Electronic Resource
[S.l.]
:
International Union of Crystallography (IUCr)
Acta crystallographica
25 (1969), S. 12-29
ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The practical significance of accurate structure factors F(S) is discussed and illustrated in terms of different sorts of detailed structural information that becomes accessible when accurate estimates of F(S) are available and analysed appropriately. The common and contrasting features of X-ray and neutron F(S) (of non-magnetic systems) are considered. Structure factor formalism is reviewed to show the simplifications that are implicit in the formalism normally adopted for operational data-treatment. Limitations in the normal formalism introduced by the `spherical atom' approximation and the Gaussian ellipsoid harmonic rectilinear vibration approximation are shown with reference to the generalized formalism which avoids these constraints. The use of the generalized formalism for studying electron distribution aspects of covalent bonding in diamond structures and anharmonic aspects of anionic motions in fluorite structures is considered in detail. The difficulties of making reliable judgements on the probable real accuracy of F(S)-estimates by referring them to `spherical atom' models are illustrated in terms of different sets of F(S) data reported for MgO. The results of applying a variety of such models to the various data sets are discussed. The limitations of conventional Gaussian vibration formalism for extended studies (by either X-rays or neutrons, or both) of organic systems possessing appreciable librational behaviour are illustrated in terms of the generalized (neutron) formalism of F(S) that is needed to take a proper account of libration in cubic hexamethylenetetramine (HMT). It is shown that limitations of the conventional formalism are such that no simple `libration correction' is strictly applicable to the atomic positional parameters of HMT as determined viathe normal structure factor formalism.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0567739469000040
Permalink
|
Location |
Call Number |
Expected |
Availability |