Single crystals of Mg 1-x Al x B 2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB 2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg 1-x Al x B 2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of T c by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB 4 domains in the structure of superconducting Mg 1-x Al x B 2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H c2 , for H//c for the samples with small x, significant reduction of the H c2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg 1-x Al x B 2 structure. 74.70.Ad, 74.62.Dh, 81.10.-h, 74.25.Ha

The effect of oxygen vacancy and spinel phase integration on anionic and cationic redox in Li-rich cathode materials was unraveled.

The oxygen redox reaction in lithium-rich layered oxide battery cathode materials generates extra capacity at high cell voltages (i.e., >4.5 V). However, the irreversible oxygen release causes transition metal (TM) dissolution, migration and cell voltage decay. To circumvent these issues, we introduce a strategy for tuning the Coulombic interactions in a model Li-rich positive electrode active material, i.e., Li1.2Mn0.6Ni0.2O2. In particular, we tune the Coulombic repulsive interactions to obtain an adaptable crystal structure that enables the reversible distortion of TMO6 octahedron and mitigates TM dissolution and migration. Moreover, this strategy hinders the irreversible release of oxygen and other parasitic reactions (e.g., electrolyte decomposition) commonly occurring at high voltages. When tested in non-aqueous coin cell configuration, the modified Li-rich cathode material, combined with a Li metal anode, enables a stable cell discharge capacity of about 240 mAh g−1 for 120 cycles at 50 mA g−1 and a slower voltage decay compared to the unmodified Li1.2Mn0.6Ni0.2O2.

Single crystals of the methylammonium (MA) lead halides MAPbI 3 , MAPbBr 3 , and MAPbCl 3 have been investigated using infrared spectroscopy with the aim of analyzing structural and dynamical aspects of processes that enable the ordering of the MA molecule in the orthorhombic crystal structure of these hybrid perovskites. Our temperature-dependent studies were focused on 2 the analysis of the CH/NH rocking, C-N stretching, and CH/NH bending modes of the MA molecule in the 800-1750 cm -1 frequency range. They deliver a direct comparison of the behavior of the three halides on crossing the orthorhombic-tetragonal phase transition in MA lead halide single crystals. Drastic changes of all vibrational modes close to the phase transition were clearly observed. Additional spectral features that were not discussed previously are pointed out. The transformation of the 2-dimensional orthorhombic hydrogen bond layers into a more 3-dimensional arrangement in the tetragonal phase seems to be an important feature providing deeper insight into the mechanisms that lead to a free-rotating MA molecule in the inorganic host structure. The change of the molecules site symmetry in the tetragonal crystal structure seems to be an important feature of the orthorhombic-tetragonal phase transition. For low temperatures it can be stated that the iodide is stronger influenced by hydrogen bonding than the bromide and the chloride. FIGURE 1 Visualization of MAPbI 3 tetragonal (a, b), MAPbI 3 orthorhombic (c, d) andMAPbBr 3 orthorhombic (e, f) crystal structure. Two hydrogen bond layers, layer A and layer B, can be identified. Layer A is shown in b), d) and f). The only difference between layer A and layer B is the orientation of the MA molecule axis. All N-H...I bond lengths are the same for both layers A and B (Table S5). Hydrogen atoms are not shown in the tetragonal structure. The colors refer to the following elements: purple -iodine, dark blue -bromide, dark grey -lead, brown -carbon, light blue -nitrogen, light orange -hydrogen. 8 X-ray diffractionSample purity was proven by X-ray powder diffraction analysis. The measurement was performed by a Panalytical X'Pert Pro MRD powder diffractometer with sample spinner (Bragg-Brentano geometry, λ = 1.5406 Å Cu Kα with 40kV/30mA, step size 0.0032 and time per step 597.72 sec, 2θ from 11-81°). The analysis was done with HighScore Plus Version 3.05. No impurities were observed. FTIR microscope spectrometryInfrared spectra were recorded on a Bruker Vertex 80v FTIR spectrometer, equipped with a globar light source, a KBr beam-splitter and a Bruker Hyperion 2000 microscope using Cassegrainian objectives and an MCT Hg-Cd-Te detector. A detector nonlinear correction routine was applied using the OPUS software package (Bruker). A Linkam THMS600 stage was used for cooling under Argon atmosphere. The thermocouple was calibrated using melting points of different salts (melting points between 580 K and 1074 K) as described elsewhere. 50 The 0°C point (freezing of H 2 O) was included in the calibration line....

O3-type NaTMO2 (TM = transition metal) as cathodes for Na-ion batteries have aroused much interest. But the structural instability during charge–discharge cycles and the inferior rate capability restricts their application. In this study, we report a synergetic modification method to simultaneously increase the rate capacity and cycling stability of O3-type NaMn0.33Fe0.33Ni0.33O2(MFN) cathode material by integrating TiO2 coating and Ti4+ doping. Moreover, the synergetic mechanism has been put forward. First, the TiO2-coating layer prevents the side reactions on the surface, which can retain the structural integrity and stability. Second, TiO2-coating induces Ti4+ doping which enlarges Na–O and increases the interslab spacing d. This raises the Na+ diffusion coefficient and improves the rate performance. Third, because of the large Ti–O bond energy, the TM–O bond shrinks when Ti4+ is doped into the transition metal site. O–O is also shortened due to the Ti doping. TMO2 slabs are compressed, which benefits the structural stability and the cyclic property. Fourth, Ti doping accompanying TiO2 coating decreases Mn3+/Mn4+ and mitigates the Jahn–Teller effect. This increases the stability of the layered structure. The understanding of the multiroles of TiO2 coating is equally instructive for the exploration of other cathode materials for Na-ion or Li-ion batteries.