RE: [sdpd] LaB6 standard sample preparation

[Van den Berghe Sven <svdbergh...@sckcen.be>]

The standard would indeed no longer be valid after annealing, but it
probably won't be valid after taking it out of its "argon-filled glass
container" either.  The moment one begins sintering of the pellet, I agree
that additional preferred orientation will be generated.  The annealing I'm
talking about would be to remove the influence of the pressing.  I seem to
remember a discussion about it somewhere, but I may be wrong.

You are right, of course.  If the LaB6 standard is only meant to provide
peak profiles, one shouldn't be concerned with preferred orientation.  We
want to use the standard to qualify our lattice parameter determination
procedures, as well as our instrumental effects.  We indeed don't need (and
I personally don't believe either) a precision of 8 digits on the lattice
constant.

Sven


> -----Original Message-----
> From: Armel Le Bail [mailto:alb...@cristal.org]
> Sent: zondag 23 augustus 2001 11:00
> To: sdpd...@yahoogroups.com
> Subject: Re: [sdpd] LaB6 standard sample preparation
> 
> 
> After annealing, a NIST standard ceases to be a standard, IMHO.
> 
> Moreover, annealing would increase the grain size, possibly, and would
> lead to more risk of preferred orientation.
> 
> If you want to measure instrumental profile shape and FWHM (etc),
> then  preferred orientation is not a real problem. You may 
> characterize
> your pattern by a Rietveld program in "structure factor extraction"
> mode (i.e. without structure constraint), or by the Pawley method.
> Who cares about the LaB6 crystal structure anyway ?
> 
> Think also to reference samples from Gem Dugout :
> http://www.thegemdugout.com/
> much less expensive than the NIST standards. Maybe
> "reference samples" can be annealed, they are not really
> "standards" after all, but they can be used if you do not
> need an accuracy with 8 digits, if not more, on the cell 
> parameters ;-).
> 
> Armel
> http://www.cristal.org/course/
> 
> 
> 
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