Vapor Pressures of the Rare-Earth Metals

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Abstract
Vapor‐pressure measurements on the rare‐earth metals were made by the Knudsen effusion technique using a quartz‐fiber microbalance over a pressure range of 10—1 to 10—4 mm of Hg. The material used was specially prepared in order to minimize impurity effects. Ytterbium is the most volatile of the rare‐earth metals, while lanthanum and cerium are the least volatile. The data were fitted to a straight line by the least‐squares method and the heats of vaporization or sublimation were calculated from the slope of the line. The heats of sublimation at 298°K were then calculated using the best available data for the enthalpies of the condensed and vapor states. For nine of the elements, measured heat content and entropy data for the condensed state as well as calculated free energy functions for the gaseous state were available, permitting a third law calculation of ΔH298°. The second‐ and third‐law methods were generally in good agreement, but a comparison of the present work and previous investigations revealed several differences, amounting to as much as 15 kcal/mole in the case of gadolinium. A possible explanation for the variations in the room temperature heats of sublimation of the different rare‐earth metals, based on the differences in the electronic structures of the gaseous atoms and the number of d electrons present in the condensed phase is presented.