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.