On the 240 K anomaly in the magnetic properties of LiNiO2

In this work, results of X-band ESR spectroscopy, ac-magnetic susceptibility and X-ray powder diffraction measurements on Li_1-xNi _1+x O₂ (x = 0.02 and x = 0.07) are presented and discussed. While the susceptibility of the compound with x = 0.02 is shown to follow a Curie-Weiss law, with a Weiss temperature of the order of 30 K, the compound with x = 0.07 is found to order ferromagnetically below K. However, an additional anomaly is observed in the magnetic properties of this latter compound at around 240 K. We attribute this anomaly to the presence of macroscopic Ni-rich regions which order ferrimagnetically below this temperature. This phenomenon is different from the bulk ferromagnetism that occurs at much lower temperatures, and allows us to discard earlier suggestions proposed in the literature in which the 240 K anomaly has been considered as denoting an intrinsic phenomenon. Received 14 May 1999 and Received in final form 5 August 1999     

Implantation as a tool for performing selective materials (application to solar energy conversion)

A selective solar material must absorb most of the solar spectrum, principally the visible light, and reflect the IR light.

Insulators are generally transparent in a wide part of the optical spectrum and the defects are revealed in these crystals by strong absorption bands. On the other hand, metals absorb much of the IR and near IR light and have a large reflection coefficient in the same region of the spectrum. In previous papers^1,2 it has been shown that metallic colloids, formed by precipitation of impurities in insulators, are responsible for a strong absorption band. Such metallic inclusions may be easily produced in most insulators by implantation. According to the nature of the implanted metal a selective absorption can be obtained. So a composite material (cermet) may be performed combining a colloidal absorption in the visible and a metallic reflection. We will discuss the different ways to achieve these properties using direct ion beam implantation.

Various cermets (LiF: Na, Au; MgO: Na, Au) have been studied as function of energy (0.1-1 MeV) and dose (10¹⁶-10¹⁷ ions/cm²). Colloids are completely developed by consecutive annealing.⁴

The modelization of these cermets requires a careful characterization by optical methods (spectrophotometry) and microscopic investigation (TEM, SEM, RBS, SIMS).⁷ These techniques are used to determine the filling factor and the concentration profile of metal in the insulating matrix.

With the help of the Maxwell-Garnett theory and using a single or multilayer model it is possible to suggest an interpretation of the optical properties.     

The P2-Na(2/3)Co(2/3)Mn(1/3)O2 phase: structure, physical properties and electrochemical behavior as positive electrode in sodium battery

Manganese substituted sodium cobaltate, Na(2/3)Co(2/3)Mn(1/3)O(2), with a layered hexagonal structure (P2-type) was obtained by a co-precipitation method followed by a heat treatment at 950 °C. Powder X-ray diffraction analysis revealed that the phase is pure in the absence of long-range ordering of Co and Mn ions in the slab or Na(+) and vacancy in the interslab space. The oxidation states of the transition metal ions were studied by magnetic susceptibility measurements, electron paramagnetic resonance (ESR) and (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The charge compensation is achieved by the stabilization of low-spin Co(3+) and Mn(4+) ions. The capability of Na(2/3)Co(2/3)Mn(1/3)O(2) to intercalate and deintercalate Na(+) reversibly was tested in electrochemical sodium cells. It appears that the P2 structure is maintained during cycling, the cell parameter evolution versus the sodium amount is given. From the features of the cycling curve the formation of an ordered phase for the Na(0.5)Co(2/3)Mn(1/3)O(2) composition is expected.     

A Simplified Wittig Synthesis Using a Solid-Liquid Transfer Process : VI - Synthesis of γ -Unsaturated Alcohols from Aromatic and Heteroaromatic Aldehydes

Very good yields can be obtained for the synthesis of γ-unsaturated alcohols without degradation of γ-hydroxylated phosphonium salts using the phase transfer technique in low hydrated solidliquid medium.     

Cesium Carbonate in Weakly Hydrated Solid/Liquid Heterogeneous Medium: A New Reagent for Anionic Activation Synthesis

Transformation of acidic hydrogen carrying aldehydes- in corresponding ethylenic esters or ethers is carried out in the presence of cesium carbonate. Cesium carbonate proves to be an efficient reagent for any transformation by anionic activation of poly-functional molecules.     

The Reimer-Tiemann Reaction in Slightly Hydrated Solid-liquid Medium: A New Method for the Synthesis of Formyl and Diformyl Phenols

Formylation of phenol in weakly hydrated heterogeneous solid-liquid medium makes it possible to transform phenol into salicyl-aldehyde and parahydroxybenzaldehyde with a ratio of 4.5/1 with excellent yield. Use of methanol as cosolvent modifies the orientation of this reaction, which leads for the first time to simultaneous synthesis of salicyladehyde and parahydroxybenzaldehyde, of 2-hydroxy 1,3-benzenedicarboxaldehyde, and of 2-hydroxy 1,3-benzenedicarbox-aldehyde. Experimental conditions make it possible to easily extract each of these molecules.     

Deprotonation-Alkylation of Alkyl Cyanides Under Sonochemical Conditions

Deprotonation-alkylation of n-alkyl cyanides can be readily effected by an alkyl halide in the presence of sodium in a one pot procedure. Yields are generally better than in the usual methods, and the overall reaction conditions have important advantages in terms of ease and simplicity compatible with preparative scale-up.     

How to prepare and stabilize very small nanoemulsions

Practical and theoretical considerations that apply when aiming to formulate by ultrasonication very small nanoemulsions (particle diameter up to 150 nm) with very high stability are presented and discussed. The droplet size evolution during sonication can be described by a monoexponential function of the sonication time, the characteristic time scale depending essentially on the applied power. A unique master curve is obtained when plotting the mean diameter size evolution as a function of sonication energy. We then show that Ostwald ripening remains the main destabilization mechanism whereas coalescence can be easily prevented due to the nanometric size of droplets. The incorporation of "trapped species" within the droplet interior is able to counteract Ostwald ripening, and this concept can be extended to the membrane compartment. We finally clarify that nanoemulsions are not thermodynamically stable systems, even in the case where their composition lies very close to the demixing line of a thermodynamically stable microemulsion domain. However, as exemplified in the present work, nanoemulsion systems can present very long-term kinetic stability.     

Reaction in Low Hydrated Solid-Liquid Heterogeneous Medium: Thiiranes Synthesis from Epoxides

A new method for the selective thiiranes synthesis from epoxides has been elaborated. A low hydrated solid-liquid heterogeneous medium is involved which contains solid thiocyanate with or without solvent.     

Homologation of Furfural and Deriuatives to Furylacetaldehydes

The homologation of furfural 1a and methyl 3-(5-formyl-2-furyl) propenoate 1b or ethyl 3-(5-formyl-2-furyl) propenoate 1c to the corresponding furylacetaldehydes was carried out in two stages:

i) preparation of the furan epoxides from 1a, 1b, 1c

ii) cleavage and rearrangement of the epoxides on sepiolite. Sepiolite is a convenient catalyst for this last stage involving substrates as labile as the furan epoxides.