Boron carbide (B4C) is one of the hardest materials known to date. The extreme hardness of B4C arises from architecturally efficient B-12 or B11C icosahedrons and strong inter-icosahedral B-C bonding. As an excellent material for use in ballistic armor, the mechanic limit of B4C and possible phase transitions under extreme stress conditions are of great interest. Here we systematically explored the post-icosahedral solid structures of B4C under high pressure, using an unbiased structure search method. A new structure composed of extended framework of B and zigzag chains of C is predicted to be stable above 96 GPa. The new structure was predicted to have a high Vickers hardness of 55 GPa and simultaneously to retain a metallic ground state. The exceptional mechanical properties found in this structure are attributed to strong sp(3) covalent network formed under extreme pressure conditions. The predicted structure represents a new type of superhard boron carbides that form under high pressure without the presence of boron icosahedrons, which encourages experimental exploration in this direction.

Controlled nanotwinning is an effective way to enhance the mechanical properties of materials. Recent discovery of nanotwinned diamond converted from carbon nano-onions with high-density defects reveals that the presence of nanotwinned structures can increase the hardness of the product to exceed that of natural diamond by a surprisingly large margin. To understand the mechanism of nanotwinning, the microscopic transformation pathway from carbon nano-onions to nanotwinned diamond was investigated in the present study. We carried out a direct high-pressure high-temperature synthesis of nanotwinned diamond from onion carbon without high-density defects. The obtained nanotwinned diamond possesses an exceptionally high Vickers hardness of 215 GPa at 4.9 N. The transformation path was analyzed using aberration-corrected transmission electron microscopy (TEM) which suggests a martensitic process strongly influenced by the pressure-temperature conditions. Specifically, the appearance of {111} nanotwinned structure and stacking faults was determined by the characteristics of the onion shells, while the accumulation of the stress due to the sliding of the shells cause the crystal to re-align along the shear direction. These findings not only clarify the direct transformation mechanism from onion-like precursors to nanotwinned diamond, but also have broad implications for further exploration of new materials with exceptional properties. (C) 2017 Elsevier Ltd. All rights reserved.

While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theoretical calculations for BaPd2As2. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, T-c is linearly suppressed, which is the typical behavior of classical phonon-mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered BaPd2As2 have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is B-0 = 40(6) GPa below 12 GPa and B-0 = 142(3) GPa below 27.2 GPa.

Amorphous sulfur was prepared by rapid compression of liquid sulfur at temperatures above the lambda-transition for to preserve the high-temperature liquid structure. We conducted synchrotron high-energy X-ray diffraction and Raman spectroscopyto diagnose the structural evolution of amorphous sulfur from room temperature to post-lambda-transition temperature. Discontinuous changes of the first and second peaks in atomic pair-distribution-function, g(r), were observed during the transition from amorphous to liquid sulfur. The average first-neighbor coordination numbers showed an abrupt drop from 1.92 to 1.81. The evolution of the chain length clearly shows that the transition was accompanied by polymeric chains breaking. Furthermore, a re-entry of the lambda-transition structure was involved in the heating process. The amorphous sulfur, which inherits the post-lambda-transition structure from its parent melts, transformed to the pre-lambda-transition liquid structure at around 391 K. Upon further heating, the pre-lambda-transition liquid transformed to a post-lambda-transition s tructure through the well-known lambda-transition process. This discovery offers a new perspective on amorphous sulfur's structural inheritance from its parent liquid and has implications for understanding the structure, evolution and properties of amorphous sulfur and its liquids.

Considerable excitement was generated by the observation of large and linear positive magnetoresistance in nonmagnetic silver chalcogenides. Renewed interest in these materials was kindled by the discovery that Ag-2 Te in particular is a topological insulator with gapless linear Dirac-type surface states. High-pressure x-ray-diffraction studies, combined with first-principles electronic structure calculations, have identified three phase transitions as the pressure is increased: an isostructural transition identified with an electronic topological transition followed by two structural phase transitions. These recent studies were carried out on nominally stoichiometric Ag2Te. For the present work we have prepared single-phase self-doped Ag2-delta Te samples with a well-characterized silver deficit (delta = 2.0 x10(-4)) for structural and electrical transport measurements over extended ranges of pressure (0-43 GPa), temperature (2-300 K), and magnetic field (0-9 T). The temperature dependence of the resistivity exhibits anomalous behavior at 2.3 GPa, slightly above the isostructural transition, which we postulate is due to Fermi surface reconstruction associated with a charge density wave (CDW) phase. The anomaly is enhanced by the application of a 9-T magnetic field and shifted to higher temperature, implying that the electronic Zeeman energy is sufficient to alter the gapping of the Fermi surface. A peak in the pressure dependence of the resistivity and a sudden drop in the pressure dependence of the mobility, occurring at 2.3 GPa, provide additional evidence for a CDW phase at pressures slightly above the isostructural transition.

Vanadium diboride (VB2) with an AlB2-type structure has been synthesized at 8 GPa and 1700 K in a D-DIA-type multianvil apparatus. The obtained bulk modulus is B-0 = 262(2) GPa with fixed B' = 4.0 for VB2 via high-pressure X-ray diffraction measurements. Meanwhile, VB2 has also been demonstrated to possess a high Vickers hardness of 27.2 +/- 1.5 GPa, a high thermal stability of 1410 K in air, among the highest for transition-metal borides, and an extremely low resistivity value (41 mu Omega cm) at room temperature. Results from first-principles calculations regarding the mechanical and electronic properties of VB2 are largely consistent with the experimental observations and further suggest that VB2 possesses simultaneously the properties of a hard and refractory ceramic and those of an excellent electric conductor.

Migration of Pb in the soil can be enhanced by acidification and frequent change of environmental condition. The paddy soil, where the environmental condition such as redox fluctuates frequently due to soil submergence and drainage, may offer a favorable condition for Pb migrating to deeper soil and further contaminate groundwater by leaching or irrigation. To date, not much is known about how quickly Pb migrates in the soil and the relevant transformation of Pb. We use long-term soil profile monitoring, sequential extraction and isotopic measurement to examine the temporal change of concentrations and isotopic ratios of Pb associated with different soil components in the paddy soil profile in the Yangtze River Delta area during 2011-2017. We find that Pb vertical migration in the paddy soil is faster. Pb in the shallow soil may migrate downward up to 60 cm during six years. The migration of Pb is dominated by the carbonate, and secondarily influenced by Fe/Mn oxides. Our results also imply that the mechanism of Pb migration in soils is changing. The mechanism which is now characterized as the carbonate-dominating will be replaced as the Fe/Mn oxides-dominating in the near future as the carbonate in shallow soil is becoming depleted.

The contributions of natural versus anthropogenic forces on temporal changes of metals in the soil of the Yangtze River delta region were successfully quantified by combining repeated soil sampling, geostatistics and the modified principal component scores and multiple linear regressions approach. The findings show that the mean concentrations of Cu, Cd, As, Hg, Cr and Ni generally exhibited a decrease trend from 2010 to 2020. The decline of soil Hg was most outstanding, decreased by 20 % as a whole. The result of the modified principal component scores and multiple linear regressions approach suggests that the decrease of Hg content was predominately driven by the geochemical processes, with 48 % contribution. The mean soil Pb concentration increased by 40 % from 2010 to 2020. Approximately 76 % of the Pb change was attributed to the strengthened development of Pb-containing battery industry in the south part of the Yangtze River delta region. The Pb battery industry development also contributed 48 % of the change of Cd. The anthropogenic activities involving Cu, Zn or fossil fuel consumption contributed 32-35 % of the changes of As, Cu and Zn concentrations in the soil, and the steel industries contributed 82 % of Cr and 60 % of Ni changes, respectively.

The use of magnetic susceptibility (& chi;) as a means of assessing heavy metal pollution in soils has been explored by researchers, yielding varying results in terms of the correlations between & chi; with heavy metals. The efficacy of & chi; as an indicator of soil heavy metal pollution remains a topic of debate. This study aims to elucidate the interrelationships between & chi;, iron oxides, and heavy metals in soil through the application of a modified 5-step sequential extraction procedure (SEP), and to identify an effective approach for assessing metal concentrations in soil using magnetic susceptibility measurements. The soil samples were collected from a typical alluvial island in the lower Yangtze River, China, and a total of 6 forms (exchangeable and acid soluble fraction, easily reducible fraction, oxidizable fraction, amorphous iron oxide, crystallized iron oxyhydroxides and residual fraction) were partitioned and their heavy metal concentrations and & chi; were analyzed. The results show that crystalline Fe oxyhydroxides and residual fractions are the two uppermost fractions of heavy metals. By combining the fractionation of elements with the variation of & chi; of the soil during the processing of SEP, it was inferred that the external input of Fe, Pb, Cr and Cd in the soil likely originated from the vicinal steel production. The correlation analysis revealed a significant correlation between heavy metal concentrations and & chi; in the residual fraction, whereas no significant correlations were observed between the concentrations of heavy metals and & chi; in the bulk soil samples. It is recommended that the evaluation of heavy metal contamination in the soil neighboring industrial sites can be conducted via magnetic susceptibility measurements subsequent to the elimination of crystalline iron oxyhydroxides.