Maria Retuerto

Nanocrystalline Ni5P4: a hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media

Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low efficiency alkaline electrolyzers that suffer 50–65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that crystalline Ni5P4 evolves H2 with geometric electrical to chemical conversion efficiency on par with Pt in strong acid (33 mV dec−1 Tafel slope and −62 mV overpotential at −100 mA cm−2 in 1 M H2SO4). The conductivity of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short term studies at current densities of −10 mA cm−2, equivalent to ∼10% solar photoelectrical conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.

Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.

Giant Magnetoresistance in the Half-Metallic Double-Perovskite Ferrimagnet Mn2FeReO6

The first transition-metal-only double perovskite compound, Mn(2+) 2 Fe(3+) Re(5+) O6 , with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Mn2FeWO6 : A new Ni3TeO6-type polar and magnetic oxide.

Mn(2+)2 Fe(2+)W(6+)O6 , a new polar magnetic phase, adopts the corundum-derived Ni3TeO6 -type structure with large spontaneous polarization (PS) of 67.8 μC cm(-2), complex antiferromagnetic order below ≈75 K, and field-induced first-order transition to a ferrimagnetic phase below ≈30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.

Polar and Magnetic Layered A-Site and Rock Salt B-Site-Ordered NaLnFeWO6 (Ln = La, Nd) Perovskites

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T(N) ≈ 25 K for NaLaFeWO6 and at ∼21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = ((1/2) 0 (1/2)) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

Stabilization and study of SrFe(1-x)Mn(x)O2 oxides with infinite-layer structure.

A series of layered oxides of nominal composition SrFe(1-x)Mn(x)O(2) (x = 0, 0.1, 0.2, 0.3) have been prepared by the reduction of three-dimensional perovskites SrFe(1-x)Mn(x)O(3-δ) with CaH(2) under mild temperature conditions of 583 K for 2 days. The samples with x = 0, 0.1, and 0.2 exhibit an infinite-layer crystal structure where all of the apical O atoms have been selectively removed upon reduction. A selected sample (x = 0.2) has been studied by neutron powder diffraction (NPD) and X-ray absorption spectroscopy. Both techniques indicate that Fe and Mn adopt a divalent oxidation state, although Fe(2+) ions are under tensile stress whereas Mn(2+) ions undergo compressive stress in the structure. The unit-cell parameters progressively evolve from a = 3.9932(4) Å and c = 3.4790(4) Å for x = 0 to a = 4.00861(15) Å and c = 3.46769(16) Å for x = 0.2; the cell volume presents an expansion across the series from V = 55.47(1) to 55.722(4) Å(3) for x = 0 and 0.2, respectively, because of the larger effective ionic radius of Mn(2+) versus Fe(2+) in four-fold coordination. Attempts to prepare Mn-rich compositions beyond x = 0.2 were unsuccessful. For SrFe(0.8)Mn(0.2)O(2), the magnetic properties indicate a strong magnetic coupling between Fe(2+) and Mn(2+) magnetic moments, with an antiferromagnetic temperature T(N) above room temperature, between 453 and 523 K, according to temperature-dependent NPD data. The NPD data include Bragg reflections of magnetic origin, accounted for with a propagation vector k = ((1)/(2), (1)/(2), (1)/(2)). A G-type antiferromagnetic structure was modeled with magnetic moments at the Fe/Mn position. The refined ordered magnetic moment at this position is 1.71(3) μ(B)/f.u. at 295 K. This is an extraordinary example where Mn(2+) and Fe(2+) ions are stabilized in a square-planar oxygen coordination within an infinite-layer structure. The layered SrFe(1-x)Mn(x)O(2) oxides are kinetically stable at room temperature, but in air at ~170 °C, they reoxidize and form the perovskites SrFe(1-x)Mn(x)O(3-δ). A cubic phase is obtained upon reoxidation of the layered compound, whereas the starting precursor SrFeO(2.875) (Sr(8)Fe(8)O(23)) was a tetragonal superstructure of perovskite.

Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni(2+) and Mn(4+) cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn(4+) and Ni(2+) moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni(2+) and Mn(4+) (Ni(2+)-O-Mn(4+) angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.

Magnetic and Structural Studies of the Multifunctional Material SrFe0.75Mo0.25O3−δ

SrFe0.75Mo0.25O3-δ has been recently discovered as an extremely efficient electrode for intermediate temperature solid oxide fuel cells (IT-SOFCs). We have performed structural and magnetic studies to fully characterize this multifunctional material. We have observed by powder neutron diffraction (PND) and transmission electron microscopy (TEM) that its crystal symmetry is better explained with a tetragonal symmetry (I4/mcm space group) than with the previously reported orthorhombic symmetry (Pnma space group). The temperature dependent magnetic properties indicate an exceptionally high magnetic ordering temperature (TN ∼ 750 K), well above room temperature. The ordered magnetic structure at low temperature was determined by PND to be an antiferromagnetic coupling of the Fe cations. Mössbauer spectroscopy corroborated the PND results. A detailed study, with X-ray absorption spectroscopy (XAS), in agreement with the Mössbauer results, confirmed the formal oxidation states of the cations to be mixed valence Fe(3+/4+) and Mo(6+).

Crystal structure and bond valence of CaH2 from neutron powder diffraction data

Abstract The crystal structure of CaH2 has been studied from neutron powder diffraction (NPD) at 295 K in a non-deuterated sample; a good quality NPD pattern was obtained in spite of the hydrogen incoherent scattering. The structure was refined by the Rietveld method in the Pnma space group (No. 62), Z = 4, with unit-cell parameters a = 5.9600(1), b = 3.6006(7) and c = 6.8167(1) Å. The two kinds of crystallographically independent H atoms, H1 and H2, are located in tetrahedral and square-pyramidal cavities, respectively, while Ca ions are nine-fold coordinated to hydrogen atoms. The average 〈Ca—H1〉 and 〈Ca—H2〉 bond lengths are 2.279 and 2.544 Å, respectively. Bond valence calculations show that Ca—H1 bonds are under compressive stress, whereas Ca—H2 bonds undergo tensile stress in a structure with a relatively high global instability index. It is also remarkable that the displacement factors for H2 are significantly larger than for H1, suggesting an increased lability for the Ca—H2 bonds. We provide with an analysis of the isotope effect, by comparing the present results on CaH2 with literature data on CaD2; we indeed observe a higher distortion of the H1 and H2 coordination polyhedra with respect to the deuteride, as observed in other isostructural dihydrides.

Pb2MnTeO6 Double Perovskite: An Antipolar Anti-ferromagnet

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to ∼120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s(2) lone-pair electrons, together with a surprising off-centering of Mn(2+) (d(5)) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN ≈ 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near ∼150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.