Oxygen Chemical Diffusion Coefficient in Manganite Thin Films by Isothermal Electric Resistivity Measurements

In this paper we report the determination of oxygen diffusion coefficient for optimally doped manganite thin films of La0.85Na0.15MnO3 (LNMO). The study was performed by means of isothermal conductivity measurements on two different epitaxially oriented thin films grown on SrTiO3 (100) and SrTiO3 (110). The values found for the oxygen diffusion coefficients are around 10-14 cm2s-1. The diffusion seems to be faster for the c-oriented film with respect to the other one (growth on SrTiO3 (110)). The activation energy for the diffusion process is 0.35 eV for the LNMO film on SrTiO3 (100) and 0.66 for the LNMO film on SrTiO3 (110). A possible reason to account for the different behaviour between the two films is proposed.Comment: 20 Pages and 4 Figures. final version to appear in J. Phys. Chem.

Oxygen content variation and cation doping dependence of (La)1.4(Sr1-yCay)1.6Mn2O7 (y = 0, 0.25, 0.5) bilayered manganites properties

The results of the synthesis and characterization of the optimally doped (La)1.4(Sr1-yCay)1.6Mn2O7 solid solution with y=0, 0.25 and 0.5 are reported. By progressively replacing the Sr with the smaller Ca, while keeping fixed the hole-concentration due to the divalent dopant, the 'size effect' of the cation itself on the structural, transport and magnetic properties of the bilayered manganite has been analysed. Two different annealing treatments of the solid solution, in pure oxygen and in pure argon, allowed also to study the effect of the oxygen content variation. Structure and electronic properties of the samples have been investigated by means of X-ray powder diffraction and X-ray absorption spectroscopy measurements. Magnetoresistivity and static magnetization measurements have been carried out to complete the samples characterization. Oxygen annealing of the solid solution, that showed a limit for about y=0.5, induces an increase of the Mn average valence state and a transition of the crystal structure from tetragonal to orthorhombic while the argon annealing induces an oxygen under-stoichiometry and, in turn, a reduction of the Mn average valence state. Along with the Ca substitution, the Jahn-Teller distortion of the MnO6 octahedra is reduced. This has been directly connected to a general enhancement of the transport properties induced by the Ca-doping. For the same cation composition, oxygen over-stoichiometry leads to higher metal-insulator transition temperatures and lower resistivity values. Curie temperatures (TC) reduce by increasing the Ca-doping. The lower TC for all the annealed samples with respect to the 'as prepared' ones are connected to the strong influence on the magnetic interaction of the point defects due to the oxygen content variation.Comment: 49 pages, 13 figure

High Pressure X-ray Diffraction Study of MgMn2O4 Tetragonal Spinel

The phase stability of the MgMn2O4 spinel has been studied by means of high-pressure X-ray diffraction for pressures up to 30 GPa. Two samples with different inversion degrees have been considered. Both spinels undergo a phase transition towards an orthorhombic structure (CaMn2O4-type). For the more inverted sample the transition pressure is at least 1 GPa lower with respect to that of the less inverted spinel. Also the volume contraction, relative compressibility and density trends are different for the two samples. These variations have been explained according to differences in the cation distribution. and electronic properties of the samples.Comment: 12 pages; 4 Figures presented at the SRMS-

Structure-property correlation in oxide-ion and proton conductors for clean energy applications: recent experimental and computational advancements

In the last decade, the field of oxide-ion and proton conductors continued to trigger a significant amount of basic research aimed at improving the properties and the comprehension of actual materials, as well as at discovering novel phases. This need comes from the current and future urgent requests of changing our energy management towards cleaner solutions such as solid oxide fuel cells. In this review article, we highlight the most recent advancements in this exciting field by putting particular emphasis on the structure-property correlations in oxide-ion and proton conductors both from an experimental and a computational perspective. Special focus is laid on developments in the area of operando and in situ spectroscopy, machine learning and high-throughput approaches to accelerate the discovery of new and advanced materials

Chemical Pressure Effect on the Magnetic Order of the La1.4Sr1.6Mn2O7 Bilayered Manganite

In this Communication is it shown the role of Ca-doping in stabilizing the A-type AFM structure of the optimally doped La1.4Sr1.6Mn2O7 bilayered manganite.Comment: 10 pages, 4 figure

Absence of Long Range Magnetic Order in the La1.4Sr0.8Ca0.8Mn2O7 Bilayered Manganite

In this work we studied, by means of high-resolution neutron diffraction as a function of temperature, the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite for two different annealing treatments. Out data allowed us to shown, for the first time, the absence of long-range magnetic order in this optimally doped bilayered manganite where the A-site of the structure is doped with equal proportions of different isovalent cations (Ca and Sr). The system, however, presents defined IM transitions which suggest that the transport properties are not linked to the evolution of long-range order and that two dimensional spin ordering in the layers of the perovskite blocks may be sufficient to 'assist' the hole hopping. Possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.Comment: 24 pages, 7 figure

Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering

Solution-based methods represent the most widespread approach used to deposit hybrid organic-inorganic perovskite films for low-cost but efficient solar cells. However, solution-process techniques offer limited control over film morphology and crystallinity, and most importantly do not allow sequential film deposition to produce perovskite-perovskite heterostructures. Here the successful deposition of CH3NH3PbI3 (MAPI) thin films by RF-magnetron sputtering is reported, an industry-tested method to grow large area devices with precisely controlled stoichiometry. MAPI films are grown starting from a single-target made of CH3NH3I (MAI) and PbI2. Films are single-phase, with a barely detectable content of unreacted PbI2, full surface coverage and thickness ranging from less than 200 nm to more than 3 {\mu}m. Light absorption and emission properties of the deposited films are comparable to as-grown solution-processed MAPI films. The development of vapor-phase deposition methods is of interest to advance perovskite photovoltaic devices with the possibility of fabricating perovskite multijunction solar cells or multicolor bright light-emitting devices in the whole visible spectrum

Sodium Doped LaMnO3 Thin Films: Influence of Substrate and Thickness on Physical Properties

In this paper we report the results about the synthesis and characterization of optimally doped La1-xNaxMnO3 thin films grown onto SrTiO3 (100), NdGaO3 (100) and NdGaO3 (110) for thickness ranging from 11 to 82 nm. The effect of substrate nature and orientation, film thickness and annealing procedure was investigated in order to optimize their magnetoresistance (MR). We obtained very smooth films displaying MR values greater than 70%, near to room temperature.Comment: 31 pages, 9 figures Final version to appear in J. Phys. Chem.

Anisotropic compression in the high pressure regime of pure and Cr-doped vanadium dioxide

We present structural studies of V$_{1-x}$Cr$_x$O$_2$ (pure, 0.7% and 2.5% Cr doped) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic $M_1$ symmetry between 4 and 12 GPa. Above 12 GPa, the monoclinic $M_1$ symmetry changes to isostructural $M_x$ phase (space group $P2_1/c$) with a significant anisotropy in lattice compression of the $b$-$c$ plane of the $M_{1}$ phase. This behavior can be reconciled invoking the pressure induced charge-delocalization