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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Properties of Nitrides Prepared by the Ammonolysis of Magnesiothermic Niobium Powders

We have studied the effects of the specific surface area of mesoporous magnesiothermic niobium powders and ammonolysis conditions on characteristics of the reaction products. Ammonolysis was carried out in the temperature range 400–800°C using powders with specific surface areas of 18, 83, and 123 m{sup 2}/g. The results demonstrate that the ammonolysis of the powders with specific surface areas of 83 and 123 m{sup 2}/g first leads to the formation of the NbON oxynitride, which then converts into NbN with a face-centered cubic lattice at a temperature of 700°C. The NbN nitride obtained by the ammonolysis of the powder with a specific surface area of 18 m{sup 2}/g has a hexagonal close-packed structure.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Ammonolysis with N 2 -diluted NH 3 suppresses Ta( IV ) defects in BaTaO 2 N and enhances photocatalytic water oxidation

BaTaO 2 N stands out among oxynitride photocatalysts because of its ability to capture visible light and to drive the photoelectrochemical water oxidation reaction. However, its solar energy conversion performance is limited by electron–hole recombination at Ta( IV ) defects in the material. These defects are formed by overreduction of the Ta(v) oxide precursor by excess ammonia under the high temperature conditions during ammonolysis. Here we show for the first time that Ta( IV ) defect concentrations can be lowered by conducting the ammonolysis reaction in mixed NH 3 /N 2 gas. The obtained BaTaO 2 N samples crystallize in the cubic CaTiO 3 structure type and form 200–300 nm faceted nanocrystals, based on X-ray diffraction, scanning electron microscopy, and HRTEM. Electron paramagnetic resonance spectra observe the Ta( IV ) defects at g = 1.999 and confirm an 11-fold reduction for the product synthesized in mixed (0.13 : 1.0 vol) NH 3 /N 2 gas, equivalent to 1.14 × 10 16 cm −3 Ta( IV ) ions. This optimized BaTaO 2 N has nearly twice the photocatalytic oxygen evolution activity (AQE of 6.78% at 400 nm) of a reference material made with 1.0 atm ammonia and 78% higher photoelectrochemical water oxidation photocurrent (0.9 mA cm −2 at 1.23 V vs. RHE) under simulated sunlight. According to X-ray photoelectron spectroscopy, remaining Ta( IV ) defects are concentrated in the surface region of the BaTaO 2 N particles, where >50% of all Ta ions are found in the +4 oxidation state. This surface Ta( IV ) population can be directly observed in Vibrating Kelvin Probe Surface Photovoltage Spectra (VK-SPV) via its 1.2–1.4 eV photovoltage onset. Here, it suggests that the surface Ta( IV ) ions contribute empty d-states 0.5–0.7 eV below the BaTaO 2 N conduction band edge. These findings highlight how the energetics and concentrations of Ta( IV ) defects influence the photoelectrochemical water oxidation ability of BaTaO 2 N. Additionally, the work establishes ammonolysis with diluted NH 3 as a new tool to minimize defects in BaTaO 2 N and to raise its solar energy conversion efficiency toward its theoretical limit. Because of its simplicity, the reduced ammonia pressure strategy will likely be applicable to other oxynitrides, which generally suffer from overreduction problems during ammonolysis.

Salmanion, Mahya [University of California, Davis,↗
DOE OSTI2025

Ammonolysis Under NH 3 –Limiting Conditions as a Pathway to Improved LaTiO 2 N Water Splitting Photoanodes

LaTiO 2 N is a promising intermediate band gap semiconductor for the water splitting reaction, a pathway to hydrogen fuel from solar energy. However, the photoelectrochemical (PEC) activity of the material is hindered by defects, particularly Ti(III) species, which promote photocarrier recombination. These defects are formed during the high-temperature ammonolysis reaction. Here we show that improved LaTiO 2 N materials can be synthesized under NH 3 -limiting conditions by introducing N 2 to lower the NH 3 partial pressure to0.13atm.This reduces the Ti(III) defect density in the material from 6.06 × 10 16 to ∼4.61 × 10 15 cm −3 , by a factor of 13, based on electron paramagnetic resonance (EPR) spectroscopy. Any remaining Ti(III) defects are localized at the LaTiO 2 N surface, according to X-ray photoelectron spectroscopy (XPS), due to the formation of a depletion layer in the semico. Optical absorption spectra of the improved LaTiO 2 N reveal a blue-shifted band gap absorption edge and a suppressed sub-band gap absorption. Defect removal also reduces a sub-band gap surface photovoltage feature visible in the 1.0 atm reference material. The improved LaTiO 2 N supports a 1.57 mA cm −2 water oxidation photocurrent at 1.23 V RHE under simulated sunlight conditions, and an enhanced quantum efficiency of 4.5% (400 nm) for photocatalytic oxygen evolution from aqueous silver nitrate solution. Stable PEC operation is observed for over 55 min. This confirms that ammonolysis under NH 3 -limiting conditions improves the solar energy conversion properties of LaTiO 2 N. The ability to control metal ion defects in oxynitrides by varying the ammonia partial pressure during ammonolysis might be generally useful for the preparation of metal nitrides and oxynitrides.

defects↗
DOE OSTI2014

Optical and transport properties of LaTi{sub 1−x}M{sub x}(O,N){sub 3±δ} (x=0; 0.1, M=Nb{sup 5+}, W{sup 6+}) thin films prepared by plasma ammonolysis

Oxynitride thin films of composition LaTi{sub 1−x}M{sub x}(O,N){sub 3±δ} with x=0; 0.1 and M=Nb{sup 5+}, W{sup 6+} were prepared by nitridation via microwave-induced plasma (MIP) ammonolysis, which allowed keeping the nitridation time short (16 min). The higher possible oxidation states of the B-site substituents (Nb{sup 5+}, W{sup 6+}) with respect to Ti{sup 4+} caused higher N content for LaTi{sub 0.9}Nb{sub 0.1}(O,N){sub 3±δ} and LaTi{sub 0.9}W{sub 0.1}(O,N){sub 3±δ} compared to LaTiO{sub 2}N due to charge-compensation. XPS O 1s and N 1s core level and valence band spectra evidenced for increasing N content in the order LaTiO{sub 2}N<LaTi{sub 0.9}Nb{sub 0.1}(O,N){sub 3±δ}<LaTi{sub 0.9}W{sub 0.1}(O,N){sub 3±δ}. All films were N deficient comparing with their stoichiometric formulas and assuming cations in highest oxidation states. Along with increasing N content, the bandgaps decreased from 2.4 to 2.2 and 2.1 eV for LaTiO{sub 2}N, LaTi{sub 0.9}Nb{sub 0.1}(O,N){sub 3±δ}, and LaTi{sub 0.9}W{sub 0.1}(O,N){sub 3±δ} and the electronic conductivities increased along with the decreased bandgaps. The Seebeck coefficients at 860 K indicated the highest charge-carrier density for LaTi{sub 0.9}W{sub 0.1}(O,N){sub 3±δ}. -- Graphical abstract: The synthesis of LaTi{sub 1−x}M{sub x}(O,N){sub 3±δ} (x=0; 0.1, M=Nb{sup 5+}, W{sup 6+}) thin films by microwave-induced plasma ammonolysis is described and changes of their optical and transport properties upon B-site substitution are discussed. Highlights: • A new synthesis method for oxynitride thin films is presented. • The bandgap is tuneable by cationic co-substitution of W{sup 6+} and Nb{sup 5+} for Ti{sup 4+} in LaTiO{sub 2}N. • Cationic co-substitution with W{sup 6+} and Nb{sup 5+} increased the electrical conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2019

LiPON and NaPON glasses: A study of the ammonolysis of lithium and sodium metaphosphate melts

Metaphosphate glasses such as LiPO 3 and NaPO 3 are known to incorporate nitrogen in the molten state under NH 3 flow to form (Li/Na)PON glasses through the reaction: (Li/Na)PO 3 + x NH 3 → (Li/Na)PO 3–(3 x /2) N x + (3 x /2)H 2 O, by partially replacing two-coordinated oxygen with two- and three-coordinated nitrogen. After nitridation, the glasses exhibit improved properties such as increased working range, chemical durability, and ionic conductivity. In this study, LiPO 3 and NaPO 3 glasses were prepared by the conventional melting and casting method and used as base glasses for the ammonolysis procedure. The nitridation processes were carried out by remelting the base glasses at temperatures up to 780°C, under a constant NH 3 flow. The effects on the nitrogen content in the resulting (Li/Na)PON glasses caused by different processing times and masses of powder and/or bulk materials were investigated. Nitridation was successfully confirmed by CNHS chemical analyses, Raman spectroscopy, and Differential Scanning Calorimetry. Mass loss measurements after the ammonolysis process and Raman spectroscopy were used to quantify the nitrogen content into the glass structure. A new approach using a specific Raman normalization, (P–N<)/(O–P–O) sym , has been demonstrated as a reliable, simple, and fast way to determine the amounts of N incorporated to metaphosphate glass structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Magnetic properties of the ammonolysis product of α-Fe powder containing a small amount of aluminum

Magnetite was prepared containing a small amount of aluminum and its nitride was generated through low temperature ammonolysis following reduction under hydrogen. The nitrided product was determined by XRD to be a mixture of “α″-Fe{sub 16}N{sub 2}” having a slightly deformed crystal structure from α″-Fe{sub 16}N{sub 2} and the residual α-Fe. Magnetic coercivity of the mixture was decreased from the value of 150 mT obtained for the nitride product made without aluminum, due to the precipitation of nonmagnetic amorphous alumina in the low temperature nitrided bcc (Fe{sub 1−x}Al{sub x}) with x≤0.03. The aluminum-doped nitride product in which the “α″-Fe{sub 16}N{sub 2}” fraction was 30 at% exhibited magnetization at 1.5 T of approximately 200 Am{sup 2}kg{sup −1} at room temperature and its magnetic coercivity was 20 mT. - Graphical abstract: Magnetic iron nitride particles were separated by nonmagnetic amorphous γ-alumina. Magnetic coercivity was decreased by reducing the magnetic interaction between the particles. - Highlights: • Magnetic coercivity decreased in α”-Fe{sub 16}N{sub 2} like compound as a soft magnet. • Small amount of Al addition was effective in its preparation. • Magnetic interaction decreased between the “α”-Fe{sub 16}N{sub 2}” particles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Controlling Nb(IV) Defects in SrNbO 2 N Oxygen Evolution Photocatalyst by Ammonolysis With Dinitrogen–Ammonia Mixtures

Strontium niobium oxynitride (SrNbO 2 N) is a promising, corrosion resistant semiconductor for the visible light-driven water splitting reaction, a non-photovoltaic pathway to green hydrogen fuel. However, SrNbO 2 N materials made by ammonolysis usually contain Nb 4+ defect states that cause electron–hole recombination. Here, in this work, we demonstrate that such defects can be minimized by synthesizing SrNbO 2 N from metal oxides in a mixed 13%:87% (vol) NH 3 /N 2 atmosphere. According to electron paramagnetic resonance (EPR), SrNbO 2 N made in pure NH 3 contains paramagnetic impurities with g = 2.002 and 2.195, which can be assigned to lattice and surface Nb 4+ defects. These states also cause broad optical absorptions centered at 800 and 1020 nm, respectively, and the lattice defect produces a 1.55–1.63 eV signal in surface photovoltage spectra. The improved SrNbO 2 N contains five times fewer lattice Nb 4+ defects (8.95 × 10 15 cm −3 ), based on the integrated EPR signal intensity, and supports a water oxidation photocurrent of 1.07 mA cm −2 at 1.23 V versus RHE under simulated sunlight and an apparent quantum efficiency of 5.1% at 400 nm during photocatalytic oxygen evolution. Based on earlier results with LaTiO 2 N and BaTaO 2 N, dilution of NH 3 during synthesis appears generally beneficial to transition metal oxynitrides.

electron paramagnetic resonance↗
DOE OSTI2021

Quantum Calculations of VX Ammonolysis and Hydrolysis Pathways via Hydrated Lithium Nitride

Recently, lithium nitride (Li 3 N) has been proposed as a chemical warfare agent (CWA) neutralization reagent for its ability to produce nucleophilic ammonia molecules and hydroxide ions in aqueous solution. Quantum chemical calculations can provide insight into the Li 3 N neutralization process that has been studied experimentally. Here, we calculate reaction-free energies associated with the Li3N-based neutralization of the CWA VX using quantum chemical density functional theory and ab initio methods. We find that alkaline hydrolysis is more favorable to either ammonolysis or neutral hydrolysis for initial P-S and P-O bond cleavages. Reaction-free energies of subsequent reactions are calculated to determine the full reaction pathway. Notably, products predicted from favorable reactions have been identified in previous experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Steam-Assisted Ammonolysis of MoO2 as a Synthetic Pathway to Oxygenated δ-MoN

A common route for the synthesis of molybdenum nitrides is through the temperature-programmed reaction of molybdenum oxides with NH3, or ammonolysis. In this work, the role of precursor phase, gas phase chemistry (impact of H2O), and temperature profile on the reaction outcome (700 °C) was examined, which resulted in varying amounts of MoO2, H2MoO5, and the nitride phases—cubic γ (nominally Mo2N) and hexagonal δ (nominally MoN). The phase fraction of the δ phase increased with precursor in the sequence MoO2 > MoO3 > H2MoO5. Steam in the reaction gas also favored the production of δ over γ, but with too much steam, MoO2 was obtained in the product. Synthesis conditions for obtaining nearly phase-pure δ were identified: MoO2 as the precursor, 2% H2O in the gas stream, and a moderate heating rate (3 °C/min). In situ X-ray diffraction provided insights into the reaction pathway. Extensive physico-chemical analysis of the δ phase, including synchrotron X-ray and neutron diffraction, electron microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and prompt gamma activation analysis, revealed its stoichiometry to be MoO0.108(8)N0.892(8)H0.012(5), indicating non-trivial oxygen incorporation. The presence of N/O ordering and an impurity phase Mo5N6 were also revealed, detectable only by neutron diffraction. Notably, a computationally predicted MoON phase (doi: 10.1103/PhysRevLett.123.236402), of interest due to its potential to display a metal-insulator transition, did not appear under any reaction condition examined.

Pandey, Shobhit↗
DOE OSTI2010

Indium and gallium oxynitrides prepared in the presence of Zn{sup 2+} by ammonolysis of the oxide precursors obtained via the citrate route

Ammonia nitridation of indium and gallium oxide precursors obtained through a soft solution route led to their oxynitrides [In{sub 0.97}{open_square}{sub 0.03}][N{sub 0.92}O{sub 0.08}] at 660 {sup o}C and [Ga{sub 0.89}{open_square}{sub 0.11}][N{sub 0.66}O{sub 0.34}] at 850 {sup o}C, respectively, where {open_square} refers to a In or Ga vacancy. Cation vacancies in their wurtzite-type lattice were eliminated in similar preparations with the co-presence of Zn{sup 2+} by forming complete solid solutions of (InN){sub 1-x}(ZnO){sub x} and (GaN){sub 1-y}(ZnO){sub y}. The optical absorption edge shape was found to be relatively steep at the solid solution limits of x {approx} 0.23 and y {approx} 0.33 compared to the case without zinc.

36 MATERIALS SCIENCE↗
DOE OSTI2010

A study of vapor-phase deposition of silicon nitride layers by ammonolysis of dichlorosilane at lowered pressure

Deposition kinetics of silicon nitride layers at lowered reactor pressures of 10-130 Pa and temperatures in the range 973-1073 K has been studied. The equilibrium constant of the bimolecular reaction of dichlorosilane with ammonia has been calculated. The apparent activation energies calculated taking into account the experimental growth rate nearly coincide with the experimental data. Recommendations for improving the quality of silicon nitride layers are made.

36 MATERIALS SCIENCE↗
DOE OSTI2022

CALPHAD modeling of uranium nitride (UN) fabrication routes enabled by first-principles calculations

The thermochemical details of fabricating uranium nitride (UN) by ammonolysis of uranium tetraflouride (UF 4 ) were determined using density functional theory (DFT) and CALculation of PHAse Diagrams (CALPHAD) computational methods. The thermochemical data of all binary, ternary, and quaternary U-H-N-F phases were computed using DFT, and the data for the phases that have not been measured experimentally, including UN 2 and NH 4 F(g), were combined with existing experimentally-determined data for CALPHAD modeling. The DFT data were benchmarked using experimental Gibbs energy of reaction and experimental thermochemical data for individual species. Phase diagrams relevant to the ammonolysis reaction are depicted, showing regions of stability for solid U-N, U-F and U-N-F phases. An unidentified phase produced in a previous experiment was identified as UN 0.95 F 1.2 (UNF) by comparing its X-ray diffraction spectrum to the experimental spectrum, and its formation during the fabrication of UN from UF 4 is supported by the simulated phase diagram. Here, it is calculated that UN 2 can be produced by the ammonolysis of UF 4 , but requires elevated temperatures, high NH 3 (g) partial pressure, and large amounts of flowing NH 3 (g) to avoid solid flu oride impurities in the uranium nitride. Likewise, U 2 N 3 can be produced instead at temperatures greater than 980 K. The use of silane (SiH 4 ) gas was investigated as a potential additive in the ammonolysis fabrication route to speed removal of fluorine. The addition of SiH 4 (g) offers little advantage to the removal of fluorine, and adds the complication of Si 3 N 4 formation. The use of DFT to fill in missing data to perform CALPHAD calculations demonstrated here allows for the determination of more comprehensive and trustworthy phase diagrams than the use of existing experimental data alone.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Bandgap tuning in SrTi(N,O,F){sub 3} by anionic-lattice variation

Polycrystalline SrTiO{sub 3} and SrTi(O,F){sub 3} powders were synthesized by a solid-state reaction. A partial substitution of oxygen by nitrogen was subsequently carried out using thermal ammonolysis resulting in SrTi(N,O){sub 3} and SrTi(N,O,F){sub 3}. Powder X-ray diffraction (XRD) revealed a cubic perovskite structure with space group Pm-3m for all samples. The thermal ammonolysis slightly increased the lattice parameters, crystallite sizes and strain. As a result from the co-substitution of oxygen with nitrogen and fluorine for SrTi(N,O,F){sub 3}, highly distorted TiO{sub 6} octahedra were detected using X-ray absorption near edge structure (XANES) spectroscopy. The weakening of all active modes of the Raman spectra after thermal ammonolysis also indicated enhanced distortions in the local crystal structure. SrTi(N,O,F){sub 3} has the largest amount of nitrogen as well as fluorine among all four samples as determined by thermogravimetric analysis (TGA), elemental analysis and X-ray photoelectron spectroscopy (XPS). In the UV–vis spectra a distinctive shift of the absorption-edge energy was observed exclusively for the SrTi(N,O,F){sub 3} sample from 390 to 510 nm corresponding to a bandgap narrowing from 3.18 to 2.43 eV. - Graphical abstract: Figure shows the shift of the absorption-edge energy for the SrTi(N,O,F){sub 3} sample from 390 to 510 nm corresponding to a bandgap narrowing from 3.18 to 2.43 eV. Display Omitted - Highlights: • Synthesis of phase-pure SrTi(N,O,F){sub 3} via solid-state reaction. • The incorporated nitrogen contents increase by the presence of fluorine in SrTi(N,O,F){sub 3}. • Co-substitution with nitrogen and fluorine is beneficial for the bandgap narrowing compared to by only nitrogen or fluorine substitution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Rare earth niobium oxynitrides, LnNbON{sub 2-{delta}} (Ln = Y, La, Pr, Nd, Gd, Dy): Synthesis, structure and properties

Graphical abstract: Ammonolysis of LnNbO{sub 4} (Ln = rare earth or Y) leads to the formation of oxynitrides of different structures depending on the size of the rare earth. Highlights: {yields} We have carried out ammonolysis of LnNbO{sub 4} (Ln = rare earth and Y) to form oxynitrides of different structures depending on the size of the rare earth. {yields} Infrared spectroscopy shows the marked difference between oxides and the corresponding oxynitrides. {yields} The oxynitrides are stable in air upto {approx}400 {sup o}C above which an intermediate phase with nitrogen molecules attached to the oxide lattice forms. {yields} Gadolinium niobium oxynitride shows paramagnetism. -- Abstract: Ammonolysis of rare earth niobates of the type LnNbO{sub 4} (Ln = Y, La, Pr, Nd, Gd, Dy) yields oxynitrides of different structures. When Ln = La, Nd and Pr, the structure is that of an orthorhombic perovskite of the general formula LnNbON{sub 2}. As the size of the rare earth decreases, the oxynitride has a nitrogen-deficient defect fluorite (Ln = Pr, Nd, Gd), or pyrochlore (Ln = Y) structure. The IR spectra of the oxynitrides and the corresponding oxides are significantly different. Thermogravimetric analysis suggests the formation of an intermediate phase wherein the N{sub 2} molecule is attached to the oxide lattice above 400 {sup o}C and decomposes to give the oxide on heating in an oxygen atmosphere. Raman spectra of the intermediate phases show evidence for the N{identical_to}N stretching vibration. Gadolinium niobium oxynitride is found to be paramagnetic.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Perovskite-type SrTi{sub 1-x}Nb{sub x}(O,N){sub 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{sub 1-x}Nb{sub x}(O,N){sub 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 {sup o}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{sub 1-x}Nb{sub x}(O,N){sub 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 {sup o}C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap's width from 3.24 eV (SrTi{sub 0.95}Nb{sub 0.05} (O,N){sub 3}) to 1.82 eV (SrTi{sub 0.05}Nb{sub 0.95}(O,N){sub 3}) caused by the increasing amount of nitrogen towards the latter composition. -- Graphical Abstract: The system SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} is described and the changes in crystal structure, nitrogen content and width of the band gap are discussed. Display Omitted Research highlights: {yields} Synthesis of SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} perovskite-type oxynitrides via thermal ammonolysis. {yields} Co-substitution of Nb{sup 5+} enabled to adjust the amount nitrogen insertion. {yields} Tuning of the optical band gap through nitrogen.{yields} Changes of crystal structure from cubic to tetragonal with increasing niobium content.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Preparation and neutron diffraction study of Dion-Jacobson type oxynitrides LiLaTa{sub 2}O{sub 7−3x}N{sub 2x} (x = 0.09, 0.29)

Highlights: ► Dion-Jacobson oxynitrides LiLaTa{sub 2}O{sub 7−3x}N{sub 2x}□{sub x} (x < 0.3) by ammonolysis of LiLaTa{sub 2}O{sub 7}. ► Enhanced thermal stability and band gap narrowing by the oxynitride modification. ► Locations of N and □ within LiLaTa{sub 2}O{sub 7−3x}N{sub 2x} by neutron diffraction and {sup 7}Li MAS NMR. -- Abstract: Ammonolytic heating of Dion-Jacobson type layered perovskite LiLaTa{sub 2}O{sub 7} produced oxynitride derivatives LiLaTa{sub 2}O{sub 7−3x}N{sub 2x}□{sub x}via the substitution, 3O{sup 2−} → 2N{sup 3−} + □. The nitridation level increased with raising the temperature and/or increasing the time of reaction, reaching the limit at x ≈ 0.3 by the ammonolysis at 650 °C for 48 h. Upon the nitridation, tetragonal pristine lattice underwent a axis expansion together with c axis contraction. Neutron Rietveld refinement of LiLaTa{sub 2}O{sub 7} and LiLaTa{sub 2}O{sub 7−3x}N{sub 2x} (x = 0.09, 0.29) revealed that the Li-centered tetrahedra are gradually compressed along the 2-fold rotation axis, with the increase of x. Solid state {sup 7}Li nuclear magnetic resonance spectroscopy suggested that the anion vacancy resides mostly on the interlayer surface of perovskite block. As measured from the diffuse-reflectance spectroscopy, the nitridation induced marked reduction in the band gap magnitude, from 4.11 eV (x = 0) to 3.35 eV (x = 0.09) to 2.76 eV (x = 0.29).

36 MATERIALS SCIENCE↗