Via Rietveld Mailing List
Thu, 26 Mar 1998

From: [email protected] (L. Cranswick)

Based on previous comments on zero shift, etc.  Following is some
feedback I have received that may be of use to people trying to
model/quantify these effects(?).

Lachlan.

-----

"On the topic of "shift". The shift in peak positions due to a zero
offset or a specimen displacement (height of sample) is given by:

2Th_shift = zero_error - 2 * (180/Pi) * Cos(Th) * specimen_displacement
/ Radius;

where Radius corresponds to the radius of the diffractometer. This
relation can be found in Klug and Alexander. 

The Cos(Th) term correcponds to the specimen displacement in mm; it
vanishes at 180 degrees 2Th. One way of determining the zero_error is to
collect some high (2Th > 120 degrees) and low angle lines of a standard
with a high linear absorption coefficient (LaB6) and then plot
zero_error versus -2*(180/Pi)*Cos(Th)/Radius. The intercept on the
y-axis would then be the zero_error and the slope of the line would be
the specimen_displacement.

This is not as easy as it sounds since other affects come into play at
high angles, namely the temperature dependence of the lattice parameters
and axial divergence at both high and low angles.

As previously mentioned, the best way of determining specimen
displacement is to align the diffractometer such that the zero error is
zero." 



-- 
Lachlan M. D. Cranswick
Collaborative Computational Project No 14 (CCP14)
    for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703  Fax: +44-1925-603173  Room C14
E-mail: [email protected]
CCP14 Webpage (Under heavy reconstruction):
   http://www.dl.ac.uk/CCP/CCP14/

Crystallographic Nexus Virtual Web and Internet on CD-ROM:
     http://www.unige.ch/crystal/stxnews/nexus/index.htm