Serge Vilminot

Influence of the sol-gel synthesis on the formation of spinel MgAl2O4

Spinel precursors have been obtained by sol-gel processing using either magnesium nitrate and chemically modified aluminum alkoxide or a double alkoxide as raw materials. The xerogels have been characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), infrared (IR) spectroscopy, and transmission electron microscopy (TEM). It is proposed that the size of the particles is related to the functionality of the aluminum alkoxide. The use of the double alkoxide allowed pure spinel nanosized materials to be obtained.

[Zn3(HCOO)6]: A Porous Diamond Framework Conformable to Guest Inclusion

We report the syntheses, crystal structures, and the thermal properties of [Zn3(HCOO)6](CH3OH)1.5(H2O)0.5 (1parent), [Zn3(HCOO)6] (2empty), and six guest-inclusion compounds [Zn3(HCOO)6](I2) (3iodine), [Zn3(HCOO)6](C4H8O) (4THF), [Zn3(HCOO)6](C4H4O) (5furan), [Zn3(HCOO)6](C6H6) (6benzene), [Zn3(HCOO)6](CH3CN) (7acetonitrile), and [Zn3(HCOO)6]((CH3)2CO) (8acetone) as well as the H2 and N2 adsorption of 2empty. The frameworks of all the compounds are similar, and consist of Zn-centred ZnZn4 tetrahedral nodes of a distorted diamond structure. It is robust by virtue of the diamond structure and it displays permanent porosity in which the pores, occupying about 30% of the volume, can be reversibly emptied and refilled with solvents and gases without loss of crystallinity. The crystal-to-crystal transformation is assured by performing the guest-inclusion process by exposure of 2empty to the vapours of the guests following the complete desolvation of 1parent. Conformity is evidenced by the change in the lattice parameters proportional to the size of the guests. The framework is thermally stable up to 140°C at which it is transformed exothermically to a more compact form, β-Zn(HCOO)2, that is stable to 270°C. Due to the flexibility and amphiphilic nature of the pores, consisting of both C–H and O arrays at the surface, 2empty can take up a wide spectrum of both polar and non-polar guests of different size. The guests are confined in zig-zag molecular arrays within the channels where weak hydrogen bonds provide the main host–guest interaction. Except for acetonitrile, which sits in the central part of the channels, all the guests appear to line the wall of the channels.

Structural characterisations of a lamellar organic–inorganic nickel silicate obtained by hydrothermal synthesis from nickel acetate and (aminopropyl)triethoxysilane

Lamellar organic–inorganic nickel silicates have been synthesised under hydrothermal conditions at 170 °C from a mixture of nickel acetate solution and (aminopropyl)triethoxysilane. The influence of the presence or absence of a mineralising agent such as HF or NH4F has been studied. Structural characterisations show that this material adopts a lamellar arrangement of nickel hydroxide layers separated by 21 A with, in the interspace, organically modified silicate entities linked to the inorganic layer. The hysteresis cycle and ac susceptibility have been carried out and reveal a paramagnetic–ferromagnetic transition below 20 K.

Synthetic transition metal phyllosilicates and organic-inorganic related phases

We present a brief overview of the first-row transition metal phyllosilicates and their organically functionalised analogues obtained under autogenous pressure conditions. Some structural features of phyllosilicates are first recalled and the magnetic properties of Co2+ and Ni2+ phyllosilicates are described. In the second part, the state-of-the-art in organically functionalised phyllosilicates is reported, including our contributions on the structural and magnetic characterisation. The impetus is to enhance the potential of these new compounds as multi-property materials. Since our interest lies in low-dimensional magnetic properties, divalent transition metal cations are involved in the layered inorganic layers, whereas 3-aminopropyltriethoxysilane is used as the silicon source.

Magnetic structure and properties of Cu3(OH)4SO4 made of triple chains of spins s=1/2

Cu3(OH)4SO4, obtained by hydrothermal synthesis from copper sulfate and soda in aqueous medium, is isostructural with the corresponding antlerite mineral, orthorhombic, space group Pnma (62), with a=8.289(1) b=6.079(1) and c=12.057(1) A ˚ , V=607.5(2) A ˚ 3 , Z=4. Its crystalline structure has been refined from X-ray single crystal and powder neutron diffraction data at room temperature. It consists of copper (II) triple chains, running in the b-axis direction and connected to each other by sulfate groups. The magnetic structure, solved from powder neutron diffraction data at 1.4 K below the transition at 5 K evidenced by susceptibility and specific measurements, reveals that, inside a triple chain, the magnetic moments of the copper ions (mB=0.88(5) at 1.4 K) belonging to outer chains are oriented along the c-axis of the nuclear cell, with ferromagnetic order inside a chain and antiferromagnetic order between the two outer chains. No long-range magnetic order is obtained along the central chain with an idle spin behavior. r 2002 Elsevier Science (USA). All rights reserved.

Evidence of Eu3+O2− associates by luminescence study of some silicates and aluminosilicates

Abstract Unusual luminescence spectra with a strong 5D0 arr 7F0 line, a high position of the 5D0 level and the occurrence of two lines close to 16550 cm−1, i.e. in a frequency domain at the border between 5D0 → 7F2 and 5D0 → 7F1 transitions, have been evidenced in the case of cordierite, Mg2Al4Si5O18:Eu, mullite, 2Al2O3.SiO2:Eu and lanthanum disilicate, La2Si2 O7:Eu. By comparison with results on apatites and some fluorides, these unusual spectra have been attributed to the presence of a strong and anisotropic crystal field due to an EuO bonding.

Magnetic Properties and Magnetic Structure of CuII3MoVI2O9

The magnetic properties and the low-temperature nuclear and magnetic structures of Cu(3)Mo(2)O(9) are reported. It consists of magnetic chains of oxygen-bridged vertex-shared Cu(4) tetrahedra connected into a 3D network by bridging MoO(4). Each chain is a segment of the pyrochlore structure. The magnetic properties are dominated by antiferromagnetic coupling and show a low-dimensional behavior with a broad maximum at 20 K. At 9.5 K, long-range antiferromagnetic ordering with a small canting is observed. The magnetic structure determination from neutron diffraction resolves only the orientations and magnitude (1.0(1) micro(B) per Cu) of the moments of two of the three crystallographically independent atoms. The third appears not to contribute to the long-range ordering. Geometric frustration may be responsible for the randomness. It is a unique antiferromagnet with eight sublattices (two sets of four) in the ac plane, leaving the canting to be only possible along the b axis.

Synthesis and characterization of YBa2Cu3O7−x. Superconducting materials☆

YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconducting materials have been obtained from oxalate coprecipitation and metalloorganic decomposition. Resistivity and magnetic measurements confirm the superconducting behavior below 90 K. Calorimetric measurements reveal the role of impurities in the specific heat evolution.

Magnetic Properties and Magnetic Structures of Synthetic Natrochalcites, NaMII2(D3O2)(MoO4)2, M = Co or Ni

The magnetic properties and magnetic structures from neutron diffraction of two synthetic natrochalcites, NaM(II)2(H3O2)(MoO4)2, M = Co (1Co) or Ni (2Ni), are reported. They are isostructural (monoclinic C2/m) and consist of chains of edge-shared MO6 octahedra connected by mu-O from H3O2(-) and MoO4(2-). These chains form a three-dimensional network with O-H-O, O-Mo-O, and O-Na-O bridging 4, 3, and 4 metal ions, respectively. Both compounds behave as canted antiferromagnets but differ in their behaviors, 1Co showing a broad maximum (28 K) above the Neel transition (21 K) and the canting taking place at 13 K, some 8 K below T(N), while for 2Ni the canting takes place at T(N) (28 K). Analyses of the neutron powder diffraction data shed some light on the geometry of D3O2(-) and suggest antiferromagnetism with a propagation vector k = (0,0,0) with the moments within each chain being parallel but antiparallel to those in neighboring chains. The difference between 1Co and 2Ni is in the orientation of the moments; they are parallel to the chain axis (b-axis) for 1Co and perpendicular to it for 2Ni with a major component along the c-axis and a small one along the a-axis. The heat capacity data peak at 20.9(3) K (1Co) and 25.1(1) K (2Ni). The derived magnetic entropies, following correction of the lattice contribution using the measured data for the nonmagnetic Zn analogue, suggest S = 1/2 for 1Co but is lower than that expected for 2Ni (S = 1). In both cases, only ca. 60% of the entropy is found below the magnetic ordering temperature, suggesting considerable short-range correlations at higher temperatures. While the temperature at which the magnetic diffraction becomes observable coincides with that of at the peak in heat capacity, it is lower than T(N) observed by magnetization measurements in both cases, and there is evidence of short-range ordering in a narrow range of temperature (T(N) +/- 5 K).

Nuclear and magnetic structures and magnetic properties of synthetic brochantite, Cu4(OH)6SO4

Cu4(OH)6SO4 (1) and Cu4(OD)6SO4 (2) were obtained by hydrothermal syntheses from copper sulfate and sodium hydroxide in H2O and D2O, respectively. They crystallize in the monoclinic system, space group P2(1)/a (14), a = 13.1206(5), b = 9.8551(3), c = 6.0295(2) Angstroms, beta = 103.432(3) degrees, V = 758.3(1) Angstroms(3), Z = 4 and a = 13.1187(5), b = 9.8552(3), c = 6.0293(2) Angstroms, beta = 103.410(3) degrees, V = 758.3(1) Angstroms(3), Z = 4, respectively. They are iso-structural to the mineral brochantite and consist of double chains of edge-sharing copper octahedra that are connected to one another by corners to form corrugated planes along bc; these planes are in-turn bridged by the unprecedented mu7-sulfate tetrahedra to give a 3D-structure. All the hydrogen atoms were precisely located from refinement of the neutron powder diffraction data of the deuterated sample. Magnetic susceptibility data reveal a low-dimensional behavior at high temperature and the presence of both ferromagnetic and antiferromagnetic super-exchanges resulting in a 3D long-range antiferromagnetic ordering at 7.5 K accompanied by a small canting of the moments. The transition is confirmed by a lambda-peak in the specific heat. The magnetic structure at 1.4 K shows the moments are oriented perpendicular to the corrugated planes with alternation along +/-a for neighboring chains within the double chains. The enhanced incoherent scattering at low-angle suggests the existence of short-range ferromagnetic clusters.