Carlos A. López

The strongly defective double perovskite Sr11Mo4O23: crystal structure in relation to ionic conductivity

Fil: Lopez, Carlos Alberto. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - San Luis. Instituto de Investigaciones en Tecnologia Quimica. Universidad Nacional de San Luis. Facultad de Quimica, Bioquimica y Farmacia. Instituto de Investigaciones en Tecnologia Quimica; Argentina

High-temperature dynamic octahedral tilting in the ionic conductor Sr11Mo4O23

This work presents the crystal structure evolution of a novel ionic conductor Sr11Mo4O23 at high temperature. The formula of this phase can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskites A2B′B′′O6. The crystal network contains oxygen-anion and strontium-cation vacancies. The structure is complex; Sr, Mo and O atoms are distributed in four, two and six distinct Wyckoff sites, respectively. It was refined from neutron powder diffraction data collected at 473, 673, 873 and 1073 K. The thermal evolution of crystallographic parameters supports the known reversible process of removal/uptake of O-atom content in the 673–873 K temperature range. Above 873 K, from difference Fourier maps, it was found that the structure exhibits an oxygen delocalization around one of the Mo sites. This delocalization was understood as a dynamical octahedral tilt of the MoO6 octahedron, yielding an increase in the ionic conductivity at high temperature.

Visualization of the Diffusion Pathway of Protons in (NH4)2Si0.5Ti0.5P4O13 as an Electrolyte for Intermediate-Temperature Fuel Cells

We demonstrate that (NH4)2Si0.5Ti0.5P4O13 is an excellent proton conductor. The crystallographic information concerning the hydrogen positions is unraveled from neutron-powder-diffraction (NPD) data for the first time. This study shows that all the hydrogen atoms are connected though H bonds, establishing a two-dimensional path between the [(Si0.5Ti0.5)P4O132-]n layers for proton diffusion across the crystal structure by breaking and reconstructing intermediate H-O═P bonds. This transient species probably reduces the potential energy of the H jump from an ammonium unit to the next neighboring NH4+ unit. Both theoretical and experimental results support an interstitial-proton-conduction mechanism. The proton conductivities of (NH4)2Si0.5Ti0.5P4O13 reach 0.0061 and 0.024 S cm-1 in humid air at 125 and 250 °C, respectively. This finding demonstrates that (NH4)2Si0.5Ti0.5P4O13 is a promising electrolyte material operating at 150-250 °C. This work opens up a new avenue for designing and fabricating high-performance inorganic electrolytes.

Ionic conductivity enhancement in Ti-doped Sr11Mo4O23 defective double perovskites

A substantially higher ionic motion can be achieved by partially replacing Mo(VI) by Ti(IV) cations in the novel Sr11Mo4−xTixO23−δ (with x = 0.0, 0.5 and 1.0) electrolyte oxides, successfully enhancing the oxygen vacancy level. These phases can be rewritten as Sr1.75□0.25(Sr)(Mo,Ti)O5.75−δ highlighting the relationship with conventional double perovskites. This original structure presents a broken corner sharing connectivity of the octahedral framework, hence leading to a complex and highly defective network. These materials have been prepared in polycrystalline form by thermal treatment up to 1300 °C. The structures were refined from X-ray and neutron powder diffraction data collected at room temperature and at 500 and 800 °C for x = 1. At high temperature this perovskite shows a phase transition to cubic symmetry and also evidences a reversible process of removal/uptake of O-atoms as observed in the undoped phase. AC-conductivity measurements from impedance spectroscopy confirm that Ti-doping increases the ionic mobility by 70%, attaining ionic conductivity values as high as 3.2 × 10−3 and 1.8 × 10−2 S cm−1 at 650 and 800 °C, respectively.

Elucidating the diffusion pathway of protons in ammonium polyphosphate: a potential electrolyte for intermediate temperature fuel cells

Ammonium polyphosphate (NH4PO3) is a potential electrolyte material for intermediate temperature fuel cells (150–250 °C). The crystal structure of NH4PO3, including the H positions, is unravelled by neutron powder diffraction (NPD) data by successive Fourier synthesis from the polyphosphate backbone. The structure consists of zig–zag chains aligned along the [001] directions of tetrahedral phosphate PO4 units that are connected through O3 atoms with a P–O3–P angle of 126.3(5)°. The proton conductivity mechanism of NH4PO3 is clarified from the thermal evolution of the structure. It shows that some H atoms subtly shift at high temperatures, resulting in a weakening of certain H-bonds, thus increasing the lability of those H atoms involved in the proton conduction mechanism. Conductivity measurements in humid air and H2 of NH4PO3 show high proton conductivities of 1.2 × 10−5 to 2.61 × 10−3 S cm−1 and 2.2 × 10−5 to 2.69 × 10−3 S cm−1, respectively, in the temperature range of 50 °C to 275 °C.

Ferromagnetic and multiferroic interfaces in granular perovskite composite xLa0.5Sr0.5CoO3-(1−x)BiFeO3

Nanopowder of ferromagnetic La0.5Sr0.5CoO3 (LSCO) and multiferroic BiFeO3 (BFO) were synthesized by spray pyrolysis method. Different compositions of multiferroic xLSCO-(1−x)BFO composites were synthesized at 800 °C for 2 h. Scanning electron microscopy and energy dispersive spectroscopy elemental mapping were performed to study the morphology of composites. Ferri/ferromagnetic responses above TC (LSCO) are observed, which are associated with the interfaces LSCO/BFO. This interface presents a different behavior compared to the original perovskites, and the magnitude of the magnetization depends on x. Electrical DC conductivity as a function of temperature for LSCO nanopowder (x = 1) presents a different behavior than that reported in bulk material. For x = 1 and 0.9, the model by Glazman and Matveev [Zh. Eksp. Teor. Fiz. 94, 332 (1988)] is proposed to describe the electrical conductivity. On the other hand, x = 0, 0.1, and 0.5 present a variable range hopping behavior. Complex impedance spectroscopy as a ...

Optical-active phonons in A3Fe2B″O9 (A=Ca, Sr; B″=Te, W) double perovskites

Raman scattering and infrared transmittance techniques are used to investigate the phonons of the Sr3Fe2TeO9 (SFTO), Sr3Fe2WO9 (SFWO), and Ca3Fe2WO9 (CFWO) double perovskites at 300 K. While SFTO and SFWO crystallize in a tetragonal structure belonging to the I4/m space with two formulas per unit cell (Z=2), CFWO crystallizes in a monoclinic structure belonging to the space group P21/n with Z=2. The observed spectra are very similar to that of the prototype cubic (Fm3¯m) double perovskite, indicating that both the tetragonal and monoclinic structures result from small distortions of the cubic cell. The assignment of the optical phonons follows that given for the prototype Fm3¯m double perovskites.

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.