Zachariasen led the way

Sixty years of structural determinations of the 5f elements . . . or, why are the actinides so named?

Sixty years ago a surprisingly succinct memo summarized our knowledge of the structure of the actinide oxides. The memo is historically significant because the author, William H. Zachariasen, wrote that based on x-ray structural determinations of thorium, uraniu shell had been obtained.

William H. Zachariasen and his 1944 memo.

Zachariasen, having examined the x-ray pattern of a sample of neptunium oxide (NpO₂) wrote in part:

The interpretation of the diffraction pattern and crystal-chemical considerations permit me to make the following statements, all of which can be regarded as quite certain: The submitted sample is NpO₂...The radius of Np⁺⁴ is thus 0.015 � larger than that of Pu⁺⁴, 0.016 � smaller than that of U⁺⁴, and nearly identical with that of Ce⁺⁴.

I believe that a new set of 'rare earth' elements has made its appearance. I believe that the persistent valence is four, so that thorium is to be regarded as the prototype; just a lanthanum is the prototype of the regular rare earth elements.

This memo predated Glenn Seaborg's own "Actinide Hypothesis" memo of July 14, 1944. The table Zachariasen included in his memo for the results of NpO₂ compared with those obtained earlier for isomorphous dioxides, coming at such an early time in the Manhattan Project, was quite sensational and also controversial. The controversy stems from the how the new series should be most appropriately labeled and viewed.

Ultimately, Seaborg's view prevailed because of the 5f electron configuration of curium and the stability of the lower-valent states at the series end. Despite this outcome, Zachariasen maintained his position over the years that the "name actinide is not acceptable because thorium is never actinium-like." He thereafter called the new series the 5f elements and pointed out that not until elements 95 and 96 (americium and curium) were the elements rare-earth-like, and further, that the metal dioxide structure (MO₂) persisted from thorium dioxide (ThO₂) to californium dioxide (CfO₂), or elements 90 through 98.

Zachariasen was a giant among early Manhattan Project scientists and was notable for many firsts. He was a world figure in x-ray structural analysis and published more than 200 papers, the first when he was just 19. The significance of his accomplishments and background is captured in the inaugural issue of Los Alamos Science, published in the summer of 1980.

In early 1942, the fledgling Metallurgical Laboratory (plutonium) project was established at the University of Chicago-away from the East and West Coasts where the work had originated-in case of attack. In late 1943, Arthur Compton, Nobel laureate and head of the physics department at Chicago, asked Zachariasen to help explain the chemistry of the new element, plutonium, then available only in microgram-scale preparations. Zachariasen's impact was immediate, demonstrating, "that satisfactory x-ray patterns of plutonium preparations on the 10-microgram scale often could provide positive identification of the phase or phases present...."

Summarizing his three years working on compounds produced by Seaborg's group, Zachariasen wrote:

For the past three years within the plutonium project, I carried out partial or complete crystal structure determinations of 140 different compounds of plutonium, neptunium, uranium, thorium or rare earth elements. My collaborator Dr. Rose Mooney made similar determinations of an additional 20 compounds.

All but three of the first 25 plutonium compounds to be identified were identified by the x-ray diffraction method. Every one of the compounds of actinium, protactinium, neptunium and americium whose existence is now established was identified by the x-ray diffraction method, and solely by that method.

In fact, even today among structural determinations of actinide oxides, Zachariasen's scientific output comprises more than half of the reported structures.

In the 1950s, Zachariasen came to Los Alamos in the summer to work out the complicated structures of plutonium metal. Of that work, A.S. Coffinberry and W.N. Miner wrote in their book, The Metal Plutonium: "It is highly improbable that any scientist other than Zachariasen could have solved the three structures as complex as those of alpha, beta and gamma plutonium from powder patterns alone."

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