Spacegroup: Pa3) structure. The compound forms only in this specific phase, suggesting that the thermodynamics of phase selection in oxynitrides may differ significantly from those in its oxide counterparts; fluorite SnO2 is a high-pressure phase that forms at above 10 GPa. In addition, the nitrogen-rich composition calls for questions in the charge balancing problem. Since tin does not form 5+ oxidation state, each nitrogen incorporation into fluorite SnO2 requires oxygen vacancy formation to meet the charge neutrality:. The formation of nitrogen-rich tin oxynitrides, suggests that additional stabilization mechanism to configurational entropy may exist for the ceramics. For 7 different O-N substitution ratios, the formation energies are compared in four well-known SnO2 polymorph structures, namely the rutile (P42/mnm), CaCl2-type (Pnnm), α-PbO2 type (Pbcn) and the fluorite-type (Pa3) structures (Fig. 1). Experimental characterization efforts reveal nitrogen-nitrogen bonding in the synthesized thin films, confirming the results of the computation
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