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

Abstract

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 baddeleyitetype 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.

Synthesis and characterization of metastable transition metal oxides and oxide nitrides

“…The Rietveld refinement was carried out using the tetragonal space group I4 1 /amd, which does not allow an ordered anion arrangement. This problem has already been discussed elsewhere multiple times [4,7,56,60]. Refined parameters are listed in Tables 5 and 6.…”

“…In our previous work we were able to synthesize a sample of anatase-type δ-TaON with a maximum yield of 43 wt % [4]. Structural refinements were performed on a sample (35 wt % δ-TaON) without γ-TaON as a side phase to avoid the above mentioned difficulties in the structure refinement.…”

“…In most cases it is very difficult to obtain phase pure materials important for an unambiguous interpretation of the data from the properties measurements. During the last years we published on synthesis and characterization of four new metastable phases: bixbyite-type V 2 O 3 [1], anosovite-type V 3 O 5 [2], VO 2 (B)-type TaON [3], and anatase-type TaON [4]. Unfortunately, only anosovite-type V 3 O 5 can be synthesized as single phase [2].…”

“…All other samples contained large amounts of side phases leading to severe problems concerning the investigation of physical properties. However, the crystal structures could be determined, also in combination with quantum-chemical methods [1][2][3][4]. Recently, we optimized the preparation techniques and in this contribution we present simple routes to phase pure bixbyitetype V 2 O 3 and VO 2 (B)-type TaON (γ-TaON) [5].…”

“…Furthermore, quantum-chemical methods allow predictions of structural, thermodynamic and electronic properties for metastable phases that have not yet been synthesized. In previous studies [4,6,7] we have calculated fundamental band gaps of TaON polymorphs with hybrid functionals in order to account for the self-interaction error of standard density functional theory (DFT) using the semilocal generalized gradient approximation (GGA). The advantages and limitations of hybrid functionals for the study of the electronic properties of solid catalysts have been discussed recently [8].…”