K2NiF4-type Structure Research Articles

Crystal structure and high-temperature properties of (Pr,Sr)2(Co,Mn)O4±δ with K2NiF4-type structure

Abstract Novel oxides PrSrCo1−yMnyO4±δ, 0.0≤y≤0.5 and Pr0.5Sr1.5Co1−yMnyO4±δ 0.3≤y≤0.5 with K2NiF4-type structure were synthesized. The crystal structure, oxygen content, thermal expansion and electrical conductivity of the obtained compounds were examined. A chemical titration showed that PrSrCo0.5Mn0.5O4±δ is slightly overstoichiometric (δ=0.03(2)), while Pr0.5Sr1.5Co0.5Mn0.5O4±δ is oxygen deficient (δ=−0.05(2)). Thermal expansion behavior was studied by both dilatometry and high-temperature X-ray powder diffraction in the temperature range of 298–1173 K in air. Pr0.5Sr1.5Co0.5Mn0.5O3.95(2) exhibited linear thermal expansion along the a- and c-axes over the studied temperature range with thermal expansion coefficients (TECs) were 17.5 ppm K−1 and 17.8 ppm K−1, respectively. For PrSrCo0.5Mn0.5O4.03(2) two regions (298–600 K and 600–1173 K) observed, where the TEC along the a-axis decreased from 12.7 ppm K−1 to 10.4 ppm K−1 whereas the TEC along the c-axis increased from 14.5 ppm K−1 to 26.7 ppm K−1. Both compounds demonstrated lower in comparison with undoped PrSrCoO4 values of electrical conductivity of 7 S/cm for PrSrCo0.5Mn0.5O4.03(2) and 23 S/cm for Pr0.5Sr1.5Co0.5Mn0.5O3.95(2) at 1173 K in air.

Improvement of microwave dielectric characteristics in SrNdAlO4 ceramics by La-substitution

Single-phase Sr(Nd1−xLax)AlO4 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1) solid solutions with tetragonal K2NiF4-type structure were obtained by a standard solid-state reaction approach in the entire composition range. Their microwave dielectric characteristics were investigated together with microstructures, and were significantly modified by La-substitution. The X-ray diffraction results presented that the samples were SrNdAlO4 and SrLaAlO4. And the dielectric constant (er) changed slightly, the temperature coefficient of resonant frequency (τf) varied from −9 to −34 ppm/°C. The significantly improved Q × f value was achieved in the present ceramics with x = 0.9 and reached the maximum of 65,900 GHz. The ionic radius difference of A-site affects the distortion in the rock-salt layer and the tolerance t affects the interlayer mismatch, both of them will predominantly affect the Q × f value. The best combination of microwave characteristics was achieved at x = 0.9: er = 18.49, Q × f = 65,900 GHz, τf = −27 ppm/°C.

Synthesis and oxygen transport properties of La2 − ySryNi1 − xMoxO4 + δ

La2NiO4+δ-based materials (LN) with tetragonal K2NiF4-type structure exhibit high electronic and oxygen ion conductivity. In this work synthesis, phase relations and oxygen transport properties are reported for the system La2−ySryNi1−xMoxO4+δ where 0.0≤y≤0.4 and 0.0≤x≤0.1. The solubility of Mo in LN is low, but becomes enhanced by Sr substitution on the A-site. La1.8Sr0.2Ni0.95Mo0.05O4+δ (LSNM) bar shaped samples were subjected to electrical conductivity relaxation (ECR) for assessment of oxygen bulk diffusion (Dchem), oxygen surface exchange (kchem) and electronic conductivity. The p-type conductivity was 50S·cm−1 at 900°C, which is lower than both LN and LSN (Sr=0.2). On the other hand, kchem was enhanced by one order of magnitude compared with LN at all temperatures, while Dchem was lower at T>700°C and higher at T<700°C.

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).

Single-crystal synthesis and structure refinement of La2Li0.5Al0.5O4 with K2NiF4-type structure

Single crystals of lanthanum lithium aluminum oxide, La2Li0.5Al0.5O4, were synthesized for the first time by a flux method at 773 K. The La2Li0.5Al0.5O4 single crystal is colorless, has an average size of 60 μm with a square platelet shape, and crystallizes in the tetragonal K2NiF4-type structure, space group I4/mmm with a = 3.7742(9) Å, c = 12.753(4) Å, V = 181.66(10) Å3, and Z = 2. The structure was determined using single-crystal X-ray diffraction data and refined to the conventional values R = 1.25% and wR = 1.28% for 524 observed reflections. The octahedral site was randomly occupied by Li and Al atoms. The corresponding polycrystalline sample was also prepared by a conventional solid-state reaction method at 1123 K.

Evaluation of La1.8−xPrxSr0.2CuO4−δ oxides as cathode materials for IT-SOFCs

Conducting properties, thermal expansion, and chemical stability of La1.8−xPrxSr0.2CuO4−δ (x = 0.2; 0.4) related to the K2NiF4-type structure have been investigated in order to estimate their appropriateness to be used as a cathode material for solid oxide fuel cells. Electrical conductivity of La1.8−xPrxSr0.2CuO4−δ demonstrates metallic-like behavior in the temperature range of 100–900 °С at variable oxygen partial pressure (pO2 = 10−3–1 atm), and reaches 50 S/cm at 750 °C in air. The cuprates display thermal expansion coefficients (TECs) of about 14·10−6 K−1 that are compatible with La0.8Sr0.2Ga0.85Mg0.15O3−δ (LSGM) and Ce0.9Gd0.1O1.95 (GDC) solid electrolytes. The electrocatalytic activity study of La1.6Pr0.2Sr0.2CuO4−δ electrode deposited on GDC as well as LSGM shows that the lowest polarization resistance is achieved in case of deposition of the electrode on GDC electrolyte.

Synthesis, oxygen nonstoichiometry and total conductivity of (La,Sr)2(Mn,Ni)O4 ± δ

The series of complex oxides, La2-xSrxNi1-yMnyO4-δ (х=0.7–1, y=0.1, 0.2, 0.3, 0.5, 0.7), were synthesized via glycine–nitrate and citrate–nitrate combustion techniques followed by sintering at 1350–1400°C in air. All single-phase compounds have the K2NiF4-type structure (sp.gr. I4/mmm). The unit cell volume increases with y and decreases with x in La2-xSrxNi1-yMnyO4±δ. At room temperature, the solid solutions LaSrNi1-yMnyO4±δ (y=0.1, 0.3, 0.5, and 0.7) show a small excess of oxygen (δ>0) which increases with the manganese doping level y. Total conductivity and Seebeck coefficient were determined in air from room temperature to 1100°C, with the maximum value ~100S/cm for La1.2Sr0.8Ni0.9Mn0.1O4±δ and La1.3Sr0.7Ni0.9Mn0.1O4±δ. The conduction is temperature activated for all phases under study. Seebeck coefficient has tendency to decrease from low temperature to high temperature and predominantly adopts slightly negative values varying from −15 to −5μV/K.

Influence of La/Pr Ratio on the Ageing Properties of La2-X Pr x NiO4+δ as Cathodes in IT-SOFCs

The present study is focused on alternative oxygen electrodes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) using Metal Supported Cells (MSCs) conditions. To prevent detrimental oxidation of the metal support, sintering of the cell components at high temperature under low pO2 is required. Ln2NiO4+δ (Ln = La, Pr) compounds with the K2NiF4-type structure act as cathode materials for IT-SOFC due to their mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ shows excellent electrochemical properties at intermediate temperature (i.e. low polarization resistance Rp value, Rp = 0.03 Ω.cm² at 700 °C), while La2NiO4+δ exhibits higher chemical stability. Thus, the properties of La2 -x Pr x NiO4+δ mixed nickelates were investigated with the aim to find best compromise between chemical stability and electrochemical performances. Herein, the chemical stability of the nickelates under air at operating temperatures as well as the evolution of the polarization resistances during ageing (recorded under air at idc = 0 and idc ≠ 0 conditions) were studied for duration up to one month. La2NiO4+δ is chemically stable whereas Pr2NiO4+δ dissociates after 1 month at 600, 700 and 800 °C. At idc = 0 condition, the ageing of the half cells during 1 month under air shows no change in Rp , despite their various degrees of chemical stability. A different behavior is observed under idc ≠ 0 conditions with a large increase in Rp for Pr-rich phases in SOFC mode while interestingly, Rp remain stable in SOEC mode. In this presentation, the electrochemical properties of La2 -x Pr x NiO4+δ under ageing and postmortem analysis will be discussed in details.

Improve electrical conductivity of reduced La2Ni0.9Fe0.1O4+δ as the anode of a solid oxide fuel cell by carbon deposition

La2Ni0.9Fe0.1O4+δ (LNF) with a K2NiF4-type structure is synthesized with a citrate complexing method and used as an anode material of a solid oxide fuel cell. It is reduced to La2O3 and Ni–Fe alloy phases in H2 atmosphere. The electrical conductivity of the anode material is enhanced by carbon deposition with CH4 and methanol as the carbon sources, respectively. The deposited carbon is characterized with a scanning electron microscope, a transmission electron microscope, thermogravimetric analysis and a Raman spectrometer. The carbon deposited from methanol has a higher degree of graphitization than that from CH4, bringing about a higher electrical conductivity. The conductivity of the reduced LNF at 800 °C increases from 0.23 S cm−1 to 2.47 S cm−1 after the carbon deposition in methanol. The increase of the anode conductivity results in an improvement of the fuel cell performance. A maximum power density of 166 mW cm−2 is obtained at 800 °C from a single cell with La0.8Sr0.2Ga0.83Mg0.17O3-δ as the electrolyte and Ba0.5Sr0.5Co0.8Fe0.2O3-δ as the cathode.

Electrochemical characterization of B-site cation-excess Pr2Ni0.75Cu0.25Ga0.05O4+δ cathode for IT-SOFCs

The B-site cation-excess K2NiF4-type structure oxide, Pr2Ni0.75Cu0.25Ga0.05O4+δ (PNCG) is investigated as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD result shows that PNCG cathode is chemically compatible with the electrolyte Gd0.1Ce0.9O2−δ (GDC) at 900oC for 5h. The PNCG material exhibits a semiconductor to metal transition around 425oC. The thermal expansion coefficient (TEC) of the PNCG sample is 12.72×10−6K−1 between 30 and 850oC in air. The polarization resistance (Rp) of PNCG cathode on GDC electrolyte is 0.105, 0.197 and 0.300Ωcm2 at 800, 750, 700°C, respectively. A maximum power density of 371mWcm−2 is obtained at 800°C for single-cell with 300μm thick GDC electrolyte and PNCG cathode. The results of this study demonstrate that PNCG can be a promising cathode material for IT-SOFCs.

Ln2NiO4+δ (Ln = La, Pr) As Suitable Cathodes for Metal Supported Cells

The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln = La or Pr) is studied in the temperature range 25 - 1400 °C, both in air and at low pO2 (down to 10-4 atm). Thermal gravimetry analysis (TGA) measurements coupled with thermal in situ X-ray diffraction (XRD) characterization show that all compounds retain their K2NiF4-type structure in these conditions, while remaining oxygen over-stoichiometric up to 1000 °C. For both nickelates, a structural transition occurs around 400 °C at pO2 = 10-4 atm in association with a variation in Dδ in the range 0.06 to 0.02 for Pr2NiO4+δ and La2NiO4+δ, respectively. Interestingly, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. EIS measurements carried out on Ln2NiO4+δ/Ce0.8Gd0.2O2-δ/8wt.%Y2O3-ZrO2 half cells revealed that the electrode are at least as efficient for the oxygen reduction reaction when sintered at pO2 = 10-4 atm, than when sintered in air. Finally, excellent matching was found between the thermal expansion coefficients of the lanthanide nickelates and SOFC electrolytes such as 8wt.% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2-δ (GDC), up to 1200 °C either in air or at pO2 = 10-4 atm, which is particularly important to avoid delamination of the half cell layers during solid oxide fuel cell long-term experiments.

Influence of La/Pr Ratio on the Ageing Properties of La2-X Pr x NiO4+δ as Cathodes in IT-SOFCs

The present study is focused on alternative oxygen electrodes for IT-SOFCs using Metal Supported Cells (MSCs) conditions. Ln2NiO4+δ (Ln = La, Pr) compounds with the K2NiF4-type structure act as cathode materials for IT-SOFC due to their MIEC properties. Pr2NiO4+δ (PNO) shows excellent electrochemical properties at intermediate temperature while La2NiO4+δ (LNO) exhibits higher chemical stability. Thus, the properties of La2 -x Pr x NiO4+δ (LPNO) mixed nickelates were investigated with the aim to find the best compromise between chemical stability and electrochemical performances. Herein, the chemical stability of the nickelates under air at operating temperatures as well as the evolution of the polarization resistances, R p, during ageing (i dc = 0 and i dc ≠ 0 conditions) were studied for duration up to one month. At i dc = 0 condition, the ageing of the half cells shows no change in R p. A different behavior is observed under i dc ≠ 0 conditions with an increase in R p in SOFC mode while interestingly, R p remains stable in SOEC mode.

Chemical and structural changes in Ln2NiO4+δ (Ln=La, Pr or Nd) lanthanide nickelates as a function of oxygen partial pressure at high temperature

The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln=La, Pr or Nd) has been studied in the temperature range 25–1300°C, either in air or at low pO2 (down to 10−4atm). Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterization have shown that all compounds retain their K2NiF4-type structure in these conditions, while remaining over-stoichiometric in oxygen up to 1000°C. Only Nd2NiO4+δ starts to decompose into Nd2O3 and NiO above 1000°C, at pO2=10−4atm. In addition, a careful analysis of the lanthanide nickelates structural features has been performed by in situ XRD, as a function of temperature and pO2. For all compounds, a structural transition has been always observed in the temperature range 200–400°C, in air or at pO2=10−4atm. In addition, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. Additional dilatometry measurements have revealed that a temperature as high as 950°C for Pr2NiO4+δ or 1100°C for La2NiO4+δ and Nd2NiO4+δ has to be reached in order to begin the sintering of the material particles, which is of primary importance to obtain an efficient electronic/ionic conduction in the corresponding designed cathode layers. Besides, excellent matching was found between the thermal expansion coefficients of lanthanide nickelates and SOFC electrolytes such as 8wt% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2−δ (GDC), at least from 400°C up to 1400°C in air or up to 1200°C at pO2=10−4atm.

Thermal expansion behavior and high-temperature electrical conductivity of A2−xAx′Cu1−yCoyO4±δ (A = La, Pr; A′ = Pr, Sr) oxides with the K2NiF4-type structure

Three different compositional series of Pr-containing cuprates with the K2NiF4-type structure have been prepared by annealing at 1273K, 48–90h in air: with Cu on B-site only in formal oxidation state +2 (La2−xPrxCuO4, x=0.1; 0.2; 0.4) (i), with partial replacement of Cu2+ by small (La2−xPrxCu1−yCoyO4±δ, 0.1⩽x⩽0.5; y=0.05; 0.1) (ii) and large (Pr2−xSrxCu1−xCoxO4±δ, x=0.5; 0.75) (iii) amount of cobalt. Partial replacement of Cu by Co, as in La2−xPrxCu1−yCoyO4±δ, leads to an increase of thermal expansion coefficients (TECs) in comparison with La2CuO4. Unusual high anisotropy of TEC along the a-(TEC(a)) and c-axes (TEC(c)) of the tetragonal unit cell is observed for Pr2−xSrxCu1−xCoxO4±δ, x=0.5 and 0.75. TEC(c)/TEC(a) ratio increases with increasing amount of cobalt from 3.29 (x=0.5) to 3.61 (x=0.75). High-temperature electrical conductivity measurements at 298–1173K in air reveal semiconductor-like behavior for all studied compositions with electrical conductivity values at 1173K in air vary from 1.8S/cm for La1.9Pr0.1Cu0.9Co0.1O4±δ to 5S/cm for La1.6Pr0.4CuO4±δ and to 25S/cm for Pr2−xSrxCu1−xCoxO4±δ, x=0.5, 0.75.

Synthesis and structure of a new layered oxyfluoride Sr2ScO3F with photocatalytic property

A new Ruddlesden–Popper type scandium oxyfluoride, Sr2ScO3F, was synthesized by a conventional solid state reaction route. The detailed structure of Sr2ScO3F was investigated using X-ray diffraction (XRD) and selected area electron diffraction (SAED). The disorder distribution pattern of fluorine anions was determined by the 19F nuclear magnetic resonance (NMR) spectrum. The compound crystallizes in a K2NiF4-type tetragonal structure (space group I4/mmm) with O/F anions disordered over the apical sites of the perovskite-type Sc(O,F)6 octahedron layers interleaved with strontium cations. Ultraviolet–visible (UV–vis) diffuse reflection spectrum of the prepared Sr2ScO3F indicates that it has an absorption in the UV–vis region. The photocatalytic activity of Sr2ScO3F was further investigated, showing an effective photodegradation of Rhodamine-B (RB) within 2h under UV light irradiation.

Phase Transitions of α-Sr2VO4 with K2NiF4-type Structure

The electronic ground state of α-Sr2VO4 has been a matter of interest for a decade because of a possible competition between a small Jahn-Teller effect, spin-orbit interactions, and exchange interactions. Nevertheless, the phase transitions of the compound have not been clearly characterized by experiments due to a difficulty in synthesizing good samples. In this study, three phase transitions were recognized at T1 = 101K, T2 = 127K, and TN = 10K by magnetic and thermal measurements for well-characterized samples. The transitions at T1 and T2 are independent structural transitions, and so the existence of an intermediate phase between them is strongly suggested. The transition at TN is a canted antiferromagentic or ferrimagnetic ordering. Some anomalous bendings were observed in magnetization curves below TN. These facts are discussed based on the theoretical models proposed.

Effects of strontium doping on the structure, oxygen nonstoichiometry and electrochemical performance of Pr2−xSrxNi0.6Cu0.4O4+δ (0.1 ≤ x ≤ 0.5) cathode materials

Pr2−xSrxNi0.6Cu0.4O4+δ (PSNCO, x = 0.1, 0.3, 0.5) oxides with K2NiF4-type structure are synthesized by a glycine–nitrate process as potential cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The influence of strontium substitution on the structure, oxygen nonstoichiometry, electrical property and electrochemical performance is investigated. Iodometric titration method is employed to study the oxygen nonstoichiometry and the formal valence of nickel. The maximum electrical conductivity reaches 109 S cm−1 at 450 °C in air for Pr1.7Sr0.3Ni0.6Cu0.4O4+δ material. Area specific resistance (ASR) values of all compositions are below 0.11 Ω cm2 at 800 °C, and the Pr1.7Sr0.3Ni0.6Cu0.4O4+δ composition has the lowest ASR value of 0.063 Ω cm2 at 800 °C. Open circuit voltage (OCV) and maximum power density of the single cell (NiO–SDC/SDC/PSNCO) are 0.767 V and 353 mW cm−2 at 800 °C, respectively. The preliminary results indicate that Pr2−xSrxNi0.6Cu0.4O4+δ materials could be good candidates for IT-SOFCs cathode materials.

Crystal growth, structure, and dielectric properties of layered cobaltates La2−xSrxCoO4 (x = 0.4, 0.5, and 0.6) single crystal

In this work, La2−xSrxCoO4 (x=0.4, 0.5, and 0.6) crystals with the Ruddlesden–Popper K2NiF4-type structure have been grown by the floating zone method. This compound crystallizes in the K2NiF4-type structure with space group I4/mmm. Temperature-dependent dielectric properties of the as-grown crystals were studied over a broad frequency range. The origin of the giant dielectric constant in this system is the mixed-valence structure between Co2+ and Co3+, and the dielectric constant at high frequency is related to the charge ordering structure. The dielectric relaxation shows a thermally-activated behavior. The corresponding activation energy (assigned to small polaronic hopping) can be obtained by Arrhenius fitting.

Comparative Study of the Layered Perovskites Pr1−xA1+xCoO4, (A = Sr, Ca)

We present structural, magnetic, and electric properties of Pr1−xCa1+xCoO4 layered perovskites and compare them with the results of recently studied Pr1−xSr1+xCoO4. The Rietveld refinements of X‐ray powder diffraction (XRD) patterns indicate that all samples crystallize in the K2NiF4‐type structure with space group symmetry I4/mmm. The parent compounds Pr(Ca, Sr)CoO4 are paramagnetic insulators. Upon increasing x, a variety of different magnetic transitions can be identified. The complex magnetic behavior results from the interplay of competing interactions. The temperature‐dependent electrical resistivity of the samples shows 2D variable range hopping (2D‐VRH). The results are discussed with respect to structure−property relationships.

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.