>> I have found that their (U,V,W) refined parameters seem to be very
correlated.
The Cagliotti formula FWHM**2=W+V.tan(theta)+U.tan**2(theta) with
parameters U,V,W represents instrument line width FWHM as a function of
scattering angle 2.theta for a conventional "neutron powder
diffractometer".
This function may approximate line broadening in other situations, but
really you should use more appropriate parameters for broadening due to
particle size and strain, or other types of diffractometer. See the
FullProf manual for alternatives. Particle size and strain may be
different for different phases, and even for different crystallographic
axes. Such effects may not be seen for low resolution neutron
diffractometers.
Since Cagliotti's formula is a quadratic equation, U,V,W will naturally be
strongly correlated if there is no clear minimum in the line width as a
function of angle. You can remove this correlation by simply fixing the
angle for this minimum i.e. by differentiation and requiring that
V=-2U.tan(theta) where 2.theta is approximately the monochromator angle
for a conventional neutron diffractometer.
______________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
<alan.he...@neutronoptics.com> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
______________________________________________
