Quoting Sung-Ho KIM <[EMAIL PROTECTED]>: > Hi, All, > 1) According to A. David Rae��s paper, Acta Cryst. > (1990), B46, 474-487, B1a1 can be transformed to P1c1 > using new axes (a+c)/2, b, c and origin relocated to > put the glide at y=0." Please help me to convert B1a1 > (non-standard setting) into P1c1 (standard setting). > In particular, how to convert atomic coordinate? > > 2) If an atom (a1) partly occupy four different sites > (b1, b2, b3 and b4) and the total amount of a1 have to > keep 1.0, how to define constraints for a1 and b1~ b4 > atoms in GSAS? > Please give me an advice about two things. >
I may at least bring an answer to the second question. This topics has already been discussed in this list. The problem is a bit tricky but can be solved unambiguously. - if two atoms A and B (one of them may be vacancy) are distributed among 4 sites 1, 2, 3, 4 - if the total amount of A and B is known - if the diffraction contrast between A and B is sufficient Then one may introduce the atoms as follows: site 1: A1 B1 site 2: A2 B2 site 3: A3 B3 site 4: A4 B4 A'4 B'4 A''4 B''4 That means that two additional A and two additional B atoms are introduced on site 4. The initial total amount of A and B should match the known composition. Then, the constraints should be introduced as follows, where d represents the shifts on occupancy parameters expressed in number of atoms (that means that a multiplicative factor should be introduced in GSAS because this program deals with site fraction contrary to Fullprof): dA1=-dB1=-dA4=dB4 dA2=-dB2=-dA'4=dB'4 dA3=-dB3=-dA''4=dB4 so that the A atoms removed from site 1 are compensated by B atoms in the same site and are transfered to site 4, and so on. The procedure can be extended whatever the number sites. A brief explanation can be found in J.-M. Joubert, R. Cern�, M. Latroche, A. Percheron-Gu�gan and K. Yvon, "Site occupancies in LaNi5 three-substituted compound determined by means of multiwavelength X-ray powder diffraction." J. Appl. Crystallogr., 31 (1998) 327-332. The problem was a little bit different since it was about the determination of the distribution of 4 atoms among 2 sites, but the solution is the same. Best regards Jean-Marc Joubert
