>Now if you have transparency effect + large asymmetry (which
>is likely to occur with some diffractometer geometry), I don't
>know what could happen if the software has not adequate
>asymmetry correction.
The trouble with asymmetric peaks is that both the peak maximum and the peak
centre of gravity are shifted with respect to the "true" Bragg position,
which coincides with the divergence of the asymmetric "kernel". These
shifts are strongly angular-dependent (being maximum at low and high angle)
but also change with resolution (i.e., peak width), and, to a lesser extent,
with sample (or spot) width (height for us neutronists). The peak maximum
is the least shifted but the most sensitive to resolution effects, whereas
the peak centre of gravity is more shifted but less sensitive. If you do a
spit-function fit, you assign the Bragg position to the peak maximum,
whereas if you do a symmetric-function fit you assign it somewhere in
between the centre of gravity and the maximum. If you want to do a test for
this effect, just generate an asymmetric profile appropriate for your
instrument, e.g., using the utility function "proftest" in GSAS, and then
try to fit it with whatever function you normally use. One may of course
consider empirical corrections, but the best it to use the
Finger-Cox-Jephcoat function (e.g., latest versions of GSAS). This
function, however, only accounts for beam divergence in an approximate way.
Best
Paolo G. Radaelli
Dr. Paolo G. Radaelli
ISIS Facility
Rutherford Appleton Laboratory, Bldg. R3
Chilton, Didcot
Oxon. OX11 0QX
United Kingdom
Phone : (+44) 1235-44 6290
FAX : (+44) 1235-44 5642
e-mail: P.G.Radaelli@rl.ac.uk
pgr@isise.rl.ac.uk