SUMMARYBy means of high temperature X-ray techniques the crystal structure of lanthanum, cerium, praseodymium, neodymium, ytterbium, and possibly gadolinium was found to be body-centered cubic at temperatures near their respective melting points. For ytterbium a hexagonal close-packed structure was also observed, which was shown to be stabilized by atmospheric impurities. Evidence for possible high temperature crystalline transformations in gadolinium, terbium, dysprosium, holmium, and lutetium was obtained by means of electrical resistance measurements; erbium gave no such evidence. X-ray data were used to derive empirical equations which describe thermal expansions coefficients of scandium, yttrium and the rare-earth metals.Europium exhibits a rapidly decreasing coefficient of e~pansion with increasing tempera,ture, which may be a consequence of a gradual promotion of one of the 4f electrons into the conduction band. The hexagonal rare-earth metals were found to have nearly the same axial ratio at their respective transformation temperatures.

The crystal structures and accurate lattice constants of scandium, yttrium and the rare earth metals are given, and the values of their calculated mole-atomic volumes, densities, axial ratios and metallic radii are graphically compared. Expected variations due to the 'lanthanide contraction' and structure differences are noted, and additional irregularities in the axial ratio and metallic radii plots are indicated.

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Electrical measurements have been made on CaB6, SrB6, BaB6, YB2, YB4, YB6, and YB12 for the purpose of testing models of the electronic structure of the boron atoms in the four boride types represented. The compounds CaB6, SrB6, and BaB6 were found to be semiconductors. The Hall coefficients of YB2, YB6, and YB12 were measured and found to correspond to one free electron per yttrium atom in each compound. The Hall coefficient of YB4 was also measured, but could not be interpreted because the free-electron theory does not provide a theoretical value for comparison with experiment. The results are in agreement with the models which have been proposed.

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.