Standards, Rietveld Bragg R and background

Armel Le Bail ( armel@fluo.univ-lemans.fr )
Sun, 10 Aug 1997 11:36:14 +0200

The Al2O3 NIST SRM 1976 is a relatively good standard
for testing your diffractometer (but LaB6, SRM 660, gives
lower FWHMs). Errors on intensities are mainly due to
the sample itself (not intercepting the whole X-ray beam,
preferred orientation, sample transparency, etc). So that I
recommend the SRM 1976, if the data are treated with the Le
Bail method, an option which should be included in any good
Rietveld program. With this option, you will obtain a fit as if
the structure had been perfect (Bragg R ~ 0.0), so that the
Rp and Rwp will be as low as possible. One advantage of
SRM 1976 is its compacted plate form.

Another standard which has already been discussed on this
list is NAC (NaCa2Al3F14), frequently used on the INEL
120 PSD for calibration. The main inconvenient is its cost.
The producer (which is in my lab) sells it for a price larger
than gold itself (but lower than diamond :-)...

About the background influence on Rp and Rwp, it seems
to me that the original Rietveld program gave exclusively the
"background subtracted" and "peak only" profile R factors
so that they are now called the "conventional Rietveld Rp and
Rwp factors".

Then, if I am not wrong, I have one question : who invented the
total Rp and Rwp factors ?-) I suspect that they were invented
by error or just for an endless discussion about their absence of
meaning regarding the crystal structure...

Armel Le Bail, back of vacations.