Hi Pat,
Not everything requires a computer, even though I have a reputation for
liking to use them.
Going back to Stout & Jenson, one finds
F(hkl) = SUM f(i) EXP[2 pi i * (hx + ky + lz)]
[since you are ignoring displacement (nee thermal) factors]
For the FCC case, you have 4 atoms at (0 0 0), (1/2 1/2 0), (0 1/2 1/2),
(1/2 0 1/2)
So if you pick one of your reflections, say the 111, then F(111) = 4
f(Ni)
For lambda= 1.5418, 2theta = 44.53 (I cheated a bit here and used a
computer to get 2theta rather than looking up a0 for Ni in a book) and
sin(theta)/lambda = 0.246 ~= 0.25,
so f(Ni) ~= 20.39, using Volume C.
so F(111) ~= 81.56, which is pretty close to the powdercell value and
would probably be closer if I interpolated to get f at
sin(theta)/lambda=0.246 rather than sin(theta)/lambda=0.25
Why then does lazy-pulverix give 77.4?
Well, looking at the printout, one sees that the default for B is 1.0,
so adding the "thermal" term, exp[-B * sin2(theta)/lambda2] = 0.94, I
get
F(111) ~= 76.78, which is pretty close to the lazy-pulverix value.
End of mystery, I hope. I will leave computing F for the other two
reflections as "an exercise for the reader."
Patrick Weisbecker wrote:
>
> Hi all,
>
> As a new rietveld user (fullprof) i wanted to check structure factors
> calculated thanks to several others softwares : Lazy-Pulverix, Powdercell
> 1.0 and Carine Crystallography 3.1.
>
> And i was surprised to obtain significants differences. For instance for
> Nickel FCC, lazy-pulverix and fullprof give for 111,200,220 planes
> :70.4,65.2,50.3 but powdercell 1.0 and Carine Crystallography give for the
> same planes 82.2,77.0,62.1. (no thermal factors had been considered in both
> cases)
>
> Such differences can have a great influence for quantitative analysis.
>
> Does someone know where differences come from? Which software is the more
> reliable?
>
> Patrick Weisbecker
> Laboratoire de Science et Genie des Materiaux metalliques
> Ecole des Mines de Nancy
> e-mail: [EMAIL PROTECTED]
--
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