Heavy-light compounds: Back of the envelope

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Heavy-light compounds: Back of the envelope

Post by fhssn1 » Mon Mar 26, 2018 11:51 pm

Can I get some rough/back-of-the-envelope idea about how the electronic states interact when a heavy element and a light element come together to form a crystal structure. Sure I can carry out a calculation and see for myself but I'm trying to develop some kind of intuition about what I'm supposed to expect.

Consider an extreme (hypothetical) case: a very heavy element, Pb (lead), and a very light element, H (hydrogen), come together to form a PbH crystal structure (I'm assuming they stabilize in a 1-to-1 proportion, to keep things simple). Is it a correct line of thinking to say that:

- H 1s1, the single electron, is sitting at a shallow depth below vacuum level (if I remember correctly it's -13.6 eV?)

- Pb 1s1, the core-most electron, is sitting at a much much deeper level compared to vacuum (maybe hundreds of eV below vacuum).

- So in PbH, there is no chance of the Pb 1s1, and H 1s1 to interact and form any sort of hybrid state. Instead we should look at what electrons we have sitting around -13.6 eV below vacuum. Most likely those are going to be the outermost valence electrons. That means one of the Pb's 6p2 electrons are more likely to form some hybridization with H's 1s1. Or there could be no hybridization and electronic states of both elements contribute indepedently to the total DoS. And so on.

Once we develop this kind of extreme case mental picture, we can try to apply it to other compound crystals that don't have this much size difference in their constituent elements, e.g., GaN. In GaN, Ga would act as the heavy element (though not as heavy as Pb), and N would act as a light element (though not as light as H). And so a higher electronic level of Ga would interact with (or lie close to) a not-too-higher level of N, roughly speaking.

However when two elements come together, they don't necessarily merge their DoS withe respect to the same vacuum level, so it's possible the "DoS of one element" slides a little bit on the energy scale compared to the "DoS of the other element" before both contribute to total DoS. (I guess this "sliding" would have a formal name/terminology, maybe energy shift? or something).

Is this a good way to think and develop intuition? Is there a resource (a paper, book, blog post, etc), where similar higher level pictures are discussed? Thanks.

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