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Large negative thermal expansion (NTE) has been discovered during the last decade in materials of various kinds, particularly materials associated with a magnetic, ferroelectric or charge-transfer phase transition. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators. Here, we report the discovery of giant NTE for reduced layered ruthenate. The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volume change reported to date. We observed a giant negative coefficient of linear thermal expansion α=−115 × 10−6 K−1 over 200 K interval below 345 K. This dilatometric NTE is too large to be attributable to the crystallographic unit-cell volume variation with temperature. The highly anisotropic thermal expansion of the crystal grains might underlie giant bulk NTE via microstructural effects consuming open spaces in the sintered body on heating.

TitleAtomically precise graphene nanoribbon heterojunctions from a single molecular precursor AbstractThe rational bottom-up synthesis of atomically defined graphene nanoribbon (GNR)heterojunctions represents a key enabling technology for the design of nanoscale electronic devices. Synthetic strategies have thus far relied on the random copolymerization of two electronically distinctive molecular precursors to yield a segmented band structure within a GNR. Here we report the fabrication and electronic characterization of atomically precise GNR heterojunctions prepared through a late-stage functionalization of chevron GNRs obtained from a single precursor that features fluorenone substituents along the convex edges. Excitation of the GNR induces cleavage of sacrificial carbonyl groups at the GNR edge, thus giving rise to atomically well-defined heterojunctions comprised of segments of fluorenone GNR and unfunctionalized chevron GNR. The structure of fluorenone/unfunctionalized GNR heterojunctions was characterized using bond-resolved STM (BRSTM) which enables chemical bonds to be imaged via STM at T = 4.5 K. Scanning tunneling spectroscopy (STS) reveals that the band alignment across the interface yields a staggered gap Type II heterojunction and is consistent with first-principles calculations. Detailed spectroscopic and theoretical studies reveal that the band realignment at the interface between fluorenone and unfunctionalized chevron GNRs proceeds over a distance less than 1nm, leading to extremely large effective fields.

International audienceHighly dispersed crystalline/amorphous LiFePO4 (LFP) nanoparticles encapsulated within hollow-structured graphitic carbon were synthesized using an in situ ultracentrifugation process. Ultracentrifugation triggered an in situ sol–gel reaction that led to the formation of core–shell LFP simultaneously hybridized with fractured graphitic carbon. The structure has double cores that contain a crystalline LFP (core 1) covered by an amorphous LFP containing Fe3+ defects (core 2), which are encapsulated by graphitic carbon (shell). These core–shell LFP nanocomposites show improved Li+ diffusivity thanks to the presence of an amorphous LFP phase. This material enables ultrafast discharge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C) as well as ultrafast charge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C). The synthesized core–shell nanocomposites overcome the inherent one-dimensional diffusion limitation in LFP and yet deliver/store high electrochemical capacity in both ways symmetrically up to 480C. Such a high rate symmetric capacity for both charge and discharge has never been reported so far for LFP cathode materials. This offers new opportunities for designing high-energy and high-power hybrid supercapacitors

Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron-doped giant tetragonal perovskite compound Pb Bi VO (x=0.2 and 0.3). A polar tetragonal (P4mm) to non-polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb La Bi VO exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb La Bi VO polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.

We study superlattices with alternate stacking of graphene and boron nitride monolayers. We propose several candidate stacking sequences of the superlattices, and optimize their geometries based on the energetics in the framework of the density functional theory. From the total energies of the superlattices with the candidate stacking sequences, we identify the most stable stacking sequence. The atomic configuration of the superlattice with the most stable stacking sequence is found to have the shortest B-C distance among all the optimized superlattice geometries, indicating a strong interaction between the carbon and boron atoms. We also study the electronic structure of the superlattices in detail. It is revealed that the most stable structure exhibits metallic electronic properties.

Copper-based sulfide is an attractive material for Earth-abundant thermoelectrics. In this study, we demonstrate the effect of Sn-substitution on the electrical and thermal transport properties of fematinite Cu 3 SbS 4 from 300 to 573 K. The carrier concentration is controlled in the range from 4 × 10 18 to 8 × 10 20 cm −3 by Sn-substitution. The density-of-states effective mass is found to be ∼3.0 m e , assuming the single parabolic band model. The direct-type optical band gap is ∼0.9 eV, which is consistent with the density functional theory calculation. The dimensionless figure of merit reaches 0.1 for Sn-doped samples at 573 K.