Details of Grant 

EPSRC Reference:	GR/H36184/01
Title:	MEASUREMENT OF CHARGE DENSITY DISTRIBUTION IN CRYSTALS BY CONVERGENT BEAM ELECTRON DIFFRACTION
Principal Investigator:	Vincent, Dr R
Other Investigators:	Bird, Professor D
Researcher Co-Investigators:
Project Partners:
Department:	Physics
Organisation:	University of Bristol
Scheme:	Standard Research (Pre-FEC)
Starts:	14 July 1992	Ends:	13 July 1995	Value (£):	165,604
EPSRC Research Topic Classifications:	Materials Characterisation
EPSRC Industrial Sector Classifications:
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Panel History:
Summary on Grant Application Form
The charge density is of fundamental importance in understanding the properties of crystalline materials. All information about the bonding is contained within the valence charge distribution and density functional theory shows that all contributions to the ground state energy of a solid are unique functionals of the density. The aim of the proposed research is to develop methods for the experimental determination of the charge density distribution in crystals. This will be achieved by fitting simulated convergent beam electron diffraction patterns to experimental data and this extracting very accurate low order structure factors from which the charge density can be reconstructed. The method to be used is based on that pioneered by Spence and co-workers at Arizone, 1,2,3,4 but, as detailed below, we propose a number of extensions to their work. A first priority will be to implement these improvements and to rigorously test the validity and accuracy of Spence's technique. The method will then be applied to a number of materials where the charge distribution is of interest. First, GaAs, other III-V semiconductors, MgO and SiC will be used as test cases where a detailed comparison can be made between the measured charge density and that calculated either by ab-initio Hartree-Fock methods, or by using norm- conserving pseudopotentials combined with the local density approximation (LDA) to density-functional theory. Although the aim here is basically to test the experimental method, there is also the possibility that the measured charge density may provide an interesting check on the accuracy of pseudopotential/LDA and Hartree-Fock calculations. Having tested the technique on these well understood materials we will then go on to apply it to a variety of crystals where the charge distribution is of interest. The materials to be studied will be detailed below, but a priority will be to examine the intermediate valence compounds SMS and SM1-xYxS, where phase transitions arise due to a 4 <-> 5d valence change in the Sm ions 5. It is clear that the charge density is highly relevant to the physical phenomena which occur, and its accurate experimental determination will provide useful insights into the behaviour of these materials.
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