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[en] Full text: The research about renewable energy sources has attracted attention since there is a high demand for clean energy around the world. In the case of solar energy, a prominent field is the investigation of multiferroic materials for applications in photovoltaic cells. In this scenario, KBiFe2O5 (KBFO) emerged as a strong candidate for such applications given mostly by its low bandgap energy. At room temperature, KBFO crystallizes in an orthorhombic structure and possesses interesting electronic properties making it suitable also for photovoltaic applications. Besides, it is reported that such structure transforms into a monoclinic one in very high temperatures and pressures. In this work, we obtained the monoclinic structure of KBFO at room temperatures and performed an analysis of the structural and vibrational properties of KBFO using Raman spectroscopy and Synchrotron X-ray powder diffraction (SXRPD) techniques under hydrostatic pressure variation. The study of Raman spectra obtained at high pressures revealed a first order phase transition in the material, characterized by a pressure hysteresis. Synchrotron X-ray powder diffraction (SXRPD) under varying pressure was employed to confirm such structural pressure-induced phase transition. An integrated evaluation of the results obtained by Raman and SXRPD techniques indicates that, possibly, the symmetry for the high-pressure crystalline phase belongs to the space group Pmmm. (author)
[en] The ternary Se79Te20Pb1 chalcogenide glass is prepared using melt quenching technique. Differential scanning calorimetry technique (DSC) is used to investigate the kinetics of crystallization of amorphous-crystallization (a-c) phase transformation under non-isothermal conditions at three different heating rates; 5, 10 and 15° C min-1. The variation of crystallized activation energy (Ec) with crystallized fraction (ϰ) and hence, with temperature (T) is investigated using five iso-conversional methods namely KAS, OFW, Friedman, Tang and Chen and Starink. It is found that Ec is not constant but vary with ϰ as well as T. Thus, the iso-conversional analysis of investigated glass indicates that the assumption of constant Ec is not appropriate. (author)
[en] Developing strong visible-light-absorbing (SVLA) earth-abundant photosensitizers (PSs) for significantly improving the utilization of solar energy is highly desirable, yet it remains a great challenge. Herein, we adopt a through-bond energy transfer (TBET) strategy by bridging boron dipyrromethene (Bodipy) and a Cu complex with an electronically conjugated bridge, resulting in the first SVLA Cu PSs (Cu-2 and Cu-3). Cu-3 has an extremely high molar extinction coefficient of 162 260 m cm at 518 nm, over 62 times higher than that of traditional Cu PS (Cu-1). The photooxidation activity of Cu-3 is much greater than that of Cu-1 and noble-metal PSs (Ru(bpy) and Ir(ppy) for both energy- and electron-transfer reactions. Femto- and nanosecond transient absorption and theoretical investigations demonstrate that a "ping-pong" energy-transfer process in Cu-3 involving a forward singlet TBET from Bodipy to the Cu complex and a backward triplet-triplet energy transfer greatly contribute to the long-lived and Bodipy-localized triplet excited state. (© 2020 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)
[en] Lithium‐ion batteries (LIBs) have become ubiquitous power sources for small electronic devices, electric vehicles, and stationary energy storage systems. Despite the success of LIBs which is acknowledged by their increasing commodity market, the historical evolution of the chemistry behind the LIB technologies is laden with obstacles and yet to be unambiguously documented. This Viewpoint outlines chronologically the most essential findings related to today's LIBs, including commercial electrode and electrolyte materials, but furthermore also depicts how the today popular and widely emerging solid‐state batteries were instrumental at very early stages in the development of LIBs. (© 2020 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
[en] Injecting hydrogen in the natural gas grid poses a number of problems. Does hydrogen hold the key to the great energy transition to come? France and other countries believe this to be the case, and have chosen to invest heavily in the sector. Such spending will be needed to solve the many issues raised by this energy carrier. One such issue is containers, since hydrogen tends to damage metallic materials. At Mines Saint-etienne, Frederic Christien and his teams are trying to answer these questions.
[fr]L'injection d'hydrogene dans le reseau de gaz naturel n'est pas sans poser certains problemes. L'hydrogene represente-t-il la cle de la future grande transition energetique? C'est la conviction de la France, ainsi que d'autres pays qui ont choisi d'investir massivement dans cette filiere. Un budget qui ne sera pas de trop pour resoudre les nombreux problemes souleves par ce vecteur energetique. Parmi eux figure la question de son conditionnement, l'hydrogene ayant tendance a endommager les materiaux metalliques. a Mines Saint-etienne, Frederic Christien et ses equipes tentent d'y apporter des elements de reponse.
[en] X-ray absorption spectroscopy represents a valuable characterization tool for complex materials like multialkali antimonides. The interpretation of such experimental results greatly benefits from state-of-the-art theoretical references. In a first-principles work based on density-functional theory and many-body perturbation theory, the fingerprints of X-ray absorption near-edge structure (XANES) of CsSb from the Cs K- and L-edge are unraveled. From the electronic structure and the orbital character of the conduction bands, the contributions to the XANES spectra from the two inequivalent Cs atoms in the adopted stoichiometric unit cell are analyzed. The predominant weight of Cs s- and d-electrons in the unoccupied region anticipates the relatively high signal yielded by the L-edge. Clear atomic signatures, in the form of a pronounced excitonic peak, are visible in the Cs K-edge spectra. In the XANES from the L-edge, both Cs atoms yield similar contributions, which are yet distinguishable in the presented calculations. Quantitative analysis enabled by the adopted methodology reveals that electron-hole correlation effects manifest themselves mainly through a redistribution of the oscillator strength to lower energies, whereas exciton binding energies are on the order of a few hundred meV. (© 2020 The Authors. Published by WILEY‐VCH Verlag GmbH and Co. KGaA, Weinheim)
[en] The vibrational and luminescence properties of the TbAl3(BO3)4 single crystal were studied in the temperature range of 5–300 K. Raman spectra of the single crystal revealed 5 of 7 A1 and all E phonon modes predicted by the group-theory analysis. The splitting energy between the LO and TO components of polar E phonons is determined. A group of intense bands associated with the 5 D4 → 7 F0 electronic transition was observed in the energy range of 14520–14680 cm–1 of the luminescence spectra. The intensity of these bands decreases upon heating. At the same time, the bands which can be assigned with 5 D4 → 7 F6, 5 D4 → 7 F5, and 5 D4 → 7 F4 transitions were revealed in luminescence spectra at room temperature. The intensity of these bands is comparable to the intensity of the Raman spectrum of TbAl3(BO3)4. The observation of luminescence from the 5 D4 multiplet (20600–20750 cm–1) upon excitation at λexc = 632.8 nm (15803 cm–1) and λexc = 532 nm (18797 cm–1) indicates strong nonlinear properties of the studied crystal. The structure of the main 7 F6 multiplet of Tb+3 in the TbAl3(BO3)4 single crystal has been studied at 5 K by Raman spectroscopy. The energies of the electronic levels of 7 F6 and 5 D4 multiplets were determined from the luminescence spectra measured at 300 K. (author)
[en] Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru=O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with Ru=(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of Ru=O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV). (© 2019 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)
[en] Lithium metal is recognized as one of the most promising anode materials owing to its ultrahigh theoretical specific capacity and low electrochemical potential. Nonetheless, dendritic Li growth has dramatically hindered the practical applications of Li metal anodes. Realizing spherical Li deposition is an effective approach to avoid Li dendrite growth, but the mechanism of spherical deposition is unknown. Herein, a diffusion-reaction competition mechanism is proposed to reveal the rationale of different Li deposition morphologies. By controlling the rate-determining step (diffusion or reaction) of Li deposition, various Li deposition scenarios are realized, in which the diffusion-controlled process tends to lead to dendritic Li deposition while the reaction-controlled process leads to spherical Li deposition. This study sheds fresh light on the dendrite-free Li metal anode and guides to achieve safe batteries to benefit future wireless and fossil-fuel-free world. (© 2020 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)
[en] The phase transition of Zr60Cu15Ni10Al10Pd5 metallic glass is examined by the application of thermal treatment employed by differential scanning calorimetry (DSC). DSC experiments were carried out on the ribbon samples of Zr60Cu15Ni10Al10Pd5 at five different heating rates i.e., 2, 4, 6, 8, 10 °C min-1. A sharp crystallization peak is observed at 740 K at a heating rate of 10°C min-1. Glass transition is not found to be prominent. To study the entire crystallization process different testing techniques are carried out to fit the experimental data. For the isothermal crystallization process, mainly two methods are used, i.e., iso-conversional and isokinetic methods. Iso-conversional methods provide activation energies at different values of crystallized fraction, whereas iso-kinetic methods provide single value of activation energy. Further, Iso-kinetic methods also provide the value of Avrami (growth) exponent which gives idea about the dimensionality of the crystal. Hence to study the complex crystallization process both the methods are equally important. (author)