Thallium vapor pressure

The volatility of the reactants is a concern in some solid state syntheses. This may be a slight problem in bismuth cuprates synthesized at high temperatures because Bi2Os has an appreciable vapor pressure at these temperatures (i.e. 900-950°C) however, chemical analyses of samples of bismuth-based superconductors before and after reactions at temperatures up to 900°C indicate no detectable loss of bismuth. This problem is much more severe in the case of thallium chemistry. 

Although the of Tl Ba2Ca iCu 02 +, +2 is very high, the research for practical applications is not so active. This is because of not only the high vapor pressure of thallium oxide when tiring the sample to synthesize Tl, Ba2Ca iCu 02 + +2, but also because of the toxicity of thallium. 

The vapor pressures of mercury in equilibrium with thallium amalgams containing various mole fractions (Ni) of mercury at 26 C were found to be as follows [Hirst and Olson, J. Am. Chem. Soc.y 51, 2398 (1929)] ... 

Thallium is a silver-gray, soft, heavy, and ductile metal having three forms. The normal close-packed hexagonal lattice transforms to a body-centered cubic structure above 230 °C and a face-centered cubic form is stable at high pressures. The triple point is at 110°C and 30 kbar. Thallium vapor is essentially monatomic, but on heating to 2000 °C, the vapor emits a visible band due to TI2. Some properties are listed in Table 1. ... 

Sensitized Fluorescence. In this type of fluorescence, an atom emits radiation after collisional activation by a foreign atom that was excited previously by absorbing resonance radiation, but which has not yet been deactivated again. An example is the sensitized fluorescence of thallium atoms in a gas mixture containing a high pressure of mercury vapor and a low pressure of thallium vapor. When irradiated at the 253.65-nm mercury line, the thallium atoms emit at 377.57 and 535.05 nm. This type of fluorescence requires a higher concentration of foreign atoms than can be obtained in flame cells, but presumably it could be observed in nonflame cells. 

Thallium—The vapour pressure data on thallium are in considerable disagreement with one another 259, Kelley 259 gives A//25°c vaporization of solid thallium as 42 78 kcal, but the reliability of this is doubtful. 

Vaues of a may be very much less than unity and be temperature dependent. Somoijai and Lester [40] comment that "all the kinetic information is contained in the evaporation coefficient and its variation with conditions of vaporization", and they recommend the avoidance of the use of ot, in describing the rates of evaporation of solids under non-equilibrium conditions. The rate of sublimation is dependent on the attaimnent of sufficient energy by suitably disposed siuface molecules (possibly accompanied by electron or proton transfer in ionic solids). The overall rate of reactant removal is sensitive to the presence of impurities at the surface. The reverse reaction may be significant if the volatile material is not immediately removed from the vicinity of the reactant particles. Arrhenius parameters measured for sublimation processes may include a term which represents a temperature dependent concentration of surface intermediates [42]. The observation that measured evaporation rates are lower than those estimated from equilibrium vapour pressures suggests that the kinetics may be determined by a surface dissociation that precedes evaporation. This view is supported by evidence that, in selected systems, specific additives can considerably promote evaporation rates. For example [40], the evaporation rate of red phosphorous between 550 and 675 K was found to be increased by three orders of magnitude by the presence of thallium. 

Osram lamps for mercury cannot be used in atomic absorption, because they contain vapor at so high a pressure that the emission line is almost completely self-absorbed, and the sensitivity is very small. It is also generally agreed that hollow-cathode lamps for thallium, zinc, and cadmium are superior to the discharge lamps. 

Magnesium reacts with cyclopentadiene vapor very readily at 500° C and gives magnesium dicyclopentadienide in excellent yield 20). This is the preferred method for the preparation of this compound. Other elements which react directly with cyclopentadiene are indium, at 460°-580° C and 3-4 atm pressure, and thallium, at 440°-470° C and 2-3 atm pressure 21),... 

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