1,721,064 research outputs found
Melting of cooperative Jahn-Teller distortion in KMg0.2Cu0.8F3
No full textKCuF3 is a Mott-Hubbard insulator with a distorted perovskite structure (space group I4/mcm). The structural distortion is due to orbital ordering (OO) associated with cooperative Jahn-Teller effect (JT) [1]. and corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion [2]. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane. Debate is open in the literature on the actual driving force (i.e. either OO or JT) of the structural distortion and of the related electronic and magnetic properties. We have recently found experimental evidence of the ideal situation in which OO is melted while the JT distortion is still present:, in fact OO is expected to be very sensitive to slight changes in the electronic structure. Electron paramagnetic resonance investigations revealed melting of OO at room temperature in the KCu1-xMgxF3 system for x=0.1 [3]. We presents here a Synchrotron Radiation X-ray powder diffraction (XRPD) study in a sample with composition KCu0.8Mg0.2F3, which at room temperature is isostructural with KCuF3, a prototypical system for studying Orbital Order (OO). This sample can be considered a realisation of the ideal situation in which OO is melted while the cooperative JT distortion is still present. The melting of the cooperative JT distortion is observed in this system for T~600 K. This result is discussed in the framework of the different energy scales for OO and cooperative JT distortion.
[1] L.F. Feiner, A.M. Oleś, J. Zaanen, Phys. Rev. Lett., 1997, 78, 2799. [2] R. Caciuffo, L. Paolasini, A. Sollier, P. Ghigna, E. Pavarini, J. van den Brink, M. Altarelli Phys.Rev., 2002, B65, 174425
[3] C. Oliva, M. Scavini, S. Cappelli, C. Bottalo, C. Mazzoli, P. Ghigna, J.Phys.Chem., 2007, B111, 597
Study of photoelectrochemical behavior of copper oxides based materials using X-ray absorption spectroscopy
No full textThe use of sunlight to convert water into fuel is very attractive and ambitious since H2 is considered to be the energy carrier of the future thanks to its high mass energy density and its environmental friendliness [1,2]. Copper oxides-based photocathodes are attractive for their absorption in the visible range, low cost, high abundance and easy synthetic protocols as well as high photoactivity [3,4].
Two p-type semiconducting copper based materials has been prepared, characterized and tested as a photocathode for H2 production: CuO and Cu2O. The first one is prepared by thermal treatment of nanocrystalline CuI, which shows high efficiency in light conversion and interesting self-protection properties [5].
Cu2O instead was prepared by electrochemical deposition from a lactate-stabilized Cu++ bath [3]. Viceversa the main drawback of Cu(I) oxide is its lack of stability during photoelectrochemical conditions. For this material the influence of a metallic underlayer (Au, Cu) between the semiconductor itself and the FTO support was studied, together with the presence of a small load of Pt catalyst.
In-situ and in-operando techniques like X-ray absorption near edge structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS) and Fixed Energy X-ray Absorption Voltammetry (FEXRAV) [6] allow us to better understanding materials behavior. We observe changes in copper oxidation states upon light and/ or applied potential. Moreover, the role of methanol as hole-scavenger during photoelectrochemical experiment has been studied. FEXRAV measurements allow following the material degradation processes and defining the stability windows. With differential light and dark XANES spectra, we investigated the local changes in electronic structure upon spectroelectrochemical conditions. These results will allow us obtaining more stable system for photoelectrochemical hydrogen production.
References
[1] G. Centi, S. Perathoner, ChemSusChem. 3 (2010) 195–208.
[2] F. Malara, A. Minguzzi, M. Marelli, S. Morandi, R. Psaro, V. Dal Santo, A. Naldoni, ACS Catal. 5 (2015) 5292–5300.
[3] A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, E. Thimsen, Nat. Mater. 10 (2011) 456–461.
[4] C. Li, T. Hisatomi, O. Watanabe, M. Nakabayashi, N. Shibata, K. Domen, J.-J. Delaunay, Energy Environ. Sci. 8 (2015) 1493–1500.
[5] T. Baran, S. Wojtyła, C. Lenardi, P. Ghigna, E. Achilli, S. Rondinini, A. Minguzzi, ACS Appl. Mater. Interfaces. (submitted).
[6] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. D’Acapito, E. Achilli. P. Ghigna. Anal. Chem. (2013), 85, 7009-7013
Electro- and photo-electrochemical water splitting as studied by In-Operando X-Rays Absorption Spectroscopy
No full textIn this work we show our most recent results obtained by in-operando X-Ray absorption spectroscopy on hydrous/amorphous [1] and on crystalline/dry [2] iridium oxide electrodes as electrocatalysts for the oxygen evolution reaction (OER). In all cases, XAS evidenced the role of Ir active sites, and the relevant oxidation states assumed during the catalytic cycle. Moreover, the local structure is not significantly influenced by the applied potential, thus suggesting a negligible reorganization energy of the catalyst.On the bases of these results, we were able to directly observe, by means of spectro-photoelectrochemical experiments, the charge transfer between a semiconductor (α-Fe2O3) and hydrous IrOx, the latter used as overlayer for generating a high performance photoanode architecture in photoelectrochemical water splitting[3]. The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of the reaction rate but to a decrease of the electron density in the hematite[4] or to the storage of photogenerates holes[5]. These effects are likely more important when hydrous overlayer, that can act as adapting catalysts[6], are considered. All these hypotheses can explain the observed improved hole lifetime and reduce recombination with electrons. The experimental approach is similar to the one adopted to study Ir oxide particles electrocatalysts[1,2]. In the present case, FEXRAV [7] and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with a 410nm diode. Thanks to this setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination.
References
[1] A. Minguzzi, O. Lugaresi, E. Achilli, C. Locatelli, A. Vertova, P. Ghigna, Rondinini S., Chem. Sci., 2014, 5, 3591-3597
[2] A. Minguzzi, C. Locatelli, O. Lugaresi, E. Achilli, G. Cappelletti, M. Scavini, M. Coduri, P. Masala, B. Sacchi, A. Vertova, P. Ghigna, S. Rondinini, submitted
[3] A. Minguzzi, O. Lugaresi, E. Achilli, F. D'Acapito, A. Naldoni, F. Malara, C. Locatelli, A. Vertova, S. Rondinini, P. Ghigna, In preparation
[4] M. Barroso, C.A. Mesa, S.R. Pendlebury, A.J. Cowana, T. Hisatomi, K. Sivula, M. Grätzel, D.R. Klug, J.R. Durrant PNAS, 2012, 109, 15640–15645
[5] L. Badia-Bou, E. Mas-Marza, P. Rodenas, E M. Barea., F. Fabregat-Santiago, S. Gimenez, E. Peris, J. Bisquert, J. Phys. Chem. C, 2013, 117, 3826−3833
[6] F. Lin, S.W. Boettcher Nature Materials, 2014, 13, 81-86
[7] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. d'Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009-7013
Magnetic phase diagram of Nd1.85Ce0.15CuO4+ delta from magnetization and muon spin relaxation measurements
No full text"Unusual Ln3+ substitutional defects": The local chemical environment of Pr3+ and Nd3+ in nanocrystalline TiO2 by Ln-K edge EXAFS
No full textThe local chemical environment of the trivalent lanthanide cations in anatase TiO(2) nanopowders doped with 1mol% of Pr or Nd, prepared via a sol-gel technique, has been studied by means of EXAFS at the Pr and Nd-K edge. Titanium dioxide can be considered an "unusual'' host for doping with Ln(3+) ions due to the large mismatch of both charge and ionic radii between the dopant and the host constituent cations. However, it can be demonstrated that the lanthanide ions enter the anatase structure as substitutional defects with respect to Ti, but that the amount of disorder around the substitutional defects is very large. For both Pr(3+) and Nd(3+) ions, the Ln-O and Ln-Ti distances have been found to increase by about 0.45 angstrom, with respect to what is found for the Ti-O and Ti-Ti distances in pure anatase. Valence-bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. Finally, no evidences for oxygen vacancies clustering around the substitutional defects have been found. Luminescence spectroscopy has shown that the lanthanide ions do not segregate in oxide or pyrochlore impurities phases
Resistivity and carrier mobility of the SmBa2Cu3O6+x superconductor with different doping levels
No full textEXAFS-XANES evidence of in operando Caesium reduction in Cs-Ru/C catalysts for ammonia synthesis
No full textWe present here a X-ray absorption spectroscopy (XAS) investigation on the local chemical order and electronic structure of Cs and Ba, promoters of the Ru/C catalysts for ammonia synthesis that attracted interest because of highly increased productivity. The role of the promoters is still largely unclear, although indirect evidence for Cs partial reduction has been obtained by this and other groups. Our XAS analysis with in situ H-2 reduction directly supports the partial Cs reduction in the promoted Ru/C catalysts, depending on the presence of Ru and on the graphitization degree of the support. Higher coordination of Ba was observed with respect to Cs in the reduced samples, without evidence of heavy atoms (Ru, Cs, and Ba) in the surroundings. Because of the strong electropositive nature of Cs, direct experimental evidence of its partial reduction is of outstanding significance also for other applications
Long range structure and local Cu environment in KMg1-xCuxF3 solid solution probed by means of XRPD and EPR
No full textKCuF3 is a Mott-Hubbard insulator with a pseudocubic perovskite structure. In its most common crystalline form, it belongs to the I4/mcm space group. The structural distortion is due to orbital ordering associated with cooperative Jahn-Teller effect [1]. CuF6 octahedra are elongated in the ab plane along a or b axis, in an antiferrodistorsive pattern. The distortion corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane; the ratio between NN-SE along and perpendicular to c is |Jc|/Ja100 [2]. For T<38 K KCuF3 shows three-dimensional antiferromagnetic order.
Melting of orbital order in KCuF3 is expected to occur well above room temperature [1], where the cooperative structural distortion should disappear and the undistorted cubic perovskite structure (space group Pm3m) should be favoured. Preliminary XRPD measurements carried out at beamline ID31 at ESRF did not detect any phase transition up to 730 °C.
A system where melting of the orbital order could be more easily observed is the complete solid solution KMg1-xCuxF3 (0<x<1). At room temperature, the structure of this system is cubic (space group Pm3m) for low Cu concentration, while it is isomorphic with KCuF3 for high Cu concentration [3]. The cubic to tetragonal phase transition with increasing x is accompanied by the onset of a cooperative Jahn-Teller distortion [3].
In the present paper we present a combined XRPD and EPR study on the relation between long range structure, local Cu environment and magnetic properties of KMg1-xCuxF3 (0<x<1) solid solution as a function of x and T.
KMg1-xCuxF3 is cubic (space group Pm3m) for x0.63. A miscibility gap is present for 0.63<x<0.73.
As shown by EPR spectroscopy, Cu environment is isotropic for x0.2 in the cubic domain. Its symmetry becomes axial and then orthorhombic with increasing Cu concentration. Collective orbital ordering becomes more and more important increasing x in the tetragonal domain, causing a symmetrisation of the EPR lineshape
Experimental estimation of the cooperative Jahn-Teller energy in orbitally ordered KCu0.8Mg0.2F3 perovskite
No full textHigh resolution synchrotron radiation X-ray powder diffraction was used to investigate the melting of the cooperative Jahn-Teller distortion (cJTd) in a perovskite of composition KCu 0.8Mg 0.2F 3. A first order phase transition relaxing the cJTd is observed at T ∼ 600 K. From the transition temperature, an estimation of kT is derived (kT = 0.05 eV) for the cJTd in the doped compound. This is the very first observation of cJTd melting in a compound of the series KCu 1-x Mg x F 3. A structural phase diagram for the Cu rich zone of the series is proposed. In principle, the extension of this experimental investigation could be used to disentangle the orbital order (OO) and cJTd energy scales in the parent compound KCuF 3, a test system for OO theories
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