Spin-crossover (SCO) is generally regarded as a spectacular molecular magnetism in 3d(4)-3d(7) metal complexes and holds great promise for various applications such as memory, displays, and sensors. In particular, SCO materials can be multifunctional when a classical light- or temperature induced SCO occurs along with other cooperative structural and/or electrical transport alterations. However, such a cooperative SCO has rarely been observed in condensed matter under hydrostatic pressure (an alternative external stimulus to light or temperature), probably due to the lack of synergy between metal neighbors under compression. Here, we report the observation of a pressure-driven, cooperative SCO in the two-dimensional (2D) honeycomb antiferromagnets MnPS3 and MnPSe3 at room temperature. Applying pressure to this confined 2D system leads to a dramatic magnetic moment collapse of Mn2+ (d(5)) from S = 5/2 to S = 1/2. Significantly, a number of collective phenomena were observed along with the SCO, including a large lattice collapse (similar to 20% in volume), the formation of metallic bonding, and a semiconductor-to-metal transition. Experimental evidence shows that all of these events occur in the honeycomb lattice, indicating a strongly cooperative mechanism that facilitates the occurrence of the abrupt pressure-driven SCO. We believe that the observation of this cooperative pressure-driven SCO in a 2D system can provide a rare model for theoretical investigations and lead to the discovery of more pressure-responsive multifunctional materials.

To avoid additional global warming and environmental damage, energy systems need to rely on the use of low carbon technologies like wind energy. However, supply uncertainties, production costs, and energy security are the main factors considered by the global economies when reshaping their energy systems. Here, we explore the potential roles of wind energy technology advancement in future global electricity generations, costs, and energy security. We use an integrated assessment model performing a series of technology advancement scenarios. The results show that double of the capital cost reduction causes 40% of generation increase and 10% of cost decrease on average in the long-term global wind electricity market. Today's technology advancement could bring us the benefit of increasing electricity production in the future 40-50 years, and decreasing electricity cost in the future 90-100 years. The technology advancement of wind energy can help to keep global energy security and stability. An aggressive development and deployment of wind energy could in the long-term avoid 1/3 of gas and 1/28 of coal burned, and keep 1/2 biomass and 1/20 nuclear fuel saved from the global electricity system. The key is that wind resources are free and carbon-free. The results of this study are useful in broad coverage ranges from innovative technologies and systems of renewable energy to the economic industrial and domestic use of energy with no or minor impact on the environment. (C) 2016 Elsevier Ltd. All rights reserved.

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0-62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2-61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low-and high-pressure structures.

Phase-selective synthesis and structure switching behavior of a functional material are essential to enable comparative studies on the structure-property relationship. Here, we report a controllable fluorination route to phase-pure erbium oxyfluorides with orthorhombic (O-ErOF) and rhombohedral (R-ErOF) structures. This facile method adopts polytetrafluoroethylene (PTFE) as the fluoridizer, and the phase selectivity can be easily achieved at specific fluorination temperatures. The phase evolution and detailed crystal structures of erbium oxyfluoride were characterized by powder X-ray diffraction (PXRD) at various sintering temperatures and Rietveld refinements, respectively. An irreversible phase transition from O-ErOF to R-ErOF was observed under heating around 600 degrees C. The upconversion (UC) luminescence properties of R-ErOF and O-ErOF were studied comparatively by means of photoluminescence, P-I, and UC decay curves. Despite their similar components and crystal structure, R-ErOF exhibits stronger (more than 20 times) red UC emission than O-ErOF. The anomalous UC behavior of the two polymorphs of ErOF was associated with the energy transfer processes dependent on their crystal structure.

Nitrogen doping via high-temperature ammonization is a frequently used strategy to extend the light harvesting capacity of wide-bandgap catalysts in the visible region. Under such a reductive atmosphere, the reduction of transition metals is supposed to occur, however, this has not been thoroughly studied yet. Here, by combining chemically-controlled doping and subsequent liquid exfoliation, ultra-thin [Nb3O8](-) nanosheets with separate N doping, reduced-Nb doping and N/reduced-Nb codoping were fabricated for comparative studies on the doping effect for photocatalytic hydrogen evolution. Layered KNb3O8 was used as the starting material and the above-mentioned three doping conditions were achieved by high-temperature treatment with urea, hydrogen and ammonia, respectively. The morphology, crystal and electronic structures, and the catalytic activity of the products were characterized thoroughly by means of TEM, AFM, XRD, XPS, EPR, absorption spectroscopy and photocatalytic hydrogen evolution. Significantly, the black N/reduced-Nb co-doped monolayer [Nb3O8](-) nanosheets exhibit the mostly enhanced photocatalytic hydrogen generation rate, indicating a synergistic doping effect of the multiple chemical-design strategy. The modified electronic structure of [Nb3O8](-) nanosheets and the role of exotic dopants in bandgap narrowing are put forward for the rational design of better photocatalysts with reduced-metal self-doping.

Hydrogen production by catalytic water splitting using sunlight holds great promise for clean and sustainable energy source. Despite the efforts made in the past decades, challenges still exist in pursuing solid catalysts with light-harvesting capacity, large surface areas and efficient utilities of the photogenerated carrier, at the same time. Here, a multiple structure design strategy leading to highly enhanced photocatalytic performance on hydrogen production from water splitting in Dion-Jacobson perovskites KCa2Nan-3NbnO3n+1 is described. Specifically, chemical doping (N/Nb4+) of the parent oxides via ammoniation improved the ability of sunlight harvesting efficiently; subsequent liquid exfoliation of the doped perovskites yielded ultrathin [Ca2Nan-3NbnO3n+1](-) nanosheets with greatly increased surface areas. Significantly, the maximum hydrogen evolution appears in the n=4 nanosheets, which suggests the most favorable thickness for charge separation in such perovskite-type catalysts. The optimized black N/Nb4+-[Ca2NaNb4O13](-) nanosheets show greatly enhanced photocatalytic performance, as high as 973 mu molh(-1) with Pt loading, on hydrogen evolution from water splitting. As a proof-of-concept, this work highlights the feasibility of combining various chemical strategies towards better catalysts and precise thickness control of two-dimensional materials.

Weyl semimetal defines a material with three-dimensional Dirac cones, which appear in pair due to the breaking of spatial inversion or time reversal symmetry. Superconductivity is the state of quantum condensation of paired electrons. Turning a Weyl semimetal into superconducting state is very important in having some unprecedented discoveries. In this work, by doing resistive measurements on a recently recognized Weyl semimetal TaP under pressures up to about 100 GPa, we show the concurrence of superconductivity and a structure transition at about 70 GPa. It is found that the superconductivity becomes more pronounced when decreasing pressure and retains when the pressure is completely released. High-pressure x-ray diffraction measurements also confirm the structure phase transition from I4(1)md to P-6m2 at about 70 GPa. More importantly, ab-initial calculations reveal that the P-6m2 phase is a new Weyl semimetal phase and has only one set of Weyl points at the same energy level. Our discovery of superconductivity in TaP by high pressure will stimulate investigations on superconductivity and Majorana fermions in Weyl semimetals.

We report on the discovery of a pressure-induced topological and superconducting phase of SnSe, a material which attracts much attention recently due to its superior thermoelectric properties. In situ high-pressure electrical transport and synchrotron x-ray diffraction measurements show that the superconductivity emerges along with the formation of a CsCl-type structural phase of SnSe above around 27 GPa, with amaximum critical temperature of 3.2 K at 39 GPa. Based on ab initio calculations, this CsCl-type SnSe is predicted to be a Dirac line-node (DLN) semimetal in the absence of spin-orbit coupling, whose DLN states are protected by the coexistence of time-reversal and inversion symmetries. These results make CsCl-type SnSe an interesting model platform with simple crystal symmetry to study the interplay of topological physics and superconductivity.

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal TiS3 up to 29.9-39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive behavior of semiconductor TiS3 becomes increasingly insulating with pressure until P-C1 similar to 12 GPa, where extremes in all three axial ratios are observed. Upon further compression to P-C2 similar to 22 GPa, the XRD data evidence a structural phase transition. Based on our theoretical calculations, this structural transition is determined to be isosymmetric, i.e., without change of the structural symmetry (P2(1)/m), mainly resulting from rearrangement of the dangling S-2 pair along the a axis.

Topological semimetal, a novel state of quantum matter hosting exotic emergent quantum phenomena dictated by the nontrivial band topology, has emerged as a new frontier in condensed-matter physics. Very recently, the coexistence of triply degenerate points of band crossing and Weyl points near the Fermi level was theoretically predicted and experimentally identified in MoP. Via high-pressure electrical transport measurements, we report here the emergence of pressure-induced superconductivity in MoP with a critical transition temperature T-c of ca. 2.5 K at ca. 30 GPa. No structural phase transition is observed up to ca. 60 GPa via synchrotron X-ray diffraction study. Accordingly, the topologically nontrivial band protected by the crystal structure symmetries and superconductivity are expected to coexist at pressures above 30 GPa, consistent with density functional theory calculations. Thus, the pressurized MoP represents a promising candidate of topological superconductor. Our finding is expected to stimulate further exploitation of exotic emergent quantum phenomena in novel unconventional fermion system.