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Register a new userHighly Boron-Doped Graphite and Diamond Synthesized From Adamantane and Ortho-Carborane under High Pressure
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This work demonstrates the effectiveness of the high-pressure method for the production of graphite and diamond with a high degree of boron doping using adamantanecarborane mixture as a precursor. At 8 GPa and $1700 ^{o}C$, graphite is obtained from adamantane $C_{10}H_{16}$, whereas microcrystals of boron-doped diamond (2{div}2.5 at.% of boron) are synthesized from a mixture of adamantane and ortho-carborane $C_{2}B_{10}H_{12}$ (atomic ratio B:C = 5:95). This result shows convincingly the catalytical activity of boron in the synthesis of diamond under high pressure. At pressures lower than 7 GPa, only graphite is synthesized from the adamantane and carborane mixture. Graphitization starts at quite low temperatures (below $1400 ^{o}C$) and an increase in temperature simultaneously increases boron content and the quality of the graphite crystal lattice. Thorough study of the material structure allows us to assume that the substitutional boron atoms are distributed periodically and equidistantly from each other in the graphite layers at high boron concentrations (>1 at.%). The theoretical arguments and model ab initio calculations confirm this assumption and explain the experimentally observed boron concentrations.
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Theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities. Hitherto the existence of a non-icosahedral boron allotrope has been one of them. Here we report on the first non-icosahedral boron allotrope, which we denoted as {zeta}-B, with the orthorhombic {alpha}-Ga-type structure (space group Cmce) synthesized in a diamond anvil cell at extreme high-pressure high-temperature conditions (115 GPa and 2100 K). The structure of {zeta}-B was solved using single-crystal synchrotron X-ray diffraction and its compressional behavior was studied in the range of very high pressures (115 GPa to 135 GPa). Experimental validation of theoretical predictions reveals the degree of our up-to-date comprehension of condensed matter and promotes further development of the solid state physics and chemistry.
This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$pm$28,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.
A first cobalt boride with the Co:B ratio below 1:1, Co5B16, was synthesized under high-pressure high-temperature conditions. It has a unique orthorhombic structure (space group Pmma, a = 19.1736(12), b = 2.9329(1), and c = 5.4886(2) {AA}, R1 (all data) = 0.037). The material is hard, paramagnetic, with a weak temperature dependence of magnetic susceptibility.
Electrotarnsport and magnetic properties of new phases in the system Cr-GaSb were studied. The samples were prepared by high-pressure (P=6-8 GPa) high-temperature treatment and identified by x-ray diffraction and scanning electron microscopy (SEM). One of the CrGa$_2$Sb$_2$ phases with an orthorhombic structure $Iba2$ has a combination of ferromagnetic and semiconductor properties and is potentially promising for spintronic applications. Another high-temperature phase is paramagnetic and identified as tetragonal $I4/mcm$.
Boron-doped single crystal diamond films were grown homoepitaxially on synthetic (100) Type Ib diamond substrates using microwave plasma assisted chemical vapor deposition. A modification in surface morphology of the film with increasing boron concentration in the plasma has been observed using atomic force microscopy. Use of nitrogen during boron doping has been found to improve the surface morphology and the growth rate of films but it lowers the electrical conductivity of the film. The Raman spectra indicated a zone center optical phonon mode along with a few additional bands at the lower wavenumber regions. The change in the peak profile of the zone center optical phonon mode and its downshift were observed with the increasing boron content in the film. However, shrinkage and upshift of Raman line was observed in the film that was grown in presence of nitrogen along with diborane in process gas.
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