Cerium vapor pressure

If two vapor pressures are indicated for a single composition, the structures must be different. Any total composition is made up of parent structure plus defects. If the lines cross, the total compositions and cadmium vapor pressures have become equal, but the cerium vapor pressures, total free energies, and structures in the two microphases are different. 

Lighter Flints and Getters. Traditionally the item most widely associated with cerium has been the pyrophoric iron-mischmetal (- 0%) alloy for lighter flints, in limited use in the 1990s. Similar low vapor pressure reactive alloys based on cerium, such as Th2Al-MM, can also be used as getters for electronic equipment and vacuum tubes (see Electronic materials Vacuumtechnology). 

Defect equilibria in intermetallic compounds are inferred from measured changes of vapor pressure with composition and from other experimental information. Equilibria analogous to those in aqueous solution are found in dissociation, com-plexing, and random solution other equilibria connected with the ordering of defects show a distinctly intermetallic flavor. Techniques for calculating the equilibria are described. Cerium-cadmium phase information is collected. 

The technique requires the measurement of some property which is proportional to a product concentration—e.g., pH, color, or electrical conductivity. In the cerium-cadmium system the cadmium vapor pressure is one such measurable property. 

At a selected alloy temperature the vapor pressure of cadmium is determined as a function of alloy composition the cerium solvent has a negligible vapor pressure. The alloy, located in one leg of a sealed inverted U-tube, is subjected to various specific pressures of cadmium from a supply of pure cadmium at selected temperatures in the second leg of the tube. The U-tube is freely suspended at its midpoint and connected to a balance, so that the transfer of cadmium from one leg of the tube to the other can be measured. This gives information as to the change in alloy composition and phase equilibrium. 

Since we do not have cadmium vapor pressure across the whole cerium-cadmium system, some standard state other than the pure metal would have to be used for cerium. 

Cerium(III) tris(/3-diketonate) complexes are readily oxidized to the cerium(IV) compounds. They are volatile with vapor pressures high enough for MOCVD use. A number of diketonates are now better characterised, prompted by the possibility of using them as CVD materials and petrol additives, as well as a source of cerium oxide as an oxygen store for catalytic converters. [Ce Me3CCOCHCOCMe2(OMe) 4], Ce(acac)4 (Figure 32), Ce(dbm)4, [Ce(pmhd)4], and Ce(tmhd)4 all have square-antiprismatic coordination of cerium (in the [Ce(catecholate)4]" ion the coordination is dodecahedral). 

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... 

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... 

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

Cerium metal is discussed in ch. 4 and only a brief mention of its high pressure behavior will be made here (for references see the list in ch. 4). Cerium can exist at atmospheric pressure in the fee (y) or dhep (iS) form and undergoes an isostructural transition near 100 K to another fcc-form referred to as o-Ce. The y-a Ce transition occurs at 7 kbar at room temperature and this transition is accompanied by about 8% volume decrease. This is one of the most widely studied transitions as a function of pressure and temperature and is believed to involve a valence change from 3 towards a higher valence state (3.7 ). The y to a transition line terminates at a critical point the very first example in which a solid - solid transition was shown to exhibit a liquid-vapor-like critical point. A pressure-induced phase transition near 50 kbar, initially reported to be yet another isostructural transition has been shown to be from fee (a-Ce) to an orthorhombic phase with the a-U structure. Stager and Drickamer (1964) have reported a pronounced resistance anomaly near 120 kbar indicative of a phase transition, but the nature of this transition is unknown. The fusion behavior of Ce is again unique in that it exhibits a minimum. 

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