Pore size distribution and supercritical hydrogen adsorption in activated carbon fibers

Pore size distributions (PSD) and supercritical H_2 isotherms have been measured for two activated carbon fiber (ACF) samples. The surface area and the PSD both depend on the degree of activation to which the ACF has been exposed. The low-surface-area ACF has a narrow PSD centered at 0.5 nm, while the high-surface-area ACF has a broad distribution of pore widths between 0.5 and 2 nm. The H_2 adsorption enthalpy in the zero-coverage limit depends on the relative abundance of the smallest pores relative to the larger pores. Measurements of the H_2 isosteric adsorption enthalpy indicate the presence of energy heterogeneity in both ACF samples. Additional measurements on a microporous, coconut-derived activated carbon are presented for reference

LiSc(BH_4)_4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH_4)_4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH_4 and ScCl_3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH_4)_4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH_4)_3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH_4)_4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB_2 and [B_(12)H_(12)]^(−2) anion containing (e.g., Li_2B_(12)H_(12)) along with residual LiBH_4. Reaction of a desorbed LiSc(BH_4)_4 + 4LiCl mixture (from 4LiBH_4/ScCl_3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li_2B_(12)H_(12) and LiBH_4. Full reversibility to reform the LiSc(BH_4)_4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications

Design of a new family of inorganic compounds Ae(2)F(2)SnX(3) (Ae = Sr, Ba; x = S, Se) using rock salt and fluorite 2D building blocks

International audienceWe could predict the structure of a new family of compounds Ae2F2SnX3 (Ae ) Sr, Ba; X ) S, Se) from the stacking of known 2D building blocks of the rock salt and fluorite types. With a high-temperature ceramic method we have then succeeded to synthesize the four compounds Ba2F2SnS3, Ba2F2SnSe3, Sr2F2SnS3, and Sr2F2SnSe3. The structure refinements from X-ray powder diffraction patterns have confirmed the structure predictions and showed their good accuracy. The structure of the four compounds results from the alternated stacking of fluorite [Ae2F2] (Ae ) Sr, Ba) and distorted rock salt [SnX3] (X ) S, Se) 2D building blocks. As shown by band structure calculations, these blocks behave as a charge reservoir and a charge acceptor, respectively. Sr2F2SnS3 and Ba2F2- SnS3 are transparent with optical gaps of 3.06 and 3.21 eV, respectively. However, an attempt to obtain a transparent conductor by substituting Ba per La in Ba2F2SnS3 was unsuccessfu

Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba ₇S(VS ₃O) ₂(S ₂) ₃ and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study.

International audienceOxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q–Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba–V–Q–O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba7V2O2S13, which can be written as Ba7S(VS3O)2(S2)3, was substituted to yield three selenide derivatives Ba7V2O2S9.304Se3.696, Ba7V2O2S7.15Se5.85, and Ba7V2O2S6.85Se6.15. They represent original multiple-anion lattices and first members in the system Ba–V–Se–S–O. They exhibit in the first layer heteroleptic tetrahedra V5+S3O and isolated Q2– anions and in the second layer dichalcogenide pairs (Q2)2– with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q2– or (Q2)2– (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO3S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V5+, together with the presence/nature of the dichalcogenides (Q2)2– and isolated Q2–, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry

Common Building Motifs in Ba2Fe3(PO4)4·2H2O, BaFe3(PO4)3, and Na3Fe3(PO4)4 Labile Fe2+/Fe3+ Ordering and Charge-Dependent Magnetism

Two new mixed-valence Fe2/3+ barium phosphates have been synthesized in hydrothermal conditions and characterized Ba2Fe2.66+3(PO4)4·2H2O (compound 1, ratio Fe3+/Fe2+ = 21, orthorhombic space group Pbca, a = 6.71240(10) Å, b = 10.6077(2) Å, c = 20.9975(5) Å, R1 = 3.39%) and BaFe2.33+3(PO4)3 (compound 2, ratio Fe3+/Fe2+ = 12, orthorhombic, space group Imma with a = 10.5236(3) Å, b = 13.4454(4) Å, c = 6.6411(2) Å, R1 = 1.63%). 1 has a two-dimensional crystal structure built of [Fe2.5+2Fe3+1(PO4)4]4- layers with charge segregation on two individual Fe crystal sites, in contrast to the single valence on these two sites found in similar layers of Na3Fe3+3(PO4)4. The crystal structure of 2 is formed of the same layers but condensed into a 3D [Fe2+2Fe3+1(PO4)3]2- framework. The complete Fe2+ vs Fe3+ charge ordering on the two available sites differs from what was found in the two previous cases and denotes a remarkable charge adaptability of the common elementary units. Compared to the antiferromagnetic Na3Fe3+3(PO4)4 the partial iron reduction into Fe2+ is responsible for strong ferromagnetic components along the c-easy axis for both 1 and 2. Additionally 1 shows multiple magnetization steps in the perpendicular direction, giving raise to atypical anisotropic magnetism into a complex magnetic phase diagram. © 2016 American Chemical Society

Common Building Motifs in Ba2Fe3(PO4)4·2H2O, BaFe3(PO4)3, and Na3Fe3(PO4)4 Labile Fe2+/Fe3+ Ordering and Charge-Dependent Magnetism

International audienceTwo new mixed-valence Fe2/3+ barium phosphates have been synthesized in hydrothermal conditions and characterized Ba2Fe2.66+3(PO4)4·2H2O (compound 1, ratio Fe3+/Fe2+ = 21, orthorhombic space group Pbca, a = 6.71240(10) Å, b = 10.6077(2) Å, c = 20.9975(5) Å, R1 = 3.39%) and BaFe2.33+3(PO4)3 (compound 2, ratio Fe3+/Fe2+ = 12, orthorhombic, space group Imma with a = 10.5236(3) Å, b = 13.4454(4) Å, c = 6.6411(2) Å, R1 = 1.63%). 1 has a two-dimensional crystal structure built of [Fe2.5+2Fe3+1(PO4)4]4- layers with charge segregation on two individual Fe crystal sites, in contrast to the single valence on these two sites found in similar layers of Na3Fe3+3(PO4)4. The crystal structure of 2 is formed of the same layers but condensed into a 3D [Fe2+2Fe3+1(PO4)3]2- framework. The complete Fe2+ vs Fe3+ charge ordering on the two available sites differs from what was found in the two previous cases and denotes a remarkable charge adaptability of the common elementary units. Compared to the antiferromagnetic Na3Fe3+3(PO4)4 the partial iron reduction into Fe2+ is responsible for strong ferromagnetic components along the c-easy axis for both 1 and 2. Additionally 1 shows multiple magnetization steps in the perpendicular direction, giving raise to atypical anisotropic magnetism into a complex magnetic phase diagram. © 2016 American Chemical Society

Across the structural re-entrant transition in BaFe₂(PO₄)₂: influence of the two-dimensional ferromagnetism

BaFe2(PO4)2 was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO6 octahedra containing high-spin Fe2+ ions (S = 2) are isolated by PO4 groups and Ba2+ cations. BaFe2(PO4)2 has a trigonal R-3 structure at room temperature but adopts a triclinic P-1 structure below 140 K due to the Jahn-Teller (JT) instability arising from the (t2g)4(eg)2 configuration. The triclinic crystal structure was refined to find significantly distorted Fe2+O6 octahedra in the honeycomb layers while the distortion amplitude QJT was estimated to 0.019 Å. The JT stabilization energy is estimated to be 7 meV per formula unit by DFT calculations. Below 70 K, very close to the ferromagnetic transition temperature Tc = 65.5 K, the structure of BaFe2(PO4)2 returns to a trigonal R-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe2+, as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above Tc , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe2(PO4)2 is uniaxial (g⊥ = 0)

Across the structural re-entrant transition in BaFe₂(PO₄)₂: influence of the two-dimensional ferromagnetism

BaFe2(PO4)2 was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO6 octahedra containing high-spin Fe2+ ions (S = 2) are isolated by PO4 groups and Ba2+ cations. BaFe2(PO4)2 has a trigonal R-3 structure at room temperature but adopts a triclinic P-1 structure below 140 K due to the Jahn-Teller (JT) instability arising from the (t2g)4(eg)2 configuration. The triclinic crystal structure was refined to find significantly distorted Fe2+O6 octahedra in the honeycomb layers while the distortion amplitude QJT was estimated to 0.019 Å. The JT stabilization energy is estimated to be 7 meV per formula unit by DFT calculations. Below 70 K, very close to the ferromagnetic transition temperature Tc = 65.5 K, the structure of BaFe2(PO4)2 returns to a trigonal R-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe2+, as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above Tc , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe2(PO4)2 is uniaxial (g⊥ = 0)

Triple Co-II,Co- (III,) (IV) charge ordering and spin states in modular cobaltites: a systematization through experimental and virtual compounds

International audienceThe series of modular compounds [BanCo2+nO3n+2][BaCo6O9] (n = 1 to 3) including experimental and hypothetical terms, was investigated using DFT calculations and several experimental results. A systematic evolution of the electronic and magnetic states was evidenced along the series leading to ordered Co-II/Co-III versus mixed Co-III/IV charge segregation in two distinct structural motifs. In essence, using different packing modes within the labile [BanCo2+nO3n+2] block, we have systematized the spin state dependence on the CoO6 connectivity, i.e. corner-sharing (HS states) against face-sharing (LS states). We also show that the electronic and magnetic features of the [BaCo6O9] blocks do not vary trough the series, (i.e. HS-Co-II and LS-Co-III charge ordering) whereas the [BanCo2+nO3n+2] blocks hold drastic changes from n = 1 to 3. In particular, the later carries a mixed III/IV cobalt charge for n >= 2. It leads to a triple valence cobalt state. For n = 2, we experimentally observe at 4 K a superstructure (2a, 2c) superstructure accompanied by a perfect Co-II/Co-III/Co-IV charge ordering. The charge ordering occurs at Tt = 160 K and is accompanied by a transition in the electronic transport leading to a 2D-VRH behaviour below Tt