Synthesis, structure and magnetic properties of R-W-O-N (R=Nd and Eu) oxynitrides

Neodymium and europium tungsten oxynitrides have been synthesized by the nitridation of corresponding R₂W₂O₉ precursor oxides, in ammonia flow at 1173 K during 24 h. The obtained polycrystalline neodymium oxynitride phase, with NdWO_3.05N_0.95 composition, crystallizes with the tetragonal symmetry of the scheelite-type structure, space group S.G. I4₁/a (#88). The analogous europium derivative, with formula EuWO_1.58N_1.42, presents the cubic perovskite-type structure, S.G. (# 221). Unit-cell parameters, a=5.255(1) Å, c=11.399(3) Å, and a=3.976(3) Å, have been established from Rietveld refinements of collected X-ray powder diffraction patterns for the Nd and Eu- oxynitrides, respectively.Magnetic susceptibility measurements show that NdWO_3.05N_0.95 behaves as paramagnetic in a wide range of temperature T ∼50-300 K. The downwards deviation from the Curie-Weiss law below 40 K reflects the splitting of the ⁴I_9/2 ground state of Nd³⁺ experienced under the influence of a S₄ crystal field CF potential, as the successful reproduction of the magnetic susceptibility χ⁻¹_m vs. T, using semi-empirical structure-derived CF parameters, indicates. EuWO_1.58N_1.42 is paramagnetic down to 20 K, and the measured effective magnetic moment 8.01 μ_B is indicative of the presence of Eu²⁺ in this oxynitride. The observed sudden jump in the magnetic susceptibility at 20 K and the value of 6 μ_B for the saturation moment is attributed to the onset of ferrimagnetic interactions in which the Eu²⁺ and W⁵⁺ sublattices appear to be involved.     

A simple urea-based route to ternary metal oxynitride nanoparticles

Ternary metal oxynitrides are generally prepared by heating the corresponding metal oxides with ammonia for long durations at high temperatures. In order to find a simple route that avoids use of gaseous ammonia, we have employed urea as the nitriding agent. In this method, ternary metal oxynitrides are obtained by heating the corresponding metal carbonates and transition metal oxides with excess urea. By this route, ternary metal oxynitrides of the formulae MTaO₂N (M=Ca, Sr or Ba), MNbO₂N (M=Sr or Ba), LaTiO₂N and SrMoO_3−xN_x have been prepared successfully. The oxynitrides so obtained were generally in the form of nanoparticles, and were characterized by various physical techniques.     

Photocatalytic properties of CoOx-loaded nano-crystalline perovskite oxynitrides ABO2N (A = Ca,Sr, Ba,La; B = Nb,Ta)

Highly crystalline niobium- and tantalum-based oxynitride perovskite nanoparticles were obtained from hydrothermally synthesized oxide precursors by thermal ammonolysis at different temperatures. The samples were studied with respect to their morphological, optical and thermal properties as well as their photocatalytic activity in the decomposition of methyl orange. Phase pure oxynitrides were obtained at rather low ammonolysis temperatures between 740 °C (CaNbO₂N) and 1000 °C (BaTaO₂N). Particle sizes were found to be in the range 27 nm–146 nm and large specific surface areas up to 37 m² g⁻¹ were observed. High photocatalytic activities were found for CaNbO₂N and SrNbO₂N prepared at low ammonolysis temperatures. CoO_x as co-catalyst was loaded on the oxynitride particles resulting in a strong increase of the photocatalytic activities up to 30% methyl orange degradation within 3 h for SrNbO₂N:CoO_x.     

A New Oxynitride Compound of Molybdenum,Na3MoO3N — Synthesis via the Azide Route and Structure

The new oxynitride compound of molybdenum, Na₃MoO₃N was prepared via the azide route. Its crystal structure was solved and refined from X‐ray powder data (orthorhombic, Pmn2₁, a = 724.63(1), b = 624.98(1), c = 563.86(1) pm, R_p: 9.04 %; R_wp: 9.83 %). The structure consists of isolated [MoO₃N]^3— tetra‐hedra which are separated by Na⁺ cations, also in a tetrahedral coordination. It is isostructural to Na₃WO₃N which is a lower symmetry derivative of the Cu₃AsS₄ structure. Due to the small difference in the scattering lengths of nitrogen and oxygen, we were unable to distinguish between fully ordered, fully disordered, or partially ordered anions. However, from the positive SHG responses, we can deduce the acentric space group being the correct one and based on the lattice energy calculations, we have been able to identify the position most probably being occupied by nitrogen.     

Surface composition and electronic properties of indium tin oxide and oxynitride films

The surface properties of indium tin oxynitride films prepared by rf-sputtering in nitrogen atmosphere were investigated by X-ray and ultraviolet photoelectron spectroscopy as well as electron energy loss spectroscopy and Auger electron spectroscopy depth profiling. The results are compared to reference measurements on conventional rf-sputtered indium tin oxide films. The incorporated nitrogen is present in different chemical environments. Employing these different spectroscopic techniques, it was found that desorption of nitrogen from the ITON structure upon annealing is the origin of the observed drastical changes in the surface composition and electronic structure. The formation of oxygen vacancies and Sn surface segregation upon annealing is linked to improvements in the physical properties (larger spectral range of transmittance and higher conductivity) of the films.     

Nitridation of niobium oxide films by rapid thermal processing

The possibility of forming niobium oxynitride through the nitridation of niobium oxide films in molecular nitrogen by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited via sputtering onto Si(100) wafers covered with a thermally grown SiO₂ layer 100 nm thick. These as-deposited films exhibited distinct texture effects. They were processed in two steps using an RTP system. The as-deposited niobium films were first oxidized under an oxygen atmosphere at 450 °C for various periods of time and subsequently nitridated under a nitrogen atmosphere at temperatures ranging from 600 to 1000 °C for 1 min. Investigations of the oxidized films showed that samples where the start of niobium pentoxide formation was detected at the surface and the film bulk still consisted of a substoichiometric NbO_x phase exhibited distinctly lower surface roughness and microcrack densities than samples where complete oxidation of the film to Nb₂O₅ had occurred. The niobium oxide phases formed at the Nb/substrate interface also showed distinct texture. Zones of niobium oxide phases like NbO and NbO₂, which did not exist in the initial oxidized films, were formed during the nitridation. This is attributed to a “snow-plough effect” produced by the diffusion of nitrogen into the film, which pushes the oxygen deeper into the film bulk. These oxide phases, in particular the NbO₂ zone, act as barriers to the in-diffusion of nitrogen and also inhibit the outdiffusion of oxygen from the SiO₂ substrate layer. Nitridation of the partially oxidized niobium films in molecular nitrogen leads to the formation of various niobium oxide and nitride phases, but no indication of niobium oxynitride formation was found. Figure Schematic representation of the phase distribution in 200 nm Nb film on SiO₂/Si substrate after two steps annealing using an RTP system. The plot below represents the SIMS depth profiles of the nitridated sample with the phase assignment     

Topotactical growth of thick perovskite oxynitride layers by nitridation of single crystalline oxides

Thick films of the perovskite-related oxynitrides LaTiO₂N, NdTiO₂N, SrNbO₂N and SrTaO₂N were synthesised by nitridation of single crystals of the corresponding oxides with general composition ABO_3.5. The oxide crystals were obtained by optical floating zone growth. They correspond to n = 4 member of the A_nB_nO_3n+2 family of layered perovskites and were reacted at temperatures between 900 °C and 1050 °C to form the oxynitrides. Electron probe microanalysis proved the presence of nitrogen in a surface layer of a few micrometer thickness. Cross-section SEM revealed additional thin stripes of oxynitride within the bulk of the crystals, indicating that nitrogen is incorporated preferably parallel to the perovskite-type layers, which in turn are connected in a zipper-type mechanism. The formation of the desired perovskite-type oxynitrides was confirmed by X-ray diffraction. Pole figure measurements proved an epitaxial orientation ABO₂N (110)[001]  ABO_3.5 (001)[100]. The mosaicity of the oxynitrides both in polar and azimuthal direction was very small (<2°) indicating a nearly single crystalline quality of the surface layer. The nitridation of the crystals results in a dramatic change in colour. Optical spectroscopy revealed shifts of the absorption edge by more than 200 nm to longer wavelengths with respect to the parent oxides, corresponding to a reduction of the band gap energies by 1.4–1.8 eV.     

A new anatase-type phase in the system Mg-Ta-O-N

Magnesium doped tantalum oxynitrides were prepared by ammonolysis of amorphous mixed oxides. An orange colored anatase-type phase with the composition Mg_0.05Ta_0.95O_1.15N_0.85 was found. It is metastable and undergoes a phase transformation to a baddeleyite-type phase between 900 and 1000 °C. X-ray diffraction measurements indicate spacegroup I4₁/amd with lattice parameters and . A possible anion ordering was examined by theoretical methods and neutron diffraction experiments. In addition, anosovite-type (Ti₃O₅) phases Mg_xTa_3−xO_3xN_5−3x; 0?x?0.3 were obtained. The electronic spectra of all phases were investigated by UV/vis spectroscopy.     

Perovskite-type SrTi1−xNbx(O,N)3 compounds: Synthesis, crystal structure and optical properties

The synthesis, crystal structure, thermal stability and absorbance spectra of perovskite-type oxynitrides with the general formula SrTi_1−xNb_x(O,N)₃ (x=0.05, 0.10, 0.20, 0.50, 0.80, 0.90, 0.95) have been investigated. Oxide samples were prepared by a polymerized complex synthesis route and post-treated under ammonia at 850 °C for 24 h to substitute nitrogen for oxygen. Synchrotron X-ray powder diffraction (XRD) evidenced that the mixed oxide phases were all transformed into oxynitrides with perovskite-type structure during a thermal ammonolysis. SrTi_1−xNb_x(O,N)₃ with compositions x≤0.80 crystallized in a cubic and samples with x≥0.90 in a tetragonal structure. The Rietveld refinement indicated a continuous enlargement of the lattice parameters towards higher niobium content of the samples. Thermogravimetric analysis (TGA) and hotgas extraction revealed the dependence of the nitrogen incorporation upon the degree of niobium substitution. It showed that more nitrogen was detected in the samples with higher niobium content. Furthermore, TGA disclosed stability for all oxynitrides at T≤400 °C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap’s width from 3.24 eV (SrTi_0.95Nb_0.05 (O,N)₃) to 1.82 eV (SrTi_0.05Nb_0.95(O,N)₃) caused by the increasing amount of nitrogen towards the latter composition.     

Nitridation of Niobium Pentoxide Films in Ammonia by Rapid Thermal Processing

The feasibility of niobium oxynitride formation through nitridation of niobium pentoxide films in ammonia by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited by sputtering on Si(100) wafers covered by a 100 nm thick thermally grown SiO₂ layer. These as‐deposited films exhibited distinct texture effects. They were processed in three steps using an RTP system. The as‐deposited niobium films were first nitridated in an ammonia atmosphere at 1000 °C for 1 min and then oxidised in molecular oxygen at temperatures ranging from 400 to 600 °C. Those samples in which a single Nb₂O₅ phase was determined after oxidation were additionally nitridated in ammonia at 1000 °C for 1 min. Investigations show that surface roughness of the samples after oxidation of niobium films first nitridated in ammonia is lower than after direct oxidation of as‐deposited films in oxygen, although the niobium pentoxide phase formed after annealing was the same in both cases. We explain this result as being due to the large expansion of the niobium lattice during the direct oxidation of the niobium film in molecular oxygen and also to the high oxidation rate of the as‐deposited niobium film in oxygen. By incorporation of oxygen in the crystal lattice of niobium and rapid formation of niobium pentoxide, substantial intrinsic stress was built up in the film, frequently resulting in delamination of the film from the substrate. Nitrogen hinders the diffusion of oxygen in nitridated films, which leads to a decrease of the oxidation rate and thus slower formation of Nb₂O₅. Nitridation of the completely oxidised niobium films in ammonia leads to the formation of niobium oxynitride and niobium nitride phases.