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Electronegativity is a key property of the elements. Being useful in rationalizing stability, structure and properties of molecules and solids, it has shaped much of the thinking in the fields of structural chemistry and solid state chemistry and physics. There are many definitions of electronegativity, which can be roughly classified as either spectroscopic (these are defined for isolated atoms) or thermochemical (characterizing bond energies and heats of formation of compounds). The most widely used is the thermochemical Pauling’s scale, where electronegativities have units of eV 1/2 . Here we identify drawbacks in the definition of Pauling’s electronegativity scale—and, correcting them, arrive at our thermochemical scale, where electronegativities are dimensionless numbers. Our scale displays intuitively correct trends for the 118 elements and leads to an improved description of chemical bonding (e.g., bond polarity) and thermochemistry. Pauling’s electronegativity scale has a fundamental value and uses accessible thermochemical data, but fails at predicting the bonding behavior for several elements. The authors propose their thermochemical scale based on experimental dissociation energies that provides dimensionless values for the electronegativity and recovers the correct trends throughout the periodic table.

Ternary hydrides are regarded as an important platform for exploring high-temperature superconductivity at relatively low pressures. Here, we successfully synthesized the hcp -(La,Ce)H 9-10 at 113 GPa with the initial La/Ce ratio close to 3:1. The high-temperature superconductivity was strikingly observed at 176 K and 100 GPa with the extrapolated upper critical field H c2 (0) reaching 235 T. We also studied the binary La-H system for comparison, which exhibited a T c of 103 K at 78 GPa. The T c and H c2 (0) of the La-Ce-H are respectively enhanced by over 80 K and 100 T with respect to the binary La-H and Ce-H components. The experimental results and theoretical calculations indicate that the formation of the solid solution contributes not only to enhanced stability but also to superior superconducting properties. These results show how better superconductors can be engineered in the new hydrides by large addition of alloy-forming elements. Recently, high-temperature superconductivity has been reported in LaH 10 and CeH 10 . Here, the authors report superconductivity in the alloy (La,Ce)H 9-10 with T c  = 176 K at 100 GPa, providing an improved compromise between high transition temperature and low pressure requirements.

At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Hydrogen-rich superhydrides are believed to be very promising high- T c superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH 5 at 130 GPa and LaH 10 at 170 GPa. With the motivation of discovering new hydrogen-rich high- T c superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH 9 at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH 9 . CeH 9 crystallizes in a P6 3 /mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH 9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides. Hydrogen-rich superhydrides are promising high-temperature superconductors which have been observed only at pressures above 170 GPa. Here the authors show that CeH 9 can be synthesized at 80-100 GPa with laser heating, and is characterized by a clathrate structure with a dense 3-dimensional atomic hydrogen sublattice.

Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. As a result of the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding (with 1:1 stoichiometry dictated by charge balance) and B1-type crystal structure. By combining theoretical predictions and diamond anvil cell experiments, we found that new materials with different stoichiometries emerge at high pressures. Compounds such as Na₃Cl, Na₂Cl, Na₃Cl₂, NaCl₃, and NaCl₇ are theoretically stable and have unusual bonding and electronic properties. To test this prediction, we synthesized cubic and orthorhombic NaCl₃ and two-dimensional metallic tetragonal Na₃Cl. These experiments establish that compounds violating chemical intuition can be thermodynamically stable even in simple systems at nonambient conditions.

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO 3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO 3 (ref. 1 ). But it has not been possible to explain many unusual properties of the lowermost ∼150 km of the mantle (the D″ layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D″ layer, MgSiO 3 transforms from perovskite into a layered CaIrO 3 -type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D″ layer: its seismic anisotropy 2 , the strongly undulating shear-wave discontinuity at its top 3 , 4 , 5 , 6 and possibly the anticorrelation between shear and bulk sound velocities 7 , 8 .

To date, trial and error strategies guided by intuition have dominated the identification of materials suitable for a specific application. We are entering a data-rich, modelling-driven era where such Edisonian approaches are gradually being replaced by rational strategies, which couple predictions from advanced computational screening with targeted experimental synthesis and validation. Here, consistent with this emerging paradigm, we propose a strategy of hierarchical modelling with successive downselection stages to accelerate the identification of polymer dielectrics that have the potential to surpass ‘standard’ materials for a given application. Successful synthesis and testing of some of the most promising identified polymers and the measured attractive dielectric properties (which are in quantitative agreement with predictions) strongly supports the proposed approach to material selection. The selection of polymeric dielectric materials for energy storage applications is not trivial, as several criteria must be satisfied simultaneously. Here, Sharma et al. present a high-throughput hierarchical strategy using the band gap and dielectric constant to screen and identify good candidates.

Following the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH 3 BH 3 , we synthesized previously unknown superhydride BaH 12 with a pseudocubic ( fcc ) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P 6 /mmm -BaH 2 and possibly BaH 10 and BaH 6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH 12 contains H 2 and H 3 – molecular units and detached H 12 chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa. Metallization of pure hydrogen via overlapping of electronic bands requires high pressure above 3 Mbar. Here the authors study the Ba-H system and discover a unique superhydride BaH 12 that contains molecular hydrogen, which demonstrates metallic properties and superconductivity below 1.5 Mbar.

Novel superhard materials, especially those with superior thermal and chemical stability, are needed to replace diamond. Carbon nitrides (C-N), which are likely to possess these characteristics and have even been expected to be harder than diamond, are excellent candidates. Here we report three new superhard and thermodynamically stable carbon nitride phases. Based on a systematic evolutionary structure searches, we report a complete phase diagram of the C-N system at 0–300 GPa and analyze the hardest metastable structures. Surprisingly, we find that at zero pressure, the earlier proposed graphitic-C 3 N 4 structure ( ) is dynamically unstable and we find the lowest-energy structure based on s-triazine unit and s-heptazine unit.

We used the machine learning technique of Li et al . (PRL 114, 2015) for molecular dynamics simulations. Atomic configurations were described by feature matrix based on internal vectors, and linear regression was used as a learning technique. We implemented this approach in the LAMMPS code. The method was applied to crystalline and liquid aluminum and uranium at different temperatures and densities, and showed the highest accuracy among different published potentials. Phonon density of states, entropy and melting temperature of aluminum were calculated using this machine learning potential. The results are in excellent agreement with experimental data and results of full ab initio calculations.