Highly active and stable OER electrocatalysts derived from Sr2MIrO6 for proton exchange membrane water electrolyzers.

Proton exchange membrane water electrolysis is a promising technology to produce green hydrogen from renewables, as it can efficiently achieve high current densities. Lowering iridium amount in oxygen evolution reaction electrocatalysts is critical for achieving cost-effective production of green hydrogen. In this work, we develop catalysts from Ir double perovskites. Sr2CaIrO6 achieves 10 mA cm-2 at only 1.48 V. The surface of the perovskite reconstructs when immersed in an acidic electrolyte and during the first catalytic cycles, resulting in a stable surface conformed by short-range order edge-sharing IrO6 octahedra arranged in an open structure responsible for the high performance. A proton exchange membrane water electrolysis cell is developed with Sr2CaIrO6 as anode and low Ir loading (0.4 mgIr cm-2). The cell achieves 2.40 V at 6 A cm-2 (overload) and no loss in performance at a constant 2 A cm-2 (nominal load). Thus, reducing Ir use without compromising efficiency and lifetime.

Mechano-Chemical Synthesis, Structural Features and Optical Gap of Hybrid CH3NH3CdBr3 Perovskite.

Hybrid methyl-ammonium (MA:CH3NH3+) lead halide MAPbX3 (X = halogen) perovskites exhibit an attractive optoelectronic performance that can be applied to the next generation of solar cells. To extend the field of interest of these hybrid materials, we describe the synthesis by a solvent-free ball-milling procedure, yielding a well crystallized, pure and moisture stable specimen of the Cd tribromide counterpart, MACdBr3, which contains chains of face-sharing CdBr6 octahedra in a framework defined in the Cmc21 (No 36) space group. The details of the structural arrangement at 295 K have been investigated by high angular resolution synchrotron x-ray diffraction (SXRD), including the orientation of the organic MA units, which are roughly aligned along the c direction, given the acentric nature of the space group. UV-vis spectra unveil a gap of 4.6 eV, which could be useful for ultraviolet detectors.

Structural Chemistry and Magnetic Properties of the Hexagonal Double Perovskite Ba2CoOsO6.

The double perovskite Ba2CoOsO6, synthesized using solid-state methods at ambient pressure, is shown as a rare example of an oxide adopting the 6L-trigonal (S.G.: P3̅m1) perovskite structure. The structure, refined using a combination of X-ray and neutron diffraction data, showed the Co and Os were ordered over the two dimer sites with additional ordering over the corner-sharing sites. Bond valence calculations show the presence of the Co(II) and Os(VI) valence states, and the latter was confirmed using X-ray absorption spectroscopy. Bulk magnetic susceptibility measurements show Ba2CoOsO6 to undergo antiferromagnetic ordering near 100 K, and neutron diffraction showed an ordered moment on the Co3, Co4, and Os2 sites; whereas the Os1/Co1 remained disordered.

Squeezing electrons out of 6s2 lone-pairs in perovskite-type oxides.

Having identified a set of conditions that predispose a solid-state ionic compound to a pressure-induced valence transition, we investigated a series of Bi(iii) perovskite oxides. We found such a transition below 10 GPa in every case, including one that we synthesised for the first time (double perovskite-type Ba2BiOsO6).

Dimensional crossover of correlated anion disorder in oxynitride perovskites.

A simple crossover from two-dimensional to three-dimensional correlated disorder of O and N atoms on a cubic lattice has been discovered within the Ba1-xSrxTaO2N series of perovskite oxynitrides. The crossover is driven by lattice expansion as x decreases, and provides a rapid increase in entropy due to a change from subextensive to extensive configurational entropy regimes.

Elucidating the Methylammonium (MA) Conformation in MAPbBr3 Perovskite with Application in Solar Cells.

Hybrid organic-inorganic perovskites, MAPbX3 (X = halogen), containing methylammonium (MA: CH3-NH3+) in the large voids conformed by the PbX6 octahedral network, are the active absorption materials in the new generation of solar cells. CH3NH3PbBr3 is a promising member with a large band gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure-property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr3 in optoelectronics.

Magnetic and Structural Studies of Sc Containing Ruthenate Double Perovskites A2ScRuO6 (A = Ba, Sr).

Ruthenium-containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba, a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure is obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show that the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal-insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction and are described by k = (0,0,1) and k = (0,0,0) for the Ba and Sr oxides, respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient-pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner-sharing BO6 sites. The magnetic structure is described by k = (1/2,0,0) with the basis vector belonging to the irreducible representation Γ3.

Structural and Magnetic Properties of the Osmium Double Perovskites Ba2-xSrxYOsO6.

The crystal and magnetic structures of double perovskites of the type Ba2-xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin-orbit coupling is constant and insignificant across the series.

Thermal expansion in BaRuO3 perovskites - an unusual case of bond strengthening at high temperatures.

The temperature dependences of the structures of three polytypes of BaRuO3 have been investigated between room temperature and 1000 °C using high resolution synchrotron X-ray diffraction. The structural studies reveal a systematic decrease of the Ru-Ru distance as the pressure required to prepare the polytype increases. The O-O distance across the shared face increases as the Ru-Ru separation decreases. The 9R and 4H polytypes undergo unexceptional changes with increasing temperature. In 6H-BaRuO3 there is an apparent increase in the Ru-Ru interaction at around 650 °C and a concurrent reduction in the O-O distance, indicating an anomalous strengthening of the Ru-Ru interactions upon heating.

Spin-Orbit Coupling Controlled Ground State in the Ir(V) Perovskites A2ScIrO6 (A = Ba or Sr).

The structural and magnetic properties of the two Ir(V) perovskites Ba2ScIrO6 and Sr2ScIrO6 were established. The structures were refined using a combined neutron and synchrotron data set. At room temperature the former has a cubic structure in space group Fm3̅m with a = 8.1450(3) Å, and the latter is monoclinic in P21/n with a = 5.6606(3) Å, b = 5.6366(3) Å, c = 7.9720(4) Å, and ß = 89.977(5)°. Magnetization measurements show both oxides have magnetic moments close to zero as a consequence of strong spin-orbit coupling that results in a Jeff ≈ 0 ground state. The distortion of the IrO6 octahedra in Sr2ScIrO6 is insufficient to generate crystal field splitting strong enough to quench the spin-orbit coupling.

Structural and Magnetic Properties of the Iridium Double Perovskites Ba(2-x)Sr(x)YIrO6.

The crystal structures of the series of ordered double perovskites Ba(2-x)Sr(x)YIrO6 (0 ≤ x ≤ 2) were refined using a combination of high-resolution synchrotron X-ray and high-intensity neutron diffraction data. The materials displayed a sequence of structures Fm3̅m(a(0)a(0)a(0)) (x = 0.6)--> I4/m(a(0)a(0)c(-)) (x = 1.0)--> I2/m(a(-)a(-)c(0)) (x = 1.4)--> P2(1)/n(a(-)a(-)c(+)) associated with increased tilting of the corner-sharing octahedra induced by increasing amount of the smaller Sr cation present. A similar sequence of transitions was induced by heating selected samples. Magnetic susceptibility measurements between 2 and 300 K showed no evidence for long-range magnetic ordering, an observation that was supported by neutron diffraction measurements, and rather strong spin-orbit coupling results in a Jeff = 0 ground state.

Correlation between the crystal structure and the Curie temperature in RCu3(Mn3Fe)O12 (R = rare-earth) complex perovskites.

New members of the family of complex-perovskite oxides with the formula RCu(3)(Mn(3)Fe)O(12) (R = Ce, Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples have been prepared from citrate precursors treated under moderate pressure conditions (2-3.5 GPa) and 1000 °C in the presence of KClO(4) as an oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at 300 K and 2 K. These oxides crystallize in the cubic space group Im3 (no. 204). Mn(4+)/Mn(3+) and Fe(3+) occupy at random the octahedral B positions of the perovskite structure. These materials have also been characterized by magnetic and magnetotransport measurements. The observed enhancement of T(C) along the RCu(3)(Mn(3)Fe)O(12) series is understood as an effect of the chemical pressure on the (Mn,Fe)-O bonds as R(3+) size decreases. The semiconducting behaviour observed in all of the samples is related with the introduction of Fe at B position. Despite the drastic change of the transport properties, significant negative magnetoresistance values are observed in the Fe-containing compounds both at 10 K and 300 K.