Source Code Deposited on CCP14 site, 17th May 1999

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  • DOS LAPOD Compiled by J. Ian Langford, 18th May 1999

LAPOD: a note on the correction of powder data for instrumental peak shifts

From: "Dr J.Ian Langford" (langfoji@novell1.bham.ac.uk)
Organization: The University of Birmingham
To: L.M.D.Cranswick@dl.ac.uk (L. Cranswick)
Date: Thu, 20 May 1999 17:02:49 BST
Subject: Re: LAPOD

LAPOD: a note on the correction of powder data for instrumental peak shifts

In my view, it is better to obtain instrumental peak shifts by using 
data from an internal or external standard, rather than to include 
relevant parameters as refinable quantities.  At Birmingham our 
correction for instrumental shifts is a two stage process.  We use 
5-10 micron silver (high purity, Koch-Light) as an external standard 
for reflections at medium and high angles (NIST SRM675 at low angles) 
and assume that shifts are due only to zero error and specimen 
surface displacement (SSD).  Transparency is a problem with Si, 
unless a smear sample is used.  We then obtain a least-squares fit of 
the difference between the observed Ag line positions and those 
calculated, at the temperature of the instrument, versus cos theta.  
This will be linear, unless some other misalignment is appreciable,  
and it gives the zero error and SSD. Microsoft EXCEL is used for this 
purpose.   We then use a standard program for producing hkl, d and 
2theta from cell parameters for the sample of interest, for a given 
system and S.G. and with allowance for instrumental shifts as 
determined from the Ag data.  This provides the hkl and corrected 
2theta input for LAPOD.

If the sample is not highly absorbing, the effective linear 
absorption coefficient, taking into account packing density, if the 
sample is a powder, can also be input.  However, at present the 
correction on;y applies to the case os a 'thick' sample 
[mu*t*cosec(theta) >> 1, where t is the sample thickness].  mu can be 
measured directly by transmission or calculated, if the packing 
density is ascertained.

The best account of sample preparation for obtaining precise cell 
parameters, and for XRD in general, is probably Chapter 9 in Jenkins 
& Snyder (1996) 'Introduction to X-ray Powder Diffractometry' (New 
York: Wiley) - and indeed of most other aspects of experimental 
powder XRD.

Ian Langford