We investigate the elastic and thermodynamic properties of nanolaminate V2 AlC by using the ab initio pseudopotential total energy method. The axial compressibility shows that the c axis is always stiffer than a axis. The elastic constants revealed the structural instability at about 500 and 732 GPa. Furthermore, elastic constants C44 reached its maximum at about 550 GPa, differing with the other four C11 , C12 , C13 and C33 constants. The Poisson's ratio investigations demonstrated that a higher ionic or weaker covalent contribution in intra-atomic bonding and the degree of ionicity increases with pressure. The G/B and B/C44 investigations revealed that V2 AlC is brittle and the brittleness decreases with pressure. Also, we found that V2 AlC is elastic anisotropic materials and the degree of anisotropy rapidly rises with pressure. Study on Debye temperature and Grüneisen parameter observed weak temperature and strong pressure responses, whereas the sensitive dependence in the thermal expansion coefficient and Helmholtz free energy are clearly seen.
YANG ZeJinLIU QiangLI JinWANG ZhaoGUO AiMinLINGHU RongFengCHENG XinLuYANG XiangDong
We investigate the elastic and the thermodynamic properties of nanolaminate V2GeC by using the ab initio pseudopotential total energy method.The axial compressibility shows that the c axis is always stiffer than the a axis.The elastic constant calculations demonstrate that the structural stability is within 0-800 GPa.The calculations of Young's and shear moduli reveal the softening behaviour at about 300 GPa.The Possion ratio makes a higher ionic or a weaker covalent contribution to intra-atomic bonding and the degree of ionicity increases with pressure.The relationship between brittleness and ductility shows that V2GeC is brittle in ambient conditions and the brittleness decreases and ductility increases with pressure.Moveover,we find that V2GeC is largely isotropic in compression and in shear,and the degree of isotropy decreases with pressure.The Grüneisen parameter,the Debye temperature and the thermal expansion coefficient are also successfully obtained for the first time.
Using Vanderbilt-type plane-wave ultrasoft pseudopotentials within the generalized gradient approximation(GGA) in the frame of density functional theory(DFT),we have investigated the crystal structures,elastic,and thermodynamic properties for Ti2SC under high temperature and high pressure.The calculated pressure dependence of the lattice volume is in excellent agreement with the experimental results.The calculated structural parameter of the Ti atom experienced a subtle increase with applied pressures and the increase suspended under higher pressures.The elastic constants calculations demonstrated that the crystal lattice is still stable up to 200 GPa.Investigations on the elastic properties show that the c axis is stiffer than the a axis,which is consistent with the larger longitudinal elastic constants(C 33,C 11) relative to transverse ones(C 44,C 12,C 13).Study on Poisson's ratio confirmed that the higher ionic or weaker covalent contribution in intra-atomic bonding for Ti2SC should be assumed and the nature of ionic increased with pressure.The ratio(B/G) of bulk(B) and shear(G) moduli as well as B/C 44 demonstrated the brittleness of Ti2SC at ambient conditions and the brittleness decreased with pressure.Moreover,the isothermal and adiabatic bulk moduli displayed opposite temperature dependence under different pressures.Again,we observed that the Debye temperature and Gru¨neisen parameter show weak temperature dependence relative to the thermal expansion coefficient,entropy,and heat capacity,from which the pressure effects are clearly seen.
Orbital responses to methyl sites in C_nH_(2n+2) (n=1-6) are studied by B3LYP/TZVP based on the most stable geometries using the B3LYP/aug-cc-pVTZ method.Vertical ionization energies are produced using the SAOP/et-pVQZ model for the complete valence space.The highest occupied molecular orbital (HOMO) investigations indicate the pelectron profiles in methane,ethane,propane,and n-butane.By increasing the number of carbon-carbon bonds in lower momentum regions,the s,p-hybridized orbitals are built and display strong exchange and correlation interactions in lower momentum space (P<0.50 a.u.).Meanwhile,the relative intensities of the isomers in lower momentum space show the strong bonding number dependence of the carbon-carbon bonds,meaning that more electrons have contributed to orbital construction.The study of representative valence orbital momentum distribution further confirms that the structural changes lead to evident electronic rearrangement over the whole valence space.An analysis based on the isomers reveals that the valence orbitals are isomer-dependent and the valence ionization energy experiences an apparent shift in the inner valence space.However,such shifts are greatly reduced in the outer valence space.Meanwhile,the opposite energy shift trend is found in the intermediate valence space.
