Synthesis and Crystal Structure of δ‑TaON, a Metastable Polymorph of Tantalum Oxide Nitride

δ-TaON
was prepared by reaction of gaseous ammonia with an amorphous tantalum
oxide precursor. As a representative of the anatase structure (aristotype)
it crystallizes in the tetragonal crystal system with lattice parameters a = 391.954(16) pm and c = 1011.32(5) pm.
At temperatures between 800 and 850 °C an irreversible phase
transformation to baddeleyite-type β-TaON is observed. While
quantum-chemical calculations confirm the metastable character of
δ-TaON, its transformation to β-TaON is kinetically controlled.
The anion distribution of the anatase-type phase was studied theoretically.
In agreement with previous studies, it was found that a configuration
with maximal N–N distances is most stable. The calculated band
edge energies indicate that δ-TaON is a promising photocatalytic
material for redox reactions, e.g., water splitting.