Nicolas Barrier

Remarkable thermal stability of Eu(4-phosphonobenzoate): structure investigations and luminescence properties

A new 3D rare-earth hybrid material Eu(p-O(3)PC(6)H(4)COO) has been synthesised by a hydrothermal route from Eu(NO(3))(3) x 5 H(2)O and the rigid precursor, 4-phosphonobenzoic acid. The structure of Eu(p-O(3)PC(6)H(4)COO) has been solved by X-ray diffraction on a powder sample and is described as an inorganic network in which both carboxylic and phosphonic acid groups are linked to Eu ions forming a three-dimensional architecture. Thermal analysis performed on this compound has underlined its remarkable stability up to 510 degrees C and an optical study has been conducted to examine its luminescence properties that have been related to the structure of the material. The structural and luminescence properties have also been compared with the related material Eu phenylphosphonate.

Precession electron diffraction tomography for solving complex modulated structures: the case of Bi5Nb3O15.

The crystal structure of the 1D incommensurately modulated phase Bi5Nb3O15 [superspace group X2mb(0b0)000, a = 5.46781(7) Å, b = 5.47381(8) Å, c = 41.9005(5) Å, and q = 0.17588(8)b*] is solved by electron diffraction using a tomography technique combined with precession of the electron beam. The (3 + 1)D structure is further validated by a refinement against powder X-ray diffraction (PXRD). A coherent picture of the true nature of this compound is obtained, conciliating experimental observations made by different groups using transmission electron microscopy and PXRD. Bi5Nb3O15 does not have a mixed-layer Aurivillius-type structure but does contain structural elements, [Bi2O2](2+) slabs, and perovskite-like blocks, characteristic of Aurivillius phases. The presence of aperiodic crystallographic shear planes (CSPs) along the modulated direction b leads to the formation of an original layered structure containing both continuous and discontinuous [Bi2O2](2+) and perovskite-like octahedral layers. Between CSPs, the stacking of these two structural elements exhibits an unprecedented nonuniform sequence referring to Aurivillius phases.

CaMn4O8, a mixed valence manganite with an original tunnel structure

A new mixed valent manganite CaMn4O8 with an original tunnel structure has been synthesized. The host lattice of this oxide has been determined by X-ray powder diffraction, using ab initio methods, and by high resolution electron microscopy. It consists of single and triple chains of edge-sharing MnO6 octahedra, interconnected through their apices. The [Mn4O8]∞ framework delimits three types of tunnels: six-sided (“H”) and eight-sided (“8”) where sit the Ca2+ cations, and empty rutile-like tunnels (“R”). The analysis of the Mn–O inter-atomic distances shows that this structure could also be described from the association of only single chains of edge-sharing MnO6 octahedra, with single chains of edge-sharing MnO5 pyramids, and evidences a high tendency of manganese to charge ordering. Nanostructural mechanisms investigated by electron microscopy also allow the structure to be described in terms of single bricks of four Mn polyhedra, and evidence twinning and intergrowth phenomena as well as local ordering of calcium in the tunnels. The magnetic measurements show strong antiferromagnetic fluctuations at low temperature, whereas the low value of TN ∼ 90 K indicates that the antiferromagnetic superexchange is weak in this oxide.

The dehydration of SrTeO3(H2O) - a topotactic reaction for preparation of the new metastable strontium oxotellurate(IV) phase ε-SrTeO3

Microcrystalline single-phase strontium oxotellurate(IV) monohydrate, SrTeO(3)(H(2)O), was obtained by microwave-assisted hydrothermal synthesis under alkaline conditions at 180 °C for 30 min. A temperature of 220 °C and longer reaction times led to single crystal growth of this material. The crystal structure of SrTeO(3)(H(2)O) was determined from single crystal X-ray diffraction data: P2(1)/c, Z = 4, a = 7.7669(5), b = 7.1739(4), c = 8.3311(5) Å, β = 107.210(1)°, V = 443.42(5) Å(3), 1403 structure factors, 63 parameters, R[F(2)>2σ(F(2))] = 0.0208, wR(F(2) all) = 0.0516, S = 1.031. SrTeO(3)(H(2)O) is isotypic with the homologous BaTeO(3)(H(2)O) and is characterised by a layered assembly parallel to (100) of edge-sharing [SrO(6)(H(2)O)] polyhedra capped on each side of the layer by trigonal-prismatic [TeO(3)] units. The cohesion of the structure is accomplished by moderate O-H···O hydrogen bonding interactions between donor water molecules and acceptor O atoms of adjacent layers. In a topochemical reaction, SrTeO(3)(H(2)O) condensates above 150 °C to the metastable phase ε-SrTeO(3) and transforms upon further heating to δ-SrTeO(3). The crystal structure of ε-SrTeO(3), the fifth known polymorph of this composition, was determined from combined electron microscopy and laboratory X-ray powder diffraction studies: P2(1)/c, Z = 4, a = 6.7759(1), b = 7.2188(1), c = 8.6773(2) Å, β = 126.4980(7)°, V = 341.20(18) Å(3), R(Fobs) = 0.0166, R(Bobs) = 0.0318, Rwp = 0.0733, Goof = 1.38. The structure of ε-SrTeO(3) shows the same basic set-up as SrTeO(3)(H(2)O), but the layered arrangement of the hydrous phase transforms into a framework structure after elimination of water. The structural studies of SrTeO(3)(H(2)O) and ε-SrTeO(3) are complemented by thermal analysis and vibrational spectroscopic measurements.

Design of salt–metal organic framework composites for seasonal heat storage applications

Porous materials are recognized as very promising materials for water-sorption-based energy storage and transformation. This study presents the first attempt to use Metal Organic Frameworks (MOFs) as host matrices of salts for the preparation of composite sorbents for seasonal heat storage. We have considered six water stable MOFs (i.e. MIL-127(Fe), MIL-100(Fe), MIL-101(Cr), UiO-66(Zr)–NH2, MIL-125(Ti)–NH2 and MIL-160(Al)) differing in their crystalline structure, hydrophilic–hydrophobic balance, pore size/shape and pore volume. The successful encapsulation of CaCl2 in the pores of MOFs leads to two series of MOFs–CaCl2 composites whose salt content could be finely tuned depending on the pore volume of MOFs and the synthesis conditions. These materials were fully characterized by combining multiple techniques (i.e. powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, X-ray energy-dispersive spectrometry elemental mapping, N2 sorption and elemental analysis). The water sorption properties of these composites were studied under conditions of a solar heat storage system (i.e. adsorption at 30 °C, desorption at 80 °C, both steps at a water vapour pressure of 12.5 mbar) in comparison to the parent MOFs. We analyze how the physico-chemical and structural properties of these host matrices impact the energy density of composite sorbents. We show that two mesoporous MOFs–CaCl2 composites (i.e. MIL-100(Fe)/CaCl2 and MIL-101(Cr)/CaCl2) with the highest salt loading (46 and 62 wt% respectively) exhibit very high energy storage capacities (up to 310 kW h m−3 (485 W h kg−1)) outperforming the best composites or physical sorbents reported so far together with very little loss upon adsorption–desorption cycling and high chemical stability upon ageing (up to 18 months).

Hydrothermal Synthesis and Dehydration of CaTeO3(H2O): An Original Route to Generate New CaTeO3 Polymorphs

CaTeO3(H2O) was obtained from microwave-assisted hydrothermal synthesis as a polycrystalline sample material. The dehydration reaction was followed by thermal analysis (thermogravimetric/differential scanning calorimetry) and temperature-dependent powder X-ray diffraction and leads to a new δ-CaTeO3 polymorph. The crystal structures of CaTeO3(H2O) and δ-CaTeO3 were solved ab initio from PXRD data. CaTeO3(H2O) is non-centrosymmetric: P21cn; Z = 8; a = 14.785 49(4) Å; b = 6.791 94(3) Å; c = 8.062 62(3) Å. This layered structure is related to the ones of MTeO3(H2O) (M = Sr, Ba) with layers built of edge-sharing [CaO6(H2O)] polyhedra and are capped of each side by [Te(IV)O3E] units. Adjacent layers are stacked along the a-axis and are held together by H-bonds via the water molecules. The dehydration reaction starts above 120 °C. The transformation of CaTeO3(H2O) into δ-CaTeO3 (P21ca; Z = 8; a = 13.3647(6) Å; b = 6.5330(3) Å; c = 8.1896(3) Å) results from topotactic process with layer condensation along the a-axis and the 1/2b⃗ translation of intermediate layers. Thus, δ-CaTeO3 stays non-centrosymmetric. The characteristic layers of CaTeO3(H2O) are also maintained in δ-CaTeO3 but held together via van der Waals bonds instead of H-bonds through water molecules. Electron localization function and dipole moment calculations were also performed. For both structures and over each unit cell, the dipole moments are aligned antiparallel with net dipole moments of 3.94 and 0.47 D for CaTeO3(H2O) and δ-CaTeO3, respectively. The temperature-resolved second harmonic generation (TR-SHG) measurements, between 30 and 400 °C, show the decreasing of the SHG intensity response from 0.39 to 0.06 × quartz for CaTeO3(H2O) and δ-CaTeO3, respectively.

Synthesis, crystal and electronic structures, and physical properties of the novel compounds LaR4Mo36O52 (R = Dy, Er, Yb, and Y) containing infinite chains of trans-edge-shared Mo6 octahedra and Mo2 pairs and rectangular Mo4 clusters with triple Mo-Mo bonds.

The novel quaternary reduced molybdenum oxides LaR(4)Mo(36)O(52) (R = Dy, Er, Yb, and Y) have been synthesized with solid-state reactions at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of LaEr(4)Mo(36)O(52) by X-ray diffraction. LaEr(4)Mo(36)O(52) crystallizes in the tetragonal space group I4 with two formula units per cell and the following lattice parameters: a = 19.8348(2) and c = 5.6594(1) A. The Mo network is dominated by infinite chains of trans-edge-shared Mo(6) octahedra, which coexist with Mo(2) pairs and rectangular Mo(4) clusters. The Mo-Mo distances within the infinite chains range from 2.5967(7) to 2.8529(8) A and from 2.239(3) to 2.667(2) A in the Mo(2) pairs and rectangular Mo(4) clusters, respectively. The Mo-O distances are comprised between 1.993(7) and 2.149(7) A, as usually observed in these types of compound. The La(3+) and Er(3+) ions are in a square-prismatic [LaO(8)] and a tricapped trigonal-prismatic [ErO(9)] environment of oxygen atoms, respectively. The La-O distances range from 2.555(6) to 2.719(6) A and the Er-O ones from 2.260(6) to 2.469(5) A. Theoretical calculations allow the determination of the optimal electron count of both motifs in the title compound. Weak interactions occur between neighboring dimetallic and tetrametallic clusters and between trans-edge-sharing infinite chains and dimers and tetramers. The presence of rectangular clusters is favored on the basis of theoretical considerations. Single-crystal resistivity measurements show that LaEr(4)Mo(36)O(52) is metallic between 4.2 and 300 K, in agreement with the band structure calculations. Magnetic susceptibility measurements indicate that the oxidation state of the magnetic rare earths is +3, and there is an absence of localized moments on the Mo network.

Rare earth site splitting in Sm3MoO7

Trisamarium molybdenum heptaoxide, Sm(3)MoO(7), is isomorphous with Ln(3)MoO(7) (Ln = La and Pr). The crystal structure consists of chains of corner-linked MoO(6) octahedra running parallel to the b axis and separated from each other by seven- or eight-coordinate Sm-O polyhedra. In contrast to La(3)MoO(7) and Pr(3)MoO(7), a splitting of one Sm site into two positions is observed.

Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light

Sub-nanometer silver clusters that exhibit discrete electronic structure with molecular-like properties are highly desirable in various technologies. However, the methods for their preparation suffer from limitations related with the reproducibility and particles uniformity and/or the possibility of the scale-up. Another critical drawback is that free sub-nanometer silver clusters tend to aggregate into larger particles. In this work, a new approach that successfully overcomes the above limitations is developed. It allows, for the first time, an ultrafast preparation of sub-nanometer silver particles with high abundance, uniformity (7 Å), and stability into the cages of nanosized zeolite crystals. The new method consists of UV excitation of a water suspension of nanozeolite containing photoactive vanadate clusters in the presence of ethanol (as an electron donor) and silver precursor. The characteristic features of sub-nanometer silver particles are presented, and the mechanism of their formation is discussed. Sub-nanometer Ag clusters exhibit exceptional photocatalytic activity and selectivity in the reforming of formic acid to H2 and CO2 under visible light.

Mn2TeO6: A Distorted Inverse Trirutile Structure

Inverse trirutile Mn2TeO6 was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P42/mnm) to a monoclinic phase (P21/c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO6 octahedra alternate with more regular TeO6 octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn-Teller origin, is driven by a single primary mode.