High-Pressure, High-Temperature Synthesis of Superhard Boron Suboxide
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H. Hubert, McMillant.
L.A.J. Garvie', K. Leinenwebert, P.R. Buseckt*, W.T. Petuskey t , and P.F.
tMaterials Research Group in High Pressure Synthesis, Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604. *Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604. 'Department of Geology, Arizona State University, Tempe, AZ 85287-1404.
ABSTRACT A multianvil device was used to investigate the formation of B.O phases produced in the 2 to 10 GPa pressure range with temperatures between 1000 and 1800 *C. Amorphous and crystalline B and BP were oxidized using B 2 0 3 and Cr0 3. Using powder X-ray diffraction and parallel electron energy-loss spectroscopy (PEELS), we were unable to detect graphitic or diamondstructured B20, reported in previous studies. The refractory boride B 6 0, which has the arhombohedral boron structure, is the dominant suboxide in the P and T range of our investigation. PEELS with a transmission electron microscope was used to characterize the boron oxides.
INTRODUCTION There is a continuing search for materials combining high hardness, low density, chemical inertness, and useful optical and electronic properties. Since the first successful laboratory synthesis of diamond in 1954 [1], attempts to develop materials with a combination of properties approaching, or even improving upon, those of diamond have generated important research efforts. Tetrahedrally coordinated compounds with highly covalent bonding (low electronegativity difference between combining atoms) are thought to provide good candidates to reproduce the characteristics of diamond. In 1965, Hall and Compton [2] introduced the concept of symmetrical and asymmetrical compounds, isoelectronic with carbon, to explain the behavior of known group IV analogs with the view of designing interesting materials. BN is classified as a symmetrical compound since it contains elements symmetrically arranged around carbon; for example, cubic BN exhibits properties close to that of diamond. Hall et al. [2] proposed an asymmetrical compound made of B and 0, and reported the synthesis of the graphite analog B20 by high-pressure and high-temperature methods. Their samples were prepared by reducing B 2 0 3 with B at pressures from 5 to 7.5 GPa and temperatures from about 1200 to 1800 'C. The light reddish-brown reactants were analyzed by powder X-ray diffraction (XRD) and assigned to a graphite-like structure with hexagonal symmetry (a=7.98 A
and c--9.09 A). In a later study, Endo et al. [3,4] reported the synthesis of B2 0 with a diamond-like structure, at lower pressure, by oxidizing BP via the thermal decomposition of Cr0 3 to Cr0 2. The precursors were placed in a gold capsule, pressurized from 3.5 to 5.5 GPa at 1000 to 1300 'C for 5 hours. The recovered dark-brown material exhibited electron diffraction patterns consistent with a trigonal space group P3, a=2.879 A and c= 7.052 A. The XRD pattern [3] matched that 191 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Socie
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