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New polycrystalline SrMo1−xMxO4−δ (M = Fe and Cr) scheelite oxides have been prepared by topotactical oxidation, by annealing in air at 500 °C, from precursor perovskites with the stoichiometry SrMo1−xMxO3−δ (M = Fe and Cr). An excellent reversibility between the oxidized Sr(Mo,M)O4−δ scheelite and the reduced Sr(Mo,M)O3−δ perovskite phase accounts for the excellent behavior of the latter as anode material in solid-oxide fuel cells. A characterization by X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) has been carried out to determine the crystal structure features. The scheelite oxides are tetragonal, space group I41/a (No. 88). The Rietveld-refinement from NPD data at room temperature shows evidence of oxygen vacancies in the structure, due to the introduction of Fe3+/Cr4+ cations in the tetrahedrally-coordinated B sublattice, where Mo is hexavalent. A thermal analysis of the reduced perovskite (SrMo1−xMxO3−δ) in oxidizing conditions confirms the oxygen stoichiometry obtained by NPD data; the stability range of the doped oxides, below 400–450 °C, is lower than that for the parent SrMoO3 oxide. The presence of a Mo4+/Mo5+ mixed valence in the reduced SrMo1−xMxO3−δ perovskite oxides confers greater instability against oxidation compared with the parent oxide. Finally, an XPS study confirms the surface oxidation states of Mo, Fe, and Cr in the oxidized samples SrMo0.9Fe0.1O4-δ and SrMo0.8Cr0.2O4-δ.

The high-temperature cubic phase of SrCoO 3− δ is a promising cathode material for solid oxide fuel cells (SOFC) due to its high electrical conductivity and oxygen permeation flux. However, this phase is not stable below 900 °C where a 3C-cubic to 2H-hexagonal phase transition takes place when the sample is slowly cooled down. In this work we have stabilized a 3C-tetragonal P4/mmm structure for SrCo 1− x Nb x O 3− δ with x = 0.05. We have followed the strategy consisting of introducing a highly-charged cation at the Co sublattice, in order to avoid the stabilization of the unwanted 2H structure containing face-sharing octahedra. The characterization of this oxide included X-ray (XRD) and neutron powder diffraction (NPD) experiments. SrCo 0.95 Nb 0.05 O 3− δ adopts a tetragonal superstructure of perovskite with a = a 0 , c = 2 a 0 ( a 0 ≈ 3.9 A) defined in the P4/mmm space group containing two inequivalent Co positions. Flattened and elongated (Co,Nb)O 6 octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). In the test cell, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3− δ (LSGM). The test cells gave a maximum power density of 0.4 and 0.6 W/cm 2 for temperatures of 800 and 850 °C, respectively, with pure H 2 as fuel and air as oxidant. The good performance of this material as a cathode is related to its mixed electronic-ionic conduction (MIEC) properties, which can be correlated to the investigated structural features: the Co 3+ /Co 4+ redox energy at the top of the O-2p bands accounts for the excellent electronic conductivity, which is favored by the corner-linked perovskite network. The considerable number of oxygen vacancies, with the oxygen atoms showing high displacement factors suggests a significant ionic mobility.

Oxides of composition SrMo1� xCrxO3� d (x ¼ 0.1, 0.2) have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers higher than 700 mW cm � 2 at 850 � C with pure H2 as a fuel. All the materials are suggested to

A polycrystalline oxygen-stoichiometric La2ZnMnO6 double-perovskite oxide has been prepared by soft-chemistry procedures, followed by annealing in air at 800 °C. A reduced specimen, with a La 2ZnMnO6-δ composition, has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600 °C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Zn and Mn and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the monoclinic P21/n space group, exhibiting an antisite Zn/Mn disorder of about 15% and 14%, respectively. The partial reduction of Mn4+ to Mn3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range. Magnetic susceptibility measurements indicate an antiferromagnetic behaviour, confirming the presence of Mn 3+ ions in the structure. The magnetic structure of the reduced phase, determined from NPD data at 3 K, shows an antiferromagnetic G-type coupling between Mn at 2c and 2d sites (promoted by the anti-site disorder); the ordered magnetic moment at Mn site is 0.789 μB at 3 K. Both phases display a semiconductor-like behaviour with a maximum conductivity of 0.052 S cm-1 for the reduced phase at 650 °C, due to the occurrence of Mn3+-Mn4+ mixed valence. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes. The reversibility and versatility of the present compounds as catalysts for oxygen reduction (cathode) or fuel oxidation (anode) were tested in single SOFC cells yielding power density spanning from 120 to 155 W/cm2 using these perovskites as anode, cathode and symmetric electrodes for SOFC. © 2013 Elsevier Masson SAS. All rights reserved., We acknowledge the financial support of the Spanish “Ministerio de Ciencia e Innovación” (MICINN) to the project MAT2010-16404.

La 0.5 Sr 0.5 Co 0.5 Ti 0.5 O 3− δ perovskite oxide has been prepared as polycrystalline powder, characterized and tested as cathode and anode material for solid-oxide fuel cells. The oxidized material is suggested to present mixed ionic-electronic conductivity (MIEC) from “in-situ” neutron powder diffraction (NPD) experiments, complemented with transport measurements; the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms suggest a large lability and mobility combined with a semiconductor-like behaviour with a maximum conductivity of 29 S cm −1 at T = 850 °C. A complete reversibility towards reduction–oxidation processes has been observed, where the reduced Pm-3m perovskite with La 0.5 Sr 0.5 Co 0.5 Ti 0.5 O 2.64 composition has been obtained by topotactical oxygen removal without abrupt changes in the thermal expansion. The oxidized material shows good performance working as a cathode with LSGM electrolyte, yielding output power densities close to 500 mW/cm 2 at 850 °C. At intermediate temperatures (800 °C) it may be used as a cathode or as an anode, yielding power densities of 220 and 170 mW/cm 2 , respectively. When used simultaneously as cathode and anode a maximum power density of 110 mW/cm 2 was obtained. Therefore, we propose the La 0.5 Sr 0.5 Co 0.5 Ti 0.5 O 3− δ composition as a promising candidate for symmetrical electrode in intermediate-temperature SOFC.

The title compound has been synthesized by the hydroxide route. The crystal structure has been investigated at room temperature from high-resolution neutron powder diffraction (NPD) data. It crystallizes in a cubic spinel structure, space group F d 3 ¯ m , Z = 8, with a = 8.3940(2) A at 295 K. The crystallographic formula is (Ni 0.202(1) Mn 0.798(1) ) 8a (Ni 0.790(1) Mn 1.210(1) ) 16d O 4 where 8 a and 16 d stand for the tetrahedral and octahedral sites of the spinel structure, respectively. There is a significant inversion degree of the spinel structure, λ = 0.80. In fact, the variable parameter for the oxygen position, u = 0.2636(4), is far from that expected ( u = 0.25) for normal spinels. From a bond-valence study, it seems that the valence distribution in NiMn 2 O 4 spinel is not as trivial as expected (Ni 2+ and Mn 3+ ), but clearly the tetrahedral Mn ions are divalent whereas the octahedral Mn and Ni are slightly oxidized from the expected +3 and +2 values, respectively. The mixed valence observed at the octahedral sites provides the charge carriers that, by a hopping mechanism between Mn 3+ /Mn 4+ adjacent sites, leads to a significant conductivity, up to 0.85 S cm −1 at 800 °C in air.

Oxides of composition SrMo 1− x Fe x O 3− δ ( x = 0.1, 0.2) have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers close to 900 mW cm −2 at 850 °C with pure H 2 as a fuel. This excellent performance is accounted for the results of an “in situ” neutron powder diffraction experiment, at the working temperature of the SOFC, showing the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that suggest a large lability and mobility, combined with a huge metal-like electronic conductivity, as high as 340 S cm −1 at T = 50 °C for x = 0.1. The magnitude of the electronic conductivity decreases with increasing Fe-doping content. An adequate thermal expansion coefficient, reversibility upon cycling in oxidizing–reducing atmospheres and chemical compatibility with the electrolyte make these oxides good candidates for anodes in intermediate-temperature SOFC (IT-SOFCs).

Magnesium base alloys are very attractive for hydrogen storage due to their large hydrogen capacity, small weight and low-cost. We have designed a new synthesis method for the ternary metal hydride perovskite system Na1−xLixMgH3, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. Well-crystallized samples were obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 °C from mixtures of MgH2, NaH and LiH enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features (GdFeO3 type). Na1−xLixMgH3 hydrides series (0 ≤ x ≤ 0.18) show an orthorhombic symmetry with space group Pnma (No. 62). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperatures.

We describe a simplified synthesis method for the metal hydride Mg2NiH4, based on the direct reaction of MgH2 hydride and Ni metal (2MgH2 + Ni → Mg2NiH4) by ball-milling under nitrogen atmosphere at room temperature. Different milling times and different MgH2:Ni atomic stoichiometries have been evaluated, in order to optimize the synthesis conditions. X-ray diffraction (XRD) analysis was used to identify the purity of the samples and quantify the present phases, through Rietveld analyses of the crystal structure, which can be defined in the cubic space group F m 3 ¯ m . Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents. Scanning electron microscopy images show an average particle size of 1–2 μm for Mg2NiH4; the microstructure presents a sponge-like aspect adequate to favor the absorption and desorption processes.

The use of pyrochlore oxides as solid-oxide fuel cells (SOFC) cathodes is a promising and rather unexplored alternative. The presence of some cationic disorder in pyrochlores promotes the Frenkel-defect formation responsible of a high-ionic conductivity in these compounds. Moreover, the use of the suitable transition metals at the B position gives rise to an adequate electronic conductivity constituting disordered pyrochlores as mixed, ionic–electronic conductors. In this work, we have synthesized and characterized a new family of pyrochlore oxides with the formula R 2 MnRuO 7 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) using the smallest rare-earth cations in order to promote disorder in the crystal lattice. Polycrystalline samples were prepared by a soft chemistry procedure involving citrates of the different metal ions, followed by thermal treatments in air or O 2 pressure. They are all cubic, space group F d 3 ¯ m . The Rietveld-refinement from neutron powder diffraction data at room-temperature evidences that the cation disorder (distribution of Mn between A and B positions) increases when the size of R decreases. This disorder is accompanied by an increment of the oxygen-vacancy concentration due to the reduction of Mn 4+ at the B position to Mn 2+ at the A position. The obtained compounds display a semiconductor-like behavior with a maximum conductivity of 8.9 S cm −1 for Er 2 MnRuO 7 at 900 °C. Moreover, the measured thermal expansion coefficients are in the range of 9.8–10.7 × 10 −6 K −1 between 100 and 900 °C that perfectly match those of the usual electrolytes, YSZ, LSGM or CGO. The obtained results present the R 2 MnRuO 7 pyrochlores as alternative cathodes for IT-SOFCs.

This work was supported by the National Science Foundation (DMR MIRT 1122603) and Robert A. Welch Foundation (F-1066) in the U.S. J.A.A., R.M.C., J.S.B., and M.T.F. thank the Spanish MINECO Projects No. MAT2013-41099-R and No. RyC-2010-06276 for financial support.

Brownmillerite-type oxides Ca2Fe2-xCoxO5-δ (x = 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) have been explored as possible cathodes for solid oxide fuel cells (SOFC). The samples have been prepared, characterized and tested as cathode materials in single solid-oxide fuel cells. As shown in a neutron powder diffraction (NPD) study at RT, for x = 0 and 0.2 the compounds crystallized in a single phase with brownmillerite-type structure (s.g. Pcmn), whereas for x = 0.4, 0.6, 0.8, 1, 1.2, and 1.4 the samples crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite (s.g Pcmb). This superstructure consists of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with (Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. In an “in situ” NPD experiment of Ca2Fe0.8Co1.2O5-δ at the working temperature of the SOFC, this compound shows the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that suggest a large lability and mobility. In single test cells these cathode materials generated a maximum power of 412 mW/cm2 at 850 °C with pure H2 as a fuel. The electrodes were supported on a 300-mm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM). The measured thermal expansion coefficients between 300 and 850 °C exhibit an excellent chemical compatibility with the electrolyte. The obtained compounds display a semiconductor-like behavior with conductivity values at the SOFCs working temperatures (650–850 °C) that are sufficient to yield a good performance in IT-SOFC., We thank the financial support of the Spanish Ministry of Education to the project MAT2013-41099-R and we are grateful to the Institut Laue-Langevin (ILL) for making all facilities available.

SrMo0.9Co0.1O3 perovskite has recently been described as a performing anode material in solid-oxide fuel cells (SOFC). In this work, we describe the structural phase transitions that this oxide undergoes below room-temperature, studied by “in-situ” neutron powder diffraction (NPD) data and differential scanning calorimetry (DSC) measurements. The oxide is defined in a cubic unit cell with space group Pmm at room temperature. Upon cooling the sample, two structural phase transitions appear, the first one from the cubic structure to a tetragonal I4/mcm structure close to 240 K, and the second one to an orthorhombic Imma phase below 150 K. By DSC measurements the first phase transition was identified to happen at 243 K., We thank the financial support of the Spanish Ministry of Economy and Competitivity (MINECO) to the project MAT2013-41099-R; we are grateful to the Institut Laue-Langevin (ILL) in Grenoble for making all facilities available.

Sr0.7Ho0.3CoO3-δ oxide has been recently described as an excellent cathode material (1274 mW cm-2 at 850°C with pure H2 as fuel1) for solid oxide fuel cells (SOFCs) with LSGM as electrolyte. In this work, we describe a detailed study of its crystal structure conducted to find out the correlation between the excellent performance as a cathode and the structural features. The tetragonal crystal structure (e.g., I4/mmm) basically contains layers of octahedrally coordinated Co2O6 units alternated with layers of Co1O4 tetrahedra sharing corners. An >in situ> neutron power diffraction (NPD) experiment, between 25 and 800°C, reveals the presence of a high oxygen deficiency affecting O4 oxygen atoms, with large displacement factors that suggest a large lability and mobility. Difference Fourier maps allow the visualization at high temperatures of the 2D diffusion pathways within the tetrahedral layers, where O3 and O4 oxygens participate. The measured thermal expansion coefficient is 16.61 × 10-6 K-1 between 300 and 850°C, exhibiting an excellent chemical compatibility with the electrolyte., We thank the financial support of the Spanish Ministry of Education to the project MAT2013-41099-R, and we are grateful to the Institut Laue-Langevin (ILL) for making all facilities available.

SrMo0.9Co0.1O3-δ oxide has been prepared, characterized and tested as anode material in single solid-oxide fuel cells (SOFC), yielding output powers close to 800 mW cm-2 at 850 C with pure H2 as a fuel. This excellent performance is accounted for the results of an >in-situ> neutron powder diffraction (NPD) experiment, at the working conditions of a SOFC, showing the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that suggest a large lability and mobility, combined with a huge metal-like electronic conductivity, as high as 386 S cm-1 at T = 50 C. Besides, the oxidation of the perovskite gives rise to a new oxygen deficient scheelite-like phase with formula SrMo0.9Co0.1O 4-δ with Mo(VI), which has been studied by NPD and thermal analysis as far as crystal structure and composition are concerned. An adequate thermal expansion coefficient for both (oxidized and reduced) phases, an excellent reversibility upon cycling in oxidizing-reducing atmospheres and a good chemical compatibility with the electrolyte (La0.8Sr 0.2Ga0.83Mg0.17O3-δ; LSGM) make this oxide a good candidate for anode in intermediate-temperature SOFC (IT-SOFCs). © 2014 Elsevier B.V. All rights reserved., The authors thanks the Robert A. Welch Foundation of Houston, TX and the Spanish “ Ministerio de Ciencia e Innovación” (MICINN-Project MAT2010-16404) for the financial support.

In this paper we have studied the effect of Cr doping on the structural and electronic transport properties in 4d perovskites SrMo1- xCrxO3 (x = 0, 0.1 and 0.2). Polycrystalline samples were prepared by a soft chemistry procedure followed by thermal treatments in a 5%H2 atmosphere. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features (GdFeO3 type). The Cr doping at B position does not change the space group of the samples; they are all cubic, space group Pm-3m, at room temperature. However, upon cooling down the samples two structural phase transitions appear, the first from cubic Pm-3m structure to a tetragonal structure defined in I4/mcm close to 240 K, and the second to an orthorhombic Imma phase below 100 K. Cr substitution drives the SrMo1-xCrxO3 system from a Pauli-paramagnetic state to a weak ferromagnetic state combined with predominant antiferromagnetic interactions; the susceptibility and the saturation magnetization increases monotonically with increasing the Cr-doping content. © 2014 Elsevier B.V. All rights reserved., The authors thanks the Robert A. Welch foundation of Houston (TX) and the Spanish “Ministerio de Ciencia e Innovación” (MICINN-Project MAT2010-16404) for the financial support.

Polycrystalline La2ZnMnO6 is prepared by sol-gel synthesis starting with stoichiometric amounts of La2O3, ZnO, and MnCO3 dissolved in citric acid and some drops of HNO3 (1.

New pyrochlore-like phases of composition R2MnTiO7 (R = Er and Y) have been synthesized by a soft-chemistry procedure involving citrates of the different metal ions followed by thermal treatments at moderate temperatures (850 C for 12 h in air). A characterization by X-ray diffraction and neutron powder diffraction (NPD) has been carried out in order to determine the crystal structure features: these phyrochlores are cubic, space group Fd-3m, defining an intrinsically frustrated three-dimensional system. The Rietveld-refinement from NPD data at room temperature evidences an antisite cation disorder (distribution of Mn between A and B positions) that is accompanied by an increment of the oxygen-vacancy concentration due to the reduction of Mn4+ at the B position to Mn2+ at the A position. Thermogravimetric analysis (TGA) was useful to evaluate the stability of these oxides in reducing conditions up to 500 C. Magnetic susceptibility measurements indicate a ferromagnetic behavior, due to the random distribution of Mn4+ ions in the octahedral sublattice. At lower temperatures there is a polarization of the R+3 magnetic moments, which also participate in the magnetic structure. Aiming to evaluate these materials as possible electrodes for solid oxide fuel cells (SOFC) we determined that the thermal expansion coefficients between 100 and 900 C perfectly match with those of the usual electrolytes; however, these pyrochlore oxides display a semiconductor-like behavior with poor conductivity values, e.g. 6 × 10-3 cm-1 at 850 C for Er, which would prevent its use as MIEC (mixed ionic-electronic conductors) oxides in SOFC devices. © 2013 Elsevier Ltd. All rights reserved., We thank the financial support of the Spanish Ministry of Science and Innovation (MICINN) to the project MAT2010-16404.

A novel ternary hydride in the Mg-Cu-H system has been identified by a new synthesis method based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. A well-crystallized sample was obtained in a piston-cylinder hydrostatic press at moderate pressures of 3.5 GPa and temperatures around 850 C starting from mixtures of MgH2 and Cu enclosed in platinum capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features. The crystal structure of Mg 3CuHx was determined by ab-initio procedures from neutron powder diffraction (NPD) data in the cubic space group F-43m (No. 216), with lattice parameter a = 6.285 Å. The hydrogen positions were determined by Fourier synthesis revealing the introduction of hydrogen atoms in the system. A final Rietveld refinement of the structure gives up a Mg3CuH 0.6 composition, which is consistent with the Fourier synthesis. Thermogravimetric analysis (TGA) has been carried out to determine the hydrogen desorption temperature, and to confirm the hydrogen contents. A destabilization with respect to MgH2 hydride, with a reduction of the desorption temperature of approximately 240 C, suggests that Cu-doping of MgH2 may find applications in the hydrogen storage field. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved., We thank the financial support of the Spanish Ministry of Science and Technology to the project MAT2010-16404.

We have designed a new synthesis method for the complex Mg 2NiH4 hydride, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. A well-crystallized sample was obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 C from mixtures of MgH2 and Ni enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features. Mg 2NiH4 hydride shows a monoclinic symmetry with space group C2/c (No. 15). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperature, the hydrogen contents and the cyclability of the compound under a hydrogen pressure of 10 atm. These studies reveal two different desorption temperatures that correspond to the loss of the labile hydrogen H3 leading to a stable hexagonal structure (Mg2NiH3) followed by the complete hydrogen desorption to give Mg2Ni. The cyclability of the hydride was studied through 10 cycles at a hydrogen pressure of 10 atm; a slight loss of reversibility was observed due the progressive oxidation to MgO due to traces of oxygen adsorbed within the DSC chamber., We thank the financial support of the Spanish Ministry of Science and Innovation (MICINN) to the project MAT2010-16404.

Title hydrides with x = 0, 0.25, and 0.5 are prepared from stoichiometric mixtures of MgH2, NaH, and LiH (gold capsule, 2 GPa, 750 °C

Sr2MMoO6 (M = Mg, Mn) double perovskites have recently been proposed as anode materials in solid-oxide fuel cells (SOFC). The evolution of their crystal structures has been followed by >in situ> temperature-dependent neutron powder diffraction from 25 °C room temperature (RT) to 930 °C by heating in ultrahigh vacuum (PO2 ≈ 10 -6 Torr) in order to simulate the reducing atmosphere corresponding to the working conditions of an anode in a SOFC. At RT, the samples are described as tetragonal (I4/m space group) and monoclinic (P21/n) for M = Mg, Mn, respectively. Sr2MgMoO6 undergoes a structural phase transition from tetragonal to cubic (Fm-3m) below 300 °C; Sr2MnMoO6 experiences two consecutive phase transitions to tetragonal (I4/m) and finally cubic (Fm-3m) at 600 °C and above. In the cubic phases, the absence of octahedral tilting accounts for a good overlap between the oxygen and transition-metal orbitals, resulting in a good electronic conductivity; a high mobility of the oxygen atoms is derived from the elevated displacement parameters, for instance 3.0 Å2 and 4.6 Å2 at 930 °C for M = Mg, Mn, respectively. Both factors contribute to the excellent performance described for these mixed ionic and electronic conductor oxides as anodes in single fuel cells. From dilatometric measurements, the thermal expansion coefficients (TEC) in the cubic region are 12.7 × 10-6 K-1 and 13.0 × 10-6 K-1 for M = Mg and Mn, respectively. These figures are comparable to those obtained from the mentioned structural analysis; moreover, the TECs for the cubic phases perfectly match those of the usual electrolytes in a SOFC. © 2013 American Institute of Physics., We acknowledge the financial support of the Spanish “Ministerio de Ciencia e Innovación” (MICINN) to the project MAT2010-16404. L.T. thanks the financial support of CONICYT for “Beca Nacional de Doctorado 2009.”

The members of the family of pyrochlore oxides with the formula R(2)MnRuO(7) (R = Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples were prepared by a soft chemistry procedure involving citrates of the different metal ions, followed by thermal treatments in air or O(2) pressure. The crystallographic and magnetic structures have been analysed from X-ray diffraction (XRD) and neutron powder diffraction (NPD) data, in complement with magnetic measurements; the evolution along the series of the crystallographic parameters (unit-cell parameters, bond distances and angles) is discussed. In R(2)MnRuO(7) pyrochlores, Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd ̅3m, which defines an intrinsic frustrated three-dimensional system. In all the cases, the low-temperature NPD data unveils an antiferromagnetic coupling of two subsets of Mn(4+)/Ru(4+) spins, indicating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites. At lower temperatures there is a polarization of the R(3+) magnetic moments, which also participate in the magnetic structure, when a magnetic rare earth is present.

A pyrochlore-like phase of composition Ho2MnRuO7 has been synthesized by a soft-chemistry procedure followed by thermal treatments at moderate temperatures up to 900 °C for 12 h in air. It has been characterized by x-ray diffraction and neutron powder diffraction (NPD), as well as dc and ac susceptibilities. As in the parent Ho2Mn2O7 oxide, the magnetic Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd3¯m, which define a potentially frustrated three-dimensional array of corner sharing (Mn,Ru)4 tetrahedra. The dc and ac magnetic susceptibilities of Ho2MnRuO7 display a sharp increase near 60 K. In addition, the field-cooled and zero-field-cooled curves diverge below 30 K. The ac data present frequency variability below 60 K and broad frequency dependent maxima at lower temperatures (∼30 K), suggesting a spin-glass like behavior similar to Ho2Mn2O7. However, a low-temperature NPD study of the magnetic structure unveils an antiferromagnetic coupling of two subsets of M...