Nonstoichiometric Strontium Ferromolybdate as an Electrode Material for Solid Oxide Fuel Cells

This review is devoted to the application of Sr2FeMoO6−δ (SFM) and Sr2F1.5Mo0.5O6−δ (SF1.5M) in La1−xSrxGa1−yMgyO3−δ (LSGM)-based SOFCs. We consider the most relevant physical properties (crystal structure, thermodynamic stability, iron and molybdenum valence states, oxygen vacancy formation and oxygen non-stoichiometry, electrical conductivity), A- and B-site ion substitution, and the performance of SF1+xM SOFCs (polarization resistance, operation with hydrogen, operation with hydrocarbons and methanol). Their properties can be tailored to a particular application by the substitution of different metal cations into their lattices. SF1+xM materials are excellent catalysts in hydrocarbon oxidation and can prevent carbon deposition due to the ability to exchange lattice oxygen with the gaseous phase. Moreover, they are sulfur tolerant. This opens the way to direct hydrocarbon-fueled SOFCs, eliminating the need for external fuel reforming and sulfur removal components. Such SOFCs can be greatly simplified and operate with much higher overall efficiency, thus contributing to the solution to the lack of energy problem in our modern world.

Structural stability and evolution of half-metallicity in Ba2CaMoO6: interplay of hole- and electron-doping

Half-metallic ferromagnetic materials have attracted a lot of attention due to their probable technological applications in spintronics. In this respect, doping plays a crucial role in tailoring or controlling the physical properties of the system. Herein, the impact of both hole and electron doping on the structural, electronic and magnetic properties of the recent high pressure synthesized non-magnetic insulator Ba₂CaMoO₆ double perovskite oxide are investigated by replacing one of the Mo ions with Nb and Tc. The structural and mechanical stability of the undoped/doped materials are analyzed by calculating the formation energies and stiffness tensors, respectively, which confirm the system's stability. Interestingly, our results revealed that Nb- and Tc-doped systems display an electronic transition from insulating to p- and n-type half-metallic ferromagnetic states, respectively. The most striking feature of the present study is that oxygen ions become spin-polarized, with a magnetic moment of ∼0.12 μ_B per atom, and are mainly responsible for conductivity in the Nb-doped system. However, the admixture of Tc 4d non-degenerate orbitals are primarily contributing to the metallicity in the Tc-doped structure, with a moment of ∼0.59 μ_B. It is also found that Nb and Tc ions remain in the 5+ and 7+ states with electronic configurations of t22g↑t22g↓e0g↑e0g↓ and t32g↑t22g↓e0g↑e0g↓, with spin states of S = 0 and S = 1/2 in the individual doped systems, respectively. Hence, the present work proposes that a doping strategy with a suitable candidate could be beneficial to tune the physical properties of the materials for their potential utilization in advanced spin-based devices.

Insulator-to-half metal transition and enhancement of structural distortions in [Formula: see text] double perovskite oxide via hole-doping

Using density functional theory calculations, we found that recently high-pressure synthesized double perovskite oxide [Formula: see text] exhibits ferrimagnetic (FiM) Mott-insulating state having an energy band gap of 0.20 eV which confirms the experimental observations (Feng et al. in Inorg Chem 58:397-404, 2019). Strong antiferromagnetic superexchange interactions between high-energy half-filled [Formula: see text]-[Formula: see text] and low-energy partially filled [Formula: see text] orbitals, results in a FiM spin ordering. Besides, the effect of 3d transition metal (TM = Cr, Mn, and Fe) doping with 50% concentration at Ni sites on its electronic and magnetic properties is explored. It is established that smaller size cation-doping at the B site enhances the structural distortion, which further gives strength to the FiM ordering temperature. Interestingly, our results revealed that all TM-doped structures exhibit an electronic transition from Mott-insulating to a half-metallic state with effective integral spin moments. The admixture of Ir 5d orbitals in the spin-majority channel are mainly responsible for conductivity, while the spin minority channel remains an insulator. Surprisingly, a substantial reduction and enhancement of spin moment are found on non-equivalent Ir and oxygen ions, respectively. This leads the Ir ion in a mixed-valence state of [Formula: see text] and [Formula: see text] in all doped systems having configurations of [Formula: see text] ([Formula: see text]) and [Formula: see text] ([Formula: see text]), respectively. Hence, the present work proposes that doping engineering with suitable impurity elements could be an effective way to tailor the physical properties of the materials for their technological potential utilization in advanced spin devices.

Study of the elastic constants in a La0.6Sr0.4MnO3 film by means of laser-generated ultrasonic wave method

Two dimensional (2D) displacement field of the laser-generated ultrasonic wave is detected in a two-layered structure of La(0.6)Sr(0.4)MnO(3) (LSMO)/MgO system by means of the optical difference detection method. In order to obtain the elastic constants of the La(0.6)Sr(0.4)MnO(3) thin film, the displacement field of the laser-generated ultrasonic wave for the La(0.6)Sr(0.4)MnO(3)/MgO system is analyzed with finite element method (FEM). We further compare the theoretical simulations with the experimental results, and the elastic constants for the LSMO film, i.e., the Young's modulus and Poisson ratio, are obtained.

Crystal and magnetic structure of the double perovskite Sr2CoUO6: a neutron diffraction study

Sr2CoUO6 double perovskite has been prepared as a polycrystalline powder by solid-state reaction, in air. This material has been studied by X-ray, neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure is monoclinic, space group P2(1)/n, Z= 2, with a= 5.7916(2), b= 5.8034(2), c= 8.1790(3) A, beta= 90.1455(6)degrees. The perovskite lattice consists of a completely ordered array of CoO6 and UO6 octahedra, which exhibit an average tilting angle phi= 11.4 degrees. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below TN = 10 K. The low-temperature magnetic structure was determined by NPD, selected among the possible magnetic solutions compatible with the P2(1)/n space group, according with the group theory representation. The propagation vector is k= 0. A canted antiferromagnetic structure is observed below TN = 10 K, which remains stable down to 3 K, with an ordered magnetic moment of 2.44(7)mu(B) for Co2+ cations. The magnetic moment calculated from the Curie-Weiss law at high temperatures (5.22 mu(B)/f.u.) indicates that the orbital contribution is unquenched at high temperatures, which is consistent with high-spin Co2+((4)T(1g) ground state) in a quasi-regular octahedral environment. Magnetic and structural features are consistent with an electronic configuration Co2+[3d(7)]-U6+[Rn].

Finding universal correlations between cationic disorder and low field magnetoresistance in FeMo double perovskite series

We search for general patterns that explain the low field magnetoresistance at low temperatures in the system A(2-x)A'xFeMoO6. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the Fe and Mo sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic SrFeO3 patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.