Novel superconducting structures of BH2 under high pressure.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

In Situ Ligand Modification Strategy for the Construction of One-, Two-, and Three-Dimensional Heterometallic Iodides.

Three heterometallic iodides featuring the novel in situ modified ligands N,N',N″-trimethyl-2,4,6-tris(4-pyridyl)-1,3,5-triazine (Me3tpt3+), N,N'-dimethyl-2,4,6-tris(4-pyridyl)-1,3,5-triazine (Me2tpt2+), and N-monomethyl-2,4,6-tris(4-pyridyl)-1,3,5-triazine (Metpt+), [Pb3CuI10(Me3tpt)] (1), [Pb5Cu2I16(Me2tpt)2] (2), and [Pb3Cu6I14(Metpt)2] (3), were synthesized. Compound 1 exhibits a chain structure, in which the Pb4I16 units are connected by the CuI3 units. The negative charge of the resulting chain is balanced by the cationic Me3tpt3+ groups. 2 features a layer structure, in which the Pb-I chains are connected by the dimeric Cu2 units. The anionic layer is decorated by the Me2tpt2+ motifs via coordinating to the intralayer Cu(I) ions. 3 displays a 3D framework structure, in which the inorganic layer with an 18-membered ring is composed of the strictly alternating arrangements of trimeric Pb3 units and hexameric Cu6 units. The adjacent inorganic layer is further connected by a Metpt+ linker to form the final 3D hybrid framework. It is notable that the in situ N-methylation reaction for tpt has taken place and the resultant motif (Me3tpt3+ for 1, Me2tpt2+ for 2, and Metpt+ for 3) is captured within the corresponding structure. More importantly, the structural diversities from low-dimensional chain and layer to high-dimensional framework is accomplished via the (partial) N-methylation of tripyridine motifs in the heterometallic iodide systems. Our studies offer a new coordination mode of tripyridine motif (N-coordination together with N-methylation) and provide a general strategy to integrate the polypyridine motifs and heterometallic halide systems to generate intriguing hybrid structure and investigate the potential structure-related properties. The UV-vis spectra show that the band gaps for 1-3 are 1.48, 1.35, and 1.34 eV, respectively. Their thermal stabilities have also been studied.

Structures and electronic properties of the SiAu(n) (n = 17-20) clusters.

The structures and electronic properties of the SiAu(n) (n = 17-20) clusters are systematically investigated using DFT calculations. The result shows that doping with silicon would significantly change the structures of the gold clusters. For the SiAu(n) (n = 17-20) clusters, the lowest-energy structures exhibit shell-like cage configuration in which the dopant Si atom binds to the cage surface and one Au atom skips to the top of the Si atom forming a SiAu5 or SiAu6 subunit except SiAu19, which is a tetrahedron-like structure with a protruding Au atom. The Au atoms of the SiAu(n) (n = 17-20) clusters carry different partial charges due to their different locations.

Lithium Diffusion in Silicon Encapsulated with Graphene.

The model of a graphene (Gr) sheet putting on a silicon (Si) substrate is used to simulate the structures of Si microparticles wrapped up in a graphene cage, which may be the anode of lithium-ion batteries (LIBS) to improve the high-volume expansion of Si anode materials. The common low-energy defective graphene (d-Gr) structures of DV5-8-5, DV555-777 and SV are studied and compared with perfect graphene (p-Gr). First-principles calculations are performed to confirm the stable structures before and after Li penetrating through the Gr sheet or graphene/Si-substrate (Gr/Si) slab. The climbing image nudged elastic band (CI-NEB) method is performed to evaluate the diffusion barrier and seek the saddle point. The calculation results reveal that the d-Gr greatly reduces the energy barriers for Li diffusion in Gr or Gr/Si. The energy stability, structural configuration, bond length between the atoms and layer distances of these structures are also discussed in detail.