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Sodium under pressure It has recently been shown that, when high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions — including (for sodium) a striking and as yet unexplained pressure-induced drop in the melting temperature. Jean-Yves Raty et al . have now identified the cause of this unusual melting behaviour: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. Intriguingly, one of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states, the first time such an effect has been seen in a liquid metal. When high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions, including a striking pressure-induced drop in the melting temperature. The cause of the unusual melting behaviour has now been identified: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. One of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states. At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1 , 2 , 3 , 4 , 5 , 6 . It was predicted from theory 1 , 2 —and later confirmed by experiment 3 , 4 , 5 —that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states—an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [NdIII (phen)3 (NCX)3] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN− by SeCN−. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.

High thermal conductivity materials are critical for advanced thermal management applications. The semiconductor cubic boron arsenide (c‐BAs) has drawn significant attention due to its ultrahigh thermal conductivity. In this study, high‐quality isotope‐enriched cubic boron arsenide (c‐ 10 BAs and c‐ 11 BAs) crystals are synthesized to further enhance the thermal conductivity of c‐BAs and measured a room temperature thermal conductivity of 1500 W m −1  K −1 for the c‐ 11 BAs. This value is the highest thermal conductivity for isotope‐enriched c‐BAs reported so far. The experimental study, together with ab initio calculation, verifies the high quality with reproducibility of the crystals. The exceptionally high thermal conductivity of the isotope‐enriched BAs, combined with their semiconductor properties, holds significant potential for improving thermal management in semiconductor devices and electronics packaging applications.

Electric field reconstruction at interfaces plays a crucial role in device performances controlling, for example, Schottky potential barrier and interfacial Rashba effect. Here, scanning transmission electron microscopy (STEM) and ab‐initio calculation are used to estimate the atomic‐scale and large‐scale potential reconstruction at the interface between a metallic oxide SrRuO3 (SRO) thin film and an insulating DyScO3 (DSO) substrate. The intensity and the symmetry of the large‐scale electrostatic reconstruction at the interface is probed by 4D‐STEM discussing the center‐of‐mass shift for different angular ranges detection. Numerical simulations indicate that thermal diffuse scattered (TDS) electrons can be sensitive to large‐scale electric field and experiments based on these diffused electrons near the interface confirm that the electric field extends more in the insulating DyScO3 (DSO) side. The magnitude of the electrostatic drop at the interface estimated by the 4D‐STEM experiment is in accordance with the ab‐initio values for a p‐type reconstruction of the interface plane. Furthermore, an atomically resolved TDS potential asymmetry is observed in real‐space at the SRO/DSO interface by 4D‐STEM. This asymmetry is associated with the formation of a local ferroelectric type dipole at the interfacial unit‐cell revealing unambiguously the balance evolution between antiferrodistortive and ferroelectric instabilities at the interface between a metallic SRO and an insulating DSO. Scanning transmission electron microscopy (STEM) and ab‐initio calculations estimate the atomic‐scale and large‐scale potential reconstruction at the interface between a metallic oxide SrRuO3 (SRO) thin film and an insulating DyScO3 (DSO) substrate. The 4D‐STEM experiments and calculations take into account the thermal diffuse scattered electrons in order to estimate the large‐scale electrostatic potential discontinuity at the SRO/DSO interface.

The performance of 23 density functionals, including one LDA, four GGAs, three meta‐GGAs, three hybrid GGAs, eight hybrid meta‐GGAs, and ten double‐hybrid functionals, was investigated for the computation of activation energies of various covalent main‐group single bonds by four catalysts: Pd, PdCl−, PdCl2, and Ni (all in the singlet state). A reactant complex, the barrier, and reaction energy were considered, leading to 164 energy data points for statistical analysis. Extended Gaussian AO basis sets were used in all calculations. The best functional for the complete benchmark set relative to estimated CCSD(T)/CBS reference data is PBE0‐D3, with an MAD value of 1.1 kcal mol−1 followed by PW6B95‐D3, the double hybrid PWPB95‐D3, and B3LYP‐D3 (1.9 kcal mol−1 each). The other tested hybrid meta‐GGAs perform less well (M06‐HF: 7.0 kcal mol−1; M06‐2X: 6.3 kcal mol−1; M06: 4.9 kcal mol−1) for the investigated reactions. In the Ni case, some double hybrids show larger errors due to partial breakdown of the perturbative treatment for the correlation energy in cases with difficult electronic structures (partial multi‐reference character). Only double hybrids either with very low amounts of perturbative correlation (e.g., PBE0‐DH) or that use the opposite‐spin correlation component only (e.g., PWPB95) seem to be more robust. We also investigated the effect of the D3 dispersion correction. While the barriers are not affected by this correction, significant and mostly positive results were observed for reaction energies. Furthermore, six very recently proposed double‐hybrid functionals were analyzed regarding the influence of the amount of Fock exchange as well as the type of perturbative correlation treatment. According to these results, double hybrids with <50–60 % of exact exchange and ∼30 % perturbative correlation perform best. Catch 23: An extensive benchmark for evaluating modern density functionals in transition‐metal‐catalyzed bond‐activation reactions is presented. The study focuses on the class of sophisticated double‐hybrid functionals and finds dispersion‐corrected PWPB95 and PBE0 as the most robust and well‐performing methods.

Cu nanoparticles with different sizes, morphology, and surface structures exhibit distinct activity and selectivity toward CO2 reduction reaction, while the reactive sites and reaction mechanisms are very controversial in experiments. In this study, we demonstrate the dynamic structure change of Cu clusters on graphite‐like carbon supports plays an important role in the multicarbon production by combining static calculations and ab‐initio molecular dynamic simulations. The mobility of Cu clusters on graphite is attributed to the near‐degenerate energies of various adsorption configurations, as the interaction between Cu atoms and surface C atoms is weaker than that of CuCu bonds in the tight cluster form. Such structure change of Cu clusters leads to step‐like irregular surface structures and appropriate interparticle distances, increasing the selectivity of multicarbon products by reducing the energy barriers of CC coupling effectively. In contrast, the large ratio of edge and corner sites on Cu clusters is responsible for the increased catalytic activity and selectivity for CO and H2 compared with Cu(100) surface, instead of hydrocarbon products like methane and ethylene. The detailed study reveals that the dynamic structure change of the catalysts results in roughened surface morphologies during catalytic reactions and plays an essential role in the selectivity of CO2 electro‐reduction, which should be paid more attention for studies on the reaction mechanisms. The dynamic evolution process of the catalyst during catalytic reactions is highlighted. For carbon‐supported Cu clusters, the dynamic structure change leads to surface structure reconstruction and proper interparticle distance, which reduces the energy barriers of CC coupling effectively and improves the selectivity of multicarbon products.

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1−xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1−xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.

Ionic liquids (ILs) are attractive stationary phases to broaden selectivity in gas chromatography (GC). The rational design, selection and application of ILs in comprehensive two‐dimensional GC is a desirable goal. In this study, methods to predict two‐dimensional chromatograms of fatty acid methyl esters (FAMEs) starting from given structures of IL stationary phases and their interactions with FAME solutes are described. Molecular parameters of the dipole moment, lowest unoccupied molecular orbital energy and molar volume of different ILs were calculated by using Gaussian 09. With established correlations between molecular simulation and linear solvation energy relationships, molecular parameters were converted to s, e and l values for stationary phase descriptors. This allowed reliable prediction of the equivalent chain length of FAMEs on each IL column (with the correlation R2=0.98). Isovolatility curves in GC×GC space of reference saturated FAMEs were then determined for each IL column to take into account the dependence of retention time on temperature in temperature‐programmed separation. The resulting predicted GC×GC chromatograms were compared with previously reported experimental results. Correlations with R2 values of 0.99 and 0.98 were achieved for the first‐ and second‐dimension retention times, respectively. Forecasting a result: Prediction of the separation performance of fatty acid methyl ester (FAME) analytes on capillary columns coated with ionic liquid phases by knowledge of their molecular structures is possible (see figure; LSER=linear solvation energy relationship). Whilst providing deeper insight into separation mechanisms, trial‐and‐error experimentation related to method development in multidimensional gas chromatography can also be reduced.

The epoxidation of cyclohexene has been investigated on a metal–organic framework MIL‐47 containing saturated V+IV sites linked with functionalized terephthalate linkers (MIL‐47‐X, X=OH, F, Cl, Br, CH3, NH2). Experimental catalytic tests have been performed on the MIL‐47‐X materials to elucidate the effect of linker substitution on the conversion. Notwithstanding the fact that these substituted materials are prone to leaching in the performed catalytic tests, the initial catalytic activity of these materials correlates with the Hammett substituent constants. In general, substituents led to an increased activity relative to the parent MIL‐47. To rationalize the experimental findings, first‐principles kinetic calculations were performed on periodic models of MIL‐47 to determine the most important active sites by creating defect structures in the interior of the crystalline material. In a next step these defect structures were used to propose extended cluster models, which are able to reproduce in an adequate way the direct environment of the active metal site. An alkylperoxo species V+VO(OOtBu) was identified as the most abundant and therefore the most active epoxidation site. The structure of the most active site was a starting basis for the construction of extended cluster models including substituents. They were used for quantifying the effect of functionalization of the linkers on the catalytic performance of the heterogeneous catalyst MIL‐47‐X. Electron‐withdrawing as well as electron‐donating groups have been considered. The epoxidation activity of the functionalized models has been compared with the measured experimental conversion of cyclohexene. The agreement is fairly good. This combined experimental–theoretical study makes it possible to elucidate the structure of the most active site and to quantify the electronic modulating effects of linker substituents on the catalytic activity. A framework for catalysis: The epoxidation of cyclohexene has been studied on functionalized versions of the metal–organic framework MIL‐47 containing V+IV sites. All the investigated substituted MIL‐47‐X frameworks (X=NH2, CH3, F, Cl, Br, OH; see figure) led to an increased conversion to cyclohexene oxide. A linear free‐energy relationship was found between centers.

Chemical and magnetic states of iron doped tin oxide (Sn[O.sub.2]) as a diluted magnetic semiconductor (DMS) at room temperature have been investigated using [sup.57]Fe Mossbauer spectrometry, XRD and magnetometery. The influence of the doping conditions of Sn[O.sub.2] with iron on the generation of oxygen defects was reviewed and discussed on the basis of ab initio calculations. The magnetic properties depended on preparation conditions, such as thermal decomposition and sol-gel processing as well as [sup.57]Fe and super-dilute [sup.57]Mn implantation. It was shown that Sb codoping in Fe doped Sn[O.sub.2] increases the saturation magnetization. Doping of Fe(Sb)-Sn[O.sub.2] with nonmagnetic Zn ions up to 7 % also increases the magnetization although there is no precipitation of crystalline magnetic phases. The codoping of two transition metal ions (Fe-Co, Fe-Mn, Fe-Ni and Fe-V) in Sn[O.sub.2] matrix enhanced the magnetization as compared with that of single metal ion doped samples. It is suggested from different valence states of doped metal ions that double exchange interactions occur through or near the oxygen vacancies in Sn[O.sub.2]. The Sn[O.sub.2] doped with dilute [sup.57]Fe may show the intrinsic and/or extrinsic DMS properties. Oxygen vacancies play an important role in the intrinsic DMS. The intrinsic nature of DMS is supported by both, experimental results and ab initio calculations. The long range interactions between diluted magnetic ions are considered to occur through electrons produced by oxygen vacancies or electrons induced by [Sb.sup.5+] doping. Keywords: diluted magnetic semiconductor (DMS), Mossbauer spectrometry, [sup.57]Fe doped Sn[O.sub.2] semiconductor, Sn[O.sub.2] co-doped with two metals, ab initio calculation.