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]
--
Brian H. Toby, Ph.D.Leader, Crystallography Team
[EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297 National Institute of Standards Technology
FAX: 301-921-9847Gaithersburg, MD 20899-8562
