RE: UVW - how to avoid negative widths?

2009-03-20 Thread Alan Hewat
matthew.row...@csiro.au said: From what I've read of Cagliotti's paper, the V term should always be negative; or am I reading it wrong? That's right. If FWHM^2 = U.tan^2(T) + V.tan(T) + W then the W term is just the Full Width at Half-Maximum (FWHM) squared at zero scattering angle (2T). FWHM^2

RE: UVW - how to avoid negative widths?

2009-03-20 Thread Michael Glazer
. Many times one can reduce R factors by playing with the diffraction geometry terms, but with little obvious improvement of the structural results. -Original Message- From: Alan Hewat [mailto:he...@ill.fr] Sent: 20 March 2009 07:13 To: rietveld_l@ill.fr Subject: RE: UVW - how to avoid

RE: UVW - how to avoid negative widths?

2009-03-20 Thread miguelhg
Hi to all!, in fact, the Cagliti's expression is just a way to show the angular variation of fwhm, as was mentioned was usef for neutron diffraction and adopted in XRD, we can also build another dependence such as FWMH vs 2theta directly and it is useful to evaluate size and strain, the problem

Re: UVW - how to avoid negative widths?

2009-03-19 Thread Jon Wright
According to Caglioti relation, the dimensions of U,V,W are as (angle)^2. Quick question - does anyone have a trick to stop the Cagliotti formula going negative? Prodd currently has a habit of bugging out on a sqrt(negative) and I'm wondering how other folks worked around that, or if I've

RE: UVW - how to avoid negative widths?

2009-03-19 Thread May, Frank
: Thu 3/19/2009 8:58 AM Cc: Rietveld Method Subject: Re: UVW - how to avoid negative widths? According to Caglioti relation, the dimensions of U,V,W are as (angle)^2. Quick question - does anyone have a trick to stop the Cagliotti formula going negative? Prodd currently has a habit of bugging

Re: UVW - how to avoid negative widths?

2009-03-19 Thread Alan Hewat
Jon Wright said: Quick question - does anyone have a trick to stop the Cagliotti formula going negative? This can happen if the resolution is relatively flat, so that there is no well defined minimum. Then the quadratic Cagliotti formula produces large correlations between U,V,W. The trick is

Re: UVW - how to avoid negative widths?

2009-03-19 Thread Jon Wright
Alan Hewat wrote: Jon Wright said: Quick question - does anyone have a trick to stop the Cagliotti formula going negative? This can happen if the resolution is relatively flat, so that there is no well defined minimum. Seems to be the problem - also rather close to zero anyway. if

RE: UVW - how to avoid negative widths?

2009-03-19 Thread David, WIF (Bill)
- and we've got the Gaussian component of the size and strain directly from the Cagliotti relationship. -Original Message- From: Jon Wright [mailto:wri...@esrf.fr] Sent: 19 March 2009 20:49 To: alan.he...@neutronoptics.com Cc: Rietveld Method Subject: Re: UVW - how to avoid negative widths

RE: UVW - how to avoid negative widths?

2009-03-19 Thread Matthew.Rowles
From what I've read of Cagliotti's paper, the V term should always be negative; or am I reading it wrong? Additionally, there is some good work on the use of the Cagliotti (and TCHZ) functions in the paper by Young and Desai; it also goes over how to incorporate sample dependent terms into the