Crystal Field Splitting of Actinides

[jman1990]

Hello everyone,

Does anyone know much about the crystal field splitting of the f orbitals in actinides, specifically uranium? I am doing research on the chemistry of uranium and have come across an interesting problem. U(IV) compounds undergo a transition to a non-magnetic singlet ground state at low temperatures, even though it should have two unpaired f electrons.

Generally, it is accepted that this is because of thermal de-excitation of higher energy states (which is a function of U(IV)'s small crystal field splitting energy).

I would like to know about the splitting of the f orbitals in different coordination environments because it seems to me that U(IV) might be able to retain it's unpaired electrons in the correct coordination environment, even at low temperatures.

Thanks in advance

P.S. any references you might have on this topic in general would also be appreciated, I can't seem to find much about it.

 

[DrDu]

I would rather try to calculate the orbital schemes ab initio with some program for relativistic ab initio calculations like DIRAC, Paragauss etc.

 

[TeethWhitener]

any references you might have on this topic in general would also be appreciated, I can't seem to find much about it.

This book might be useful, especially the chapter entitled "Electronic structure of f-block compounds." The author gives a few Tanabe-Sugano type diagrams for f² configurations.

You might also just want to Google "spin crossover," which is what it sounds like you're after. In general, whether your compound is high-spin or low-spin depends on the balance between the crystal field splitting energy and the Racah electron repulsion (this is what Tanabe-Sugano gives you). In order to maintain a high-spin state all the way down to extremely low temperatures, you have to have a really really small crystal field splitting energy. My intuition says that if you want to build a solid state system like this, you should go for ligands way down in the spectrochemical series (like bromide or iodide), but I don't know enough about actinide chemistry to say whether those species are stable or not. Maybe a uranium salt with an extremely weakly coordinating anion?

I would rather try to calculate the orbital schemes ab initio with some program for relativistic ab initio calculations like DIRAC, Paragauss etc.

Unfortunately, this might end up being what you have to do. If the spin-orbit interaction scrambles the f-orbital state with the spin states of the electrons, then Hund's rules no longer apply. This regime doesn't have a particularly nice intuitive picture, and you'll probably need a computer to sort out the ugly details.