J. Coutures

Etude à haute température du diagramme d'equilibre du Système formé par le sesquioxyde de lanthane avec le sesquioxyde d'Yttrium

en The study at high temperature up to the melting point of the system formed by lanthanum sesquioxide with yttrium sesquioxide shows the formation of several very extended solid solutions, each corresponding to a form given in these conditions by rare earth sesquioxides. The phases determined in situ at high temperature are very different from those observed with products cooled down to room temperature. The influence of quenching and annealing is demonstrated, particularly with LaYO 3 .

Etude par rayons X à haute temperature des transformations polymorphiques des pérovskites LnAlO3 (Ln = élément lanthanidique)

Abstract The temperatures of phase transitions of the rare earth aluminates have been determined by high temperature X-ray diffractometry. All of the transitions are reversible and occur respectively for Rh ⇌ C at 500°C (LaAlO 3 ), 1330°C (PrAlO 3 ), 1550°C (NdAlO 3 ), and 1950°C (SmAlO 3 ) and for O ⇌ Rh at 770°C (SmAlO 3 ), 1330°C (EuAlO 3 ), and 1700°C (GdAlO 3 ). Ln AlO 3 perovskites from TbAlO 3 up to LuAlO 3 are orthorhombic without any phase transition.

Sm2O3Ga2O3 and Gd2O3Ga2O3 phase diagrams

Abstract Four definite compounds exist in the Sm2O3Ga2O3 binary phase diagram, namely: Sm3GaO6, Sm4Ga2O9, SmGaO3, and Sm3Ga5O12. The 3 1 compound is orthorhombic (space group Pnna - Z.4) with the cell parameters: a = 11.400A, b = 5.515A, c = 9.07A and belongs to the oxysel family. Sm3GaO6 and SmGaO3 melt incongruently at 1715 and 1565°C; Sm4Ga2O9 and Sm3Ga5O12 have a congruent melting point at 1710 and 1655°C. With regard to the Gd2O3Ga2O3 system three definite compounds have been identified: Gd3GaO6, Gd4Ga2O9, and Gd3Ga5O12. Only the garnet melts congruently at 1740°C with the following composition: Gd3.12Ga4.88O12. Gd3GaO6, and Gd4Ga2O9 melt incongruently at 1760 and 1700°C. GdGaO3 is only obtained by melt overheating which may yield an equilibrium or a metastable phase diagram.

Etude a haute température des systèmes formés par les sesquioxydes de lanthane avec les sesquioxydes de lanthanides II. Influence de la trempe sur la nature des phases obtenues à la température ambiante

Splat cooling from the solar-furnace melt of mixed-oxide solutions of lanthanide oxides in La 2 O 3 changes the range of solid solution or produces phases of crystal structure not observed for the individual oxides in the pure state. Significant inertness to hydration has been obtained for type A hexagonal solutions enriched in Yb 2 O 3 by the effect of quenching.

A structural and magnetic study of the CaLa1−xYxCrO4 system (0 ≤ x ≤ 1)

Abstract Using a solar concentration furnace the CaLa 1− x Y x CrO 4 solid solutions were prepared with composition range0 ≤ x ≤ 1. Depending on the yttrium content they belong either to the ideal or to distorted K 2 NiF 4 -type structures. The magnetic properties are characterized mostly by bidimensional antiferromagnetic couplings due to half-filled t 2 g orbitals of Cr 3+ , but for x = 1 the rather covalent Y O bonds give rise to three-dimensional-type interactions.

Etude a haute temperature des systemes formes par le sesquioxyde de neodyme avec les sesquioxydes d'yttrium et d'ytterbium

Abstract The systems formed by neodymium sesquioxide with respectively yttrium sesquioxide and ytterbium sesquioxide showed several very extended solid solutions at high temperature, near the melting point. The phases determined “in situ” at high temperature are often different of those obtained in annealed or quenched samples.

Etude des solutions solides “CeLnO3” et des perovskites CeLnO3 et PrLnO3 (Ln = élément lanthanidique)

Abstract Interlanthanide perovskites CeTmO 3 , CeYbO 3 , CeLuO 3 , PrYbO 3 , PrLuO 3 , and Ce Ln O 3 solid solutions have been prepared by cofusion techniques. Thermal stability and the optical properties of these materials have been investigated. A very important nephelauxetic effect ( ΔE = 3600 cm −1 ) is observed. The thermal stability of the compounds or of the solid solutions increases when the ionic radius of the Ln 3+ decreases. The behavior of these compounds under oxidizing conditions is reported.Interlanthanide perovskites CeTmO/sub 3/, CeYbO/sub 3/, CeLuO/sub 3/, PrYbO/sub 3/, PrLuO/sub 3/, and CeLnO/sub 3/ solid solutions have been prepared by cofusion techniques. Thermal stability and the optical properties of these materials have been investigated. A very important nephelauxetic effect (..delta..E = 3600 cm/sup -1/) is observed. The thermal stability of the compounds or of the solid solutions increases when the ionic radius of the Ln/sup 3 +/ decreases. The behavior of these compounds under oxidizing conditions is reported.

Obtention par trempe a partir de l'etat liquide d'une nouvelle phase dans le systeme Nd2O3.Al2O3. etude comparative avec Nd4Ga2O9

Abstract By using splat cooling devices associated with laboratory solar furnace a new phase is obtained in the neodymium rich part of the Al 2 O 3 Nd 2 O 3 system. X ray and optical absorption studies and comparisons with Nd 4 Ga 2 O 9 show that the two phases are isostructural and monoclinic (space group P 21/C ) with Al 3+ in tetrahedral coordination. Optical examinations show that substitution of Al 3+ by Si 4+ induced a disorder on the neodymium site.

Caracterisation d'une phase metastable dans les solutions alumine-oxyde des lanthanides

Using peculiar quenching experiments a metastable phase may be obtained from liquid solutions xAl2O3. (1−x)Ln2O3 with Ln = Dy through Lu and 0, 5<x<1. This stoichiometric compound has an hexagonal unit cell and its cell parameters were determined. It probably corresponds to a LnAlO3 equimolecular composition. Such a phase appears when the Ln3Al5O12 garnet-like compound is disappearing, Furthermore, a metastable LnAlO3 (Ln = Tm, Yb, Lu) perovskite-type compound was observed.

Obtention et etudes des verres refractaires a base d'alumine et de sesquioxyde de neodyme

Abstract The amorphous phase obtained by rapid quenching with an hammer-anvil device of solar furnace melted binary mixture (1−x)Al2O3-xNd2O3 was investigated by thermal analysis and optical methods. By this way the domain of this amorphous phase obtention is determined. It is concluded that the amorphous phase is indeed a glass exhibiting no short range order which recristallizes circa 900°C in the two equilibrium phases βAl2O3 and NdAlO3.