Magnetic properties of the spinel system Mg_xMn_3-xO₄ (0 ≤ x ≤ 2)

Temperature-dependent studies of the low-field magnetization of the polycrystalline spinel oxide MgxMn3-xO4 (0 ≤ x ≤ 2) are reported. With the use of Lotgering’s model, which is equivalent to the Néel’s two-sublattice model, a set of molecular field constants λAB, λBB and λAA, has been obtained for 0 ≤ x ≤ 0.4 from the fit of χ between TC and room temperature. Moreover, this model fits consistently the low-temperature canted-spin angles ψ of the B sublattice. The Curie temperatures TC, as well as the exchange parameters JAB, JBB and JAA, show that the BB interactions are much greater than AA and AB, which are of the same order, in contrast to the situation usually found in magnetic spinels. As the concentration of the non-magnetic ion Mg2+ increases at the tetrahedral site (A), TC decreases from 42K (for x = 0) while the ferromagnetic behaviour diminishes at the expense of a magnetic frustration, thus highlighting the great influence of the non-magnetic ions located at the A and B-sites on the magnetic order.Se presentan estudios de la variación térmica de la magnetización a bajo campo de espinelas policristalinas de la solución sólida MgxMn3-xO4 (0 ≤ x ≤ 2). Con la utilización del modelo de Lotgering, que equivale al modelo de Néel de dos subredes, se ha obtenido un conjunto de constantes de campo molecular λAB, λBB y λAA, para 0 ≤ x ≤ 0.4 a partir del ajuste de χ entre TC y temperatura ambiente. Además, este modelo ajusta consistentemente los ángulos ψ de espín canteado de la subred B a baja temperatura. La temperatura de Curie TC, así como los parámetros de canje JAB, JBB and JAA, muestran que las interacciones BB son mucho mayores que las AA y AB que son ambas del mismo orden, en contraste con lo hallado usualmente en espinelas magnéticas. Cuando la concentración del ión no-magnético Mg2+, aumenta en los lugares tetraédricos, TC decrece desde 42 K (para x = 0), mientras que el comportamiento ferromagnético se debilita a expensas de una frustración magnética, resaltando así la gran influencia de los iones no-magnéticos localizados en lugares A y B, sobre el orden magnético

Structural and magnetic properties of frustrated GaxMn(3-x)O4(1.2 ≤ x ≤ 1.6) spinels

International audienceWe report a systematic study of the structural and magnetic properties of frustrated compounds of GaxMn(3−x)O4 (1.2 ≤ x ≤ 1.6) prepared by solid-state reaction. Using Rietveld refinement of X-ray diffraction patterns and O'Neill-Navrotsky model, we demonstrate that the system GaxMn(3−x)O4 (1.2 ≤ x ≤ 1.6) is an inverse spinel with low inversion parameter, in which Ga3+ replaces Mn3+ cations located in B-sites. The inverse magnetic susceptibility, the shape of ZFC/FC magnetization curves at low temperatures, the existence of hysteresis in all compounds, the frustration parameter and the spontaneous magnetization analysis show that the compounds with x = 1.2–1.4 exhibit a non-collinear ferrimagnetic order and the compounds with x = 1.5–1.6 exhibit a frustrated non-collinear ferrimagnetic order. Spin wave stiffness parameters were determined for each composition using the fitting results of spontaneous magnetization curves. It is demonstrated that for the compounds x = 1.2–1.4 with a non-frustrated ferrimagnetic order, the change of spontaneous magnetization Ms(T) obeys to Bloch's law (T3/2). For x = 1.5–1.6, the compounds exhibit a frustrated ferrimagnetic order, and the Ms(T) shows a deviation from Bloch's law

Exsolution of Co-Fe Alloy Nanoparticles on the PrBaFeCoO(5+δ) Layered Perovskite Monitored by Neutron Powder Diffraction and Catalytic Effect on Dry Reforming of Methane

International audienceReversible exsolution and dissolution of metal nanoparticles (NPs) in complex oxides have been investigated as an efficient strategy to improve the performance and durability of the catalysts for thermal and electrochemical energy conversion. Here, in situ exsolution of Co-Fe alloy NPs from the layered perovskite PrBaFeCoO(5+δ) (PBFC) and their dissolution back into the oxide host have been monitored for the first time by in situ neutron powder diffraction and confirmed by X-ray diffraction and electron microscopy. Catalytic tests for dry reforming of methane showed stable operation over ∼100 h at 800 °C with negligible carbon deposition (<0.3 mg/g(cat) h). The CO(2) and CH(4) conversions are among the highest achieved by layered double perovskites. The cyclability of the PBFC catalyst and the potential to improve the catalytic activity by adjusting the composition, size, and the NP distribution would pave the way for highly efficient energy conversion applications

Ce- and Ni-Codoped Double PrBaMn2O5 Perovskite as a Ceramic SOFC Anode

International audienceThis study explores the efficacy of cerium introduction, both on its own and in combination with nickel, into PrBaMn2O5+δ (PBM) structures to enhance solid oxide fuel cell (SOFC) anodes. We synthesized Pr1–xCexBaMn2O5+δ compositions for x values of 0.05 (PrCe5) and 0.1 (PrCe10), as well as a nickel-doped variant (PrCe5Ni), assessing their performance under H2–3% H2O reducing conditions pertinent to SOFC anode operations. Our findings reveal that the PrCe5 composition exhibits a thermal expansion coefficient (TEC) that not only improves upon that of the Ce-free counterpart but also aligns closely with the TEC standards of prevalent SOFC electrolytes. A notable advancement was achieved with the application of a 3.5 μm gadolinia-doped ceria (GDC) buffer layer through physical vapor deposition, effectively mitigating chemical interactions between PBM-based anodes and yttria-stabilized zirconia (YSZ) electrolytes, a concern highlighted by in situ neutron diffraction analyses. Electrochemical impedance spectroscopy, conducted over 220 h in a H2–3% H2O atmosphere at 750 °C, demonstrated that the optimal 3.5 μm thickness of the GDC buffer layer significantly minimizes the area-specific resistance (ASR) degradation rate to 0.002 Ω cm2/h, markedly outperforming both thinner (1 μm) and thicker (8 μm) GDC layers, which showed higher degradation rates of 0.15–0.2 Ω cm2/h due to the diffusion of Ba ions or delamination. Moreover, cerium doping fosters superior microstructural stability and obviates barium diffusion, thereby suggesting an enhanced durability of the doped anodes over their lifespan. Integrating nickel into the PrCe5 structure halved the ASR to 0.5 Ω cm2 at 750 °C, situating it well within the ideal performance range for SOFC anodes. The enhancement brought about by simultaneously doping PBM with cerium and nickel, which fundamentally relies on the critical contributions of defect chemistry and crystal structure, highlights the significance of these fields in creating sophisticated materials for energy related applications

Exsolution of Ni Nanoparticles from A-Site-Deficient Layered Double Perovskites for Dry Reforming of Methane and as an Anode Material for a Solid Oxide Fuel Cell

International audienceExsolution is a promising technique to design metal nanoparticles for electrocatalysis and renewable energy. In this work, Ni-doped perovskites, (PrBa)MnNiO with = 0, 0.05, 0.1, and 0.2 (S-PBMNx), were prepared to design exsolution systems as solid oxide fuel cell anodes and for catalysis applications. X-ray diffraction and transmission electron microscopy (TEM) analyses demonstrated that correlating A-site deficiency with Ni content can effectively induce exsolution of all Ni under H atmosphere at ∼ 875 °C, yielding the reduced (exsolved) R-PBMNx materials. On heating the exsolution systems in air, metal incorporation in the oxide lattice did not occur; instead, the Ni nanoparticles oxidized to NiO on the layered perovskite surface. The lowest area-specific resistance (ASR) under wet 5% H/N in symmetrical cells was observed for -PBMN0.2 anode (ASR ∼ 0.64 Ω cm at 850 °C) due to the highest Ni particle density in the -PBMNx series. The best performance for dry reforming of methane (DRM) was also obtained for -PBMN0.2, with CH and CO conversion rates at 11 and 32%, respectively, and the highest production of H (37%). The DRM activity of -PBMN0.2 starts at 800 °C and is sustained for up to at least 5 h operation with little carbon deposition (0.017 g·gcat·h). These results clearly demonstrate that varying Ni-doping in layered double perovskite oxides is an effective strategy to manipulate the electrochemical performance and catalytic activity for energy conversion purposes

β-Adrenoceptor subtype expression and function in rat white adipocytes

1. The pharmacological features of rat white adipocyte β-adrenoceptor subtypes were investigated by saturation and β-agonist competition studies with [(3)H]-CGP 12177 and by lipolysis induced by β-agonists as well as their inhibition by CGP 20712A (selective β(1)-antagonist) and ICI 118551 (selective β(2)-antagonist) in an attempt to establish a relationship between the functionality and binding capacity of β-adrenoceptor subtypes. 2. Two populations of binding sites were identified on adipocyte membranes, one with high affinity (0.22±0.07 nM) and the other with low affinity (23±7 nM). The low affinity binding sites constituted 90% of the total binding sites. 3. The competition curves, with 15 nM [(3)H]-CGP 12177, for the β-agonists, isoprenaline (Iso), noradrenaline (NA) and adrenaline (Ad), and the selective β(3)-agonist, BRL 37344 (BRL), were clearly biphasic (P<0.001). The rank orders of agonist potency (pK(i)) in competing for [(3)H]-CGP 12177 high affinity and low affinity binding sites, respectively, were Iso (9.28±0.24)>NA (8.90±0.12)>Ad (8.65±0.12)>>BRL (4.53±0.17) and BRL (7.38±0.19)>>Iso (2.96±0.26)⩾NA (2.80±0.17)>Ad (2.10±0.11) indicating the expression of β(1)- and β(3)-adrenoceptor subtypes on rat white adipocytes, respectively. Inversely, competition studies with the selective β(1)-agonist, xamoterol (Xam), provided evidence for a single homogeneous population of binding sites with low density (81±9 fmol mg(−1)) and high pK(i) value (7.23±0.26) confirming the presence of β(1)-adrenoceptors. 4. To assess a possible contribution of the β(2)-subtype, procaterol (Proc), a selective β(2)-agonist, was used to compete with 2 nM [(3)H]-CGP 12177. A single low affinity (4.61±0.07) population of binding sites was identified. The density of these sites (71±12 fmol mg(−1)) was similar to the one obtained with Xam, suggesting that Proc displaced [(3)H]-CGP 12177 from the β(1)-subtype. 5. The functional potency (pD(2)) order with BRL (9.07±0.20) and catecholamines (Iso: 7.26±0.06, NA: 6.89±0.02 and Ad: 6.32±0.07) was the same as that found for the low affinity binding sites in competition studies. Xam induced lipolysis with greater potency than dobutamine (Dob), 6.31±0.06 and 5.66±0.10, respectively. Proc stimulated lipolysis with a low potency (5.59±0.21). 6. The lipolytic response to 0.001 μM BRL was inhibited by both, selective β(1)- and β(2)-antagonist, in a monophasic manner with low potencies (CGP 20712A pK(i): <4.5 and ICI 118551 pK(i): 5.57±0.13). Similar monophasic profiles were obtained for inhibition of Xam- and Dob-induced lipolysis. In this case, CGP 20712A was more potent (>10 times) than ICI 118551. The monophasic inhibition was also observed with ICI 118551 in the presence of 0.05 μM Iso or 0.13 μM NA. In contrast, two populations of sites were identified with CGP 20712A in the presence of Iso as well as NA. The pK(i) values for the first sites were 8.41±0.09 and 8.58±0.17, respectively, and for the second population of sites 4.73±0.22 and 4.27±0.27, respectively. The proportion of the first sites was low: 19±4 and 22±5%, respectively. Biphasic curves were obtained with both antagonists using 2.5 μM Proc (CGP 20712A: pK(i)1: 8.17±0.08, site1: 23±6%, pK(i)2: 4.77±0.14; ICI 118551: pK(i)1: 7.78±0.03, site1: 37±2%, pK(i)2: 5.35±0.25). 7. Our results show that the radioligand [(3)H]-CGP 12177 allows the characterization of β(1)- and β(3)-adrenoceptor subtypes on rat white adipocytes. Lipolysis is highly dependent on β(1)- and β(3)-adrenoceptors. Finally, binding and functional studies confirm that lipolysis is mainly driven by the β(3)-subtype