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1.
J Chem Phys ; 154(13): 134708, 2021 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-33832239

RESUMO

Elemental copper and potassium are immiscible under ambient conditions. It is known that pressure is a useful tool to promote the reaction between two different elements by modifying their electronic structure significantly. Here, we predict the formation of four K-Cu compounds (K3Cu2, K2Cu, K5Cu2, and K3Cu) under moderate pressure through unbiased structure search and first-principles calculations. Among all predicted structures, the simulated x-ray diffraction pattern of K3Cu2 perfectly matches a K-Cu compound synthesized in 2004. Further simulations indicate that the K-Cu compounds exhibit diverse structural features with novel forms of Cu aggregations, including Cu dimers, linear and zigzag Cu chains, and Cu-centered polyhedrons. Analysis of the electronic structure reveals that Cu atoms behave as anions to accept electrons from K atoms through fully filling 4s orbitals and partially extending 4p orbitals. Covalent Cu-Cu interaction is found in these compounds, which is associated with the sp hybridizations. These results provide insights into the understanding of the phase diversity of alkali/alkaline earth and metal systems.

2.
Dalton Trans ; 48(38): 14299-14305, 2019 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-31453996

RESUMO

A thorough structural exploration was performed for MgB6 combining the global structure searching method with first-principles calculations. Besides the known Cmcm phase, new phases, i.e. I4/mmm, C2/m-I, C2/m-II and P21/m, were predicted to be stable in the pressure range of 18-100 GPa. Unexpectedly, Cmcm-MgB6 was found to be a semiconductor with an indirect band gap of 0.38 eV with the HSE06 functional, in good agreement with the experimental finding. I4/mmm-MgB6 stabilized above 18 GPa exhibits semimetallic behaviour with a topological node-line near the Fermi level. Consequently, C2/m-I MgB6 with a sandwich structure similar to MgB2 is predicted to be a superconductor with a critical temperature (Tc) of 9.5 K. By analysing the electronic structure, the intriguing semiconductor-semimetal-superconductor transition may be ascribed to the delocalization of more B-p electrons in the boron sublattice. The novel functions uncovered for MgB6 may inspire more efforts to discover materials with intriguing properties.

3.
J Colloid Interface Sci ; 554: 650-657, 2019 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-31351335

RESUMO

Transparent and flexible supercapacitors (TFSCs) could diversify the future wearable electronics owing to the fascinating optoelectronic and electrochemical performances. Herein, we report symmetric TFSCs assembled by reduced graphene oxide (rGO)@Ag nanowire/poly (ethylene terephthalate) (PET) transparent electrodes for capacitive storage, in which the interfacial structure of rGO film can be tuned by a facile freeze drying technique. The enlarged interlayer spacing of rGO film deteriorated the electronic migration derived from the loose layer structure, whereas about 33-52% of the areal capacitance of TFSCs was boosted as compared with the ones without freeze drying at the same transmittance. It is concluded that the enlarged inter-distance of rGO film could facilitate diffusion and transport of ions in the electrolyte, furthermore, the expanded rGO film could provide more interface to accommodate more ions for storage. The simulation results also confirmed the lower diffusion barrier and larger band gap of rGO with larger interlayer distance. The mechanically robust TFSCs exhibit the maximum energy density of 89.2 nWh cm-2, and the maximum power density of 4.63 µW cm-2 with remaining energy density of 41.1 nWh cm-2, as well as 3000 cyclic stability, demonstrating an efficient strategy toward high performance TFSCs.

4.
Adv Sci (Weinh) ; 5(11): 1800666, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30479920

RESUMO

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure-induced structural and electronic evolutions of Ca2N by in situ synchrotron X-ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca2N undergoes structural transforms from R 3 ¯ m symmetry to I 4 ¯ 2d phase via an intermediate Fd 3 ¯ m phase, and then to Cc phase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal-to-semiconductor transition in Ca2N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure-induced metal-to-semiconductor switching in Ca2N and offer a possible strategy for producing new electrides under moderate pressure.

5.
Inorg Chem ; 57(15): 9385-9392, 2018 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-30024155

RESUMO

Transition-metal (TM) phosphides attract increasing attention with applications for energy conversion and storage, due to their outstanding physical, chemical, and electronic properties. The 3d transition metal tetraphosphides (TMP4, TM = V, Cr, Mn, and Fe) possess multiple allotropies and rich electronic properties. Here, we perform the investigations of the structural, electronic, and elastic properties for 3d-TMP4 (TM = V, Cr, Mn, and Fe) using density functional theory (DFT) calculations. These compounds are featured with alternating buckled phosphorus sheets with ten-numbered phosphorus rings and varied transition-metal layers. Hybrid DFT calculations reveal that TMP4 compounds exhibit a wide range of electrical properties, ranging from metallic behavior for VP4 to semiconducting behavior for CrP4 with the narrow direct band gap of 0.63 eV to enlarged semiconducting MnP4 and FeP4 with band gap of 1.6-2.1 eV. The bonding analysis indicates that P-P and TM-P covalent interactions dominate in the phosphorus sheets and TMP6 octahedrons, which are responsible for the structural and electronic diversity.

6.
J Phys Chem Lett ; 7(23): 4898-4904, 2016 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-27934060

RESUMO

Unique multicenter bonding in boron-rich materials leads to the formation of complex structures and intriguing properties. Here global structural searches are performed to unearth the structure of beryllium hexaboride (BeB6) synthesized decades ago. Three BeB6 phases (α, ß, and γ) were predicted to be stable at ambient and high pressures. The ground state at ambient pressure, α-BeB6, consists of a strong and uniformly distributed covalent B-B network, which results in exceptional elastic properties and a hardness of 46 GPa comparable to γ-B. Even more surprisingly, α-BeB6 retains credible electron phonon coupling in the boron sublattice, and is predicted to be superconducting at 9 K. Above 4 GPa, ß-BeB6 is stabilized with alternating boron slabs and triangular beryllium layers analogous to the structure of MgB2. The ß-BeB6 is predicted to be superconducting at 24 K, similar to Nb3(Al,Ge). The γ-BeB6 is stable above 340 GPa. The understanding of intrinsic multicenter-bonding mechanism and related properties demonstrated in the very example of BeB6 provides new insights for the design of tunable multifunctional materials.

7.
Sci Rep ; 6: 33506, 2016 09 15.
Artigo em Inglês | MEDLINE | ID: mdl-27627856

RESUMO

First-principles calculations were performed to understand the structural stability, synthesis routes, mechanical and electronic properties of diverse ruthenium nitrides. RuN with a new I-4m2 symmetry stabilized by pressure is found to be energetically preferred over the experimental NaCl-type and ZnS-type ones. The Pnnm-RuN2 is found to be stable above 1.1 GPa, in agreement with the experimental results. Specifically, new stoichiometries like RuN3 and RuN4 are proposed firstly to be thermodynamically stable, and the dynamical and mechanical stabilities of the newly predicted structures have been verified by checking their phonon spectra and elastic constants. A phase transition from P4/mmm-RuN4 to C2/c-RuN4 is also uncovered at 23.0 GPa. Drawn from bonding and band structure analysis, P4/mmm-RuN4 exhibits semi-metal-like behavior and becomes a semiconductor for the high-pressure C2/c-RuN4 phase. Meanwhile the P21/c-RuN3 shows metallic feature. Highly directional covalent N-N and Ru-N bonds are formed and dominating in N-enriched Ru nitrides, making them promising hard materials.

8.
Phys Chem Chem Phys ; 18(4): 2361-8, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26692374

RESUMO

Employing particle swarm optimization (PSO) combined with first-principles calculations, we systemically studied high-pressure behaviors of hard CrB4. Our predictions reveal a distinct structural evolution under pressure for CrB4 despite having the same initial structure as FeB4. CrB4 is found to adopt a new P2/m structure above 196 GPa, another Pm structure at a pressure range of 261-294 GPa and then a Pmma structure beyond 294 GPa. Instead of puckering boron sheets in the initial structure, the high-pressure phases have planar boron sheets with different motifs upon compression. Comparatively, FeB4 prefers an I41/acd structure over 48 GPa with tetrahedron B4 units and a P213 structure above 231 GPa having equilateral triangle B3 units. Significantly, CrB4 exhibits persistent metallic behavior in contrast with the semiconducting features of FeB4 upon compression. The varied pressure response of hard tetraborides studied here is of importance for understanding boron-rich compounds and designing new materials with superlative properties.

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