Vapor pressure rare earth metals

For further purification of the rare earth metals electrotransport [or solid-state electrolysis (SSE) as it is sometimes called] has been the most successful technique. But it has only been demonstrated on the low vapor pressure rare earth metals (Sc,... 

Figure 4 shows vapor pressure curves of rare-earth metals[24], clearly showing that there is a wide gap between Tm and Dy in the vapor pressure-temperature curves and that the rare-earth elements are classified into two groups according to their volatility (viz.. Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu, non-volatile elements, and Sm, Eu, Tm, and Yb, volatile elements). Good correlation between the volatility and the encapsulation of metals was recently... 

TABLE 7. Vapor Pressures, Boiling Points, and Heats of Sublimation of Rare Earth Metals... 

As to the partial pressures of the monolanthanum and monocerium carbides, LaC and CeC , in the equilibrium vapor above the rare-earth-metal-platinum-metal-graphite systems, the relative ion currents, which represent more than 90% ionization from the parent neutral molecules, and thus, the relative partial pressures of the latter, have been measured by means of the mass spectrometric method-... 

In summaries of work by Beaudry and Gschneidner (1978) and Gschneidner (1980a), conditions for the distillation/sublimation of rare earth metals are given batch sizes are 0.5—1kg, vacua of 10" —10 Torr are maintained and transport rates of 1 to 4 grams per hour are achieved at temperatures that correspond to vapor pressures in the range of 10 to 10" Torr for the metal being purified. 

For metals with high vapor pressures, the alloys are best prepared by melting in sealed tantalum or tungsten crucibles heated inductively or in a resistance furnace. Fortunately these high vapor pressure non-rare earth metals do not react with the crucible materials. 

Compounds of the high vapor pressure non-metallic elements and the gaseous elements (X) can be prepared by direct combination of the gas with solid rare earth metals at fairly low temperatures. Many times the reactions do not go to completion because of the formation of an impervious RX coating which prevents or slows down the reaction. For the sulfides and selenides a few milligrams of I2 helps break up this coating and the reaction goes to completion in the matter of a few hours, compared to weeks or months when no I2 is present (Takeshita et al. 1982). 

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... 

The anomalous variation of the heats of sublimation (AZ/J and boiling points (Tb) as a function of the atomic number was immediately recognized by Habermann and Daane (1961) in their extensive study of the vapor pressure of all of the rare earth metals. The nearly parallel variation of these two quantities follows from Trouton s rule, which states that... 

The vapor pressure of the rare earth metal at its melting point determines the method chosen to melt the metal. The low melting metals. La, Ce, Pr and Nd, which have low vapor pressures can be melted by any of the common methods,... 

Melting and boiling points, vapor pressures and heats of sublimation of the rare earth metals. 

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