K2NiF4-type Oxides Research Articles

Synthesis of nano-crystalline K2NiF4-type phases La1+xSr1-xCoO4 (x = 0.0, 0.2, and 0.5): Effect of mixed valence state of Co on structural, magnetic and photocatalytic properties

A lot of work has already been reported on bulk K2NiF4-type layered perovskite oxides but hardly is known about their nano-crystalline state as they are generally prepared at high temperatures. In this connection, we have synthesized nanocrystalline La1+xSr1-xCoO4 layered perovskites oxides via sol-gel Pechini method. Efforts were also done to synthesize phases with x > 0.5 but remained unsuccessful. The Rietveld refinement investigation has confirmed the successful formation of single-phase compounds that crystallize in the I4/mmm space group with tetragonal symmetry. The X-ray photoelectron spectroscopy (XPS) analysis revealed that Co in La1+xSr1-xCoO4 has mixed valence states of +2 and + 3. Dominant anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions were observed in all the synthesized nanophases and the existence of weak ferromagnetic (FM) interactions due to the generation of double-exchange Co2+―O―Co3+ interaction because of mixed valent state of Co. The increase in Néel temperature (TN) values with La doping has been attributed by the increased prevalence of anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions. The band gap energy values (Eg) for all the prepared nano samples vary from 1.57 to 1.64 eV, rendering them highly competent catalysts for degradation of dyes. Additionally, the photocatalytic performance of the prepared nano photocatalysts was evaluated using cationic (MB & RhB) and anionic (MO) dye contaminants as well as their ternary mixture. LaSrCoO4 photocatalyst has been found to efficiently and swiftly degrade all three dye pollutants separately as well as in mixtures, with outstanding recyclability and durability. It has been observed that the x = 0.0 photocatalyst showed best degradation efficacy for RhB dye, i.e., ∼99 % in 100 min, compared to ∼91 % and ∼84 % for the x = 0.2 and 0.5 photocatalysts in the same time interval, respectively. These results suggest that LaSrCoO4 nano powder can be considered a promising photocatalyst in the dyes degradation.

Sub-ppb mixed potential H2S gas sensor based on YSZ and Nd2AO4 (A=Cu, Ba and Ni) as a new type of sensing electrode

In this paper, for the detection of hazardous gas hydrogen sulfide (H2S), new type YSZ (yttria stabilized-zirconia)-based mixed potential gas sensor is developed and K2NiF4 type oxides Nd2AO4 (A=Cu, Ba and Ni) are successfully synthesized as sensing electrode (SE). XRD results shows the materials have pure phase K2NiF4-type oxides. Compared with other sensors, the sensor based on Nd2NiO4 sensing electrode has the highest response value to 1 ppm H2S. Polarization curve results show that the reason for the highest response value is that it has the strongest electrochemical catalytic activity to H2S. Moreover, the response of the sensor based on Nd2NiO4-SE to 1 ppm H2S is − 28 mV with fast response/recovery time (70 s/135 s). The low detection range is 0.05 ppm, meeting the requirement of security monitoring field. The sensitivities of the sensor to 0.05–0.2 ppm and 0.2–10 ppm H2S are − 5 mV/decade and − 63 mV/decade respectively. Besides, the present device also performs good repeatability, excellent selectivity and acceptable stability during 30 days continuous measurement, which demonstrated that the sensor we fabricated has a good application prospect for the field of real-time and in-situ security monitoring of hydrogen sulfide.

Defect chemistry and proton uptake of La2-xSrxNiO4±δ and La2-xBaxNiO4±δ Ruddlesden-Popper phases

Due to their layered structure, Ruddlesden-Popper oxides possess intriguing properties and appear promising for application in a protonic ceramic fuel cell (PCFC) as an air electrode. In this work, two series of K2NiF4-type oxides, namely La2-xSrxNiO4±δ with 0.75 ≤ x ​≤ ​1.4 and La2-xBaxNiO4±δ with 0.25 < x ​≤ ​1.1, were thoroughly investigated in terms of structure, oxygen nonstoichiometry, and proton incorporation at elevated temperatures. For the Sr-substituted samples, only an oxygen-deficient regime is observed, while for La2-xBaxNiO4±δ at a low temperature, as well as with a low Ba content (x ​= ​0.25) an oxygen excess regime can be noticed. The proton uptake of La2-xSrxNiO4±δ remains below 0.005 protons per formula in 16.7 ​mbar ​H2O. In contrast, La2-xBaxNiO4±δ, with a stronger basic character, incorporates up to 0.06 protons per formula unit at 400 ​°C in humid atmosphere. The proton uptake of the La2-xBaxNiO4±δ raises with an increase of Ba content. The proton concentrations in LaBaNiO4±δ and La0.9Ba1.1NiO4±δ exceed those of (Ba,La)FeO3-δ perovskites, which is tentatively attributed to the higher A/B site cation ratio in the Ruddlesden-Popper structure.

K2NiF4 type oxides, Ln2-xSrxNiO4+δ (Ln = La and Pr; x = 0–1.4) as an oxygen electrocatalyst for aqueous lithium–oxygen rechargeable batteries

The structural and electrochemical characteristics of the Ln2-xSrxNiO4+δ (Ln = La and Pr; x = 0–1.4) system were investigated to clarify the relationship between the electrocatalytic activity and various characteristics such as the structure, composition, electrical conductivity, and oxygen content. Rietveld analysis of the powder X-ray diffraction (XRD) data showed that the tetragonal distortion of the NiO6 octahedra decreased monotonically with an increase of x, while the Ln (Sr)–O (//c-axis) in the rock-salt block was expanded with an increase in x, although the tetragonality, c/a showed a maximum at x = 0.6. Cyclic voltammetry (CV) measurements showed that the activity for the oxygen evolution reaction (OER) increased with x, while that of the oxygen reduction reaction (ORR) was independent of x. The characteristic peak due to oxygen insertion into the lattice was observed at ca. 1.45 V vs. RHE, the intensity of which increased with the Sr content, which reflects the increase in the internal chemical diffusion of oxygen species. X-ray photoelectron spectroscopy (XPS) measurements for the constituent elements confirmed the presence of OH− groups inside the catalyst particles, especially in the Sr-rich region. It is suggested that the reversible intercalation ability of the OH− group into the rock-salt layer in the Ruddlesden-Popper (RP) phase (An+1BnO3n+1) contributes significantly to the improvement of the OER activity.

Phase Stability and Electrochemical Characterization of Solid Oxide Fuel Cell Cathodes Containing K2NiF4 Structured Oxides

K2NiF4 -structured oxides, e.g. La2NiO4, exhibit facile kinetics for oxygen surface exchange and comparable electrical conductivity to perovskite-type oxides. Even though they exhibit great potential for replacing the perovskite-type alternatives in SOFC cathodes due to their superior oxygen exchange kinetics, K2NiF4 type-oxides suffer from phase stability issue when in contact with doped-cerias, commonly used as the ionic conducting component in many composite cathode materials. In this study, we reveal a very effective methodology to stabilize the chemical composition of composite cathodes containing K2NiF4 -structured oxides containing doped-ceria. Phase stability studies were conducted to verify the retention of the K2NiF4 -structured oxides after high temperature processing of the electrode. Electrical conductivity measurements, dilatometry analysis, and X-ray diffractometry as functions of temperature, oxygen partial pressure were conducted over wide compositional windows of interest to identify new high performance cathode compositions. Button cells featuring the compositions identified through these studies were prepared via tape casting and sintering. The electrochemical performance of these button cells were analyzed through polarization modelling and impedance spectroscopy in the temperature range 700 to 900°C and cathode side oxygen partial pressures between 5x10-2 and 1 atm. The effect of cathode microstructure and thickness on the cell performance is also discussed.

Effect of A cation size on structural, magnetic and electrical properties of K2NiF4-type oxide LaSrFeO4

K2NiF4-type oxides La0.8R0.2SrFeO4 (R=La, Nd, Gd, Dy) have been synthesized by Pechini method and their structures were established by Rietveld refinements of the XRD data in the space group I4/mmm. Analysis of cell parameters shows that the decrease in the c from La to Dy compound is much larger than that in a. All the samples show antiferromagnetic behavior with increase in value of Θ with decreasing average <La/R/Sr> site radius. For all compounds, semiconducting behavior persists in the whole temperature range and can be expressed by two conduction mechanisms – Arrhenius model and polaron hopping model. The results suggests that the transport properties are dominated by the adiabatic small polaron hopping mechanism. Both activation energy (Ea) and polaron hopping energy (Ep) decrease with decreasing average ionic radius of rare earth ion at A-site.

Tuning the high-temperature properties of Pr2NiO4+δby simultaneous Pr- and Ni-cation replacement

Novel Pr2−xSrxNi1−xCoxO4±δ (x = 0.25; 0.5; 0.75) oxides with the tetragonal K2NiF4-type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x = 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x = 0.25 (δ = 0.09(2)) and a stoichiometric one for x = 0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient (TEC) along the a- and c-axis of the crystal structure, which increases with increasing the Co content from TEC(c)/TEC(a) = 2.4 (x = 0.25) to 4.3 (x = 0.75). High-temperature NPD (HT NPD) study of the x = 0.75 sample reveals that a very high expansion of the axial (Ni/Co)–O bonds (75.7 ppm K−1 in comparison with 9.1 ppm K−1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low- and high-spin states of Co3+. This scenario has been confirmed by high-temperature magnetization measurements on a series of Pr2−xSrxNi1−xCoxO4±δ samples. For compositions with high Ni content (x = 0.25 and 0.5) we synthesised K2NiF4-type oxides Pr2−x−ySrx+y(Ni1−xCox)O4±δ, y = 0.0–0.75 (x = 0.25); y = 0.0–0.5 (x = 0.5). The studies of the TEC, high-temperature electrical conductivity in air, chemical stability of the prepared compounds in oxygen and toward interaction with Ce2−xGdxO2−x/2 (GDC) at high temperatures reveal optimal behaviour of Pr1.35Sr0.65Ni0.75Co0.25O4+δ. This compound shows stability in oxygen at 900 °C and does not react with GDC at least up to 1200 °C. It features low TEC of 13 ppm K−1 and high-temperature electrical conductivity in air of 280 S cm−1 at 900 °C, thus representing a promising composition for use as a cathode material in intermediate temperature solid oxide fuel cells (IT-SOFC).

Structural origin of the anisotropic and isotropic thermal expansion of K2NiF4-Type LaSrAlO4 and Sr2TiO4.

K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al-O2) being higher than that between Al and equatorial oxygen O1 α(Al-O1) [α(Al-O2) = 2.41(18)α(Al-O1)]. The higher α(Al-O2) is attributed to the Al-O2 bond being longer and weaker than the Al-O1 bond. Thus, the minimum electron density and bond valence of the Al-O2 bond are lower than those of the Al-O1 bond. For Sr2TiO4, the Ti-O2 interatomic distance, d(Ti-O2), is equal to that of Ti-O1, d(Ti-O1) [d(Ti-O2) = 1.0194(15)d(Ti-O1)], relative to LaSrAlO4 [d(Al-O2) = 1.0932(9)d(Al-O1)]. Therefore, the bond valence and minimum electron density of the Ti-O2 bond are nearly equal to those of the Ti-O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B-O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B-O2)/d(B-O1) of A2(2.5+)B(3+)O4 compared with A2(2+)B(4+)O4, A2(2.5+)B(3+)O4 compounds have higher α(B-O2), and A2(2+)B(4+)O4 materials exhibit smaller α(B-O2), leading to the anisotropic thermal expansion of A2(2.5+)B(3+)O4 and isotropic thermal expansion of A2(2+)B(4+)O4. The "true" thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition.

Preparation and characterization of Pd loaded Sr-deficient K2NiF4-type (La, Sr)2MnO4 catalysts for NO–CO reaction

Pd-loaded K2NiF4-type perovskite oxides, Pd/La0.2Sr1.8−xMnO4 were prepared by liquid-phase adsorption process, and their structural and catalytic properties were investigated. Pd deposition was carried out by mixing La0.2Sr1.8MnO4 with Pd(NO3)2 in aqueous solution, followed by filtration, drying, and calcination at 800°C. During the Pd deposition process, 5% of Sr in La0.2Sr1.8MnO4 oxides was dissolved into the aqueous phase, whereas the perovskite structure retained. The content of Pd loaded on La0.2Sr1.8MnO4 was 0.24wt%. X-ray absorption spectroscopy studies showed that highly dispersed PdO nanoparticles were supported on Pd/La0.2Sr1.8MnO4 prepared by the liquid-phase adsorption process, whereas aggregated PdO particles were formed on Pd/La0.2Sr1.8MnO4 prepared by an impregnation method. The Pd/La0.2Sr1.8MnO4 catalyst prepared by the liquid-phase adsorption process exhibited higher catalytic activity for NO–CO reaction than the 0.24wt% Pd/La0.2Sr1.8MnO4 catalyst prepared by the impregnation method.

First-principle investigations of K2NiF4-type double perovskite oxides La4B′B″O8 (B′B″ = Fe, Co, Ni)

The K2NiF4-type structure La4CoNiO8 (LCNO), La4FeCoO8 (LFCO), and La4FeNiO8 (LFNO) are studied by using the first-principle electronic structure calculations. Our results indicate that the ground state of LCNO is a ferrimagnetism (FiM) with a large energy gap about 1.9 eV, LFCO and LFNO are antiferromagnetism with energy gaps about 1.3 and 1.4 eV, respectively. Their orthorhombic distortions, out-of-plane elongation, and tilting of octahedron are discussed. It is indicated that LFCO and LFNO have stronger crystal distortion than LCNO. Our calculations indicate that the in-plane magnetic exchange interaction of LCNO is much stronger than LFCO and LFNO, thus LCNO should have much higher magnetic ordering temperature than LFCO and LFNO.

Role of Ga3+ and Cu2+ in the High Interstitial Oxide-Ion Diffusivity of Pr2NiO4-Based Oxides: Design Concept of Interstitial Ion Conductors through the Higher-Valence d10 Dopant and Jahn–Teller Effect

We have investigated the crystal structure, nuclear- and electron-density distributions, electronic structure, and oxygen permeation rate of three K2NiF4-type oxides of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ, Pr2Ni0.75Cu0.25O4+δ, and Sr2Ti0.9Co0.1O4–e, in order to study the role of d10 Ga3+, Jahn–Teller Cu2+, and interstitial oxygen O3 in the high oxygen diffusivity of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ. The composition Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ has a larger amount of interstitial oxygen O3 atoms (δ = 0.31 at room temperature (RT)) compared with Pr2Ni0.75Cu0.25O4+δ (δ = 0.19 at RT) and the oxygen deficient Sr2Ti0.9Co0.1O4–e (e = 0.02 at RT). The interstitial O3 atom is stabilized by (1) the substitution of (Ni,Cu)2+ by higher valence Ga3+, (2) static atomic displacements of the apical O2 oxygen, and (3) local relaxation near d10 Ga3+. Nuclear-density distributions of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ and Pr2Ni0.75Cu0.25O4+δ at high temperatures have visualized the −O2–O3–O2– diffusional pathway of oxide ions, w...

Electrical Conductivity and Thermoelectric Power of La2NiO4+δ

The electrical conductivity, Seebeck coefficient, and thermal chemical expansion properties of La2NiO4+δ were measured as a function of temperature and oxygen partial pressure (pO2). The electronic conductivity was analyzed in relation to the thermoelectric power (thermopower) data to elucidate the positive deviation of the activity coefficient of hole on the basis of the delocalized electron model by using the Joyce-Dixon approximation of the Fermi-Dirac integral from the partition function in the quasi-free-particle approximation with the regular solution model. The electrical conductivity and thermopower vs oxygen activity were all explained consistently in the logical frame of the degenerate p-type conductor across the entire experimental range investigated. Like other K2NiF4–type oxides, the chemical diffusion coefficient was slightly higher than the surface exchange coefficient, suggesting the need for further work to enhance the sluggish surface exchange kinetics of oxygen. The best-estimated mobility values were in reasonable agreement with the reported values and the very weak temperature dependence of hole mobility suggested a band conduction.

On the role of lattice dynamics on low-temperature oxygen mobility in solid oxides: a neutron diffraction and first-principles investigation of $$ {\hbox{L}}{{\hbox{a}}_2}{\hbox{Cu}}{{\hbox{O}}_{{4 + \delta }}} $$

The structure of oxygen-intercalated La2CuO4.07 has been investigated at 20 and 300 K by neutron diffraction on an electrochemically oxidized single crystal. At 20 K, reconstruction of the nuclear density by maximum entropy method shows strong displacements of the apical oxygen atoms towards [100] with respect to the F-centred unit cell, whilst displacements towards [110] and [100] were both found to be present at ambient temperature. Combining structural studies with first-principles lattice dynamical calculations, we interpret the displacements of the apical oxygen atoms to be at least partially of dynamic origin already at ambient temperature. Strong displacements of the apical oxygen atoms of stoichiometric and oxygen-doped \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) and corresponding associated lattice instabilities, i.e. low-energy phonon modes, are considered as a general prerequisite of low-temperature oxygen diffusion mechanisms. Lattice dynamical calculations on \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) suggest that the oxygen species diffusing at low temperature are not the interstitial but, more prominently, the apical oxygen atoms. The presence of interstitial oxygen atoms is, however, important to amplify via specific, low-energy phonon modes, a dynamic exchange mechanism between apical and vacant interstitial oxygen sites, thus allowing a dynamically triggered, shallow potential oxygen diffusion pathway. The crucial role of lattice dynamics to enable low-temperature oxygen mobility in K2NiF4-type oxides is discussed on a microscopic scale and compared to similar low-temperature oxygen diffusion mechanisms, recently proposed for non-stoichiometric oxides with Brownmillerite-type structure.

In situ neutron diffraction study of the high-temperature redox chemistry of Ln3−xSr1+xCrNiO8−δ (Ln = La, Nd) under hydrogen

The chemical reduction of the K2NiF4-type oxides, Ln2Sr2CrNiO8−δ (Ln = La, Nd) and Nd2.25Sr1.75CrNiO8−δ, has been investigated in situ under a dynamic hydrogen atmosphere at high temperature using neutron powder diffraction. The high count-rate and high resolution of the D20 diffractometer at ILL, Grenoble allowed real-time data collection and structure refinement by full-pattern Rietveld analysis with a temperature resolution of 1 °C. Excellent agreement was obtained with the results of thermogravimetric analysis of these materials, which are potential fuel-cell electrodes. The neutron study revealed that oxygen is lost only from the equatorial anion site; the reduction of La2Sr2CrNiO8−δ yields a pure Ni(II) phase, La2Sr2CrNiO7.5en route to a mixed Ni(II,I) oxide, La2Sr2CrNiO7.40, whereas hydrogen reduction of Nd2Sr2CrNiO8−δ and Nd2.25Sr1.75CrNiO8−δ proceeds continuously from Ni(III) to an average oxidation state of 1.80 for the nickel ion. The data collected throughout a subsequent heating/cooling cycle in air indicated that the reduced phases intercalate oxygen reversibly into the equatorial vacancies of the K2NiF4-type structure. The retention of I4/mmm symmetry, along with the absence of the formation of any impurities throughout the heating/cooling cycles under reducing and oxidizing atmospheres, demonstrates both the reversibility and the strongly topotactic character of the oxygen deintercalation/intercalation chemical redox process and establishes the excellent structural stability of these layered mixed-metal oxides over a wide range of oxygen partial pressures.

Crystal Structure and Oxide Ion Conductivity of the In-Containing K2NiF4-type Oxides

Crystal Structure and Oxide Ion Conductivity of the In-Containing K2NiF4-type Oxides

Preparation, Crystal Structure, and Reducibility of K2NiF4Type Oxides Eu2−xSrxNiO4+δ

K2NiF4type compounds Eu2−xSrxNiO4+δ(0.5≤x≤1.2) have been prepared by a citric acid complex decomposition method. Rietveld refinement of the powder X-ray diffraction data showed that there was a crystal system transformation from orthorhombicFmmmto tetragonalI4/mmmat aroundx=0.6–0.7. The Sr substitution caused a drastic shift of the OIIions along thec-axis from Eu(Sr) toward Ni while scarcely affecting the NiO4network in the basal plane. The Jahn–Teller distortion of the NiO6octahedron decreased while the percentage of Ni3+increased with increasingx. The variation of the higher peak temperature on the TPR profiles withxare opposite to the variation of the cell parametersc.

Preparation, crystal structure, and reducibility of K2NiF4 type oxides Sm2−xSrxNiO4+δ

K2NiF4 type compounds Sm2–xSrxNiO4+δ(0.4≤x≤1.2) have been synthesized by a citric acid complex decomposition method. Rietveld refinement of the powder X-ray diffraction data showed that there was a crystal system transformation from orthorhombic Fmmm to tetragonal I4/mmm at around x=0.6. The Sr substitution caused a drastic shift of the OII ions along the c axis from Sm(Sr) towards Ni whilst scarcely affecting the NiO4 network in the basal plane. The distortion of the NiO6 octahedron decreased, while the average bond valence of Ni increased with increasing x.

Synthesis and catalytic properties of perovskite-related phases in the La–Sr–Co–Cu–Ru–O system

A series of oxides of the composition La0.8Sr0.2Co1–2yCuyRuyO3 with the perovskite structure were synthesized and characterized by their X-ray diffraction (XRD) patterns and by element analysis in a transmission electron microscope (TEM) equipped with an energy-dispersive spectrometer (EDS). The XRD and TEM–EDS studies confirmed that almost monophasic samples with the perovskite structure having rhombohedral (hexagonal) symmetry were obtained for 0 ⩽y⩽ 0.20, although the simultaneous formation of a by product with the K2NiF4-type structure was observed for high y values. The lattice parameters and rhombohedral distortion increased monotonically with increasing y value due to the fact that the average ionic radius of the substituting Cu/Ru pair is larger than that of the host Co ion. Temperature-programmed desorption (TPD) studies showed that the substitution of the Cu/Ru pair for Co caused a decrease in the amount of oxygen desorbed from the bulk of the oxide, and the catalytic activity for CO oxidation and CO–NO reactions decreased. The Cu/Ru substitution did, however, selectivity increase the N2 formation and decreased that of N2O in the CO–NO reactions. Synthesis of K2NiF4-type oxides with the overall compositions (La0.8Sr0.2)2Co0.50Cu0.50–zRuzO4 with z= 0.05 and 0.10 was also attempted. In both cases, tetragonal K2NiF4-type oxides having a composition close to (La0.8Sr0.2)2(Co,Cu)0.97Ru0.03O4 were obtained, implying that the solubility of Ru into the given K2NiF4-type framework is very low.

Madelung Constant of the Tetragonal K2NiF4Type Oxides

Madelung Constant of the Tetragonal K2NiF4Type Oxides