Studies in X-Ray Crystallography: I. The Allotropic Forms of Cerium. II. The Structure of Urea Oxalate

Citation

Schuch, Adam Frank (1950) Studies in X-Ray Crystallography: I. The Allotropic Forms of Cerium. II. The Structure of Urea Oxalate. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ck0c-vr70. https://resolver.caltech.edu/CaltechTHESIS:07282025-203451269

Abstract

I. The Allotropic Forms of Cerium

The crystal structure of elementary cerium has been investigated by X-ray diffraction at temperatures from room temperature to that of liquid nitrogen. Despite a variety of heat treatments, I have been able to obtain at room temperature only a cubic closest-packed structure with a₀ = 5.139 Kx. No evidence could be found to confirm the existence of a hexagonal closest-packed structure.

When stresses are produced in cerium, aa for example by quenching from 250°C. or higher, and it is then rapidly cooled with liquid air, a new form is obtained which also has the cubic closest-packed arrangement but with a₀ = 4.82 Kx. If the cerium is annealed or free of stresses then no allotropic change takes place.

In the new structure it is believed that the 4f electron has moved into hybridized 5d 6s orbitals to take part in bond formation. This experimental work confirms in part the predictions of Tromba.

II. The Structure of Urea Oxalate

Urea oxalate crystallizes monoclinic prismatic, P 2₁/c, A₁ = 5.218 Å., A₂ = 12.404 Å., A₃ = 11.576 Å., and β = 143°1'. There are two formula. weights per unit cell. The structure is a layer one with the molecules almost parallel to (100). The layers are held together by van der Waals forces. There is no discrete urea oxalate molecule. Each layer is a network of urea and oxalic acid molecules held together by hydrogen bonds. One extremely short O - H · O distance of 2.47 Å. was found.

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