Mona Bahout

Spin reversal and ferrimagnetism in (Gd,Ca)MnO3

The gadolinium-based manganite GdMnO3, partially substituted by calcium at the cationic site, has been investigated through its crystallochemical and magnetic properties. The Gd1−xCaxMnO3 solid solution crystallises in an orthorhombic perovskite-type structure (S.G. Pbnm) in the range 0 ≤ x ≤ 0.45. With increasing x, the structure evolves towards a quasi-tetragonal symmetry, with c/√2 ≈ a < b, while the orthorhombicity factor b/a decreases. The magnetisation in the ordered state was studied as a function of temperature and applied field. ZFC + FC cycles show that the solid solution can be described as a ferrimagnetic-like system, in which negatively-polarised gadolinium moments behave as free spins under the internal field of the ordered Mn sublattice. As a result, the spontaneous magnetisation changes sign upon cooling (FC process), reaching similar values as those obtained upon reversal of the magnetic field. Results are explained in terms of two interacting magnetic sublattices: a Mn-based ferromagnetic one and a negatively-aligned gadolinium network. The local field at a given site depends on the exchange interaction between these two sublattices, leading to a spin reversal when the magnetic moment of the gadolinium sublattice is larger than the ferromagnetic network. This interpretation is confirmed through the thermal evolution of the remanent magnetisation Mrem(T) after freezing the spins using a high magnetic field.

Oxygen diffusion mechanism in the mixed ion-electron conductor NdBaCo2O5+x

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

Critical behavior in the perovskite-like system Y(Ni,Mn)O3

Abstract The electrical and magnetic properties of the perovskite-type solid solution YNi x Mn 1− x O 3 have been studied at different temperatures and magnetic fields. Electrical conductivity measurements have shown a semiconductor behavior throughout the solid solution. The room temperature conductivity increases with the Ni concentration until x =1/3, and then decreases for higher Ni contents. The effective moment in the paramagnetic state shows a clear break at x = x crit =1/3, presenting a constant value at x ≤ x crit and decreasing at higher nickel concentration. In the ordered state, other magnetic parameters show similar discontinuities versus the nickel content, such as a sudden increase of the magnetization at high field or a maximum in the coercive strength of the ferromagnetic-like M ( H ) loop. By charge-conservation arguments we can readily identify such critical value as an optimum conversion of Mn 3+ into Mn 4+ , leading to the charge-balance formula Y 3+ [Ni 1/3 2+ (Mn 3+ 0.5 Mn 4+ 0.5 ) 2/3 ]O 3 2− , that is, equal amounts of Mn 3+ and Mn 4+ in the sample. Magnetic data shows that, at x ≤ x crit , the solid solution behaves as a spin-glass and/or antiferromagnetic system, while at x > x crit , full ferromagnetic characteristics are observed.

High temperature structural stability, electrical properties and chemical reactivity of NdBaCo2−xMnxO5+δ (0 ≤ x ≤ 2) for use as cathodes in solid oxide fuel cells

The effects of Mn substitution for Co on the crystal chemistry, oxygen content, thermal expansion and electrical conductivity of the NdBaCo2−xMnxO5+δ perovskites (0 ≤ x ≤ 2) have been investigated. The NdBaCo2−xMnxO5+δ samples exhibit structural changes with increasing Mn contents from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) then to cubic (1.5 ≤ x ≤ 2.0) symmetry. All the samples lose oxygen when heated in air at T > 400 °C although the degree of oxygen loss and kinetics of oxygen exchange between the gas phase and oxide decrease with increasing Mn contents. The thermal expansion coefficients evaluated from ex situ XRD and electrical resistivity decrease with increasing Mn substitution and the values for the x = 1.5 and 2.0 compositions match with those of the Ce0.8Gd0.2O1.95 (GDC) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) electrolytes. With electrical conductivity values of >100 S cm−1 at 800 °C and good chemical stability with GDC and LSGM, the Mn-substituted perovskites are promising cathode materials for SOFCs.

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.

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.

Use of in situ neutron diffraction to monitor high-temperature, solid/H2-gas reactions

For the first time, the chemistry in H(2) gas of a perovskite-like material, Pr(2)Sr(2)CrNiO(8), has been monitored at temperatures up to approximately 700 degrees C, in situ, by neutron powder diffraction.

Effects of substitution at the manganese site in RE(Ni,Mn)O3 perovskites (RE=Y, Eu)

The magnetically ordered states of the YNixMn1−xO3 and EuNixMn1−xO3 solid solutions (0.0≤x≤0.50) have been studied at different temperatures and magnetic fields. The magnetization as a function of temperature M(T) shows features due to canted-antiferromagnetism (x=0.25 and 0.33) and ferromagnetism (x=0.50), while YNi0.20Mn0.80O3 presents a spin-glass behavior. A large field-dependence of the susceptibility maximum at Tmax is found in the nickel-poor region, except for the spin-glass compound, for which Tmax stays constant at TSG∼20 K. For the nickel-rich samples the paramagnetic-to-ferromagnetic transition at Tc is field-independent, with Tc∼80 and 150 K, for YNi0.50Mn0.50O3 and EuNi0.50Mn0.50O3, respectively. The coercive field Hcoerc and the high-field data obtained from magnetization loops M(H) show a well-defined frontier at x=xcrit=1/3, between canted-antiferromagnetism (x≤xcrit) and full ferromagnetism (x>xcrit). This critical value corresponds to equal amounts of Mn3+ and Mn4+, leading to a maximum number of pairs Mn3+/Mn4+ and an optimum in the double-exchange interactions.

Redox behavior of the SOFC electrode candidate NdBaMn2O5+δ investigated by high-temperature in situ neutron diffraction: first characterisation in real time of an LnBaMn2O5.5 intermediate phase

The structural behavior of the tetragonal NdBaMn2O5 phase, a member of the family of A-site ordered layered manganites that have been recently suggested as possible mixed ionic and electronic conductors, has been investigated by means of in situ neutron powder diffraction. Considering applications in energy production and storage devices and use of NdBaMn2O5+δ as an electrode in symmetrical cells, the study was carried out in relevant atmosphere conditions, i.e. dilute hydrogen (wet and dry) and dry air in the temperature range 25–800 °C. Neutron data under flowing hydrogen allowed monitoring of the structural phase transition from the charge-ordered to the charge-disordered state as a function of temperature. Slow reduction of the fully oxidised phase, NdBaMn2O6, previously formed from quick oxidation of the pristine material, enabled real-time observation of the intermediate NdBaMn2O5.5 phase and its crystal characterization up to 700 °C in the course of its conversion to NdBaMn2O5. Oxygen vacancy ordering within the Nd layers of NdBaMn2O5.5 correlated with antiferrodistortive orbital ordering of the Jahn–Teller Mn3+ ion in the square pyramids and octahedra results in large thermal expansion and relatively slow anisotropic oxygen diffusion occurring in the NdO layer. The four heating/cooling cycles evidenced no oxygen miscibility between the three distinct phases detected in the NdBaMn2O5+δ system with δ ∼ 0, 0.5 and 1 and clearly demonstrated that reversible oxygen intercalation/deintercalation underpins the phase stability of the LnBaMn2O5+δ materials to redox cycling and to wet atmosphere in high temperature electrochemical devices.

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

The n = 1 Ruddlesden-Popper (RP) oxides La1.6Sr0.4Ni1-xCoxO4+δ (x = 0.6 and 0.8) have been prepared by the citrate-gel method and studied using thermal analysis and in situ neutron powder diffraction (NPD) under oxygen (x = 0.6) and subsequent 5%H2 flow. On heating under O2, the x = 0.6 sample loses oxygen from the interstitial site until it is emptied at 475 °C. Subsequent heating in 5%H2 results in reduction which proceeds within two-steps; the first one occurs between ~170 and 300 °C and involves removal of the interstitial oxygen (Oint), the second step starting at ~ 400 °C and going on up to 600 °C (the highest temperature reached) involves oxygen deintercalation from the equatorial position (Oeq). After 20 min of isothermal heating at 600 °C, unless a thermodynamic equilibrium state was not reached there was no sign of impurity related to the decomposition of the sample and the composition refined to La1.6Sr0.4Ni0.4Co0.6O3.85(3). The x = 0.8 sample exhibits similar behavior under hydrogenwith slight shifts in temperature and the composition reached after the reducing cycle refined to La1.6Sr0.4Ni0.2Co0.8O3.85(1). This study demonstrates that the Co-rich La1.6Sr0.4Ni1-xCoxO4+δ oxides can withstand hydrogen reducing conditions with topotactic deintercalation of Oint and Oeq but are not stable enough at 600 °C in comparison to La1.5Sr0.5Ni0.5Co0.5O4+δ owing toincreased Sr/La ratio in the latter.