Deposition and dielectric characterization of strontium and tantalum-based oxide and oxynitride perovskite thin films

We have synthesized the composition x = 0.01 of the (Sr_1-xLa_x)₂(Ta_1-xTi_x)₂O₇ solid solution, mixing the ferroelectric perovskite phases Sr₂Ta₂O₇ and La₂Ti₂O₇. Related oxide and oxynitride materials have been produced as thin films by magnetron radio frequency sputtering. Reactive sputter deposition was conducted at 750 °C under a 75 vol.% (Ar) + 25 vol.% (N₂,O₂) mixture. An oxygen-free plasma leads to the deposition of an oxynitride film (Sr_0.99La_0.01) (Ta_0.99Ti_0.01)O₂N, characterized by a band gap E_g = 2.30 eV and a preferential (001) epitaxial growth on (001) SrTiO₃ substrate. Its dielectric constant and loss tangent are respectively Epsilon' = 60 (at 1 kHz) and tanDelta = 62.5 × 10⁻³. In oxygen-rich conditions (vol.%N₂ ≤ 15%), (110) epitaxial (Sr_0.99La_0.01)₂(Ta_0.99Ti_0.01)₂O₇ oxides films are deposited, associated to a larger band gap value (E_g = 4.55 eV). The oxide films permittivity varies from 45 to 25 (at 1 kHz) in correlation with the decrease in crystalline orientation; measured losses are lower than 5.10⁻³. For 20 ≤ vol.% N₂ ≤ 24.55, the films are poorly crystallized, leading to very low permittivities (minimum Epsilon' = 3). A correlation between the dielectric losses and the presence of an oxynitride phase in the samples is highlighted.     

Enhanced Photoactivity from Single‐Crystalline SrTaO2N Nanoplates Synthesized by Topotactic Nitridation

There are few methods yielding oxynitride crystals with defined shape, yet shape‐controlled crystals often give enhanced photoactivity. Herein, single‐crystalline SrTaO₂N nanoplates and polyhedra are achieved selectively. Central to these synthetic advances is the crystallization pathways used, in which single‐crystalline SrTaO₂N nanoplates form by topotactic nitridation of aerosol‐prepared Sr₂Ta₂O₇ nanoplates and SrTaO₂N polyhedra form by flux‐assisted nitridation of the nanoplates. Evaluation of these materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) showed improved performance for the SrTaO₂N nanoplates, with a record apparent quantum efficiency (AQE) of 6.1 % for OER compared to the polyhedra (AQE: 1.6 %) and SrTaO₂N polycrystals (AQE: 0.6 %). The enhanced performance from the nanoplates arises from their morphology and lower defect density. These results highlight the importance of developing new synthetic routes to high quality oxynitrides.     

Structural and thermal investigation of Ta–25 mass% Cu alloy prepared by mechanosynthesis route

A mixture of Ta and 25 mass% Cu elemental powders was subjected to mechanical alloying in a high-energy ball mill up to 60 h. The results are composite particles formed by nanocrystalline Cu and amorphous Ta phases. Thermal stability of amorphous was investigated by DSC. The XRD, FTIR and EDX analyses of Ta–25 mass% Cu powder milled for 60 h performed after DSC at 800 and 900 °C have revealed large amounts of Ta nitride and Ta oxides even though the milling process was done in Ar atmosphere. This is due to high reactivity of Ta fine particles with oxygen and nitrogen from air. During manipulations of the powder (taking samples from vials and its investigation), the adsorption phenomena on its surface occur, and both surface-adsorbed N₂ and O₂ are processed with powder and embedded in it. While heating of Ta–25% Cu milled powder in DSC, nitrogen and oxygen diffusion into tantalum is activated, and Ta₂N and TaO₂/Ta₂O₅ compound forms.

     

Low temperature high-pressure synthesis of LnNiO3 (Ln = Eu,Gd) in molten salts

We report a new low temperature method for the synthesis of LnNiO₃ (Ln = Eu, Gd) at 400 °C under 180 bar oxygen pressure with the flux method. Utilization of the LiCl/KCl flux allowed for a decrease of the reaction temperature from 1000 °C and resulted in the synthesis of pure phase compounds. These materials have been characterized by powder X-ray diffraction and thermogravimetric analysis. LnNiO₃ (Ln = Eu and Gd) compounds crystallize in the orthorhombic GdFeO₃-type perovskite structure (space group: Pbnm). Both materials decompose to Ln₂O₃ and NiO at 775 °C under a nitrogen atmosphere and undergo reduction to Ln₂O₃ and Ni metal (at 385 °C and 340 °C for Eu and Gd, respectively) under a hydrogen atmosphere (10% H₂/N₂). Attempts to prepare the first T′-type infinite layer compound with Ni²⁺, EuNiO₂, by low temperature reduction of EuNiO₃ were unsuccessful.     

Strukturuntersuchungen an den Oxidnitriden SrTaO2N,CaTaO2N und LaTaON2 mittels Neutronen‐ und Röntgenbeugung

Structural Investigations on the Oxidenitrides SrTaO₂N, CaTaO₂N and LaTaON₂ by Neutron and X‐ray Powder Diffraction The crystal structures of the perovskite related oxidenitrides SrTaO₂N, LaTaON₂ and CaTaO₂N have been determined with special regard to the structures of the respective anionic partial structure. The structure refinements were performed by individual Rietveld analyses of both X‐ray and neutron powder diffractograms and in addition by joint refinements in order to confirm the results. Both refinement methods yield consistent structure solutions. At least the first two compounds have fully ordered anionic sublattices. The crystal structure of SrTaO₂N has been solved in the space group I4/mcm (a = 5.7049(3) Å, c = 8.0499(5) Å, R_p = 0.0706, R_wp = 0.0904, reflections: 70 (neutrons)/36 (X‐ray), R(F²)(n) = 0.147, R(F²)(X) = 0.0952), with an ordered anionic partial structure. LaTaON₂ crystallizes monoclinic (C2/m, a = 8.0922(3) Å, b = 8.0603(2) Å, c = 5.7118(2) Å, β = 134.815(1)°, R_p = 0.0592, R_wp = 0.0766, reflections: 235(n)/113(X), R(F²)(n) = 0.0944, R(F²)(X) = 0.165) and also shows a totally ordered distribution of the anions. In the case of CaTaO₂N (Pnma, a = 5.6239(3) Å, b = 7.8954(4) Å, c = 5.5473(3) Å, R_p = 0.0503, R_wp = 0.0656, reflections 206(n)/110(X), R(F²)(n) = 0.0985, R(F²)(X) = 0.0405) slightly unbalanced displacement parameters (neutron data, ordered O/N distribution model) hint at a partial exchange of oxygen and nitrogen.     

Strontium tantalum oxides with perovskite-type structure: synthesis and dye photodecomposition properties

In this work the preparation and photocatalytic properties of strontium tantalum oxides with perovskite-type structures are presented. The perovskite-type oxides were prepared by the sol–gel method and annealed at 800, 900 and 1,000 °C for 36 h. Before and after annealing the solids were characterized by XRD, N₂ adsorption (BET), UV–visible (diffuse reflectance), FTIR, TGA–DTA, and SEM-EDS techniques. The X-ray diffraction patterns of the samples showed the coexistence of three strontium tantalate oxides, Sr₂Ta₂O₇, SrTa₄O₁₁, and Sr₅Ta₄O₁₅, the relative amounts of which were highly dependent on the annealing temperature. It has been proposed that the photoactivity of the oxides in the decomposition of crystal violet dye could be related to the proportion of the Sr₂Ta₂O₇ phase in the annealed samples.     

Structure and electrical properties of Y,Fe-based perovskite mixed conducting composites fabricated by a modified polymer precursor method

In this work, samples of Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ with 20, 40, 60 and 80 mol% of iron amount were prepared by a low-temperature polymer precursor method. The SEM-EDS analysis proved that analyzed Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ samples were composites of two Ti- and Fe-rich perovskite samples. This kind of composite consists of two phases in which one has a good ionic and the other electronic conductivity, which makes such a composite a potential mixed ionic and electronic conductors (MIECs) material. The total electrical conductivities of analyzed samples were measured in air atmosphere (cathode conditions in Solid Oxide Fuel Cell). The values changed from ∼10⁻³ to 10⁻¹ S cm⁻¹ and depended on the ratio between two observed perovskite phases. The 0.12 S cm⁻¹ conductivity value at 800 °C for sample with the highest amount of Fe-rich perovskite in the structure makes this composite material a candidate for air electrode in electrochemical devices.     

Nitrido-silicate. II [1]. Hochtemperatur-Synthesen und Kristallstrukturen von Sr2Si5N8 und Ba2Si5N8

Nitrido-Silicates. II. High Temperature Syntheses and Crystal Structures of Sr₂Si₅N₈ and Ba₂Si₅N₈ Pure Sr₂Si₅N₈ and Ba₂Si₅N₈ were obtained by reaction of silicon diimide with metallic strontium and barium, respectively. The reactions have been carried out under nitrogen atmosphere in a specially developed high-frequency furnace at temperatures between 1 550 and 1 650°C. Sr₂Si₅N₈ (Pmn2₁, a = 571.0(2), b = 682.2(2), c = 934.1(2) pm, Z = 2, R = 0.037, wR = 0.021) and Ba₂Si₅N₈ (Pmn2₁, a = 578.3(2), b = 695.9(2), c = 939.1(2) pm, Z = 2, R = 0.022, wR = 0.018) are isotypic and contain M²⁺ ions as well as a three-dimensional covalent network structure of corner-sharing SiN₄ tetrahedra. Two sorts of N occur with molar ratio 1 : 1 which are bonded to two and three Si, respectively. Predominantly, the N which are bonded to two Si belong to the coordination spheres of the M²⁺ ions.     

Polymorphie von SrTa2O6

Polymorphism of SrTa₂O₆ Orthorhombic SrTa₂O₆ is a new low temperature modification related to orthorhombic CaTa₂O₆. SrTa₂O₆(orh.) was obtained when the wellknown modification SrTa₂O₆(TTB) which is related to the tetragonal tungsten bronzes was heated in the presence of a transporting agent (chlorine) or a mineralizer (melt of B₂O₃) at temperatures below 1150°C. It could be prepared by the reaction of a 1:1 mixture of Sr(NO₃)₂ or SrCO₃ with Ta₂O₅ in a sealed quartz glass tube as well. SrTa₂O₆(orh.) also occurred as an intermediate phase of the reaction of the corresponding 1:2 mixture at temperatures below 900°C (e. g. 840°C). Indexing of Guinier powder patterns led to the following unit cell: a = 11.006 Å, b = 7.638 Å, c = 5.622 Å. At temperatures above 1220°C SrTa₂O₆(orh.) changes (in air) to SrTa₂O₆(TTB). A reversal of this transition could not be achieved without the presence of a mineralizer or a transporting agent. Ca_xSr_1?xTa₂O₆ solid solutions of the low temperature form could not definitely be established. However, at 1300°C solid TTB solutions of Ca_xSr_1?xTa₂O₆ were formed. For x > 0.05 the TTB unit cells are orthorhombically distorted. For x ≥ 0.85 the x-ray powder patterns of the solid solutions looked like the one of CaTa₂O₆(orh.) and no TTB-structure was observed at 1300°C.     

N2O and NO emissions during wastewater denitrification step: Influence of temperature on the biological process

The denitrification process occurring in wastewater treatment plants (WWTPs) is responsible for nitrous oxide (N₂O) and nitric oxide (NO) emissions. These compounds indirectly lead to the global warming. In this study, we investigated the impact of the temperature on N₂O and NO emissions. Experiments were achieved at PH 7 in a batch reactor with acetate as the carbon source. The nitrogen source was nitrates (NO₃⁻) and the COD/N ratio was set to three. Results showed that NO and N₂O emissions increased when the temperature decreased. NO emissions appeared only at 10 °C and 5 °C, with respectively 8% and 18% of the total denitrified nitrogen. N₂O emissions increased from 13 to 40 then 82% of the total denitrified nitrogen, respectively at 20, 10 and 5 °C. Several hypotheses were suggested to explain these results: a general enzymatic slow down, enzymatic inhibitions, electron donor competition between the different enzymes and metabolic pathway alterations.     

Enhanced photocatalytic degradation of rhodamine B under visible light irradiation on mesoporous anatase TiO2 microspheres by codoping with W and N

Mesoporous anatase TiO₂ microspheres were prepared via solvothermal method. Ammonium tungstate was used as the W source, and ammonia gas flowing in an ammonothermal reactor as the N source for codoping. TiO₂:(W,N) mesoporous microspheres, which were prepared from solvothermal treatment at 160 °C for 16 h and thermal ammonolysis at 500 °C for 2 h after calcination, have high specific surface area of 106 m² g⁻¹. XPS results indicate the presence of N_O, N_i and W⁶⁺ in the codoped mesoporous TiO₂ microspheres. Monodoping with N shifts the absorption band edge of anatase TiO₂ from ultraviolet region to visible region. Although codoping with W makes the visible light absorbance decrease a little, the photocatalytic degradation of a cationic dye rhodamine B (RhB) on mesoporous TiO₂:(W,N) microspheres is increased to 1.7 times of that on mesoporous TiO₂:N microspheres. This may due to decreasing recombination centers by W-doping charge compensation.     

Real structure of SrSi2O2N2

SrSi₂O₂N₂ is an important host lattice for Eu²⁺ doped phosphors. Its crystal structure (space group P1, a = 7.0802(2) Å, b = 7.2306(2) Å, c = 7.2554(2) Å, α = 88.767(3)°, β = 84.733(2)°, γ = 75.905(2)° and V = 358.73(2) Å³, Z = 4) is isotypic with EuSi₂O₂N₂: highly condensed silicate layers are separated by Sr²⁺. The samples are characterized by pronounced real structure effects owing to pseudosymmetry of partial structures. Polysynthetic twinning with domains of various sizes is ubiquitous and oriented intergrowth of domains with different orientations has also been observed and analysed in detail by means of electron diffraction and high-resolution electron microscopy. These effects also affect the X-ray powder pattern and were taken into account in a Rietveld refinement.     

Sr2(OLi2Sr4)[CrN4]2, ein Nitridochromat(VI)‐Oxid mit Sauerstoff in tetragonal‐bipyramidaler Koordination durch Lithium und Strontium

Sr₂(OLi₂Sr₄)[CrN₄]₂, a Nitridochromate(VI)‐Oxide with Oxygen in Tetragonal‐Bipyramidal Coordination by Lithium and Strontium Green gleaming crystals of Sr₂(OLi₂Sr₄)[CrN₄]₂ were prepared by reaction of Li, Sr and CrN/Cr₂N (approximate 1 : 1 mixture) with flowing nitrogen at 900 °C (molar overall composition Li : Sr : Cr = 6 : 1 : ∼3). The oxygen content results from a leak in the gas supply. The crystal structure was determined by single crystal methods (triclinic; P1; a = 615.87(9) pm, b = 682.50(10) pm, c = 754.30(8) pm, α = 82.302(14)°, β = 75.197(10)°, γ = 70.133(13)°; Z = 1) and contains distorted tetragonal bipyramids (OLi₂Sr₄)⁸⁺ and [Cr^VIN₄]^6–‐tetrahedra besides Sr²⁺.     

Synthese,Kristallstruktur und Eigenschaften eines neuen Sialons – SrSiAl2O3N2

Synthesis, Crystal Structure, and Properties of a New Sialon – SrSiAl₂O₃N₂ The sialon SrSiAl₂O₃N₂ was obtained as a coarsly crystalline solid by reaction of silicon diimide, aluminum nitride, and strontium carbonate under N₂ atmosphere in a high-frequency furnace at 1650 °C. According to the single-crystal structure determination the title compound is isotypic with LnSi₃N₅ (Ln = La, Ce, Nd, Pr). SrSiAl₂O₃N₂ (P2₁2₁2₁, a = 491.98(6), b = 789.73(7), c = 1134.94(18) pm, Z = 4, R1 = 0.0439, wR2 = 0.0939). In the solid a three-dimensional network structure of corner sharing SiON₃, AlO₃N, and AlO₂N₂ tetrahedra occurs. Lattice energetic calculations using the MAPLE concept confirm an unequivocally correct crystallographic differentiation between N and O as well as Al and Si atoms, respectively (Al–O: 167.4(5)–170.6(6); Al–N: 175.4(6)–179.4(6); Si–O: 171.2(6); SiN: 176.7(6)–179.6(6) pm). The Sr²⁺ ions are located in the voids of the (SiAl₂O₃N₂)^2– framework (Sr–O: 250.4(6)–304.2(6); Sr–N: 287.4(6) 318.2(6) pm).     

A cobalt-free electrode material La0.5Sr0.5Fe0.8Cu0.2O3 − δ for symmetrical solid oxide fuel cells

Cobalt-free perovskite oxide La_0.5Sr_0.5Fe_0.8Cu_0.2O_3 − δ (LSFC) was applied as both anode and cathode for symmetrical solid oxide fuel cells (SSOFCs). The LSFC shows a reversible transition between a cubic perovskite phase in air and a mixture of SrFeLaO₄, a K₂NiF₄-type layered perovskite oxide, metallic Cu and LaFeO₃ in reducing atmosphere at elevated temperature. The average thermal expansion coefficient of LSFC in air is 17.7 × 10^− 6 K^− 1 at 25 °C to 900 °C. By adopting LSFC as initial electrodes to fabricate electrolyte supported SSOFCs, the cells generate maximum power output of 1054, 795 and 577 mW cm^− 2 with humidified H₂ fuel (~ 3% H₂O) and 895, 721 and 482 mW cm^− 2 with humidified syngas fuel (H₂:CO = 1:1) at 900, 850 and 800 °C, respectively. Moreover, the cell with humidified H₂ fuel demonstrates a reasonable stability at 800 °C under 0.7 V for 100 h.     

Oxygen permeation and phase structure properties of partially A-site substituted BaCo0.7Fe0.225Ta0.075O3-δ perovskites

Ba_0.9R_0.1Co_0.7Fe_0.225Ta_0.075O_3-δ (BRCFT, R = Ca, La or Sr) membranes were synthesized by a solid-state reaction. Metal cation Ca²⁺, La³⁺ or Sr²⁺ doping on A-site partially substituted Ba²⁺ in BaCo_0.7Fe_0.225Ta_0.075O_3-δ oxides, and its subsequent effects on phase structure stability, oxygen permeability and oxygen desorption were systematically investigated by XRD, TG-DSC, H₂-TPR, O₂-TPD techniques and oxygen permeation experiments. The partial substitution with Ca²⁺, La³⁺ or Sr²⁺, whose ionic radii are smaller than that of Ba²⁺, succeeded in stabilizing the cubic perovskite structure without formation of impurity phases, as revealed by XRD analysis. Oxygen-involving experiments showed that BRCFT with A-site fully occupied by Ba²⁺ exhibited good oxygen permeation flux under He flow, reaching about 2.3 mL·min⁻¹ ·cm⁻² at 900 °C with 1 mm thickness. Of all the membranes, BLCFT membrane showed better chemical stability in CO₂, owing to the reduction in alkalinity of the mixed conductor oxide by La doping. In addition, we also found the stability of the perovskite structure under reducing atmospheres was strengthened by increasing the size of A-site cation (Ba²⁺>La³⁺>Sr²⁺>Ca²⁺).     

Synthese und Kristallstruktur des ersten Oxonitridoborates — Sr3[B3O3N3]

Synthesis and Crystal Structure of the First Oxonitridoborate — Sr₃[B₃O₃N₃] The cyclotri(oxonitridoborate) Sr₃[B₃O₃N₃] was synthesized at 1450 °C as coarsely crystalline colourless crystals by the reaction of SrCO₃ with poly(boron amide imide) using a radiofrequency furnace. The structure was solved by single‐crystal X‐ray diffractometry (Sr₃[B₃O₃N₃], Z = 4, P2₁/n, a = 663.16(2), b = 786.06(2), c = 1175.90(3) pm, η = 92.393(1)°, R1= 0.0441, wR2 = 0.1075, 1081 independent reflections, 110 refined parameters). Besides Sr²⁺ there are hitherto unknown cyclic [B₃O₃N₃]^6— ions (B—N 143.7(10) — 149.1(9) pm, B—O 140.5(8) — 141.4(8) pm).     

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.     

Perovskite related phases in the Sr5Nb4O15 - SrTiO3 - Sr4Nb2O9 system

Phase relations have been investigated within the Sr₅Nb₄O₁₅−SrTiO₃−Sr₄Nb₂O₉ region of the SrO−Nb₂O₅−TiO₂ system with a view to clarifying the occurrence of fully oxidised perovskite related phases. Overall phase analysis was carried out by powder X-ray diffraction and microstructures were clarified by transmission electron microscopy. There is only one main composition triangle in this area at 1350°C. The tie-line between Sr₅Nb₄O₁₅ and SrTiO₃ contains a homologous series of hexagonal layered perovskite phases including Sr₆Nb₄TiO₁₈ and Sr₇Nb₄Ti₂O₂₁. The phase Sr₄Nb₂O₉ is a nonstoichiometric phase with a disordered perovskite structure. There is some extension of this phase along the Sr₄Nb₂O₉−SrTiO₃ tie-line, but SrTiO₃ does not show a significant composition range. Samples with a composition Sr₄Nb₂O₉, when heated at 900°C show several ordered modifications. Samples along the Sr₄Nb₂O₉−SrTiO₃ tie-line which are annealed at 900°C contain these ordered materials together with samples showing considerable short range order which increases as the Ti content increases.     

Atmospheric control of gel-oxide transformation in sol–gel derived Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> fibers

Via sol–gel processing metal–organic fibers were produced and dried up to 140 °C. For these gel fibers the influence of a treatment in different atmospheres was investigated for the temperature range of 200–850 °C. The atmospheres were nitrogen, water vapor, evaporated nitric and hydrochloric acid and evaporated hydrogen peroxide. In the presence of moisture and especially with acidic moisture fibers were transformed almost completely to their oxide composition (82 mol% Al₂O₃·18 mol% Y₂O₃). In these inorganic amorphous structures considerable differences were observed on several structural levels. On the atomic scale, the coordination of Al ions was investigated by ²⁷Al MAS NMR and skeletal density by He-pycnometry. Porosity in the nm scale was characterized by N₂-sorption. As a macroscopic effect of different treatment atmospheres, the longitudinal shrinkage was observed. For fibers treated at 500 °C the relative shrinkage varied by 100% (comparing water vapor and nitrogen atmosphere). No simple correlation between the release of organic constituents, the formation of porosity and the shrinkage could be found. These aspects were controlled by the rigidity of the inorganic network against atomic reconstitution. The kind of atmosphere was found to be an effective parameter to control various aspects of the xerogel structure.