The structural,electronic and elastic properties of solid nitromethane are investigated under pressure by performing first-principles density functional theory(DFT)calculations within the generalized gradient approximation(GGA)and the local density approximation(LDA).The obtained ground state structure properties are found to be consistent with existing experimental and theoretical results.The pressure-induced variations of structure parameters(a,b,c and V)indicate that the solid nitromethane has an anisotropic compressibility,and the compression along the c direction is more difficult than along a and b directions.From the vibration curves of intermolecular bond length and bond angle,we find that the C—N bond is the most sensitive among these bonds under pressure,suggesting that the C—N bonds may be broken first under external loading.The influence of pressure on the electronic properties of solid NM has been studied,indicating that solid NM is an insulating compound with a large indirect band gap and tends to be a semiconductor with increasing pressure.Finally,we predict the elastic constants and their pressure dependence for the solid NM with the bulk modulus,Young’s modulus,shear modulus and the Poisson’s ratio derived.
First-principle simulations have been applied to investigate the effect of copper（Cu） or aluminum（Al） content on the ductility of Al3Ti,AlTi,AlCu,and AlTiCu2 alloys.The mechanical stable and elastic properties of Al-based intermetallic compounds are researched by density functional theory with the generalized gradient approximation（DFT-GGA）.The calculated lattice constants are in conformity with the previous experimental and theoretical data.The deduced elastic constants show that the investigated Al_3Ti,AlTi,AlCu,and AlTiCu2 structures are mechanically stable.Shear modulus,Young＇s modulus,Poisson＇s ratio,and the ratio B/G have also been figured out by using reckoned elastic constants.A further analysis of Young＇s modulus and Poisson＇s ratio reveals that the third added element copper content has significant effects on the Al-Ti-based ICs ductile character.
The structures and the phase transitions of ScH3 under high pressure are investigated using first-principles calcula- tions. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp （GdHa-type）→ C2/m →fcc→4hcp （YH3-type）→Cmcm of ScH3 is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH3. On the other hand, via the analysis of the structures of hexagonal SCH2.9, cubic ScH3, and cubic ScH2, we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.
First-principles calculations of structural, electronic, optical, elastic, mechanical properties, and Born effective charges of monoclinic HfO2 are performed with the plane-wave pseudopotential technique based on the density-functional theory. The calculated structural properties are consistent with the previous theoretical and experimental results. The electronic structure reveals that monoclinic HfO2 has an indirect band gap. The analyses of density of states and Mulliken charges show mainly covalent nature in Hf-O bonds. Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function, and optical conductivity each as a function of photon energy are calculated and show an optical anisotropy. Moreover, the independent elastic constants, bulk modulus, shear modulus, Young＇s modulus, Poisson＇s ratio, compressibility, Lam6 constant, sound velocity, Debye temperature, and Born effective charges of monoclinic HfO2 are obtained, which may help to understand monoclinic HfO2 for future work.
Combining a low temperature liquidizing system with a transient Raman spectroscopy, a new experimental technique is established for the first time on a two-stage light-gas gun, and it is employed to study shock-compressed fluid silane. With this experimental technique, we first obtain a Raman peak shift relating to the Si-H stretching vibration mode of molecular liquid silane under shock loading conditions. The Raman peak of 2184 cm^-1 at an initial state of 0 GPa and 85 K moves to 2223.4 cm^-1 at a shocked state of 10.5 GPa and 950 K, and its full width of half maximum broadens from 33 cm^-1 to 118 cm^-1. The shocked temperature, calculated by the thermodynamic equation of state, is well consistent with that estimated by the Doppler broadening function.
