Lanthanum vapor pressure

Uranium(III) chloride, as obtained by this procedure, is a dark purple, crystalline compound. Other procedures may yield products with varying colors. Uranium(III) chloride has a hexagonal lattice and is isomorphous with cerium(III) chloride and lanthanum bromide. The compound melts at 842° and has a density of 5.51. The vapor pressure (600 to 1000°) is given by the expression... 

Alternative processes for preparing metallic americium are the reduction of AmFs with barium vapor in high vacuum at about 1300°C, reduction of AmF4 with calcium, and reduction of Am02 with lanthanum or thorium at about ISOO C in high vacuum. The vapor pressure of americium is much higher than that of lanthanum or thorium, so that pure americium is condensed in the colder parts of the apparatus [K2, L2]. Metallic americium dissolves readily in mineral acids. 

The major breakthrough occurred in 1953 when the Ames Laboratory team (Daane et al. 1953) reported the preparation of samarium, europium and ytterbium in high purity and high yields by the reduction of their oxides with lanthanum metal in a vacuum. With the preparation of samarium metal, finally, 126 years after the first rare earth element was reduced to its metallic state, all of the naturally occurring rare earths were now available in their elemental state in sufficient quantity and purity to measure their physical and chemical properties. The success of this reaction is due to the low vapor pressure of lanthanum and the extremely high vapor pressures of samarium, europium and ytterbium (Daane 1951, 1961, Habermann and Daane 1961). It is interesting to note that this same technique has been the method of choice for the preparation of some transplutonium metals (Cunningham 1964). 

Specific studies of the vapor pressures of rare earth metals were not carried out until the late 1940 s. However, various earlier manipulations of rare earth metals, including vacuum melting of cerium and lanthanum gave a clear indication that these metals had relatively low vapor pressures, compared to the alkaline earth metals for instance. The first concerted study of the vapor pressure of a rare earth metal was that of Ahmann (1950) who used a radioactive tracer modification of the Knudsen technique to show that cerium had a vapor pressure of 10" Torr at 1735 C. Daane (1951) measured the vapor pressures of lanthanum using a direct weight loss Knudsen technique to show that lanthanum has a vapor pressure of... 

Lead is poisonous, so that in addition to controlling the lead loss, the evaporated lead must be contained. In practice, this is achieved by surrounding the sample with lead-based powder compositions (31), such as a mixture of PbO and PbZrOs for lead-lanthanum-zirconium titanate (PLZT), to provide a positive vapor pressure in a closed AI2O3 crucible (Fig. 12.22). With the controlled atmosphere apparatus, PLZT can be sintered to full density (Fig. 12.23), yielding materials with a high degree of transparency. 

The final step, vaporization of calcium, does not work for metals with high vapor pressure, as the metal itself would vaporize with the calcium. Instead the oxide is reduced with metallic lanthanum, which has a very low vapor pressure. Lanthanum oxide and a melt of the actual RE metal are formed. The reduction reaction occurs in a tantalum container. The reduced RE metals e.g. samarium, are vaporized and deposit on the walls of the tantalum container. This method is used for [boiling point (°C) in parenthesis] Sm (1794), Eu (1529), Tm (1950), Yb (1196). 

Laboratory reagents, preparation of, 8-1 to 4 Laboratory Solvents and Other Liquid Reagents, 15-13 to 22 Laguerre polynomials, A-83 to 85 Lanthanum see also Elements electrical resistivity, 12-39 to 40 electron configuration, 1-18 to 19 heat capacity, 4-135 history, occurrence, uses, 4-1 to 42 ionization energy, 10-203 to 205 isotopes and their properties, 11-56 to 253 magnetic susceptibility, 4-142 to 147 molten, density, 4-139 to 141 physical properties, 4-133 to 134 thermal properties, 12-201 to 202 vapor pressure, 6-61 to 90 vapor pressure, high temperature, 4-136 to 137... 

Novikov GI, Baev AK (1962) Vapor pressure of chlorides of trivalent lanthanum, cerium, praseodymium, and neodymium. Zh Neorg Khim 7 1340-1352... 

Oxides. Decomposition pressure measurements on the TbO system by Eyring and his collaborators (64) have been supplemented by similar and related studies on the PrO system (46) and on other lanthanide-oxygen systems (43, 44). Extensive and systematic studies of vaporization processes in lanthanide-oxide systems have been undertaken by White, et al. (6, 188,196) using conventional Knudsen effusion measurements of the rates of vaporization of the oxides into high vacuum. Combination of these data with information on the entropies and Gibbs energy functions of reactants and products of the reaction yields enthalpies of reaction. In favorable instances i.e., if spectroscopic data on the gaseous species are available), the enthalpies of formation and the stabilities of previously undetermined individual species are also derived. The rates of vaporization of 17 lanthanide-oxide systems (196) and the vaporization of lanthanum, neodymium, and yttrium oxides at temperatures between 22° and 2700°K. have been reported (188). 

Pressure sintering techniques have also been used to fabricate transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics in air and in oxygen-gas atmosphere [31]. The microstructure of the sintered body was not uniform it was completely dense near the surface, but it was porous at the center. The thickness of the dense layer increased with sintering time and oxygen-gas pressure in the sintering atmosphere. Vaporization of PbO from the specimen surface and resultant formation of lattice vacancies were attributed to this microstructural evolution. Diffusion of the gas trapped in the pores was also important in determining the thickness of the dense layer. When the PLZT specimen was sintered in air at 1200 °C for 8 h, the thickness of the dense layer was 0.25 mm. Therefore, if the specimen thickness was 0.5 mm, the whole specimen was dense and transparent. When the specimen was sintered in an oxygen-gas atmosphere under the same conditions, the specimen thickness increased markedly. 

Vapor Phase Formation and Condensation In order to grow whiskers by vapor phase formation and condensation, the bulk SiC is first vaporized by heating to very high temperatures (>2200 °C), usually under reduced pressure. Upon cooling, a-SiC whiskers form on the nucleation sites. The addition of lanthanum, yttrium, neodymium, or zirconium leads to an increase in the growth rates. However, whiskers are no longer produced commercially using this sublimation process. 

---