Guo Yun-Dong

Evidence of the stability of Mo2TiAlC2 from first principles calculations and its thermodynamical and optical properties

The elastic, thermodynamic, and optical properties of Mo_{2}TiAlC_{2} are investigated by first-principles calculations. Our results indicate that the a axis is stiffer than c axis within 0~100 GPa. Elastic constants calculations predict the large stability range of Mo_{2}TiAlC_{2} under pressure. Several important thermodynamic properties are discussed detailedly, including the Debye temperature, thermal expansion coefficient, and heat capacity etc. The bonding properties are studied from the elastic quantities and electronic properties. The electronic properties are investigated, including the energy band structure, density of states, and so on. The evidence of the instability of Mo_{3}AlC_{2} and stability of Mo_{2}TiAlC_{2} are successfully obtained.

First-principles calculation of the lattice compressibility, elastic anisotropy and thermodynamic stability of V2GeC

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 Youngs 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 Gruneisen parameter, the Debye temperature and the thermal expansion coefficient are also successfully obtained for the first time.

Electronic structure and optical properties of rutile RuO2 from first principles

The systematic trends of electrionic structure and optical properties of rutile (P42/mnm) RuO2 have been calculated by using the plane-wave norm-conserving pseudopotential density functional theory (DFT) method within the generalised gradient approximation (GGA) for the exchange–correlation potential. The obtained equilibrium structure parameters are in excellent agreement with the experimental data. The calculated bulk modulus and elastic constants are also in good agreement with the experimental data and available theoretical calculations. Analysis based on electronic structure and pseudogap reveals that the bonding nature in RuO2 is a combination of covalent, ionic and metallic bonds. Based on a Kramers–Kronig analysis of the reflectivity, we have obtained the spectral dependence of the real and imaginary parts of the complex dielectric constant (1 and 2, respectively) and the refractive index (n); and comparisons have shown that the theoretical results agree well with the experimental data as well. Meanwhile, we have also calculated the absorption coefficient, reflectivity index, electron energy loss function of RuO2 for radiation up to 30 eV. As a result, the predicted reflectivity index is in good agreement with the experimental data at low energies.

Origin of the c-axis ultraincompressibility of Mo2GaC above about 15 GPa from first principles

The mechanical properties and structural evolution of Mo2GaC are calculated by first-principles under pressure. Our results unexpectedly found that the c axis is always stiffer than a axis within 0–100 GPa. An ultraincompressibility of c axis within 15–60 GPa is observed, with a contraction of about 0.2 A, slightly larger than that of a axis (0.14 A). The abnormal expansion of c axis and the fast decrease in a axis above about 15 GPa and 70 GPa failed to induce the structural instability, whereas such behavior caused the elastic softening in many mechanical quantities. The shrinkage anomaly of c axis is closely reflected by the internal coordinate (u) shift of Mo atom as it shows three different slopes within 0–15 GPa, 20–60 GPa, and 70–100 GPa, respectively. The longest Mo-Mo bond is responsible for the unusual shrinkage of c-axis under pressure as they experience nearly identical pressure dependences, whereas the a axis presents certain response with the variation of C-Mo bond particularly at 70 GPa. The electronic properties are investigated, including the energy band and density of states, and so on. At G point of K-M line, the energy decreases at 10 GPa first and increases at 30 GPa subsequently, the critical point is at about 15 GPa, with respective values of −0.17 of 0 GPa, −0.18 of 10 GPa, −0.16 of 15 GPa, and −0.13 of 30 GPa, respectively. This alternative energy change of G point, which is the symmetry center of the rhombic parallelogram of Ga atoms and the midpoint of the two bonded Mo atoms, convincingly reveal the origin of the anomalous ultraincompressibility of c axis as the Mo-Mo bond length shrinkage has to overcome the increasing energy barrier height. The Mo-Mo bond population and the electronegativity investigations of the Mo atom further reveal the most likely origin of the ultraincompressibility of c axis. This interesting result expects further experimental confirmation as this is the first nanolaminate ceramics compound presenting quite low-pressure axial ultraincompressibility.

Magnetic moment collapse induced axial alternative compressibility of Cr2TiAlC2 at 420 GPa from first principle

The electronic structure and thermodynamical properties of Cr2TiAlC2 are studied by first principles under pressure. The obtained results observed that the ferromagnetic order is the most stable ground state and the magnetic moment will collapse at about 50 GPa. As a result, the lattice a axis becomes stiffer above about 420 GPa, ultimately presenting the same axial compressibility trends with those of nonmagnetic compounds Mo2TiAlC2 and hypothetical Cr2TiAlC2. The elastic constants and phonon dispersion curves demonstrate the structural stability during the disappearance of magnetic moment and occurrence of axial alternative compressibility. The density of states and energy band calculations confirmed the existence of magnetic moment of Cr2TiAlC2 at 0 GPa and disappearance at high pressures above 50 GPa. Evolutions of magnetic moment collapse with pressure are confirmed by a variety of properties. The obtained grüneisen parameter and thermal expansion coefficients show the maximum value among the known MAX phases, to date and to the author’s knowledge.

Electronic, thermodynamic and elastic properties of pyrite RuO2

This paper calculates the elastic,thermodynamic and electronic properties of pyrite (P a3ˉ) RuO2 by the plane-wave pseudopotential density functional theory (DFT) method.The lattice parameters,normalized elastic constants,Cauchy pressure,brittle–ductile relations,heat capacity and Debye temperature are successfully obtained.The Murnaghan equation of state shows that pyrite RuO2 is a potential superhard material.Internal coordinate parameter increases with pressure,which disagrees with experimental data.An analysis based on electronic structure and the pseudogap reveals that the bonding nature in RuO2 is a combination of covalent,ionic and metallic bonding.A study of the elastic properties indicates that the pyrite phase is isotropic under usual conditions.The relationship between brittleness and ductility shows that pyrite RuO2 behaves in a ductile matter at zero pressure and the degree of ductility increases with pressure.This paper calculates the elastic, thermodynamic and electronic properties of pyrite (Pa) RuO2 by the plane-wave pseudopotential density functional theory (DFT) method. The lattice parameters, normalized elastic constants, Cauchy pressure, brittle–ductile relations, heat capacity and Debye temperature are successfully obtained. The Murnaghan equation of state shows that pyrite RuO2 is a potential superhard material. Internal coordinate parameter increases with pressure, which disagrees with experimental data. An analysis based on electronic structure and the pseudogap reveals that the bonding nature in RuO2 is a combination of covalent, ionic and metallic bonding. A study of the elastic properties indicates that the pyrite phase is isotropic under usual conditions. The relationship between brittleness and ductility shows that pyrite RuO2 behaves in a ductile matter at zero pressure and the degree of ductility increases with pressure.

High Temperature Spectrum for ν3 Band of Carbon Dioxide

The total internal partition sums (TIPS) are calculated at the temperature up to 6000 K for 12C16O2. Using the calculated partition functions, we produce the line intensities of ν3 band of 12C16O2 at several high temperatures. The results show that the calculated line intensities are in very good agreement with those of HITRAN database at the temperature up to 3000 K, which provides a strong support for the calculations of TIPS and line intensities at high temperature. Then the calculation is extended to further high temperature, and the simulated spectra of ν3 band of 12C16O2 at 5000 and 6000 K are reported.

Study on High-Temperature Spectra of Asymptotic Asymmetric-Top Molecule O3

The line intensities of 001-000 transition of the asymptotic asymmetric-top O3 molecule at several temperatures are calculated by directly calculating the partition functions and regarding the rotationless transition dipole moment squared as a constant. The calculated values of the total internal partition sums (TIPS) are consistent with the data of HITRAN database with −0.61% at 296 K. The calculated line intensity data at 500 K and 3000 K are also in excellent agreement with the data in HITRAN database with less than 0.659% and 5.458% at 500 K and 3000 K, which provide a strong support for the calculations of partition function and line intensity at high temperature. Then we extend the calculation to higher temperatures. The line intensities and simulated spectra of ν3 band of the asymptotic asymmetric-top O3 molecule at 4000 and 5000 K are reported. The results are of significance for the studying of the molecular high-temperature spectrum including experimental measurements and theoretical calculations.