Patrick Maestro

Industrial applications of rare earths: which way for the end of the century

Abstract Several examples of the potential of rare earths for innovation are reviewed, through either the optimization of the starting materials for existing applications or the use of intrinsic properties of specific compounds. The influence of the quality of starting materials on applications like phosphors and catalysis is illustrated, as well as the potentiality of rare earth sulphides in the new pigment application.

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.

Optical properties of Ce1−xPrxO2 powders and their applications to the coloring of ceramics

Abstract The development of red pigments with high temperature stability is of great importance for the ceramic industry. Stabilization of praseodymium (IV) in the refractory oxide CeO 2 leads to cerium-rich solid solutions Ce 1− x Pr x O 2 in the red coloration range. Crystallographic and optical properties have been investigated. The detected color is consistent with Jorgensens model. Thermal stability at high temperature has been studied for various glazes.

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.

Study of the reduction/reoxidation cycle in a La/Ce/Tb mixed oxide

Abstract The redox behavior of a La/Ce/Tb mixed oxide used as a precursor for preparing green emitting phosphors has been investigated by means of temperature programmed reduction mass spectrometry (TPR-MS), oxygen pulse/temperature programmed oxidation (TPO), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The experimental conditions ensuring the complete reduction of the starting oxide to a Ce III and Tb III containing phase have been established. In accordance with the XRD study, the reduced phase consists of a hexagonal, A-type, mixed sesquioxide. The quantitative O 2 pulses/TPO data for the reoxidation of this sesquioxide phase are consistent with the formation of a fully oxidized: La 2 O 3 /CeO 2 /TbO 2 mixed oxide. Neither the XRD study nor the HRTEM images suggest the occurrence of phase segregation phenomena.

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.