The structural, elastic and electronic properties of BiI3: First-principles calculations

The structural, elastic and electronic properties of BiI₃ are investigated using the first-principles pseudopotential calculations within the framework of density functional theory. The calculated equilibrium structural parameters agree well with the experimental values. The results show that rhombohedral R-3 structure is low enthalpy structure at zero pressure. R-3 structure will transform into SbI₃-type structure (space group P2₁/c) at about 7.0 GPa. At zero pressure, BiI₃ with R-3 symmetry meets the mechanical stability criteria, but BiI₃ with P-31 m symmetry is an unstable one mechanically. For R-3 structure, the obtained bulk, shear, and Young’s moduli are 25.6, 15.3 and 38.3 GPa, respectively. BiI₃ presents large elastic anisotropy. Debye temperature of R-3 structure calculated is 181 K. The metallization pressure of R-3 structure is about 133 GPa and that of predicted high pressure phase P2₁/c structure is about 61 GPa, indicating BiI₃’s potential application as a nuclear radiation detector under high pressure environment.     

First-principles study of the structural, elastic, electronic and optical properties of the monoclinic BiScO3

The structural, elastic, electronic and optical properties of the monoclinic BiScO₃ are investigated in the framework of the density functional theory. The calculated structural parameters are in agreement with the experimental values. Moreover, the structural stability of BiScO₃ system has been confirmed by the calculated elastic constants. The band structure, density of states, charge transfers and bond populations are also given. The results indicate that BiScO₃ has a direct band gap of 3.36 eV between the occupied O 2p states and unoccupied Bi 6p states, and its bonding behavior is a combination of covalent and ionic nature. Finally, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss function and dielectric function of the monoclinic BiScO₃ are calculated. In addition, the variation of the static dielectric constants ε₁(0) as a function of pressure for BiScO₃ is also discussed.     

Comparative study of the magnetism of SrTcO3 and Ca(Sr)MnO3

By the first-principles calculations, we studied the electronic structures and the magnetic properties of SrTcO₃ and Ca(Sr)MnO₃. We found the strikingly high Néel temperature of SrTcO₃ is mostly due to the strong Tc(4d)–O(2p) hybridizations, since the Tc-4d states are more extended than the Mn-3d states. Such Tc(4d)–O(2p) hybridizations increase the super-exchange constants, hence increased the Néel temperatures.     

Theoretical studies on the structural, electronic, and optical properties of Cu2CdGeSe4

We investigate the electronic structure for Cu₂CdGeSe₄ in stannite structure with the first-principles method. This crystal is the direct band gap compound. In addition, the dielectric function, absorption coefficient, reflectivity, and energy-loss function are studied using the density functional theory within the generalized gradient approximation. We discuss the optical transitions between the valence bands and the conduction bands in the spectra of the imaginary part of the dielectric function at length. We also find a very high absorption coefficient and wide absorption spectrum for this material. The prominent structures in the spectra of reflectivity and energy-loss function are discussed in detail.     

First-principles calculations for elastic and electronic properties of ZnSnO3 under pressure

The LiNbO₃ (LN)-type structure and the ilmenite (IL)-type structure of ZnSnO₃ are investigated with the ultrasoft pseudopotential scheme in the frame of the local density approximation (LDA). The calculated lattice parameters of ZnSnO₃ under zero pressure and zero temperature are in very good agreement with the existing experimental data. The pressure dependences of the elastic constants, Debye temperatures, Poisson's ratio, sound velocity, mechanical stability and mechanical anisotropy of the LN-type structure of ZnSnO₃ have also been investigated. We find that the LN-type structure of ZnSnO₃ is a mechanically stable phase under pressures up to 21 GPa; however, the mechanical anisotropy weakens with the increasing pressures. In addition, the calculated band structure indicates that LN-ZnSnO₃ has a direct band gap of 1.669 eV, and the total and partial densities of states, under diverse pressures of the LN-type structure ZnSnO₃ have also been obtained.     

Structural and electronic properties of YPd5Al2

We present a study of structural and electronic properties of YPd₅Al₂. Specific-heat measurements reveal a relatively low Sommerfeld coefficient of the electronic contribution γ=4.1 mJ mol⁻¹ K⁻². No superconductivity is found down to 0.4 K. First-principles electronic-structure calculations based on density-functional theory have been performed and have been compared with available experimental structural and electronic data.     

Structural,electronic and optical properties of Ca3Bi2: First-principles investigation

In this work by applying first principles calculations structural, electronic and optical properties of Ca₃Bi₂ compound in hexagonal and cubic phases are studied within the framework of the density functional theory using the full potential linearized augmented plane wave (FP-LAPW) approach. According to our study band gap for Ca₃Bi₂ in hexagonal phase are 0.47, 0.96 and 1?eV within the PBE-GGA, EV-GGA and mBJ-GGA, respectively. The corresponding values for cubic phase are 1.24, 2.08 and 2.14?eV, respectively. The effects of hydrostatic pressure on the behavior of the electronic properties such as band gap, valence bandwidths and anti-symmetry gap are investigated. It is found that the hydrostatic pressure increases the band widths of all bands below the Fermi energy while it decreases the band gap and the anti-symmetry gap. In our calculations, the dielectric tensor is derived within the random phase approximation (RPA). The first absorption peak in imaginary part of dielectric function for both phases is located in the energy range 2.0–2.5?eV which are beneficial to practical applications in optoelectronic devices in the visible spectral range. For instance, hexagonal phase of Ca₃Bi₂ with a band gap around 1?eV can be applied for photovoltaic application and cubic phase with a band gap of 2?eV can be used for water splitting application. Moreover, we found the optical spectra of hexagonal phase are anisotropic along E||x and E||z.     

Structural and elastic properties of LaTiO3 under pressure

We investigate the structural and elastic properties of LaTiO₃ by the plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are obtained. These properties in the equilibrium phase are well consistent with the available experimental data. The pressure dependence of the elastic constants, ductility, mechanical stabilities, sound velocity and Debye temperatures are investigated for the first time. From the ratio G/B, we conclude that LaTiO₃ is ductile at 0 GPa and becomes more ductile at high pressure. In addition, the anisotropy factors for every symmetry plane and axis as well as linear bulk modulus at diverse pressures have been obtained.     

Ab initio study of structural, electronic and optical properties of MnHg(SCN)4 and FeHg(SCN)4

 The structural, electronic and optical properties of MnHg(SCN)₄ and FeHg(SCN)₄ were studied by means of quantum-mechanical calculations based on the density-functional theory and pseudopotential method. The lattice constants can be compared with the experimental values when the effects of temperature are considered. The peaks of partial density of states of S, C, N and Hg of FeHg(SCN)₄ have a tendency of shifting to the higher energy levels relative to those of MnHg(SCN)₄. The distributions of the 3d electronic states in the transition metal atoms show quite large difference and decide different optical properties. We found that absorptional peaks of FeHg(SCN)₄ lag behind those of MnHg(SCN)₄ and the peak in the infrared range has a higher absorptional intensity, which are in accord with the experimental results. By analyzing the distributions and transitions of the 3d electronic states, we explained the different absorption phenomena.     

Structural,electronic and optical properties of cubic SrTiO3 and KTaO3: Ab initio and GW calculations

We present first-principles VASP calculations of the structural, electronic, vibrational, and optical properties of paraelectric SrTiO₃ and KTaO₃. The ab initio calculations are performed in the framework of density functional theory with different exchange-correlation potentials. Our calculated lattice parameters, elastic constants, and vibrational frequencies are found to be in good agreement with the available experimental values. Then, the bandstructures are calculated with the GW approximation, and the corresponding band gap is used to obtain the optical properties of SrTiO₃ and KTaO₃.     

Effects of aluminum vacancies on electronic structure and optical properties of Ta4AlC3: A first principles study

We investigated the effect of aluminum vacancies (V_Al) on the structural, electronic and optical properties of Ta₄Al_1−xC₃ (x=0, 0.25, 0.5, 0.75) based on the first-principle calculation using density functional theory. We found that the lattice constant a remains almost unchanged with the variation of V_Al concentration, while c and c/a ratio decrease with increasing V_Al concentration. Moreover V_Al induced local distortions have significant influence on the electronic and optical properties of Ta₄AlC₃, especially beyond the critical V_Al concentration (x=0.5). On the other hand, the presence of V_Al can improve the dielectric properties of Ta₄AlC₃. From the optical properties analysis, we predicted that Ta₄Al_1−xC₃ is not suitable as a coating material to avoid solar heating.     

First-principles study on the structural and electronic properties of AlNCx nanosheet

The structural and electronic properties of a hydrogen terminated hexagonally AlN nanoribbon with 6 zigzag Al-N chains across the ribbon width (6-ZAlNNR) and the hexagonally bonded hetero-sheets AlNC_x (x=2,4,6) consisting of AlN and graphite strips with zigzag shaped borders have been investigated systemically by using the first-principles. The results show that in 6-ZAlNNR, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. Introducing the graphite strip C_x and increasing its width lead to the LUCB and HOVB getting closer with each other especially in flat dispersion region around the zone boundary J_y, thus decreasing in the energy gap of the hetero-sheets AlNC₂, AlNC₄ and AlNC₆ successively. Similar to the edge states existing in zigzag edged AlNNR, the flat dispersion border states also exist in the zigzag borders of hexagonally networked hetero-sheets AlNC_x. Unlike the edge states whose charges are localized at one of the edge atoms, the border states are localized at two atoms of the borders with either bonding or antibonding character.     

Thermoelectric transport coefficients of n-doped CaTiO3, SrTiO3 and BaTiO3: A theoretical study

Boltzmann transport equations and density functional theory calculations were employed to calculate the thermoelectric transport coefficients of CaTiO₃, SrTiO₃ and BaTiO₃. It was found that BaTiO₃ has the largest Seebeck coefficient and power factor. Then the transport coefficients were analyzed using the ‘Tight Binding Model’. The band narrowing, caused by the increasing lattice constants from CaTiO₃ to BaTiO₃, was the main reason for the increasing Seebeck coefficients and the decreasing electrical conductivity. The calculated electrical conductivity and electronic thermal conductivity were in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 2.45 for these oxides as degenerate semiconductors. Our theoretical results are helpful for seeking high performance thermoelectric oxides.     

Structure, electronic and magnetic properties of Cr-doped (ZnS)12 clusters: A first-principles study

We have studied the structural, electronic, and magnetic properties of (ZnS)₁₂ clusters doped with one (monodoped) and two (bidoped) Cr atoms in terms of a first-principles method. Substitutional, exohedral, and endohedral doping are considered. The substitutional isomer is found to be most favorable in energy for monodoped clusters, while the exohedral isomers are found to be most favorable for bidoped clusters. The magnetic coupling between the Cr atoms is mainly governed by the competition between direct Cr-Cr antiferromagnetic (AFM) interaction and the ferromagnetic (FM) interaction between two Cr atoms via S atom due to strong p-d hybridization. Finally, we show that the exohedral bidoped (ZnS)₁₂ clusters favor the FM state, which has potential applications in nanoscale quantum devices.     

DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

Effects of N adsorption on the structural and electronic properties of SrTiO3(0 0 1) surface

The atomic configurations, bonding characteristics, and electronic structures of the N-adsorbed (directly and substitutionally) SrTiO₃(0 0 1) surface are studied by using first-principles method based on the density functional theory. From the analysis of the energetics and density of states, it is found that the stability of the directly adsorbed N depends on the relative position of N atom to the surface. To better understand the effects of the substitutionally adsorbed N on the surface, as an example, the behavior of Au atoms adsorbed on the N-substituted surface is discussed in detail. There is clearly a synergy effect between the substitution of N for O_s(I) and the adsorption of Au atoms on the SrTiO₃(0 0 1) surface.     

Theoretical studies of structural and magnetic properties of cubic perovskites PrCoO3 and NdCoO3

The structural and magnetic properties of cubic perovskites, PrCoO₃ and NdCoO₃, are studied using the full potential linearized augmented plane wave (FP-LAPW) method within the frame work of density functional theory (DFT). The structural parameters are also investigated by analytical techniques. The calculated structural parameters are consistent with the experimental results. The strong hybridization of the O-2p, Co-3d and Pr/Nd-4 f states around the Fermi level reveals that these compounds are metallic. It is also found that the origin of ferromagnetism in these compounds is double-exchange interaction between Co-3d states via O-2p states (Co-O-Co).     

DFT study of structural, electronic and vibrational properties of pure (Al2O3)n (n = 9, 10, 12, 15) and Ni-doped (Al2O3)n (n = 9, 10) clusters

The geometrical, electronic and vibrational properties of pure (Al₂O₃)_n (n = 9, 10, 12, 15) clusters and Ni-doped (Al₂O₃)_9-10 clusters are investigated by density functional theory. There are four different Ni-doped (Al₂O₃)₉ clusters and one Ni-doped (Al₂O₃)₁₀ cluster taken into account. Compared with the pure clusters, the Ni-doped (Al₂O₃)_9-10 clusters have narrower HOMO-LUMO energy gaps. The results indicate that the impurity of Ni atom is mainly responsible for the reduction of the HOMO-LUMO energy gap. One characteristic vibration band at about 1030 cm⁻¹ is found in the vibrational frequencies of the Ni-doped (Al₂O₃)_9-10 clusters, which is caused by the asymmetric Al-O-Al stretching vibration. Another band at around 826 cm⁻¹ involving the characteristic vibration of Ni-O bond is in good agreement with experimental results.     

三方和四方相PbZr0.5Ti0.5O3 的结构稳定性和电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理超原胞方法和虚晶近似方法, 在局域密度近似和广义梯度近似下系统研究了三方相和四方相 PbZr_0.5Ti_0.5O₃的能量稳定性、原子结构以及电子结构. 计算结果表明三方相的能量比四方相低, 说明三方相结构更加稳定, 并且发现利用广义梯度近似计算的结构参数与实验值符合得更好. 电子结构表明, 两种相的Ti/Zr的3d电子和O的2p电子间存在明显的轨道杂化, 并且Ti-O之间的作用比Zr-O作用更强;Pb的6s和5d电子与O的2s和2p电子也分别存在轨道杂化. 而三方相中Pb的5d电子与O的2s电子杂化比四方相更强, 进一步说明三方相比四方相结构更加稳定.     

First-principles calculations on structural, elastic, electronic, optical and thermal properties of CsPbCl3 perovskite

The structural, elastic, electronic, optical and thermal properties of the semiconductor perovskite CsPbCl₃ were investigated using the pseudo-potential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA) and local density approximation (LDA). The computed lattice constant agrees reasonably with experimental and theoretical ones. The CsPbCl₃ crystal behaves as ductile material. The valence bands are separated from the conduction bands by a direct band gap R-R. We distinguished hybridization between Pb-p states and Cl-p states in the valence bonding region. Under compression at P=30 GPa, this material will have a metallic character. The thermal effect on the lattice constant, bulk modulus, Debye temperature and heat capacity C_V was predicted using the quasi-harmonic Debye model. To the author's knowledge, most of the studied properties are reported for the first time.