Ian Tickle wrote:
OK, limiting the vote to people whom I think we can assume know what
vaguely they're talking about, i.e. Acta Cryst. / J. Appl. Cryst.
authors, and using the IUCr search engine we get 553 hits for "structure
amplitude" and 256 for "structure factor amplitude"
But be warned that not all Acta Cryst. authors give the term
"structure amplitude" the meaning that you think they do, i.e. a
shortcut version for "structure factor amplitude" !
In particular, P.P. Ewald (no less an authority than the ones you
quote), uses the term "structure amplitude" for the complex number
F(hkl). See e.g. Acta Cryst. A35 (1979), page 8.
To my surprise, M. von Laue in his treatise
"Rontgenstrahlinterferenzen" also uses the term structure amplitude
("Strukturamplitude") for the complex quantity F. He defines the
structure factor ("Strukturfaktor") as the square-modulus of F. This
seems to go back to early papers by P.P. Ewald. Both of these
quantities are also defined in exactly the same way by Hosemann &
Bagchi in their 1962 textbook on X-ray diffraction. In optics it makes
perfect sense to speak about complex amplitudes.
We thus have the "historic" definitions :
"structure amplitude" = complex F
"structure factor" = square-modulus of F
This comes from the fact that the intensity formulae which these
authors derive, and which remain valid for finite crystals and for
paracrystals, there is a neat factorization into a lattice-factor
("Gitterfaktor") on one hand and a structure factor ("Strukturfaktor")
on the other hand. The lattice factor only depends on the number and
spatial arrangement of unit cells within the crystal, whereas the
structure factor only depends on the atomic structure of one unit
cell. The latter is of course equal to the square-modulus of F.
To add to the confusion: Current-day small-angle scattering (SAXS)
specialists call "structure factor" the quantity which von Laue would
have called "lattice factor" (and they call "formfactor" the quantity
which von Laue called "structure factor") .....
Seems that there will be little agreement....
--
Marc SCHILTZ http://lcr.epfl.ch
Ian Tickle wrote:
OK, limiting the vote to people whom I think we can assume know what
vaguely they're talking about, i.e. Acta Cryst. / J. Appl. Cryst.
authors, and using the IUCr search engine we get 553 hits for "structure
amplitude" and 256 for "structure factor amplitude"
But be warned that not all Acta Cryst. authors give the term "structure amplitude" the meaning that you think they do, i.e. a shortcut version for "structure factor amplitude" !
In particular, P.P. Ewald (no less an authority than the ones you quote), uses the term "structure amplitude" for the complex number F(hkl). See e.g. Acta Cryst. A35 (1979), page 8.
To my surprise, M. von Laue in his treatise "Rontgenstrahlinterferenzen" also uses the term structure amplitude ("Strukturamplitude") for the complex quantity F. He defines the structure factor ("Strukturfaktor") as the square-modulus of F. This seems to go back to early papers by P.P. Ewald. Both of these quantities are also defined in exactly the same way by Hosemann & Bagchi in their 1962 textbook on X-ray diffraction. In optics it makes perfect sense to speak about complex amplitudes.
We thus have the "historic" definitions :
"structure amplitude" = complex F
"structure factor" = square-modulus of F
This comes from the fact that the intensity formulae which these authors derive, and which remain valid for finite crystals and for paracrystals, there is a neat factorization into a lattice-factor ("Gitterfaktor") on one hand and a structure factor ("Strukturfaktor") on the other hand. The lattice factor only depends on the number and spatial arrangement of unit cells within the crystal, whereas the structure factor only depends on the atomic structure of one unit cell. The latter is of course equal to the square-modulus of F.
To add to the confusion: Current-day small-angle scattering (SAXS) specialists call "structure factor" the quantity which von Laue would have called "lattice factor" (and they call "formfactor" the quantity which von Laue called "structure factor") .....
Seems that there will be little agreement....
--
Marc SCHILTZ http://lcr.epfl.ch
Ian Tickle wrote:
OK, limiting the vote to people whom I think we can assume know what
vaguely they're talking about, i.e. Acta Cryst. / J. Appl. Cryst.
authors, and using the IUCr search engine we get 553 hits for "structure
amplitude" and 256 for "structure factor amplitude"
Well, then you may be warned that not all Acta Cryst. authors give the term "structure amplitude" the meaning that you think they do, i.e. a shortcut version of "structure factor amplitude" !
In particular, P.P. Ewald (no less an authority than the ones you quote), uses the term structure amplitude for the complex number F(hkl). See Acta Cryst. A35 (1979), page 8.
To my surprise, M. von Laue in his (german) treatise on Rontgenstrahlinterferenzen also uses the term structure amplitude ("Strukturamplitude") for the complex quantity F(hkl). He defines the structure factor ("Strukturfaktor") as the square-modulus of F. This comes from the fact that the intensity formula which he derives and which remains valid for finite crystals, neatly separates into two factors, a lattice-factor ("Gitterfaktor") and a structure factor ("Strukturfaktor") which is of course equal to the square-modulus of F.
To add to the confusion: Current-day small-angle scattering (SAXS) specialists call "structure factor" the quantity which von Laue called "lattice factor" .....
--
Marc SCHILTZ http://lcr.epfl.ch
|