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Why do thorium through plutonium have unusual metallic crystal structures?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

The lighter actinoid elements, ranging from thorium through plutonium, exhibit unusual and varied metallic crystal structures primarily due to the electronic configuration of their atoms.1

In these specific elements, the 5f and 6d orbitals are close enough in energy that they mix, causing some electrons to occupy the unfilled 6d orbitals rather than the expected 5f orbitals. This interaction leads to the formation of multiple hybrid electronic states that possess nearly identical energy levels. Consequently, these metals can adopt a variety of different crystal forms, contributing to their complex and unique structural characteristics.1

This behavior contrasts with heavier actinoids, such as americium and beyond, where the electron energy levels are sufficiently separated to prevent this orbital mixing, resulting in more consistent crystal structures.1