First principles vibrational dynamics of magnesium telluride

The lattice dynamics of large-gap semiconductor MgTe compound at various crystallographic phases; rocksalt (B1), zincblende (B3), NiAs (B8₁) and wurtzite (B4), has been investigated from first principles calculations based on density functional theory (DFT) within plane-wave pseudopotential method and generalized gradient approximation (GGA) of the exchange-correlation functional. The static equation of states of the compound has been studied with Vinet equation of states. The ground state of the compound is a fourfold coordinated wurtzite structure, which is consistent with experiments and recent theoretical calculations. Full phonon dispersion spectra of all related phases of the MgTe have been calculated using density functional perturbation theory within the linear-response approach. In view of the total energy calculations and the obtained vibrational spectra, it can be emphasized that the MgTe polymorphs with tetrahedral coordination (zincblende and wurtzite structures) are of covalent character rather than ionic. The large TO-LO splitting of phonon branches of rocksalt and NiAs phases reflect the high ionicity of these phases.     

Neutron Spectroscopy of Norbornane

Results of inelastic incoherent neutron scattering (IINS) on norbornane are reported. The IINS spectra in the low-temperature phases display low-frequency internal vibration modes very well. The assignment of these modes is proposed in reference to the results of the calculations of the structure and dynamics of isolated molecules by semi-empirical quantum chemistry methods.     

Interfacial activity of styrene-butadiene block copolymers in low-density polyethylene/polystyrene blends

The effect of molecular structure of styrene-butadiene (SB) block copolymers on their interfacial activity in low-density polyethylene/polystyrene (LDPE/PS) (4/1) blends was studied. It was found that addition of some SB copolymers, which are localized in brittle PS particles, leads to a decrease in the blend impact strength in spite of the fact that these SB improve the toughness of both the blend components. Comparison with our previous results showed that the distribution of SB copolymers between the interface and bulk phases and their supermolecular structure in LDPE/PS (4/1) blends strongly differs from those in LDPE/PS (1/4) blends.     

一种适用于原子团簇的多体展开新方案: 以氮团簇为例

多体展开方法虽然已经广泛地用于估算弱相互作用体系的能量,但是其并不适用于计算共价团簇和金属团簇的能量. 本文提出了一种适用于计算共价体系能量的相互作用多体展开(IMBE)方法. 在相互作用多体展开方法中,体系的能量表示为孤立原子的能量及该原子与其他周围原子间相互作用的和. 首先将该方法应用于计算氮团簇的能量,且多体展开截断至四体项. 结果表明：与传统的多体展开方法相比,相互作用多体展开方法可以显著地降低能量误差. 另外,以密度泛函理论计算结果为参考,相互作用多体展开方法估算能量的误差不依赖于体系的大小和结构,说明相互作用多体展开方法比较适合用于估算共价相互作用大体系的能量.     

Theoretical study of the helio hydrogen cyanide dication HeCNH2+

The structure and stability of the helio hydrogen cyanide molecular ion, HeCNH²⁺, is investigated by standard quantum chemical methods. Single reference calculations are carried out using second-order perturbation theory (MP2), the coupled cluster expansion in the CCSD approximation, and the hybrid approach using a perturbative estimate of the triple excitation energy component designated CCSD(T). Multireference calculations using a complete active space (CASSCF) and a second-order perturbation theory estimate of correlation effects (CASPT2) are reported.     

Ab initio study of the EFG at the N sites in imidazole

We study the nuclear quadrupole interaction at the nitrogen sites in the molecular and crystalline phases of the imidazole compound. We use PAW which is a state-of-the-art method to calculate the electronic structure and electric field gradient at the nucleus in the framework of the density functional theory. The quadrupole frequencies at both imino and amino N sites are in excellent agreement with measurements. This is the first time that the electric field gradient at crystalline imidazole is correctly treated by an ab initio theoretical approach.     

Trap-recharging waves versus damped,forced charge-density oscillations in hexagonal silicon carbide

The structural parameters and hydrostatic pressure coefficients of CdS_xTe_1-x in the two phases, namely zinc-blende and NaCl as well as the transition pressures from zinc-blende to NaCl structures at various S concentrations are presented. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA), and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al. for the structural properties and Engel-Vosko for the band structure calculations. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The present results regarding the studied quantities for compositions x in the 0–1 range (0 < x < 1) and for the NaCl phase are predictions and may serve as a reference for experimental work.     

Phase stability and electronic properties of silver halides

In this work, we study the phase stability and electronic properties of silver halides ( AgBr, AgCl and AgI) using the full-potential linearized augmented plane wave method within the density functional theory. In this approach, the Wu–Cohen generalized gradient approximation was used for the exchange–correlation potential. Moreover, the modified Becke–Johnson approximation was also used for band-structure calculations. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition, we have studied the band structures of the stable phases of these compounds which reveal that the three compounds exhibit semiconducting behavior. The results obtained are compared with other calculations and experimental measurements.     

Hafnium nitride with thorium phosphide structure: physical properties and an assessment of the Hf-N,Zr-N,and Ti-N phase diagrams at high pressures and temperatures

The physical properties of the new cubic phase of Hf3N4 as well as of isomorphic Zr3N4 and Ti3N4 are studied using first-principles calculations. Hf3N4, Zr3N4, and Ti3N4 are semiconductors with band gaps of 1.8, 1.1, and 0.6 eV, respectively. The band structure is characterized by the simultaneous presence of steep and extremely flat bands. The calculated shear modulus G indicates that the cubic Hf3N4 will be harder than the mononitride HfN. At ambient conditions, the cubic modifications of M3N4 (M=Hf, Zr, Ti) are metastable with respect to orthorhombic M3N4 phases, but the orthorhombic phases of Hf3N4 and Zr3N4 are stable with respect to the mononitrides and nitrogen.     

BeO高压相变和声子谱的第一性原理计算

采用第一性原理方法计算了BeO在零温时的高压相变和三种结构在零温零压时的声子谱.相变的计算表明,在122GPa左右的压力下BeO会发生从纤锌矿(B4)结构到氯化钠(B1)结构的相变,而闪锌矿(B3)结构在零温零压下是一种可能的亚稳态结构.采用冷声子方法计算了这三种结构的BeO在零温零压下的声子谱.计算结果表明:B1结构在零温零压下是一种不稳定的结构;尽管B4结构和B3结构具有明显的相似性,仍然可以通过声子谱来很好的区分.最后根据准简谐近似理论计算得到了BeO的高温高压相图.     

Phase stability and electronic behavior of MgS,MgSe and MgTe compounds

Ab initio calculations based on density functional theory using the full-potential linearized augmented plane wave method have been carried out to find the structural stability of different crystallographic phases, the pressure-induced phase transition and the electronic properties of the magnesium chalcogenides MgS, MgSe and MgTe. The zinc blende (B3), wurtzite (B4), rock salt (B1), CsCl (B2), NiAs (B8), β-BeO, 5-5 and TiP crystal structures are considered and the exchange and correlation potential is treated by the generalized-gradient approximation using the Perdew–Burke–Ernzerhof parameterization. Moreover, the modified Becke-Johnson (mBJ) scheme is also applied to optimize the corresponding potential for the band structure calculations. Results show that the wurtzite phase is the stable structure in the ground state adopted by MgSe and MgTe compounds while MgS adopts the rock-salt one. Moreover, the band structure calculations reveal a metallic behavior in the CsCl structure for all the compounds, whereas for the other structures, a semiconducting behavior is observed.     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

Nanoscale ab-initio calculations of optical and electronic properties of LaCrO3 in cubic and rhombohedral phases

We report nanoscale ab-initio calculations of the linear optical and electronic properties of LaCrO₃ in nonmagnetic cubic and rhombohedral phases using the full potential linear augmented plane wave (FP-LAPW) method. In this work the generalized gradient approximation is used for exchange-correlation potential. The dielectric tensor is derived within random-phase approximation. We present results for the band structure, density of states, imaginary and real parts of dielectric tensor, electron energy loss spectroscopy, sum rules, reflectivity, refractive index and extinction coefficient. The regions of transparent, absorption and reflection are discussed. We are not aware of any published experimental or theoretical data for these phases, so our calculations can be used to cover this lack of data for these phases.     

Modeling of heterogeneous surfaces and characterization of porous materials by extending density functional theory for the case of amorphous solids

A method of characterization of carbonaceous materials using nongraphitized carbon black as a reference is considered. The Tarazona density functional theory was applied to amorphous solids to describe nitrogen adsorption on nongraphitized carbon black. This allows us to describe energetic heterogeneity without the need to invoke any energy distribution functions. To derive the pore size distribution (PSD) of porous carbon whose pore walls are non-graphitized, we used the entropy concept in the regularization method. With this approach PSD is more well-behaved than that obtained with the usual means. We applied this new theory to study the effects of technological parameters on porous structure of a series of activated carbon.     

Structural stability of complex hydrides: LiBH4 revisited

A systematic approach to study the phase stability of LiBH4 based on ab initio calculations is presented. Three thermodynamically stable phases are identified and a new phase of Cc symmetry is proposed for the first time for a complex hydride. The x-ray diffraction pattern and vibrational spectra of the Cc structure agree well with recently reported experimental data on LiBH4. Calculations of the free energy at finite temperatures suggest that the experimentally proposed P6(3)mc phase is unstable at T>0 K.     

A tight-binding approach to the electronic structure of the silver halides—II: The silver iodide polymorphs

We have applied the modified tight binding approach developed in the first paper of this series to calculate the electronic structure of the silver iodide polymorphs. The four phases which we have investigated are the α, β and γ modifications and the high pressure polymorph having the NaCl structure. The ionicity values obtained for these different phases agree well with the values of Phillips derived from the dielectric data and the computed bandstructures are in good agreement with independent pseudopotential calculations.     

First-Principles Study on Native Defect Complexes in InN

We present first-principles calculations of the formation energy of different native defects and their complexes in wurtzite InN using density-functional theory and the pseudopotential plane-wave method. Our calculations are aimed in the three cases： N/In = 1, N/In 〉 1 （N-rich）, and N/In 〈 1 （In-rich）. Our results indicate that the antisite defect has the lowest formation energy under N/In = 1. The formation energy of nitrogen interstitial （nitrogen vacancy） defect is significantly low under the N-rich （In-rich） condition. Thus the antisite defect is an important defect if N/In = 1, and the nitrogen interstitial （nitrogen vacancy） defect is a vital defect under the N-rich （In-rich） condition. The atomic site relaxation around the nitrogen interstitial and vacancy is investigated. Our calculations show that the nitrogen vacancy cannot be observed although it is one of the most important defects in InN. Our results are confirmed by experiments.     

Phonon lifetimes from first-principles self-consistent lattice dynamics

Phonon lifetime calculations from first principles usually rely on time-consuming molecular dynamics calculations, or density functional perturbation theory where the zero-temperature crystal structure is assumed to be dynamically stable. Here is presented a new and effective method for calculating phonon lifetimes from first principles. This method is not limited to crystallographic phases stable at 0 K and provides a scheme more effective than most corresponding molecular dynamics calculations. The method is based on the recently developed self-consistent ab initio lattice dynamical method and is here tested by calculating the bcc phase phonon lifetimes of Li, Na, Ti and Zr as representative examples.     

Electronic structure calculations using dynamical mean field theory

The calculation of the electronic properties of materials is an important task of solid-state theory, albeit particularly difficult if electronic correlations are strong, e.g., in transition metals, their oxides and in f-electron systems. The standard approach to material calculations, the density functional theory in its local density approximation (LDA), incorporates electronic correlations only very rudimentarily and fails if the correlations are strong. Encouraged by the success of dynamical mean field theory (DMFT) in dealing with strongly correlated model Hamiltonians, physicists from the bandstructure and the many-body communities have joined forces and developed a combined LDA + DMFT method recently. Depending on the strength of electronic correlations, this new approach yields a weakly correlated metal as in the LDA, a strongly correlated metal or a Mott insulator. This approach is widely regarded as a breakthrough for electronic structure calculations of strongly correlated materials. We review this LDA + DMFT method and also discuss alternative approaches to employ DMFT in electronic structure calculations, e.g., by replacing the LDA part with the so-called GW approximation. Different methods to solve the DMFT equations are introduced with a focus on those that are suitable for realistic calculations with many orbitals. An overview of the successful application of LDA + DMFT to a wide variety of materials, ranging from Pu and Ce, to Fe and Ni, to numerous transition metal oxides, is given.