Plane-wave Pseudopotential Approach Research Articles

Performance of the modified Becke–Johnson potential employing the pseudopotential plane-wave approach for band structure calculations

The modified Becke–Johnson exchange potential combined with local-density approximation correlation (mBJLDA) has recently attracted interest because it provides highly improved band gaps at a very low computational cost. In this work we performed an extensive investigation of the performance of the mBJLDA potential employing a norm-conserving pseudopotential plane-waves approach (mBJLDA@PP), as implemented in the abinit code, using a test set of 83 solids representing a wide range of semiconductors and insulators. Our results confirm the conclusion of our previous study that the number of electrons treated as valence in the pseudopotentials of the cations can have a significant impact on the calculated mBJLDA@PP band gaps. More specifically, while the use of typical pseudopotentials leads to accurate band gaps of certain systems, it yields significantly underestimated band gaps for other systems compared to experiment and to those of the all-electron mBJLDA (mBJLDA@AE) approach. The classes of the latter systems are identified, and this problem is resolved by including some outer core states as valence. The resulting mean absolute error in the calculated band gaps (compared to experiment) is of 0.46 eV, which is comparable to that of the mBJLDA@AE band gaps, reflecting the accuracy and reliability of the mBJLDA@PP approach for the band gap calculations.

Engineering the hydrogen storage properties of the perovskite hydride ZrNiH3 by uniaxial/biaxial strain

In the present work, the bonding length, electronic structure, stability, and dehydrogenation properties of the Perovskite-type ZrNiH3 hydride, under different uniaxial/biaxial strains are investigated through ab-initio calculations based on the plane-wave pseudo-potential (PW-PP) approach. The findings reveal that the uniaxial/biaxial compressive and tensile strains are responsible for the structural deformation of the ZrNiH3 crystal structure, and its lattice deformation becomes more significant with decreasing or increasing the strain magnitude. Due to the strain energy contribution, the uniaxial/biaxial strain not only lowers the stability of ZrNiH3 but also decreases considerably the dehydrogenation enthalpy and decomposition temperature. Precisely, the formation enthalpy and decomposition temperature are reduced from −67.73 kJ/mol.H2 and 521 K for non-strained ZrNiH3 up to −33.73 kJ/mol.H2 and 259.5 K under maximal biaxial compression strain of ε = −6%, and to −50.99 kJ/mol.H2 and 392.23 K for the maximal biaxial tensile strain of ε = +6%. The same phenomenon has been also observed for the uniaxial strain, where the formation enthalpy and decomposition temperature are both decreased to −39.36 kJ/mol.H2 and 302.78 K for a maximal uniaxial compressive strain of ε = - 12%, and to −51.86 kJ/mol.H2 and 399 K under the maximal uniaxial tensile strain of ε = +12%. Moreover, the densities of states analysis suggests that the strain-induced variation in the dehydrogenation and structural properties of ZrNiH3 are strongly related to the Fermi level value of total densities of states. These ab-initio calculations demonstrate insightful novel approach into the development of Zr-based intermetallic hydrides for hydrogen storage practical applications.

Effect of Sr - Transition Metal Substitution on Electronic and Mechanical Properties of Mg2Si: A DFT Study

Abstract: Recently, magnesium alloys have attracted scientific interest due to their technological importance in thermoelectric, piezoelectric, photo-voltaic and infrared photonics applications. The electronic and elastic properties of MgXSi (X = Mg, Sr) compounds were investigated in this work, using the density functional theory (DFT) with pseudo-potential plane-waves (PPW) approach as implemented in Quantum Espresso code. The results of the elastic constants of Mg2Si are in agreement with the previous theoretical results and favourably compared with experimental data. The electronic band structures of these semiconductors were calculated to give narrow indirect and direct band gaps of Mg2Si and MgSrSi, respectively. Our results show that the two compounds are mechanically stable. The Pugh’s ratio, B/G, indicated that Mg2Si and MgSrSi are brittle and ductile in nature. The estimated anisotropy parameter, A, shows that Mg2Si has a higher degree of elastic isotropy in comparison to MgSrSi. Three-dimensional (3D) projection of Young’s modulus and area modulus of the compounds was presented. Keywords: Electronic structure, Elastic constants, Mechanical properties, Mg2Si, MgSrSi. PACS: 31.15.A-, 62.20.F-, 71.55.-i, 71.20.Be.

Ab initio study of the structure, elastic, and electronic properties of Ti3(Al1\u2212nSin)C2 layered ternary compounds

The MAX phase materials such as layered ternary carbides that simultaneously exhibit characteristics of metallic and ceramic materials have received substantial interest in recent years. Here, we present a systematic investigation of the electronic, structural stabilities, and elastic properties of Ti3(Al1−nSin)C2 (n = 0,1) MAX phase materials using the ab initio method via a plane-wave pseudopotential approach within generalized-gradient-approximations. The computed electronic band structures and projected density of states show that both Ti3SiC2 and Ti3AlC2 are metallic materials with a high density of states at the Fermi level emanating mainly from Ti-3d. Using the calculated elastic constants, the mechanical stability of the compounds was confirmed following the Born stability criteria for hexagonal structures. The Cauchy pressure and the Pugh’s ratio values establish the brittle nature of the Ti3SiC2 and Ti3AlC2 MAX phase materials. Due to their intriguing physical properties, these materials are expected to be suitable for applications such as thermal shock refractories and electrical contact coatings.

Effect of spin-orbital coupling on the electronic, mechanical, thermoelectric, and vibrational properties of XPtBi (X = Sc and Y): A first-principles study

Heusler compounds constitute a versatile class of materials, as evidenced by their numerous technological applications. Herein, we report a comparative study of the electronic, mechanical, vibrational, and thermoelectric properties of half-Heusler compounds XPtBi (X = Sc and Y) based on first-principles calculations, with and without spin-orbital coupling (SOC). The plane-wave pseudo-potential approach within density functional theory (DFT) and the Boltzmann transport equations have been used to compute these properties. SOC has a considerable effect on the electronic band structures of XPtBi (X = Sc and Y), leading to band inversion without breaking any symmetry. This band inversion results in the emergence of a topological phase in ScPtBi and YPtBi. Phonon dispersion curves and phonon densities of state have been calculated for these compounds, with and without SOC, for comparison. The calculations also reveal that, with the inclusion of SOC, these materials have improved thermoelectric properties, with figures of merit (ZT) of 0.41 for ScPtBi and 0.35 for YPtBi at 1200 K. Hence, these are promising thermoelectric materials for use at higher temperatures.

Modified Becke-Johnson calculations using norm-conserving pseudopotential plane-wave approach: Systematic analysis

The modified Becke-Johnson potential for exchange and local density approximation for correlation (mBJLDA) approach is proved to provide a highly improved description of the bandgap. This work presents the results of a systematic investigation of the mBJLDA electronic band structure calculations using the norm-conserving pseudopotential (PP) approach, as implemented in the ABINIT code. The considered systems represent different classes of semiconductors and insulators. Moreover, all-electron (AE) mBJLDA potential and screened hybrid functional YS-PBE0 calculations are also performed for comparison purposes. We show that the mBJLDA@PP bandgaps obtained using typical PP's (i.e., valence and semicore electrons are allowed to relax) are generally speaking smaller than that of the mBJLDA@AE approach. This artifact cannot be attributed to a small “c” parameter of the mBJLDA, since we found that this parameter is quite insensitive to the adopted computational method. Interestingly, the number of electrons treated as valence, especially for the cations, can be used as an extra adjustable parameter to improve the mBJLDA@PP bandgaps. In particular, the best agreement with the mBJLDA@AE bandgaps is obtained by including the outer core p electrons as valence. The optimal PP's for mBJLDA@PP calculations are transferable. A similar agreement is also achieved for the binding energies of the semicore d electrons (in the IIB-VI and IIIB-V compounds), upper valence bandwidths and electron effective masses. Sound explanations of the mBJLDA@PP results for the considered electronic structure properties are provided.

Structural, Electronic, Lattice Dynamic, and Elastic Properties of SnTiO3 and PbTiO3 Using Density Functional Theory

The structural, electronic, and elastic properties of tetragonal phase of SnTiO3 and PbTiO3 are investigated using first principle calculations. The unknown exchange-correlation functional is approximated with generalized gradient approximation (GGA) as implemented in pseudopotential plane wave approach. The convergence test of total energy with respect to energy cutoff and k-point sampling is preformed to ensure the accuracy of the calculations. The structural properties such as equilibrium lattice constant, equilibrium unit cell volume, bulk modulus, and its derivative are in reasonable agreement with the previous experimental and theoretical works. From elastic constants, mechanical parameters such as anisotropy factor A, shear modulus G, bulk modulus B, Young’s modulus E, and Poison’s ratio n are determined by using Voigt–Reuss–Hill average approximation. In addition, Debye temperature and longitudinal and transversal sound velocities are predicted from elastic constants. The electronic band structure and density of states of both compounds are obtained and compared with the available experimental as well as theoretical data. Born effective charge (BEC), phonon dispersion curve, and density of states are computed from functional perturbation theory (DFPT). Lastly, the spontaneous polarization is determined from the modern theory of polarization, and they are in agreement with the previous findings.

Projector augmented-wave pseudopotentials for uranium-based compounds

Electronic structure density functional theory calculations, within the plane-wave pseudopotential approach, have become an important method for the study of material properties from first-principles. This approach relies on the availability of pseudopotentials suitable for the accurate description of atoms in condensed matter models. Actinide-based compounds are of great scientific and technological interest. However, the availability of pseudopotentials for the actinide atoms, and in general for heavy elements, is significantly limited. In this work, we develop a set of pseudopotentials, for uranium, oxygen, nitrogen and carbon atoms, within the projector augmented-wave (PAW) formalism. We compared the accuracy of the generated PAW datasets with pseudopotentials from the pslibrary and VASP, and data available in the literature. The reported results provide substantial verification for their use in diverse chemical environments.

High-temperature and high-pressure thermophysical properties of AlP semiconducting material: A systematic ab initio study

The finite temperature thermophysical quantities are one of the most important properties of solids and have become the subject of significant interdisciplinary interest in recent years. We report a detailed theoretical investigation on the equation of state (EoS) parameters, electronic polarizability and finite temperature thermophysical properties of aluminum phosphide using plane-wave pseudopotential approach in the framework of the density functional theory within the generalized gradient approximation. Our results regarding EoS parameters and electronic polarizability are found to be in good accord with previous data reported in the literature. Moreover, using the model of the quasi-harmonic Debye approximation plus empirical energy corrections, the thermophysical properties for temperatures up to 2000 K are examined and discussed. Our findings show that the thermophysical properties vary monotonically with either temperature or pressure which is consistent with the trends previously reported in the literature.

Superconductivity in single-crystalline aluminum- and gallium-hyperdoped germanium

Superconductivity in group IV semiconductors is desired for hybrid devices combining both semiconducting and superconducting properties. Following boron doped diamond and Si, superconductivity has been observed in gallium doped Ge, however the obtained specimen is in polycrystalline form [Herrmannsd\"orfer et al., Phys. Rev. Lett. 102, 217003 (2009)]. Here, we present superconducting single-crystalline Ge hyperdoped with gallium or aluminium by ion implantation and rear-side flash lamp annealing. The maximum concentration of Al and Ga incorporated into substitutional positions in Ge is eight times higher than the equilibrium solid solubility. This corresponds to a hole concentration above 10^21 cm-3. Using density functional theory in the local density approximation and pseudopotential plane-wave approach, we show that the superconductivity in p-type Ge is phonon-mediated. According to the ab initio calculations the critical superconducting temperature for Al- and Ga-doped Ge is in the range of 0.45 K for 6.25 at.% of dopant concentration being in a qualitative agreement with experimentally obtained values.

EOS parameters and elastic properties of cubic rock-salt BP

The present work aims to study the equation of state (EOS) under compression up to 100 GPa, and the elastic properties of cubic rocksalt Boron phosphide (BP) material. The EOS parameters and the elastic constants of our material of interest were predicted using plane wave-pseudopotential (PW-PP) approach in the framework of the density functional theory (DFT) and the density functional perturbation theory (DFPT) with the generalized gradient approximation (GGA) for the exchange-correlation functional. The Young modulus, the Poisson's ratio, the sound velocity, the Debye temperature and the melting temperature of the aggregate material were also presented. The results obtained are in general in good agreement compared to other data of the literature.

Finite temperature thermophysical properties of MgCu intermetallic compound from quasi-harmonic Debye model

Abstract The influence of pressure and temperature on the thermodynamic properties of MgCu intermetallic compound was investigated by quasi-harmonic Debye model approximation. The equation of state (EoS) parameters has performed using plane-wave pseudopotential (PW-PP) approach in the framework of the density functional theory (DFT) and the generalized gradient approximation (GGA) for the exchange-correlation functional. Our results agree well with other data of the literature. The finite temperature thermophysical properties under pressure up to 16 GPa and high temperature up to 800 K, respectively were determined. Our results of the thermophysical properties are also agree very well with other data of the literature, where for example at ambient temperature, the deviation between our obtained value (11.05 Cal mol−1 K−1) of CV, and the theoretical value (11.21 Cal mol−1 K−1) reported in the literature is only around 1.44%. The finite temperature thermophysical properties were found varied monotonically with either temperature or pressure. Compared with other materials previously studied, similar behaviors were observed.

An ab-initio Investigation: The Physical Properties of ScIr2 Superconductor

Using ab initio technique the physical properties of ScIr2 superconductor have been investigated with Tc 1.03 K with a MgCu2-type structure. We have carried out the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented within the CASTEP code. The calculated structural parameters confirm a good agreement with the experimental and other theoretical results. Using the Voigt-Reuss-Hill (VRH) averaging scheme the most important elastic properties including the bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν of ScIr2 are determined. At ambient condition, the values of Cauchy pressure and Pugh’s ratio exhibit ductile nature of ScIr2. The electronic and optical properties of ScIr2 were investigated for the first time. The electronic band structure reveals metallic conductivity and the major contribution comes from Ir-5d states. In the ultraviolet region the reflectivity is high up to 50 eV as evident from the reflectivity spectrum.

First principles study on the structural, electronic and phonon properties of monolayer B2Si

Using first principles density functional theory, we predict a monolayer B2Si structure with space group Pmm2 in the present work. This structure is confirmed to be dynamically stable. Based on the plane wave pseudopotential approach, the charge density, electron localization function, density of states, energy band, phonon property and thermal conductivity of Pmm2-B2Si are systematically studied. It is interesting that the sp2 hybridization and coordination bond of Si are found in Pmm2-B2Si, which is the most important factor for its structural stability. The density of states and energy band analysis reveals that Pmm2-B2Si is metallic because of the partial occupied Si 3pz and B 2pz states. Moreover, the acoustic-optical coupling is important for phonon transport in Pmm2-B2Si, and the contribution of optical modes to the lattice thermal conductivity along the [100] and [010] directions is 13% and 12%, respectively. This study gives a fundamental understanding of the structural, electronic and phonon properties in Pmm2-B2Si.

First principle study of the structural, electronic, vibrational, thermodynamic, linear and nonlinear optical properties of zinc-blende ZnSe and ZnTe semiconductors

The first principle investigations of the structural, electronic, linear and nonlinear response properties of the zinc-blende ZnSe and ZnTe are performed based on the density functional theory using a plane-wave pseudopotential approach. The Born effective charges, piezoelectric tensor, phonon frequencies, LO–TO splitting and thermal properties of these compounds are calculated using a linear response method within the density functional theory framework. The macroscopic dielectric function is calculated in the response of the many body effects within the Bethe-Salpeter approach using the Tam-Dancoff approximation. The nonlinear response properties to atomic displacements and electric fields are investigated within the density functional perturbation theory framework based on the 2n+1 theorem as implemented by the ABINIT software. After the calculation of the Kohn–Sham electronic band structures, the correction of the energy band gap is computed using the GW approximation within the many-body perturbation theory framework (MBPT). The results are in excellent agreement with the available experimental and other theoretical results.

An ab-initio Investigation: The physical properties of ScIr-=SUB=-2-=/SUB=- Superconductor -=SUP=-*-=/SUP=-

AbstractUsing ab initio technique the physical properties of ScIr_2 superconductor have been investigated with T _c 1.03 K with a MgCu_2-type structure. We have carried out the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented within the CASTEP code. The calculated structural parameters confirm a good agreement with the experimental and other theoretical results. Using the Voigt-Reuss-Hill (VRH) averaging scheme the most important elastic properties including the bulk modulus B, shear modulus G , Young’s modulus E and Poisson’s ratio ν of ScIr_2 are determined. At ambient condition, the values of Cauchy pressure and Pugh’s ratio exhibit ductile nature of ScIr_2. The electronic and optical properties of ScIr_2 were investigated for the first time. The electronic band structure reveals metallic conductivity and the major contribution comes from Ir-5 d states. In the ultraviolet region the reflectivity is high up to 50 eV as evident from the reflectivity spectrum.

Elastic and thermophysical properties of BAs under high pressure and temperature

The pseudopotential plane-wave approach in the framework of the density functional theory, and the density functional perturbation theory with the generalized gradient approach for the exchange-correlation functional has been used to calculate the structural phase stability, elastic constants and thermodynamic properties of boron-arsenide (BAs) compound. The BAs compound transforms from the zincblende phase to rock-salt structure; the phase transition pressure was found to be 141.2 GPa with a volume contraction of around 8.2%. The thermodynamic properties under high pressure and temperature up to 125 GPa and 1200 K respectively were also determined, analyzed and discussed in comparison with other data of the literature. The systematic errors in the static energy were corrected using the bpscal EEC method. Our results agree well with those reported in the literature, where for example, our calculated melting temperature (2116 K) deviates from the theoretical one (2132.83 K) with only 0.8%, and the deviation between our result (1.86) of the Grüneisen parameter and the theoretical one (1.921) is only around 3.2%.

First−principles study: Structural, mechanical, electronic and thermodynamic properties of simple−cubic−perovskite (Ba0.62K0.38)(Bi0.92Mg0.08)O3

A new simple cubic perovskite superconductor (Ba0.62K0.38)(Bi0.92Mg0.08)O3 with a Tcmag ∼30 K has been recently synthesized by utilizing facile hydrothermal conditions at 220 °C. Here, we have employed the pseudo-potential plane-wave (PP–PW) approach based on the density functional (DFT) theory, within the generalized gradient approximation (GGA) to calculate and investigate the physical and electronic properties of this new superconductor. The calculated elastic constants satisfy the Born mechanical stability criterion. Based on the calculated data of Cauchy pressure, Pugh ratio and Poisson's ratio, the compound under investigation is found to be brittle nature. The calculated Peierls stress indicates easier movement of dislocations between glide planes. The energy bands crossing the Fermi level (EF) in electronic band structure reveal metallic behavior of the newly synthesized compound. The contribution of O-2p orbital electrons at the EF is higher than those of other orbital electrons as seen from partial density of states. The formation of saddle point owing to the flatness of the energy band may enhance the electron pairing which subsequently leads to high-TC superconductivity. The valence electron charge density map shows the isotropic nature of charge distribution. Coexistence of electron and hole like pockets form the multi sheets Fermi surfaces and exhibit the multi-band nature of the compound. Importantly, Fermi surfaces with flat faces are considered as favorable condition for Fermi surface nesting. The calculated value of electron-phonon coupling constant (λ = 1.61) implies that the studied compound can be treated as a strongly coupled superconductor. Further, the thermodynamic properties at elevated temperature and pressure are calculated and analyzed for the first time by using quasi-harmonic model.

Structural and piezoelectric coefficients of AlP under pressure

The present work aims to investigate the structural parameters and the piezoelectric coefficients of cubic zinc-blende Aluminum phosphide (AlP) under high pressure up to 21 GPa, using plane wave-pseudopotential (PW-PP) approach in the framework of the density functional theory (DFT) and the density functional perturbation theory (DFPT) with the generalized gradient approximation (GGA) for the exchange-correlation functional. The results obtained are analyzed and compared with other data of the literature. The structural parameters and the piezoelectric coefficients calculated here agree well with other data of the literature. We found also that both the direct and converse piezoelectric coefficients increase with increasing pressure up to 21 GPa.Â

Investigations of Uranyl Fluoride Sesquihydrate (UO2F2·1.57H2O): Combining 19F Solid-State MAS NMR Spectroscopy and GIPAW Chemical Shift Calculations.

High-resolution 19F magic-angle spinning (MAS) NMR spectra were obtained for the uranium-bearing solid uranyl fluoride sesquihydrate (UO2F2·1.57H2O). While there are seven distinct crystallographic fluorine sites, the 19F NMR spectrum reveals six peaks at -33.3, 9.1, 25.7, 33.0, 39.0, and 48.2 ppm, with the peak at 33.0 ppm twice the intensity of all the others and therefore corresponding to two sites. To assign the peaks in the experimental spectra to crystallographic sites, 19F chemical shifts were calculated using the gauge including projector augmented waves (GIPAW) plane-wave pseudopotential approach for a DFT-optimized crystal structure. The peak assignments from DFT are consistent with two-dimensional double-quantum 19F MAS NMR experiments.