What is the disordered crystal structure? Who are A and B? And how would the coordination of Si change if A and B order? Do you see signals that are characteristic of both end members in the same intermediate composition samples, or do you just see one end member or the other? Cheers, Ben

It is a (Zr,Hf) SiO4 in any case the Si coordination is four oxigen

It is not the Si-coordination by Oxygen that matters, it is the Si coordination by Zr,Hf.

The point is that you won't see changes in the NMR spectra from Zr:Hf ordering or segragation unless the ordering (segregation) changes the environment around the Si.

If you send me the crystal structure data on the end-members, I can set up a first-principles phase diagram calculation (FPPD) and see if ordering or segregation is predicted:

[email protected]

If stable ordered phases are predicted, we can see how those modes of ordering might change Si-enviroments, or not.

Cara Maria

What NMR technique are you using? If single pulse or something similar, you can hardly identify whether or not segregation is occurring, unless the Zr or Hf form strong bonds with the NBOs (non-oxygens bridges). Because this would be the only way to have an "indication" of phase segregation. One way to check would be to carry out an experiment that to obtain information about the dipolar interaction among these compounds, however this is only possible in silicates enriched 29Si and also depends on whether Zr or Hf are observable by NMR.

Maybe a way to check if the behavior is real solid solution is using vibrational spectroscopy (Raman for example) and verify that the system is behaving the "one" or "two" modes.

"one" mode behavior so that the vibrational spectrum preserves the same number of Raman active modes, but whose frequencies vary with the concentration of the replacement atom, that is, if we compositions of silicates extreme pure Zr and Hf, the vibrational spectrum of mixed silicates must vary continuously with atomic substitution

two modes behavior , in which the two sub-networks can vibrate almost independent but coupled, or even mixed silicates should be possible to identify the vibrational spectrum on each compound were mixed.

Hi Maria,

Zr and Hf are chemically so similar that it is almost impossible to separate them and most Zr compounds are isostructural with Hf compounds. Unless you have different crystal structures for ZrSiO4 and HfSiO4 (or different NMR spectra for the same purpose) you will almost for sure find Zr/HfSiO4 solid solutions in the whole Zr/Hf range. It may be hard to see phase separation by XRD if your less abundant element is less than a few percent (