HIGH-RESOLUTION POWDER X-RAY-DIFFRACTION MEASUREMENTS IN GUINIER GEOMETRY USING A DIAMOND-ANVIL CELL

In this paper we describe a unique apparatus for carrying out high pre ssure powder X-ray diffraction measurements. Guinier geometry provides a truly focussed and an almost monochromatic beam of X-rays, thereby offering a significantly high signal-to-noise ratio diffraction peaks of a powdered sample. The diffractometer is a combination of a curved quartz-crystal monochromator, a home made long body diamond anvil cell capable of generating pressure in excess of 60 GPa, a fiat position s ensitive detector (or an X-ray imaging plate) and a goniometer with co mpound stages. FWHM of the direct beam, derived from a molybdenum targ et and a Johansson-Guinier type curved quartz crystal monochromator is similar to 0.04 degrees, when a scan is made with a scintillation cou nter detector. Performance of this state-of-the-art diffractometer sys tem is discussed. Problems such as the solutions of the crystal struct ures of the high pressure phases of UAl2, ThAl2, Ti3Al, BaFBr and BaFC l have been studied. Contact with the 3-parameter structural phase dia gram has been made to make a qualitative account of the phase transiti ons in the above systems. It is found that a revisit to the band struc ture calculations, especially in the case of Ti3Al, is necessary. This paper aims to address the aspects mentioned above.