The lithium-ion battery electrode material TiSnSb shows excellent electrochemical performance related to its high capacity (550 mA h g–1) and rate capability over 210 cycles. To discriminate between ...the role of active material and the role of the electrode formulation in the good electrochemical features of the TiSnSb electrodes, a full study comparing the electrochemical mechanisms of TiSnSb and a Ti/Sn/Sb composite vs Li is undertaken by combining X-ray diffraction (XRD), 121Sb, 119Sn Mössbauer and 7Li NMR spectroscopic in situ measurements. During the first discharge, TiSnSb undergoes a direct conversion reaction while Ti/Sn/Sb composites proceed by a stepwise alloying process, both leading to a mixture of lithium antimonide, lithium stannides, and titanium. More surprisingly the charge occurs differently with a reformation of the “TiSnSb” phase in the first case and the formation of Sn and Sb in the second case. The key role of the interfaces in conversion type reactions is discussed. The nature of the interfaces is linked to the long-range order of elements in the starting material. Furthermore, the length scales of the interfaces between Li3Sb, Li x Sn and Ti appear to control the reactions that occur on charge.
Display omitted
► For the rare silicide–carbide U3Si2C2, experimental data are best reproduced with GGA+U=4eV. ► High linear incompressibility along the c-axis is identified with Si–C alignment ...leading to anisotropy. ► Chemical bonding is specific of the uranium sites with dominant U1–Si and U2–C. ► Major Si–C bonding contributions characterize the compound.
The electronic structure of U3Si2C2, with the rare SiC unit is examined from ab initio with an assessment of the properties of chemical bonding. We show that plain GGA fails describing the experimental lattice parameters and the electronic structure. A better agreement with experiment (crystal determination and magnetic properties) is obtained with the GGA+U method and U=4eV. The energy–volume equation of state and the set of elastic constants are obtained showing incompressibility along the c-axis with U–C–Si alignment and a brittle material. Bonding of U1 and U2 selectively with Si and C and Si–C bonds are remarkable
The electronic structure of the ternary nitride Li{sub 2}ZrN{sub 2} is examined from ab initio with DFT computations for an assessment of the properties of chemical bonding. The compound is found ...insulating with 1.8 eV band gap; it becomes metallic and less ionic upon removal of one equivalent of Li. The chemical interaction is found mainly between Zr and N on one hand and Li and N on the other hand. While all pair interactions are bonding, antibonding N-N interactions are found dominant at the top of the valence band of Li{sub 2}ZrN{sub 2} and they become less intense upon removal of Li. From energy differences the partial delithiation leading to Li{sub 2-x}ZrN{sub 2} (x={approx}1) is favored. - Graphical abstract: Trigonal structure of Li{sub 2}ZrN{sub 2} showing the Zr-N-Li layers along the c-axis. Highlights: Black-Right-Pointing-Pointer Li{sub 2}ZrN{sub 2} calculated insulating with a 1.8 eV gap in agreement with its light green color. Black-Right-Pointing-Pointer Lithium de-intercalation is energetically favored for one out of two Li equivalents. Black-Right-Pointing-Pointer Li plays little role in the change of the structure, ensured by Zr and N binding. Black-Right-Pointing-Pointer Similar changes in the electronic structure as for various intercalated phases of ZrN.
Valence modulations in CeRuSn Feyerherm, R.; Dudzik, E.; Prokeš, K. ...
Physical review. B, Condensed matter and materials physics,
07/2014, Letnik:
90, Številka:
4
Journal Article
Recenzirano
Odprti dostop
CeRuSn exhibits an extraordinary room temperature structure at 300 K with the coexistence of two types of Ce ions, namely trivalent Ce super(3+) and intermediate-valent Ce super(4- delta +), in a ...metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the c axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl-type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the c axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L sub(3) absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to our previous report, the new data exhibit small but significant variations as a function of temperature that are consistent with a transition of a fraction of Ce super(3+) ions to the intermediate valence state, analogous to the gamma arrow right alpha transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.
Two different types of fluxes, namely sodium based and chloride based fluxes were used to grow Cr substituted barium and strontium hexaferrite ferrite crystals, (Sr,Ba)Fe12 − xCrxO19 at comparatively ...low temperatures of about 1300 °C. The sodium based flux led to growth of larger crystals up to 5 mm, but with only minor Cr contents x ≤ 0.07. From the chloride based flux the obtained Cr contents are significantly higher with x = 5.7 (Sr) and x = 3.4 (Ba), however, crystals reach only sizes in the sub-mm range. X-ray absorption spectroscopy data support exclusively isovalent substitution of Fe3+ by Cr3+ even for very low Cr contents. 57Fe Mößbauer spectroscopy reveals Cr to preferentially occupy the six-fold by oxygen coordinated site at 12k and, to a lower degree, 2a and 4f2 in space group P63/mmc. All characteristic magnetic properties drop upon Cr substitution, e. g., the Curie temperature from 728 K for pure BaFe12O19 to 465 K for BaFe8.6Cr3.4O19, the saturation magnetization from 71 emu/g to 29.7 emu/g and the coercive field from 363 Oe to 45 Oe.
Display omitted
•Growth of large crystals of Cr substituted (Ba,Sr)Fe12O19 from two different fluxes.•Cr-substitution in a large range of compositions.•Cr/Fe order revealed by Mößbauer spectroscopy.•All Cr and Fe in oxidation state +3 present.•Anisotropic magnetic properties determined.
We report on the atomic pair distribution function determination in the ternary compound CeRuSn using high-energy synchrotron x-ray diffraction. Above room temperature CeRuSn orders in a monoclinic ...structure that is related to the CeCoAl type of structure by a doubling of the c-axis parameter. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to an ill-defined structure with a main component three times larger than the c parameter. The pair distribution functions collected between 90 and 315 K suggest that the local symmetry type does not change significantly during the phase transformation. However, anomalously large atomic displacement factors suggest that a wide distribution of interatomic distances exists in the sample. Limited agreement between the data at 90 K and the best fit suggests that the structure with a three times larger c lattice parameter with respect to the CeCoAl type can only be regarded as an approximation to the real crystal structure of CeRuSn at low temperatures.
A metastable polymorph of vanadium sesquioxide was prepared by the reaction of vanadium trifluoride with a water-saturated gaseous mixture of 10 vol % hydrogen in argon. The new polymorph ...crystallizes in the bixbyite-type structure. At temperatures around 823 K a transformation to the well-known corundum-type phase is observed. Quantum-chemical calculations show that the bixbyite-type structure is about 9 kJ/mol less stable than the known corundum-based one. This result, in combination with the absence of imaginary modes in the phonon density of states, supports the classification of the bixbyite-type phase as a metastable V2O3 polymorph. At ∼50 K a paramagnetic to canted antiferromagnetic transition is detected.
PDF
