To all: The mathematical description of "crystals" is valid for the bulk of the volume. However, the description suffers from a physical "termination of series" effect at the surface of the crystal. For very large crystals, the amount of surface is much greater relative to the total volume of the crystal. However, when the crystalline material approaches what we now refer to as the "nano" state (perhaps less than 40 Angstroms - 8 to 10 unit cells), the amount of surface becomes significant. The chemical composition - oxidation state of surface atoms - is intuitively different at the surface of a material than within the bulk. How does one deal with that situation? The effects are present in the powder pattern; how does one model them? Frank May Research Investigator Department of Chemistry & Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 U.S.A 314-516-5098 - office 314-623-4524 - cell
________________________________ From: Larry Finger [mailto:[EMAIL PROTECTED] Sent: Wed 4/2/2008 9:47 AM To: Jae-Ho Chung Cc: [email protected] Subject: Re: Question on Rietveld with x-ray Jae-Ho Chung wrote: > Dear experts on Rietveld with x-ray, > > > > When it comes to Rietveld, I have exclusively been working with > neutrons. Now I am just trying Rietveld with x-rays, and immediately > came up with a question. Since x-ray scatters from electrons, it seems > to me that we always have to use, for instance, O-2 instead of O, and so > on, for ionic compounds. But I have an impression that O is still used > in many examples, including certain example files given in GSAS, such as > Y2O3.EXP. Why is it? I suggest that you plot the scattering curves for O and O-2, and then decide how much difference it makes. In addition, you should also postulate a theory concerning the differences. Hint: The electron cloud is not a "point object". > As a naturally following question, since no material is perfectly ionic, > the oxidation numbers may become another "refinable" parameters. But I > suppose they are never refinable in GSAS or in FullProf. Therefore, I > suppose the fractional occupation may be compromised instead? Is this in > any way discussed among x-ray experts? See above. What experiment would you propose to make the oxidation number be refinable? Larry
