Hello Feng,
I used WEBATOMS(version 1.8), to calculated the feff.inp for Si-K of alpha-SiO2 (quartz phase), using the crystal structure (as listed below) from Wyckoff's Crsytal Structures(P312-313), however, I could not get correct atomic configuration, such as, I did not get the tetrahedron (of 4 nearest neighboring oxygen atoms), I got two n.n. oxygen atoms only, please refer to the results on the bottom. As suggested by Dr. Ravel, I tried the atoms in Artemis, but still I could not get correct feff.inp.Would you please help me to check and suggest, thank you so much.
The answer to your question lies in one of those cristallographic oddeness, namely that you can describe the crystallographic structure of quartz in two distinct sets of axis, the obverse (a,b,c) and the reverse (a', b', c), with the latter being obtained by rotation of the former by pi/3 around c. The caveat is that the cristalloraphic data you used are relevant to the reverse axes (and usually is described so in the litterature), whereas the P31 2 1 is relevant to the obverse. Thus you have to make a slight coordinate change; we had to deal with that before (Schlegel et al, Geochim. Cosmochim. Acta, vol. 66, p 3037, 2002), and we came up with the following cristallographic data P 32 2 1 a = 4.9137 b = 5.4047 x(O) = 0.5867 y(O) = 0.7328 Z(O) = 0.7855 x(Si) = 0.5303 y(Si) = 0 z(Si) = 2/3 As a structural check, all Si-O distances must be within 1.60-1.62 angstroms (160-162 pm for purists) Hope this will help; let me know if not Best regards, Michel