HI Don,
On Fri, Jun 8, 2018 at 8:55 AM Don Baker
Hello All,
First let me apologize if the question below is inappropriate for this group, but I don't know of a better set of experts that I can ask.
Great to hear from you, hope you're doing well. And, to be clear, this sort of question is completely appropriate here.
So, here is some background:
I have some old S K-edge XANES data from NSLS X15B for S-bearing silicate glasses that was collected by a Ph.D. student who decided to leave for a good job rather than finishing their degree. During my current sabbatical year I have been looking at the data and reading papers on the topic. I think that I now understand, at least qualitatively, the changes in the spectra as a function of iron concentration in the glasses, but to perform quantitative modelling I need XANES spectra of MgS and CaS, which we did not think to collect all those years ago. I have searched for XANES spectra of MgS and CaS on the CARS XAFS Data Library and on the XAFS Database at IIT without success. I am unaware of other databases available.
Now for for my questions:
1. Does anyone know of a XANES database that contains MgS and CaS spectra? (Or did I miss it in the databases I searched?)
There is also a good-sized set of S XANES from ESRF ID21 at http://www.esrf.eu/home/UsersAndScience/Experiments/XNP/ID21/php.html though I do not see CaS or MgS spectra there. We have lots of data that are not (yet?) in the CARS database, including quite a bit of data on S K edges, taken with various groups. Much of this data *should* be on the CARS XAFS database, but isn't yet. For CaS and MgS in particular, we have recently measured S XANES (in total fluorescence yield) on these and there is sort of a story here. Following up on Riti's comments, these spectra can definitely have over-absorption effects. In addition, these materials are apparently challenging to synthesize well, and keep as stable sulfides. We've been working with Stephen Parman at Brown and students on this, and have some spectra that we think are good. The MgS spectra has been published in a recent paper with Elizabet Head (Head et al GCA226, p149 2018 https://doi.org/10.1016/j.gca.2018.01.033). There are also MgS spectra from Mike Fleet, though we are unsure of how to explain the differences. Because some of this data is unpublished and still very new, I'm slightly reluctant to make it public, but will send some of the data we have privately.
2. If I cannot find measured MgS and CaS spectra, what do you think about using calculated spectra (e.g., Zheng et al.,2012, NPJ Computational Materials, 12 -- which uses FEFF) for quantitative modelling?
In my humble opinion,XANES calculations are useful for identifying features, but are less useful for Linear combination analysis with experimental spectra. Cheers, --Matt Newville