Florent Boucher

α- to β-(dmes)BiI5 (dmes = Dimethyl(2-ethylammonium)sulfonium Dication): Umbrella Reversal of Sulfonium in the Solid State and Short I···I Interchain Contacts—Crystal Structures, Optical Properties, and Theoretical Investigations of 1D Iodobismuthates

Syntheses, X-ray structural characterization, optical properties, and electronic structures of 1D metal(III) iodide hybrids, namely, alpha-((CH(3))(2)S(CH(2))(2)NH(3))BiI(5) (1a), beta-((CH(3))(2)S(CH(2))(2)NH(3))BiI(5) (1b), ((CH(3))(2)S(CH(2))(2)NH(3))SbI(5) (2), and (HO(2)C(C(6)H(4))CH(2)NH(3))BiI(4) (3), are reported. According to the results of single-crystal X-ray diffraction analyses, the 1D inorganic chains are constructed by corner-shared M(III)I(6) octahedra in 1a, 1b, and 2 and by edge-shared ones in 3. In polymorphs 1a, 1b, and 2, the polymeric BiI(5)(2-) anionic chains are charge-balanced by the dimethyl(2-ethylammonium)sulfonium (dmes) dications. Complex 1a crystallizes in the polar space group of P2(1)cn. A spectacular umbrella reversal of half sulfonium parts together with the conformational change of half polymeric anions in the crystal structure of 1a occurs at moderate temperature (73 degrees C), leading to the beta-phase 1b, through a reversible single-crystal-to-single-crystal process. Complex 1b, as well as the isotype structure of 2, crystallize in the nonpolar acentric space group of P2(1)2(1)2(1). Because of their acentric structural characteristic, second harmonic generation (SHG) optical properties are observed in the polycrystalline powder samples of 1a, 1b, and 2. It is notable that the SHG signal of 1a is much stronger than that of 1b and 2 owing to the polarity of 1a. Remarkably, the peculiar dissymmetrical dication of dmes is able to modify the bonding features of the inorganic frameworks through shortening I...I distances between adjacent chains (d(I...I) < 4A). The structure of 3, which crystallizes in the triclinic space group P1, features a polymeric anionic chain constructed from edge-shared BiI(6) octahedra. The charge is balanced by the pairs of trans-4-(ammoniummethyl)-cyclohexane-carboxylic acid, which are linked together via the H bonding between the carboxylic groups to form a pseudodication. The results of DFT calculations based on the structures of 1a and 3 indicate that the narrower band gap in 1 appears to be associated on the one hand with a sigma* I-p/Bi-s interaction that moves the Fermi level to higher energy and on the other hand with the interchain I...I contacts.

Elucidation of the Na2/3FePO4 and Li2/3FePO4 Intermediate Superstructure Revealing a Pseudouniform Ordering in 2D

Based on TEM, synchrotron X-ray diffraction, DFT calculations, and Mössbauer spectroscopy, a unified understanding of the Na and Li intercalation process in FePO4 is proposed. The key to this lies in solving the highly sought-after intermediate A(2/3)FePO4 (A = Na, Li) superstructures that are characterized by alkali ions as well as Fe(II)/Fe(III) charge orderings in a monoclinic three-fold supercell. Formation energies and electrochemical potential calculations confirm that Na(2/3)FePO4 and Li(2/3)FePO4 are stable and metastable, respectively, and that they yield insertion potentials in fair agreement with experimental values. The 2/3 Na(Li) and 1/3 vacancy sublattice of the intermediate phases forms a dense (101)(Pnma) plane in which the atom/vacancy ordering is very similar to that predicted for the most uniform distribution of 1/3 of vacancies in a 2D square lattice. Structural analysis strongly suggests that the key role of this dense plane is to constrain the intercalation in the diffusion channels to operate by cooperative filling of (bc)(Pnma). From a practical point of view, this generalized mechanism highlights the fact that an interesting strategy for obtaining high-rate FePO4 materials would consist in designing grains with an enhanced (101) surface area, thereby offering potential for substantial improvements with respect to the performance of rechargeable Li and Na batteries.

Origin of the 3.6 V to 3.9 V voltage increase in the LiFeSO4F cathodes for Li-ion batteries

Recently, the LiFeSO4F material has been reported as the highest potential Fe-based cathode material for Li-ion batteries. Its working voltage vs. Li+/Li0 jumps from 3.6 V to 3.9 V when LiFeSO4F is synthesized with the fully ordered tavorite structure and the fully disordered triplite structure, respectively. The present study aims at rationalizing this voltage increase by means of DFT + U calculations combined with crystallographic and electrostatic analyses. We show that the triplite polymorph, although characterized by two distinct edge-shared crystallographic sites, locally exhibits corner-sharing connections between consecutive FeO4F2 octahedra, exactly as in the tavorite polymorph. As a consequence, edge-sharing connections exist in the lithiated triplite structure between consecutive FeO4F2 and LiO4F2 polyhedra. We then demonstrate that the origin of the voltage increase lies in the difference in the anionic networks of the two polymorphs, and more specifically in the electrostatic repulsions induced by the configuration of the fluorine atoms around the transition metal in the two polymorphs (trans- vs. cis-configurations in tavorite vs. triplite polymorphs). Such a finding should help in the design of novel high potential fluorosulphate materials, which beyond enhanced performances present sustainability attributes as they can be made from abundant elements and via low temperature eco-efficient processes.

Structures and phase transitions of the A7PSe6 (A = Ag, Cu) argyrodite-type ionic conductors. III. α-Cu7PSe6

The crystal structure of the third polymorph of the Cu(7)PSe(6) argyrodite compound, alpha-Cu(7)PSe(6), heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-temperature form, i.e. below the last phase transition, alpha-Cu(7)PSe(6) crystallizes in orthorhombic symmetry, space group Pna2(1), with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) A, V = 1028.590 (9) A(3) (deduced from powder data, T = 173 K) and Z = 4. Taking into account a twinning by reticular merohedry, the refinement of the alpha-Cu(7)PSe(6) structure leads to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu(+)d(10) element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu(7)PSe(6), to be compared with only one for Ag(7)PSe(6), is attributed to the d(10) element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.

Fast determination of phases in LixFePO4 using low losses in electron energy-loss spectroscopy

Experimental valence electron energy-loss spectra, obtained on different phases of LixFePO4, are analyzed with first principles calculations based on density functional theory. In the 4–7 eV range, a large peak is identified in the FePO4 spectrum but is absent in LiFePO4, which allows the easy formation of energy filtered images. The intensity of this peak, nonsensitive to the precise orientation of the crystal, is large enough to rapidly determine existing phases in the sample and permit future dynamical studies.

DFT-NMR Investigation and 51V 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]·4H2O

This work shows that the combination of first-principles calculations and (51)V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs(4)[H(2)V(10)O(28)].4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs(4)[H(2)V(10)O(28)].4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature (1)H MAS NMR spectra. First-principles calculations of (51)V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five (51)V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the (51)V quadrupolar parameters. These results are fully consistent with (51)V 3QMAS NMR spectra recorded with and without (1)H decoupling. Finally, correlations are established between local octahedral VO(6) deformations and (51)V NMR parameters (C(q) and Deltadelta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

Strong anisotropic influence of local-field effects on the dielectric response ofα-MoO3

Dielectric properties of \ensuremath{\alpha}-MoO

Density functional theory investigation of 3d transition metal NMR shielding tensors in diamagnetic systems using the gauge-including projector augmented-wave method

{}_{3}

Synthesis and structure resolution of RbLaF4.

are investigated by a combination of valence electron-energy-loss spectroscopy and ab initio calculation at the random-phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO

Quantitative use of electron energy-loss spectroscopy Mo-M2,3 edges for the study of molybdenum oxides.

{}_{3}