Partially Anion-Ordered Cerium Niobium Oxynitride Perovskite Phase with a Small Band Gap

A new
phase of black color, cerium niobium oxynitride with general
formula CeNb­(O,N)3, was synthesized by thermal ammonolysis,
and its crystal structure and physical and chemical properties are
investigated. Powder X-ray and neutron diffraction has confirmed the
perovskite-structured CeNbO1.49(7)N1.51(7) phase
crystallized with the orthorhombic Pnma symmetry
with unit cell parameters of a = 5.7137(5) Å, b = 8.0567(5) Å, c = 5.7209(5) Å,
presenting a partially ordered anionic arrangement. The incorporation
of nitrogen into the oxygen sublattice was confirmed by X-ray photoelectron
spectroscopy. Complex multistage redox processes of this material
at elevated temperatures in air, including full amorphization above
200 °C, recrystallization at 650 °C, decomposition at 950
°C, and a final oxidation into the stoichiometric cerium niobium
oxide CeNbO4, were observed by simultaneous thermal analysis
and in situ X-ray powder diffraction. The band gap of the oxynitride
material was investigated by UV–vis spectroscopy and, due to
the incorporation of nitrogen into the oxygen sublattice, a significant
decrease from Eg = 2.67 eV for the precursor
oxide CeNbO4+δ to Eg =
1.79 eV was observed. The simulation by density functional theory
further confirms that the partially ordered anionic arrangement widens
the band gap (1.78 eV) and stabilizes the structure compared to the
completely ordered (layered) structure (1.22 eV).