BUSCADOR RECOLECTA

[EN] NiFe-carbon magnetic nanocomposites prepared using hybrid sebacate intercalated layered double hydroxides (LDHs) as precursors are shown to be of interest as supercapacitors. Here, the low-temperature formation mechanism of these materials has been deciphered by means of a combined study using complementaryin situ(temperature-dependent) techniques. Specifically, studies involving X-ray powder diffraction, thermogravimetry coupled to mass spectrometry (TG-MS), statistical Raman spectroscopy (SRS), aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) have been carried out. The experimental results confirm the early formation of FeNi(3)nanoparticles atca.200-250 degrees C, preceding the concerted collapse of the starting NiFe-LDH laminar structure over just 50 degrees C (from 350 to 400 degrees C). At the same time, the catalytic interactions between the metallic atoms and the organic molecules permit the concomitant formation of a graphitic carbon matrix leading to the formation of the final FeNi3-carbon nanocomposite. Furthermore,in situtemperature-dependent experiments in the presence of the intrinsic magnetic field of the STEM-EELS allow observing the complete metal segregation of Ni and Fe even at 400 degrees C. These results provide fundamental insights into the catalytic formation of carbon-based nanocomposites using LDHs as precursors and pave the way for the fine-tuning of their properties, with special interest in the field of energy storage and conversion, Financial support from the European Union (ERC Advanced Grant Mol-2D 788222, ERC Starting Grant 2D-PnictoChem 804110, ERC Proof of Concept Grant Hy-MAC 713704, and COST-Action on Molecular Spintronics (MOLSPIN CA15128)), the Spanish MINECO (Projects MAT2017-89993-R, MAT2015-66888-C3-3-R, RTI2018-097895-B-C43 co-financed by FEDER, and the Unit of Excellence "Maria de Maeztu" MDM-2015-0538), and the Generalitat Valenciana (Prometeo Program and iDiFEDER/2018/061 co-financed by FEDER) is gratefully acknowledged. G. A. acknowledges the support by the Deutsche Forschungsgemeinschaft DFG (FLAG-ERA AB694/2-1) and the Generalitat Valenciana (CIDEGENT/2018/001 grant). G. A. received financial support through the Postdoctoral Junior Leader Fellowship Programme from "la Caixa" Banking Foundation. J. R. thanks the Spanish MINECO for his predoctoral grant. Electron microscopy observations were carried out at the ICTS ELECMI node at Centro Nacional de Microscopia Electronica at the Universidad Complutense de Madrid. The authors thank Dr Maria Dolores Jordan Martin for her kind assistance with the XPS measurements.

The reduction of the lattice thermal conductivity is one of the crucial steps in improving thermoelectric materials. In skutterudites, a well-known approach is to reduce the thermal conductivity by filling the structural cage with rare-earth atoms. In this work, we show that it is not just the amount of such filling itself but its nanoscale structuration that lowers the thermal conductivity. A straightforward synthesis procedure under high pressure yields Ce- and Yb-filled CoSb3 skutterudites, with and without an inhomogeneous distribution of the filler atoms. The composition of the phases is evaluated from synchrotron Xray diffraction (SXRD) data; the highly nanostructured morphology is verified by high-resolution transmission electron microscopy (TEM). The filling fluctuation, i.e., the uneven distribution of filling atoms in the sample originating a phase segregation, brings about low lattice thermal conductivity, as a strong source of phonon scattering. This effect is prominent in the Ce-filled compound, where Ce is segregated into Ce-rich and Ce-poor regions, and the lattice contribution of the thermal conductivity kappa(L), shows a concomitant reduction, approaching values as low as 1.6 W m(-1) K-1 at 800 K. Although the level of filling is much higher in YbxCoSb3, its lattice thermal conductivity remains larger. Overall, though, its power factor is enhanced due to charge transfer from the Yb-filler. We thus define a new paradigm for the design of filled skutterudites with exceptionally low thermal conductivities, based on the nanoscale mixing of two phases with different filling factors, spontaneously induced by high-pressure synthesis conditions, which can be considered as pseudoamorphous structures with significant reduction in kappa(L).

Thermoelectric mischmetal-filled Mm(x)Co(4)Sb(12) (Mm: natural cocktail of rare-earth elements, mostly Ce and La) skutterudites have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. Synchrotron X-ray diffraction patterns display a splitting of the diffraction peaks ascribed to purely Ce-, and Mm-filled skutterudite phases, which have been analyzed and confirmed by high-resolution TEM and EELS. A total thermal conductivity () of 1.51 W m(-1) K-1 is measured at 773 K for Mm(0.5)Co(4)Sb(12), below that of other filled skutterudites, which is promoted by the enhanced phonon scattering over a broad range of the phonon spectrum due to the inhomogeneous and nanoscale mischmetal inclusion. Compared to undoped CoSb3 skutterudite synthesized by conventional methods, is reduced by a factor of 3, while the power factor is also substantially enhanced.

Sb-doped Bi2Te3 is known since the 1950s as the best thermoelectric material for near-room temperature operation. Improvements in material performance are expected from nanostructuring procedures. We present a straightforward and fast method to synthesize already nanostructured pellets that show an enhanced ZT due to a remarkably low thermal conductivity and unusually high Seebeck coefficient for a nominal composition optimized for arc-melting: Bi0.35Sb1.65Te3. We provide a detailed structural analysis of the Bi2−xSbxTe3 series (0 ≤ x ≤ 2) based on neutron powder diffraction as a function of composition and temperature that reveals the important role played by atomic vibrations. Arc-melting produces layered platelets with less than 50 nm-thick sheets. The low thermal conductivity is attributed to the phonon scattering at the grain boundaries of the nanosheets. This is a fast and cost-effective production method of highly efficient thermoelectric materials.

In this chapter we will review a few examples of applications of atomic resolution aberration corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to complex oxide materials. These are most challenging systems where subtle changes in structure or chemistry may result in colossal responses in macroscopic physical behavior. Here, we will review how atomic resolution compositional mapping can be achieved in manganite thin films and single crystals, highlighting the importance of considering artifacts during quantification. Besides, minor changes in near edge fine structure may take place when the crystalline environment, and hence nearest neighbor configuration, is modified. These can also be tracked by atomic resolution EELS, as will be shown through the study of binary Fe oxides. Also, examples regarding the study of distributions of point defects such as 0 vacancies in cobaltite thin films will be discussed. In these materials, a combination of epitaxial strain and defects may promote physical behaviors not present in bulk, such as the stabilization of unexpected spin state superlattices. Last, a study of extended defects such as dislocation lines will be reviewed. In particular, we will show how chemical segregation at dislocation cores in yttria-stabilized zirconia grain boundaries results in the generation of static O vacancies that affect the local electrostatic potential and hence, the macroscopic ionic conduction properties.

The multifunctional (ferromagnetic and ferroelectric) response at room temperature that is elusive in single phase multiferroic materials can be achieved in a proper combination of ferroelectric perovskites and ferrimagnetic spinel oxides in horizontal heterostructures. In this work, lead-free CoFe 2 O 4 /BaTiO 3 bilayers are integrated with Si(001) using LaNiO 3 /CeO 2 /YSZ as a tri-layer buffer. They present structural and functional properties close to those achieved on perovskite substrates: the bilayers are fully epitaxial with extremely flat surface, and exhibit robust ferromagnetism and ferroelectricity at room temperature.

This work reports on magnetocapacitance (MC) effects in epitaxial heterostructures of nominally 15 unit cells (u.c.) LaMnO(LMO) and 2 u.c. SrTiO(STO) with an alternating layer-repetition rate of 8: (LMO/STO). Epitaxial multilayer growth at high temperatures (900 °C) activates a selective inter-diffusion of Laand Srcations across the interfaces, which gives rise to Sr p-doping of the LMO and La n-doping of the STO layers. MC effects at the buried LaSrMnO/SrLaTiO(LSMO/SLTO) interfaces are probed by frequency, temperature and magnetic field dependent AC impedance spectroscopy. The technique is shown to be appropriate to account for the separate analysis of different resistance and capacitance contributions at the buried interfaces. As a result of the La/Sr inter-diffusion process, Schottky barriers are formed at the LSMO/SLTO interfaces, which give rise to massive MC of up to ≈ −200% in the out-of-plane film direction. The capacitance of the manganite-titanate LSMO/SLTO interfaces may be coupled indirectly to the resistance of the LSMO layers, because the Schottky space-charge layers and their capacitance can be modulated by varying the concentration of highly mobile charge carriers in the LSMO with a magnetic field., We acknowledge financial support by the Spanish MICINN/MINECO through grants MAT2014‐52405‐C2 and Consolider Ingenio 2010‐CSD2009‐00013 (Imagine), and by CAM through grant S2013/MIT‐2740. R.S. and J.G.‐B. acknowledge Ramón y Cajal fellowships from the MICINN/MINECO in Spain. Research at ORNL (M.V.) was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Research at UCM (M.V.) was supported by Spanish MINECO MAT 2015‐66888‐C3‐3‐R.

We report on the evolution of the microstructure of Tb-Fe-Ga films deposited by co-sputtering from TbFe and FeGa targets. The sputtering power was fixed (90 W) in the FeGa whereas it was increased from 50 to 90 W in the TbFe target resulting on TbFeGa layers with 7 ≤ x ≤ 11. The local structure was determined by means of x-ray absorption fine structure spectroscopy at Fe-K, Ga-K and Tb-L edges. The increase of Tb in the alloy promotes the phase segregation that produces a larger amount of the TbFe structural phase. The structural results have been correlated with the magnetic characterization that shows the enhancement of the out-of-plane component of the magnetization., This work has been financially supported through projects MAT2015-66888-C3-3-R and PIE-2010-OE-013-200014 of the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) and through the project PR26/16-3B-2 of Santander and Universidad Complutense de Madrid.

We report on the evolution of uncapped FeGa layers deposited by sputtering and post-growth annealed in oxygen atmosphere in a temperature range from 500 °C to 800 °C. We have investigated the morphology, structure and magnetic properties of films with a thickness of 200 nm deposited on Mo buffer layers on glass substrates. X-ray diffractometry shows a decrease of the lattice parameter up to 600 °C whereas a further increase of the temperature up to 800 °C promotes the transformation to FeO. We have observed by x-ray absorption fine structure the partial oxidation of Ga and the formation of Ga aggregates at 600 °C. These aggregates form Ga-rich bubbles that can be observed on the sample surface from which Ga evaporates leaving a Ga-poor layer that is later oxidized into FeO. The thermal treatment on oxygen atmosphere has also a clear impact on the magnetic properties of the layers. The uniaxial in-plane magnetic anisotropy of the as-grown film evolves to magnetic isotropy when annealed at 600 °C probably due to the segregation and formation of Ga-rich areas. After Ga evaporates from the sample, Fe is fully oxidized and only a weak ferromagnetism related to FeO is detected., This work has been financially supported through the projects MAT2015-66888-C3-3-R of the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) and PR26/16-3B-2 of Santander and Universidad Complutense de Madrid.

[EN] New doped inorganic nanocrystals (NC) consisting on iron oxide and other metal integrated into the structure have been synthesized in one-step by adapting the oxidant precipitation synthesis route for magnetite. Different metals have been chosen to confer extra and unique properties to the resulting magnetic hetero-nanostructure: Co and Gd for enhancing transversal and longitudinal relaxivities for magnetic resonance imaging and Bi and Au for achieving X-ray absorption for computed tomography imaging. Apart of that, gold optical properties are interesting for photothermal therapy and iron oxides for magnetic hyperthermia. All metals have been incorporated into the magnetite structure in different ways during the synthesis: by forming a solid solution, by modifying the surface of the NCs, or by co-crystallization with the magnetite. The nanostructure formed in each case depends on the ionic radius of the secondary metal ion and the solubility of its hydroxide that control the co-precipitation in the initial steps of the reaction. Magnetic properties and imaging capabilities of the hetero-nanostructures have been analyzed as a function of the element distribution. Due to the synergistic combination of the different element properties, these magnetic hetero-nanostructures have great potential for biomedical applications., Thanks to the general services from ICMM (X-ray, ICP) and URJC, in particular to Jesus Gonzalez for TEM images, and to the NAP group of Institute of Nanoscience of Aragon (INA), in particular to Laura Asín and Lucía Gutiérrez for the SAR measurement assistance. The work has been supported by the Spanish Ministerio de Ciencia, Innovación y Universidades under the following projects: Project MAT2017-88148-R and PIE-201760E007. STEM/EELS observations at Oak Ridge
National Laboratory (ORNL) U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and
Engineering Division and through the Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, DOE-BES. Research at UCM supported by MINECO-FEDER MAT2015-66888-C3-3-R. Thanks to SAF2016-79593-P project and to Nanobioapcluster of excellence supported by the Ministry of Economy and Competitiveness of Spanish Government through MAT2016-81955-REDT.

Group 15 elements in zero oxidation state (P, As, Sb and Bi), also called pnictogens, are rarely used in catalysis due to the difficulties associated in preparing well–structured and stable materials. Here, we report on the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state, suspended in an ionic liquid, with the native atoms ready to interact with external reagents while avoiding aerobic or aqueous decomposition pathways, and on their use as efficient catalysts for the alkylation of nucleophiles with esters. The few layer pnictogen material circumvents the extremely harsh reaction conditions associated to previous superacid–catalyzed alkylations, by enabling an alternative mechanism on surface, protected from the water and air by the ionic liquid. These 2D catalysts allow the alkylation of a variety of acid–sensitive organic molecules and giving synthetic relevancy to the use of simple esters as alkylating agents., We thank the European Research Council (ERC Starting Grant 804110 to G.A., and ERC Advanced Grant 742145 B-PhosphoChem to A.H.) for financial support. The research leading to these results was partially funded by the European Union Seventh Framework Program under grant agreement No. 604391 Graphene Flagship. G.A. has received financial support through the Postdoctoral Junior Leader Fellowship Program from “la Caixa” Banking Foundation (LCF/BQ/PI18/11630018). G.A. thanks support by the Deutsche Forschungsgemeinschaft (DFG; FLAG-ERA AB694/2-1), the Generalitat Valenciana (SEJI/2018/034 grant) and the FAU (Emerging Talents Initiative grant #WS16-17_Nat_04). Financial support by MINECO through the Excellence Unit María de Maeztu (MDM-2015-0538), Severo Ochoa (SEV-2016-0683) and RETOS (CTQ2014-55178-R) program is acknowledged. M.A.R.-C. thanks MINECO for the concession of a FPU fellowship. We also thank the DFG (DFG-SFB 953 “Synthetic Carbon Allotropes”, Project A1), the Interdisciplinary Center for Molecular Materials (ICMM), and the Graduate School Molecular Science (GSMS) for financial support. Research at UCM sponsored by Spanish MINECO/FEDER grant MAT2015-066888-C3-3-R and ERC-PoC-2016 grant POLAR-EM. H.-P.S. thanks the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program for financial support, in the context of an Advanced Investigator Grant granted to him (Grant Agreement No. 693398-ILID). B.S.J.H. and S.S. acknowledge financial support by the DFG within the Cluster of Excellence “Engineering of Advanced Materials” (project EXC 315, Bridge Funding). F.M. acknowledges R. Ransom for very helpful discussions.

The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films., This work has been financially supported through the projects MAT2015-66888-C3-3-R and MAT2015-66726-R (MINECO/FEDER) of the Spanish Ministry of Economy and Competitiveness, RTI2018-097895-B-C43 of the Spanish Ministry of Science, Innovation, and Universities, and Gobierno de Aragón (Grant E10-17D) and Fondo Social Europeo. A.M.-N. is thankful for the contract from Universidad Complutense and Comunidad de Madrid “Atracción de Talento” program 2018-T1/IND-10360, and A.B. thanks MINECO for the Ph.D. grant BES-2016-076482., Peer reviewed

Thermoelectric mischmetal-filled MmxCo4Sb12 (Mm: natural cocktail of rare-earth elements, mostly Ce and La) skutterudites have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston–cylinder hydrostatic press. Synchrotron X-ray diffraction patterns display a splitting of the diffraction peaks ascribed to purely Ce-, and Mm-filled skutterudite phases, which have been analyzed and confirmed by high-resolution TEM and EELS. A total thermal conductivity (κ) of 1.51 W m−1 K−1 is measured at 773 K for Mm0.5Co4Sb12, below that of other filled skutterudites, which is promoted by the enhanced phonon scattering over a broad range of the phonon spectrum due to the inhomogeneous and nanoscale mischmetal inclusion. Compared to undoped CoSb3 skutterudite synthesized by conventional methods, κ is reduced by a factor of 3, while the power factor is also substantially enhanced., This work was supported by the Spanish Ministry of Science, Innovation and Universities through grants MAT2017-84496-R, MAT2017-87134-C2-2-R and MAT2015-66888-C3-3-R co financed by FEDER. JPG would also like to thank this Ministry for granting a “Juan de la Cierva” fellowship and to Community of Madrid for granting an “Atracción de Talento” fellowship (2017-T2/I ND-5597). Financial support from the ERC grant PoC2015-MAGTOOLS is also acknowledged. The authors wish to express their gratitude to ALBA technical staff for making the facilities available for the synchrotron X-ray diffraction experiment number 2017072260. Transmission electron microscopy studies were performed at the ICTS ELECMI node at Centro Nacional de Microscopía Electrónica (CNME) at the Universidad Complutense de Madrid (UCM)., We acknowledge support of the publication fee by the CSIC Open Access Support Initiative through its Unit of Information Resources for Research (URICI)

NiFe-carbon magnetic nanocomposites prepared using hybrid sebacate intercalated layered double hydroxides (LDHs) as precursors are shown to be of interest as supercapacitors. Here, the low-temperature formation mechanism of these materials has been deciphered by means of a combined study using complementary in situ (temperature-dependent) techniques. Specifically, studies involving X-ray powder diffraction, thermogravimetry coupled to mass spectrometry (TG-MS), statistical Raman spectroscopy (SRS), aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) have been carried out. The experimental results confirm the early formation of FeNi3 nanoparticles at ca. 200-250 °C, preceding the concerted collapse of the starting NiFe-LDH laminar structure over just 50 °C (from 350 to 400 °C). At the same time, the catalytic interactions between the metallic atoms and the organic molecules permit the concomitant formation of a graphitic carbon matrix leading to the formation of the final FeNi3-carbon nanocomposite. Furthermore, in situ temperature-dependent experiments in the presence of the intrinsic magnetic field of the STEM-EELS allow observing the complete metal segregation of Ni and Fe even at 400 °C. These results provide fundamental insights into the catalytic formation of carbon-based nanocomposites using LDHs as precursors and pave the way for the fine-tuning of their properties, with special interest in the field of energy storage and conversion. This journal is, Financial support from the European Union (ERC Advanced
Grant Mol-2D 788222, ERC Starting Grant 2D-PnictoChem
804110, ERC Proof of Concept Grant Hy-MAC 713704, and
COST-Action on Molecular Spintronics (MOLSPIN CA15128)),
the Spanish MINECO (Projects MAT2017-89993-R, MAT2015-
66888-C3-3-R, RTI2018-097895-B-C43 co-nanced by FEDER,
and the Unit of Excellence “Maria de Maeztu” MDM-2015-0538),
and the Generalitat Valenciana (Prometeo Program and
iDiFEDER/2018/061 co-nanced by FEDER) is gratefully
acknowledged. G. A. acknowledges the support by the Deutsche
Forschungsgemeinscha DFG (FLAG-ERA AB694/2-1) and the
Generalitat Valenciana (CIDEGENT/2018/001 grant). G. A.
received nancial support through the Postdoctoral Junior
Leader Fellowship Programme from “la Caixa” Banking Foundation.
J. R. thanks the Spanish MINECO for his predoctoral
grant. Electron microscopy observations were carried out at the
ICTS ELECMI node at Centro Nacional de Microscop´ıa Electr
´onica at the Universidad Complutense de Madrid. The authors
thank Dr Mar´ıa Dolores Jord´an Mart´ın for her kind assistance
with the XPS measurements.

The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films.