Stability of NdBaCo2−xMnxO5+δ (x = 0, 0.5) layered perovskites under humid conditions investigated by high-temperature in situ neutron powder diffraction

The double perovskites NdBaCo2−xMnxO5+δ (x = 0 and 0.5) were investigated using in situ high temperature neutron powder diffraction in dry argon and wet atmospheres (40% D2O/argon and 40% D2O/air) in order to assess their stability as cathodes in proton conducting fuel cells. The x = 0 oxide loses oxygen on heating in dry argon at T > 400 °C and exhibits an oxygen vacancy order–disorder transition as evidenced by the orthorhombic Pmmm to tetragonal P4/mmm transition. Refinement of site occupancy factors suggests that the oxygen vacancies mainly form in the Nd layers and to a lesser extent at the equatorial positions of the transition metal polyhedra; at 800 °C, δ ∼ 0. When the gas was changed to wet argon at 800 °C and the sample cooled to 260 °C, no structural modification or change in the oxygen content was detected and no impurity phases formed, highlighting the excellent stability of the sample in wet atmospheres. On switching the gas to wet air at 260 °C, thermal analysis and neutron powder diffraction data together reveal that the sample intercalates mainly oxygen rather than proton defects within a two-phase process involving two orthorhombic phases, reflecting the symmetry of the reduced and oxidised materials. On heating, the sample transforms at T ≥ 600 °C to a single tetragonal phase whose symmetry is retained up to 800 °C and on subsequent cooling. The x = 0.5 material prepared in argon adopted a tetragonal P4/mmm structure at RT with δ ∼ 0. Its symmetry remains tetragonal on heating/cooling in wet argon. On changing the gas to wet air at 260 °C, it takes up oxygen via a two-phase process involving two tetragonal phases. Since fast oxidation is the main process that fills the oxygen vacancies of these double perovskites in wet air, a large oxygen deficiency seems to be not the only requirement for effective proton incorporation in this family of materials with basic characteristics

Propiedades magnéticas de las espinelas (Co,Ni,Mn)3O4

[EN] Magnetic properties of new materials, based on the general formula CoxNiyMnzO4 (x+y+z= 3), have been investigated as a function of magnetic field and temperature. The behavior observed in the paramagnetic regime (220 K ≤ T ≤ 400 K) shows a direct correlation with the nominal cation concentration. The paramagnetic-ferrimagnetic transition which takes place at T = Tc depends on the overall composition, going from Tc = 120 K (for Co0.2NiMn1.8O4) up to Tc = 210 K (for Co1.2Ni0.3Mn1.5O4). A second transition is observed at lower temperatures, corresponding to a second ordered magnetic sublattice. This second transition takes place at about 60 K (for Co0.6NiMn1.4O4), increasing with the cobalt content up to about 160 K. Under an external magnetic field, both transitions merge into a single one, with a characteristic temperature Tmax, which rapidly decreases with increasing field. Magnetization loops M(H) obtained at 5 K show a typical behavior of soft magnetic materials, with low coercive fields. Low conductivity values were observed at room‑temperature, increasing by a factor of 200-1000 at high temperatures (400 C), which make these compounds to be very interesting materials for potential applications as NTCR thermistors.[ES] Se han investigado las propiedades magnéticas de materiales de fórmula CoxNiyMnzO4 (x+y+z+ = 3), en función del campo magnético aplicado y de la temperatura. El comportamiento observado en el régimen paramagnético (220 K ≤ T ≤ 400 K) está en relación directa con la concentración catiónica nominal. Se observa una transición paramagnética-ferrimagnética a T = Tc, cuyo valor depende de la composición global del compuesto, variando entre Tc = 120 K (Co0.2NiMn1.8O4) y Tc = 210 K (Co1.2Ni0.3Mn1.5O4). Se ha observado una segunda transición a una temperatura inferior, relacionada con una segunda subred magnéticamente ordenada. Esta segunda transición ocurre a T = 60 K (Co0.6NiMn1.4O4), aumentando progresivamente con la concentración del cobalto hasta alcanzar aproximadamente 160 K. En presencia de un campo magnético externo, ambas transiciones se confunden en una sola, cuya temperatura característica Tmax decrece rápidamente en función de campos magnéticos crecientes. Se realizaron ciclos de magnetización M(H) a T = 5 K, obteniéndose ciclos típicos de materiales magnéticos blandos, con campos coercitivos pequeños. Con el objeto de estudiar posibles aplicaciones de estos materiales como termistores NTCR, se realizaron igualmente medidas eléctricas por sobre la temperatura ambiente : los valores de conductividad aumentan de un factor 200-1000 entre la temperatura ambiente y 400 C.This work has been supported by the Project CAM 07N/0096/2002, of the Autonomous Community of Madrid.Peer reviewe

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

Structure and Reactivity of the La_2-<i>x</i>Sr_<i>X</i>MnO_4±δ (0.7≤ <i>X ≤</i>1.0) Solid Solutions in Oxidizing Condition

International audienc

Structural chemistry and magnetic properties of Pr3-xSr1+xCrNiO8

Polycrystalline samples of the n=1 Ruddlesden-Popper system Pr3-xSr1+xCrNiO8 have been synthesized over the composition range 0.00.1 adopt the tetragonal space group I4/mmm; Pr2.9Sr1.1CrNiO8 adopts the orthorhombic space group Fmmm. There is no evidence of Cr/Ni cation ordering in any composition. A maximum in the zero-field cooled magnetic susceptibility is observed at a temperature Tf that decreases with increasing Sr content; 52≤Tf (K)≤13. The frequency dependence of Tf observed in a.c. susceptibility measurements, together with the analysis of neutron diffraction data, suggests that the atomic magnetic moments in these compositions adopt a spin-glass-like state below Tf. © 2008 Elsevier Inc. All rights reserved.</x≤1.0

Magnetic properties of the spinel system MgxMn3-xO4 (0 x 2)

Bol. Soc. Esp. Ceram. Vidrio, 2008, 47, 143-14

In situ neutron diffraction study of the high-temperature redox chemistry of Ln(3-x)Sr(1+x)CrNiO(8-delta) (Ln = La, Nd) under hydrogen

The chemical reduction of the K2NiF4-type oxides, Ln2Sr2CrNiO8-δ (Ln = La, Nd) and Nd 2.25Sr1.75CrNiO8-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. © 2010 The Royal Society of Chemistry

Cation distribution and ferromagnetic exchange in the YMn0.5Co0.5O3 perovskite investigated by neutron powder diffraction

J. Solid State Chem., 2007, 180, 1737-174

Redox behaviour of potential SOFC cathode materials La1.6Sr0.4Ni1-​xCoxO4+δ (x = 0.6 and 0.8) determined from in situ neutron powder diffraction under flowing O2 and 5​%H2

International audienc