Pierre Macaudiere

Low-temperature process of the cubic lanthanidesesquisulfides:remarkable stabilization of theγ-Ce2S3 phasei

Upon treating the corresponding oxalates with carbon disulfide (p CS2 =130 Torr) at a heating rate of 5 °C min -1 , it is shown that the cubic γ-phase of pure rare-earth sesquisulfides (γ-Ln 2 S 3 ) can be obtained at 800 °C from samarium to holmium (also yttrium) and at 1000 °C from neodymium to dysprosium. By working on ternary sulfides Ce 2-x Ln x S 3 , it has been shown that the stabilization of the γ-phase occurs when the average ionic radius ranges between 1.015 and 1.104 A at 800 °C. However, the nature of the observed sulfide phase also depends greatly on the experimental conditions, i.e. the nature of the precursor and the heating rate. At 800 °C and 5 °C min -1 , cerium oxalate leads to the β-phase while cerium nitrate leads to the α-phase. On the other hand, with cerium oxalate, the lower the heating rate the higher the amount of cubic γ-phase obtained.

The stabilization of γ-Ce2S3 at low temperature by heavy rare earths

Abstract The use of some heavy lanthanide(III) elements allows the stabilization of the γ-phase of Ce 2 S 3 synthesized in H 2 S at low temperatures (600°C–800°C). Among the lanthanides used, the best element is dysprosium followed by holmium, erbium and terbium. The stabilization is more pronounced when using mixed-precursors, either the crystallized oxalate or other less crystallized complexes. However this stabilization is only transitory: heating above 800°C yields the β form. It is the first time that the γ phase is observed at a lower temperature than the β form. Due to the low temperatures and the short treatment time no change of morphology is observed between the starting precursor and the final sulphide.

Color and chemical heterogeneities of γ-[Na]-Ce2S3 solid solutions

Abstract A quantitative estimation of the concentration distribution of sodium in the γ-Ce 2 S 3 phase doped with sodium has been obtained for the first time. A precise determination of the phase homogeneity and of the sodium volume distribution in the micrometric grains has been carried out using a new chemical method, differential dissolution, combined with XRD and Raman spectroscopy. Sodium high surface concentration and impurity phases such as sodium or cerium polysulfides in small amounts were found in the samples prepared under different conditions. It was shown that these heterogeneities can be the cause of a significant modification of the color of the material.

Characterization of Ce3(SiS4)2I, a compound with a new structure type

The first cerium iodothiosilicate, Ce3(SiS4)2I, has been synthesized from the reaction of cerium sulfide with silicon, iodine and sulfur at high temperature. This compound crystallizes in the monoclinic symmetry (space group C2/c and Z=4) with the powder refined cell parameters: a=15.9634(5), b=7.8502(2), c=10.8664(3) A, β=97.931(2)°. The crystal structure was refined to R(%)=2.17 and Rw(%)=2.60 from single crystal X-ray diffraction data. Ce3(SiS4)2I presents tunnels in which are located the iodide anions surrounded by three ceriums, in [ICe3 ] isosceles triangular entities. The tunnels are constituted of a three dimensional network made of (CeIS8) polyhedra (trigonal prisms of sulfur tricapped by two sulfur and one iodine atom) linked to (SiS4) tetrahedra. Magnetic susceptibility measurements and UV–VIS diffuse reflectance spectroscopy are consistent with the occurrence of CeIII ions, whereas band structure calculations indicate that the phase is a semiconductor. The charge balance in the material can be written as : CeIII3SiIV2I–IS–II8 .

Preparation and characterization of alkali- and alkaline earth-based rare earth sulfides

Various alkali- and alkaline earth-based cerium and neodymium sulfides (A, Ln) 3 S 4 (A=Li, Na, Ca, Sr) have been prepared by solid state routes. XRD and Ln L III -edge EXAFS studies showed that the first and second coordination shells corresponding to the two LnS 4 tetrahedra remain quasi-identical whatever the A cations, while the further coordination shells around 4 A change drastically due to the presence of A cations. Such a cationic distribution may influence the chromatic properties of these compounds. Furthermore, fluorination treatment of Na 0.5 Ce 2.5 S 4 sulfide leads to an improvement of the reflectance at λ>640 nm attributed to the presence of CeF 3 at the grains surface and to a modification of the microstructure of this material.