[Ifeffit] Question on GDS values: amp

[Scott Calvin]

Hi Nic,

The delr and ss values may adopt reasonable values with a negative  
amp, but they're almost certainly not the right values. A negative amp  
turns the chi(k) upside-down. To make it ft, the other parameters then  
have to shift the graph over by half an oscillation, yielding values  
of E0 and delR that are wrong.

It is unusual that a first-shell fit to a standard doesn't seem to be  
giving a qualitatively reasonable result with an amp of 1. What  
happens if you do a sum of the most important paths without a fit?  
Does the fit look qualitatively correct over a large range of R? (The  
amplitudes will of course be substantially off without a  
fit.)            It sounds like something substantial is wrong: either  
the material isn't actually dadolinium oxide, or the structure you're  
using for the feff calculation isn't right (is there more than one  
crystallographic setting?), or there was serious distortion in the  
measurement (perhaps a thick, concentrated standard was measured in  
fluorescence without correction).

--Scott Calvin
Sarah Lawrence College

On Aug 1, 2011, at 12:36 AM, Nicholas.Tse at csiro.au wrote:

> Dear All,
>
> Just wondering what could I do to make the amp guess value as a  
> positive number? If I run it and let it float (i.e. guess), the amp  
> becomes negative in the resulting fits, but the fit has nice delr  
> and ss values (i.e. make sense). If I restrict the amp value to 1,  
> everything else doesn’t fit. I am only fitting the first nearest  
> neighbour in a measured standard; Gadolinium oxide.
>
> Regards,
>
> Nic
>