Emmanuelle Suard

Crystal structure of the new FeSe1−x superconductor

The newly discovered superconductor FeSe(1-x) (x approximately 0.08, T(c)(onset) approximately 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure-the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.

Evidence for structural defects in non-stoichiometric HT-LiCoO2 : electrochemical, electronic properties and 7Li NMR studies

HT-Lix0CoO2 phases were synthesised via a solid state reaction with nominal Li/Co ratios x0=0.90, 1.0, 1.05 and 1.10. These materials were studied by X-ray and neutron diffraction, galvanostatic lithium intercalation/deintercalation and electrical measurements as well as 7Li MAS NMR spectroscopy. While the galvanostatic charge curve of the stoichiometric material shows a voltage plateau at ca. 3.93 V for 0.75≤x≤0.94 and a particular feature for Li0.5CoO2 (x0=1.0) due to a monoclinic distortion, the charge curves of materials with x0=1.05 and 1.10 do not present the voltage plateau nor the monoclinic distortion. XRD studies show the occurrence of a solid solution in the 0.60≤x≤1.10 domain. 7Li MAS NMR spectra of Lix0CoO2 materials with x0>1 exhibit three new signals at 190, −18 and −40 ppm in addition to the signal at 0 ppm resulting from the presence of diamagnetic CoIII. A 7Li MAS NMR study of the materials obtained by electrochemical deintercalation and reintercalation of lithium from Li1.10CoO2 shows that these new features do not arise from lithiated surface species or from interstitial Li+. On the basis of our general knowledge of Li NMR in comparable materials with electron spins, we suggest the presence of paramagnetic low-spin CoII and of a simultaneous cobalt and oxygen deficiency in samples with x0>1.0.

On the structure of Li3Ti2(PO4)3

The structure of the Nasicon-type phase Li3Ti2(PO4)3, obtained by chemical lithiation of LiTi2(PO4)3, has been characterised using neutron diffraction for the long range structure and 7Li NMR for more local information. The lithium atoms were precisely located from the neutron diffraction data, using nuclear difference Fourier maps. The lithium ions, which were known to be in the large M2 cavity, occupy two M3 and M′3 subsites (distorted tetrahedra) with occupation factors of 2/3 and 1/3, respectively. From these two intermediate sites, it was shown that the diffusion pathway between two M1 sites in these Nasicon-type structures consists of a set of seven face-sharing tetrahedra. A variable temperature 7Li MAS NMR study showed a set of signals due to a distribution of environments for a given Li+ ion, in terms of occupied or vacant M3/M′3 sites in its vicinity. Elevation of the temperature to 353 K leads to a reversible exchange of these signals, due to fast hopping of Li between the two sites within a given M2 cavity.

Negative linear compressibility and massive anisotropic thermal expansion in methanol monohydrate

At low temperatures, a simple molecular crystal can shrink along one axis when heated and expand along it when compressed. The vast majority of materials shrink in all directions when hydrostatically compressed; exceptions include certain metallic or polymer foam structures, which may exhibit negative linear compressibility (NLC) (that is, they expand in one or more directions under hydrostatic compression). Materials that exhibit this property at the molecular level—crystalline solids with intrinsic NLC—are extremely uncommon. With the use of neutron powder diffraction, we have discovered and characterized both NLC and extremely anisotropic thermal expansion, including negative thermal expansion (NTE) along the NLC axis, in a simple molecular crystal (the deuterated 1:1 compound of methanol and water). Apically linked rhombuses, which are formed by the bridging of hydroxyl-water chains with methyl groups, extend along the axis of NLC/NTE and lead to the observed behavior.

On the metastable O2-type LiCoO2

Abstract The exchange of sodium for lithium in P2-Na0.70CoO2 leads to an unusual O2-LiCoO2 variety. Rietveld refinement of the neutron diffraction pattern of the O2-LiCoO2 phase confirms the stoichiometry and the structural parameters previously reported and shows that the strong Li+–Co3+ electrostatic repulsion occurring through the common face of the CoO6 and LiO6 octahedra leads to small displacements of the lithium and cobalt ions from the center of their octahedra. Magnetic measurements and 7Li MAS NMR confirm a composition very close to the ideal one for the O2 phase. The O2-LiCoO2 phase is metastable and transforms to well-crystallized O3-LiCoO2 upon heating. Even at intermediate temperatures, the formation of LT-LiCoO2 phase is never observed. Various morphologies are obtained for the O3-LiCoO2 phases, depending on the thermal treatment used. A preliminary electrochemical study shows that almost all the lithium ions can be reversibly deintercalated from the O2-LiCoO2 phase.

Effect of Y-Ca substitution upon superconductivity in the oxide YBa2Cu3-xCoxO7-δ

Abstract The coupled substitution Y-Ca / Cu-Co in YBa 2 Cu 3 O 7 has been studied. The oxides Y 1 - y Ca y Ba 2 Cu 3 - x Co x O 7 - δ have been synthesized for 0≤ x ≤0.45 and 0≤ y ≤0.30. They were found tetragonal for x ≥0.09. The oxygen stoichiometry was determined by TGA showing δ values ranging from 6.78 to 7.08. The study of the superconducting properties of these phases shows that for a fixed cobalt content T c increases dramatically as the calcium content increases, leading for instance for the oxide Y 0.64 Ca 0.36 Ba 2 Cu 2.64 Co 0.36 O 6.87 to a T c of 82 K, whereas the non-substituted phase YBa 2 Cu 2.64 Co 0.36 O 7 - δ does not superconduct. As a result, the “compensated oxide” Y 1 - x Ca x Ba 2 Cu 3 - x Co x O 7 - δ , exhibits an almost x -independent T c ranging from 86 to 82 K for 0.15≤ x ≤0.36. A correlation is observed between the T c value and the c / a ratio. This behaviour is interpreted in terms of mobile holes concentration related to bond length changes induced by calcium, assuming that the cobalt is trivalent and sits in Cu 1 sites in agreement with previous neutron diffraction studies of YBa 2 Cu 3 - x Co x O 7 - δ .

Topotactic reduction of YBaCo2O5 and LaBaCo2O5: square-planar Co(I) in an extended oxide.

The low-temperature reduction of YBaCo(2)O(5) and LaBaCo(2)O(5) with NaH to form YBaCo(2)O(4.5) and YBaCo(2)O(4.25), respectively, demonstrates that the structures of anion-deficient materials formed by such topotactic reductions can be directed by the ordering and identity of the A-site cations. YBaCo(2)O(4.5) adopts a structure consisting of a corner-shared network of square-based pyramidal CoO(5) and distorted tetrahedral CoO(4) units. The structure of LaBaCoO(4.25) is more complex, consisting of an array of square-based pyramidal CoO(5), distorted tetrahedral CoO(4), and square planar CoO(4) units. Magnetic susceptibility and variable-temperature neutron diffraction data reveal that YBaCo(2)O(4.5) adopts a G-type antiferromagnetically ordered structure below T(N) approximately 280 K. LaBaCo(2)O(4.25) also adopts antiferromagnetic order (T(N) approximately 325 K) with ordered moments consistent with the presence of square-planar, low-spin, s = 0, Co(I) centers. A detailed analysis reveals that the different anion vacancy ordered structures adopted by the two REBaCo(2)O(5-x) phases are directed by the relative sizes and ordering of the La(3+) and Y(3+) cations. This suggests that ordered arrangements of A-cations can be used to direct the anion vacancy order in topotactically reduced phases, allowing the preparation of novel metal-oxygen networks containing unusual transition metal coordination environments.

Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li7La3Zr2O12 Solid Electrolytes

Several “Beyond Li-Ion Battery” concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10–4 S cm–1 to 1.2 × 10–3 S cm–1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure–properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

How to induce red persistent luminescence in biocompatible Ca3(PO4)2

Highly biocompatible β-TriCalcium Phosphate (β-TCP) β-Ca3(PO4)2 was tailored to emit intense red persistent luminescence suitable for small animal in vivo imaging. Mn2+ substituting the octahedral Ca5 site was selected as the luminescent dopant showing 4T1 (4G) → 6A1 (6S) emission at ∼660 nm. TCP:Mn2+ annealed in Ar–H2 showed only low temperature thermally stimulated luminescence (TSL) and therefore persistent luminescence at room temperature (RT) was weak. Ln3+ (Ln = Tb, Dy) co-doping strongly enhanced Mn2+ X-ray excited optical luminescence while totally annihilating TSL. A subsequent Ar–H2 annealing of (Ln3+, Mn2+) codoped TCPs restored intense TSL. The TSL mechanism was described in terms of deep hole trapping at Mn2+ and shallow electron trapping at defects created by reduction, i.e. most probably diphosphates. While Tb3+ co-doping promoted below RT TSL peaks, Dy3+ co-doping induced intense TSL between 320 and 450 K. TCP:Mn2+,Dy3+ annealed in Ar–H2 therefore showed highly enhanced X-ray excited persistent luminescence relative to the silicate reference used for in vivo imaging.

Neutron diffraction structure determination of the “1212”-series TlBa2Ca1−xNdxCu2O7−δ

Abstract The structure of three oxides of the 1212 type series, TlBa 2 Ca 1−x Nd x Cu 2 O 7−δ , has been determined by neutron diffraction for x =0.2, 0.5 and 1. It is found that the two superconductive oxides x =0.2 ( T c =100 K ) and x =0.5 ( T c =40 K ) exhibit a significant oxygen deficiency in the [TlO] ∞ layers, δ ≈0.14, whereas the non-superconducting phase is almost stoichiometric δ ≈0. The [TlO]∞ layers are characterized by a disordering of the Tl and O (3) atoms, suggesting a distorted tetrahedral coordination of thallium. The Cu-O equatorial distances of the CuO 5 pyramids increase as x increases, whereas the apical distance decreases. This structural evolution is compared to that of other layered cuprates involving similar double pyramidal copper layers and the relationships with superconductivity are discussed.