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Giorgio Flor

Publications and source records attributed to Giorgio Flor.

At least 19 recordsLinked to original sources
arXiv2008

New Insight into the Properties of Proton Conducting Oxides from Neutron Total Scattering

In this paper we investigated the most important family of proton conducting oxides, i.e. cerates, by means of pair distribution function analysis (PDF) obtained from total neutron scattering data. The results clearly demonstrates that the local structure plays a fundamental role in the protonation process. Oxygen vacancy creation by acceptor doping reduces the local structure symmetry which is then restored upon water uptake. This mechanism mainly involves the Ba-O shell which flexibility seems to be at the basis of the proton conduction mechanism, thus providing a direct insight on the design of optimal proton conducting materials.

cond-mat.mtrl-sci↗
arXiv2007

Chemical Pressure Tuning of Magnetic Coupling in the La1.4Sr1.6Mn2O7 Bilayered Manganite

In this paper the effect of the chemical pressure induced by the Sr replacement for Ca in the La1.4Sr1.6Mn2O7 lattice is directly probed by means of neutron diffraction as a function of temperature. The results show a progressive change of the magnetic coupling from ferromagnetic to antiferromagnetic by increasing the Ca content,. indicating that the orbital order of the Mn ions is affected in the same way by the application of an external pressure and by the chemical pressure.

cond-mat.mtrl-sci↗
arXiv2007

Correlation between transport properties and lattice effects in the NdCoO3 based catalysts and sensor materials

This study presents correlations between the structural and transport properties of pure and doped neodymium cobaltate, a compound of great interest for its foreseen applications as catalyst, sensor and thermoelectric material. Neutron and x-ray powder diffraction data have been combined to carefully determine lattice constants and atomic positions and four probe direct current conductivity and thermoelectric power measurements allowed us to follow the thermal evolution of the transport properties of these compounds. The dramatic improvement of the room temperature conductivity of Nd0.8Ca0.2CoO3 with respect to the pure and the Na-doped compound is explained in terms of a different spin-state for the Co ions within this structure. The higher conductivity and the absence of anomalies in the thermal expansion makes the Ca-doped compound more attractive than the pure NdCoO3 in view of possible applications. The experimental data and the Co environment analysis here discussed, in particular bond lengths distortion and bending angles, are fully consistent with a spin state (low to intermediate) transition in NdCoO3

cond-mat.mtrl-sci↗
arXiv2007

Nature of the Monoclinic to Cubic Phase Transition in the Fast Oxygen Ion Conductor La2Mo2O9 (LAMOX)

La2Mo2O9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities comparable to those of yttria-sabilized zirconia (YSZ) . LAMOX is subject to a structural phase transition from the non-conductive monoclinic form to the highly-conductive cubic form at about 580C. The origin of the conductivity in cubic LAMOX has been suggested to be due to a "disorder" in the O sub-lattice without any insight into the real distribution of the oxygen ion. In this paper, thanks to the application of the neutron atomic pair distribution function (PDF) analysis, we provided evidences that the local structure of the cubic polymorph of LAMOX is exactly the same of that of the monoclinic phase thus indicating that the structural phase transition is actually a transition from a static to a dynamic distribution of the oxygen defects. This work represent the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor and clearly indicates that a more reliable and detailed description of their local structure, particularly in the highly conductive phases, and can lead to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.

cond-mat.mtrl-sci↗
arXiv2006

RF Sputter Deposition of Epitaxial Nanocrystalline Nd1-xSrxCoO3 Thin Films

In this paper we report the deposition of epitaxial thin films of Nd1-xSrxCoO3 with x=0, 0.2 and 0.5 on single crystalline substrates (SrTiO3 and LaAlO3) carried out by means of rf-magnetron sputtering. The deposited films are all completely oriented and epitaxial and characterized by a nanocrystalline morphology. As-deposited films have an average roughness around 1 nm while after the thermal treatment this increases up to 20 nm while preserving the nanocrystalline morphology. All the films deposited on SrTiO3 have shown to be under a certain degree of tensile strain while those on the LaAlO3 experience a compressive strain thus suggesting that at about 50 nm the films are not fully relaxed, even after the thermal treatment. For the x=0.2 composition three different thickness have been investigated revealing an increased strain for the thinner films.

cond-mat.mtrl-sci↗
arXiv2006

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.

cond-mat.mtrl-sci↗
arXiv2006

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.

cond-mat.mtrl-sci↗
arXiv2005

RF-Sputtering Deposition of Nd1-xsrxcoo3 Oriented Thin Films

In this paper we reported, to the best of our knowledge, the first deposition of highly oriented thin films (with thickness of about 90 nm) of NdCoO3 and Nd0.8Sr0.2CoO3 cobaltites on single-crystalline STO and LAO substrates. Our investigation has shown that highly oriented single phase thin films of NCO and NSCO can be successfully deposited by means of rf-sputtering if the substrates is heated at high temperatures (700C); lower substrate temperature has shown to lead to multi-phase materials with a low crystallinity degree . LAO substrate showed to give origin to a prefect match of the out-of-plane lattice constant of the NSCO target material.

cond-mat.mtrl-sci↗
arXiv2005

Doping effects in single-layered La0.5Sr1.5MnO4 manganites

In this paper we report the results of the synthesis and structural, transport and magnetic characterization of pure La0.5Sr1.5MnO4 (LSMO) and 5% doped samples, i.e. La0.5Sr1.5Mn0.95B0.05O4, where B = Ru, Co and Ni. It is shown that even a light doping is successful in suppressing the charge and orbital ordering found for the pure LSMO. In general, doping favours the carrier motion and, from a magnetic point of view, the set-up a spin-glass state. Moreover, structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the relative nature of the orbital character of the eg electrons along with the overall cation size.

cond-mat.mtrl-sci↗
arXiv2005

Influence of Ru-doping on structure, defect chemistry, magnetic interaction and carrier motion of the La1-xNaxMnO3+delta manganite

In this work we report a structural, electrical and magnetic characterization of the La1-xNaxMn1-yRuyO3+d (LNMRO) system with x = 0.05, 0.15 and y = 0, 0.05, 0.15, also comprising an investigation of the role of the oxygen content on the related redox properties. The experimental investigation has been realized with the aid of X-ray powder diffraction, electron microprobe analysis, thermogravimetry, electrical resistivity and magnetization measurements, and electron paramagnetic resonance. We demonstrate that the effect of ruthenium doping on the studied LNMRO compounds is not only directly related to the Ru/Mn substitution and to the Ru oxidation state but also indirectly connected to the oxygen content in the sample. Our data show that ruthenium addition can improve electrical and magnetic properties of non-optimally (low) cation doped manganites, causing an increase of the TC value and the insurgence of MR effect, as observed for the x = 0.05 and y = 0.05 sample.

cond-mat.mtrl-sci↗
arXiv2005

Effects of Cation Vacancy Distribution in Doped LaMnO3+d Perovskites

In this paper we report studies on the correlation between the presence and distribution of cation vacancies in doped manganites (La,M)MnO3+delta (where M = Na, Ca) and their magnetic properties. Results indicate that cation vacancies are distributed differently for the different crystal structures and dopant ion type. In particular it is shown that knowledge of the total vacancies concentration alone is not enough to fully characterize the physical properties of manganites and that their distribution between the A and B sites of the perovskite structure plays a crucial role which should be taken into account in future studies.

cond-mat.mtrl-sci↗
arXiv2005

High Temperature Neutron Diffraction Study of the La1.4Sr1.6mn2o7 Bilayered Manganite

In this paper we present the results of a high temperature (300 K /Mn-Oequatorial parameter was found: it first decreases up to 500 K and then increases up to the highest T measured. At the same time, a significant shortening of the apical Mn-O(2) bond is observed in the range where the J-T distortion is reduced. The overall data gained by this study may suggest a shift of electronic density from axial to planar eg orbitals with T. This trend is explained considering of the presence of short range magnetic interaction well above TC.

cond-mat.mtrl-sci↗
arXiv2004

Role of Point Defects on the Properties of Manganites

La1-xCaxMnO3+delta and La1-xNaxMnO3+delta samples with well defined cation molecularity and oxygen contents are analyzed with XRPD, electrical conductivity, electron paramagnetic resonance and static magnetization measurements. Point defects are introduced in the structure by: i) aliovalent cation doping (substitutional defects), ii) oxygen over-stoichiometry (cation vacancies) and iii) oxygen under-stoichiometry (oxygen vacancies). The cation doping mainly influences the material by affecting the tolerance factor and the oxidation of the Mn ions; the cation vacancies affect the magnetic properties by interrupting the interaction paths between Mn4+-Mn3+ ions whereas the oxygen vacancies have a stronger influence on the structural and electrical properties and act on the magnetic properties by the overall decrease on the Mn4+ amount and by the creation of sample regions with different magnetic features. We can state that the basic magnetic and electrical behaviors of the Ca- and Na-doped lanthanum manganite compounds are driven from the Mn4+-Mn3+ ratio and all the peculiar behaviors are indeed caused by defects, concentration gradient and, more generally, by lattice disorder.

cond-mat.mtrl-sci↗
arXiv2004

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.

cond-mat.mtrl-sci↗
arXiv2004

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.

cond-mat.mtrl-sci↗
arXiv2004

Unexpected effect of Ru-substitution in lightly doped manganites

In this Communication we report about the unexpected effect of ruthenium doping in sodium ligthly-doped manganites. This effect seems to be in contrast with the usual model applied to describe the effect of this magnetic ion into the manganite structure. We propose a possible compensation mechanism which seems also able to describe other peculiar features encountered in these materials.

cond-mat.mtrl-sci↗
arXiv2003

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.

cond-mat.mtrl-sci↗