Energy Dispersive X-Ray Diffraction from Germanium, Gallium-Arsenide Gallium-Phosphide and Aluminum-Antimony at High Pressures Using Synchrotron Radiation.

The practicality of energy dispersive X-ray diffraction from high pressure powder specimens using synchrotron radiation has been demonstrated. Reasonable quantitative agreement has been obtained between the experimental diffraction data and the theoretical relative integrated intensities of the diffraction lines for known structures under rather hydrostatic pressure conditions. Pressure-induced structural phase transitions in Ge, GaAs, GaP, and AlSb have been studied in some detail with this energy dispersive difraction method. Ge transforms to the (beta)-Sn tetragonal structure as previously observed by Jamieson, but the transition pressure is 80 (+OR-) 5 kbars, a somewhat lower value than generally reported. GaAs exhibits an orthorhombic structure above 172 (+OR-) 7 kbars, GaP a tetragonal structure above 215 (+OR-) 8 kbars, and AlSb an orthorhombic structure above 77 (+OR-) 5 kbars. Although the space groups of these latter three high pressure polymorphs have not been determined unequivocally, mainly due to the apparent presence of crystalline defects, some possible models are described for these high pressure structures. Lastly, a comparison of the existing phase transition data with the theoretical diagrams of Zunger, based on pseudopotential length scales, indicates that it may be possible to predict the high pressure polymorphs of crystals with diamond or cubic zincblende structures at ambient conditions.