Iso-electronic compounds and elements under high pressure

• Main Author: Maclean, John R.
• Published: University of Edinburgh 1998
• Subjects: 530.41
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654370

High pressure directly affects interatomic distances, offering a method of studying interatomic potentials, and the differing factors governing structural adoption. Using a combination of angle-dispersive x-ray powder diffraction and total energy quantum mechanical calculations, high pressure phase transitions and the structures of iso-electronic materials have been investigated. Synchrotron radiation and image-plate area detectors were used to collect data on high pressure phases of Si, Sb, Bi, PhS, PbSe and PbTe. A new method of extracting information, employing Monte-Carlo techniques from powder diffraction data was developed and employed in some of the structural solution. Both full-potential and pseudopotential <I>ab-initio</I> calculations have been used to compute electronic and structural properties of experimentally observed high pressure phases of Si, CuCl, CuI, CuBr and Sb. From the results of these calculations and experiments, a new high pressure phase of silicon (Si-XII or R8) has been observed, existing between the high pressure metallic β-tin structure and the metastable ambient pressure structure Si-III, also known as BC8. The pressure dependence, electronic structure and stability of this, the most complex structure of silicon, and all the relevant high pressure phases have been studied. Calculations were performed on structures adopted by the iso-electronic copper halide compounds, CuX (X=Cl,Br,I). The results of these calculations agree with results from previous experiments that the binary analogue of the BC8 structure observed in Si is a stable high pressure form, and also provide details on the nature of bonding and electronic structure exhibited by each material. Experiments and calculations performed on the group V elements Sb and Bi show that a monoclinic structure, obtained by distorting a previously proposed tetragonal structure, is adopted by both. Rietveld refinements of a further monoclinic high pressure form of Bi are given, supporting earlier experimental analysis.