ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The problem of the interatomic bond in molecules and crystals is first of all that of a fairly precise determination of the electron distribution. The electron density distribution in crystals can be considered as the distribution of the square of the wave function. Thus an experimental determination of structure factors is one of the most direct methods of experimental determination of wave functions. Calculations of physical and physical-chemical properties of crystals can be made by using quantum mechanical methods, making use of wave functions obtained experimentally. The question arises of the necessary limits of accuracy of experimental F2 values for solving various quantum mechanical problems as well as for comparing them with theoretical values. It should be noted that the theoretical structure factors are approximate since the exact solutions of the Schrödinger equation is possible for the hydrogen atom only. Experimental determination of F2 values on powders poses very complicated problems including the measurement of absolute X-ray scattering intensities. Results of experimental determinations of F2 values on powder of metals, alloys and compounds with covalent character, including semiconductors, are analysed. Possible reasons for deviations, including the role of extinction, particle size of the powders under investigation, possibility of preferred orientation, change of state and composition of the surface, the role of polarization of the monochromated beam and lattice dynamics, are discussed. The difference between the data of different authors, which seems to be due to the degree of the accuracy of scaling to absolute intensity values, is also discussed. Different parts of the f-curves give different information on the electron density distribution of the crystal. The amount of information which F2values contain on the outer electron distribution depends to a great extent on the type and the parameters of the crystal lattice, and on the degree of anisotropy. There are some differences mainly in absolute values between the data of different authors. Some problems of reduction of the reflexion intensities are also discussed. the results of some measurements made on powders of the elements and on the semiconducting-type compounds with covalent bonds are analysed as well. A large number of measurements were made on AIIIBV compounds. The results show the differences in F2 values due to the change of the effective charge of ions. In conclusion some experimental determinations of f-curves are quoted and the necessary accuracy and possibilities of correction of the measured intensity of Debye reflexions are evaluated.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0567739469000386
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