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Register a new userGraphene Synthesis via the High Pressure - High Temperature Growth Process
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We report on a new method for graphene synthesis and assessment of the properties of the resulting large-area graphene layers. Graphene was produced by the high pressure - high temperature growth from the natural graphitic source by utilizing the molten Fe-Ni catalysts for dissolution of carbon. The resulting large-area graphene flakes were transferred to the silicon - silicon oxide substrates for the spectroscopic micro-Raman and scanning electron microscopy inspection. The analysis of the G peak, D, T+D and 2D bands in the Raman spectra under the 488-nm laser excitation indicate that the high pressure - high temperature technique is capable of producing the high-quality large-area single-layer graphene with a low defect density. The proposed method may lead to a more reliable graphene synthesis and facilitate its purification and chemical doping.
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The phase diagram of Zn has been explored up to 140 GPa and 6000 K, by combining optical observations, x-ray diffraction, and ab-initio calculations. In the pressure range covered by this study, Zn is found to retain a hexagonal close-packed crystal symmetry up to the melting temperature. The known decrease of the axial ratio of the hcp phase of Zn under compression is observed in x-ray diffraction experiments from 300 K up to the melting temperature. The pressure at which the axial ratio reaches the square root of 3 value, around 10 GPa, is slightly affected by temperature. When this axial ratio is reached, we observed that single crystals of Zn, formed at high temperature, break into multiple polycrystals. In addition, a noticeable change in the pressure dependence of the axial ratio takes place at the same pressure. Both phenomena could be caused by an isomorphic second-order phase transition induced by pressure in Zn. The reported melt curve extends previous results from 24 to 135 GPa. The pressure dependence obtained for the melting temperature is accurately described up to 135 GPa by using a Simon-Glatzel equation. The determined melt curve agrees with previous low-pressure studies and with shock-wave experiments, with a melting temperature of 5060 K at 135 GPa. Finally, a thermal equation of state is reported, which at room-temperature agrees with the literature.
We report a combined experimental and theoretical study of the melting curve and the structural behavior of vanadium under extreme pressure and temperature. We performed powder x-ray diffraction experiments up to 120 GPa and 4000 K, determining the phase boundary of the bcc-to-rhombohedral transition and melting temperatures at different pressures. Melting temperatures have also been established from the observation of temperature plateaus during laser heating, and the results from the density-functional theory calculations. Results obtained from our experiments and calculations are fully consistent and lead to an accurate determination of the melting curve of vanadium. These results are discussed in comparison with previous studies. The melting temperatures determined in this study are higher than those previously obtained using the speckle method, but also considerably lower than those obtained from shock-wave experiments and linear muffin-tin orbital calculations. Finally, a high-pressure high-temperature equation of state up to 120 GPa and 2800 K has also been determined.
High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.
Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic read-out operation. The great challenge is to create multiferroic materials that operate at room-temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because of its large P ~ 10^{2} {mu}C.m^{-2}, but is unfortunately only multiferroic in a temperature range of 20 K, from 210 to 230 K. Here, using a combination of density functional theory and Monte Carlo calculations, we establish that pressure-driven phase competition induces a giant stabilization of the multiferroic phase of CuO, which at 20-40 GPa becomes stable in a domain larger than 300 K, from 0 to T > 300 K. Thus, under high-pressure, CuO is predicted to be a room-temperature multiferroic with large polarization.
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.
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