Full-potential Local Orbitals Method Research Articles

Electronic Structure of Sr(Pt;Pd)2Ge2 Superconductors by Ab Initio Calculations

Electronic structures of SrPt 2Ge 2 and SrPd 2Ge 2 superconductors have been calculated with the use of the full-potential local-orbital method within the density functional theory. The investigations were focused on analyses of differences between electronic properties of both studied intermetallics that could explain the enhancement of superconducting properties in the former one. The results indicate that the band structures in the vicinity of the Fermi level for both considered compounds are indistinguishable. Meanwhile, the calculated value of the density of states at the Fermi level for SrPt 2Ge 2 is lower than that of SrPd 2Ge 2, thus the significantly higher value of Tc in the Pt-based system should be related to an anomalous enhancement of electron-phonon coupling.

Mixed alkali and alkaline-earth borate Li2Sr4B12O23 single crystal

A comprehensive theoretical investigation of the electronic band structure, density of states, electron charge density distribution and the optical properties for mixed alkali and alkaline-earth borate Li2Sr4B12O23 (LSBO) single crystals were performed. The experimental geometrical structure was optimized by minimizing the forces acting on each atom. Calculations were performed using the full potential linear augmented plane wave plus local orbitals (FPLAPW+lo) method within the local density approximation (LDA), generalized gradient approximation (GGA) and the recently modified Becke–Johnson potential (mBJ). Our calculations show that LSBO crystal is a direct band gap semiconductor. The calculated band gap is 4.64eV (LDA), 4.92eV (GGA) and 5.51eV (mBJ). An earlier calculation using the CASTEP code within LDA obtained a band gap of about 4.66eV. To overcome the well-known LDA underestimation of the energy gap we have used GGA and mBJ. We find that mBJ succeed by large amount in bringing the calculated bond lengths in good agreement with the experimental data. Also we found that using mBJ to calculate the optical properties gives a birefringence of about 0.068 (at λ=586.5nm) in excellent agreement with the experimental data (0.068 at λ=586.5nm). Therefore, we believe that the mBJ calculations reported here show excellent agreement with the experimental data.

Theoretical investigation of some specific features of the electronic structure and optical properties of Benzoic Acid 2-Amino-4,6-Dimethylpyrimidine (1:1) co-crystals

Benzoic Acid 2-Amino-4,6-Dimethylpyrimidine (1:1) co-crystal have been comprehensively investigated by means of density functional theory. The electronic band structure show that the conduction band minimum (CBM) and the valence band maximum (VBM) are situated at the center of the Brillouin zone resulting in a direct band gap. Calculation were performed using the full potential linear augmented plane wave plus local orbitals (FPLAPW+lo) method in a scalar relativistic version as embodied in the WIEN2k code within the local density approximation (LDA), gradient approximation (PBE-GGA), Engel–Vosko generalized gradient approximation (EV-GGA) and the recently modified Becke–Johnson potential (mBJ). The calculated density of states explore that the VBM is mainly formed by N-p state while the CBM is formed by the strongly hybridized N-p and C-p states. There exists a strong hybridizations between C-s/p, H-s, N-s/p and O-s/p states above and below the Fermi level (EF). Which may led to covalent bonding between the states. To visualizes the charge transfer and the chemical bonding characters, the valence band’s electronic charge density distribution were extensively investigated. The optical properties helps to get deep insight into the electronic structure therefore, details analysis to the calculated optical properties were performed. The optical properties confirm the existence of the band gap and the lossless regions.

Electronic structure and thermodynamic properties of Cu3V2O8 compound

The electronic structure and thermodynamic properties of Cu3V2O8 compound for three structures (P-1, P21/c and Cmca) are reported. The calculations are performed by using full-potential local orbital minimum basis method. The total electronic densities of states for all structures have the similar shape but the details are different. The thermodynamic properties (the bulk modulus B, Gibbs free energy, Debye temperature ΘD) are calculated in quasiharmonic Debye–Grüneisen model using the equation of states in the form of Murnaghan, Birch–Murnaghan, Poirier–Tarantola and Vinet. Our ab initio results indicate that α(P-1) phase is stable below 839 K, β(P21/c) and γ(Cmca) phases can exist in the region of 839 K < T < 875 K, however above T = 875 K only γ(Cmca) phase is observed.

Theoretical investigation of structural, elastic, electronic, and thermal properties of KCaF3, K0.5Na0.5CaF3 and NaCaF3 Perovskites

All-electron self consistent Full Potential Augmented Plane Waves plus local orbital (FP-APW+lo) method within PBE and PBEsol Generalized Gradient density approximations are used to investigate structural, elastic, electronic and thermal properties of the cubic KCaF3, NaCaF3 and K0.5Na0.5CaF3 Perovskites. The calculated structural parameters such as equilibrium lattice constants, bulk moduli and their pressure derivatives are well consistent with the experimental and theoretical data. Using Mehl method, the elastic constants and their related parameters such as Young modulus, shear modulus and Poisson ratio are calculated for these materials. Electronic band structure investigation shows that LDA approximation within mBJ potential provides an enhancement band gap value of 55.49%, 43.64% and 47.81% respectively for KCaF3, NaCaF3 and K0.5Na0.5CaF3 compared to GGA-PBE. Therefore, these compounds exhibit clearly an indirect wide-gaps from R to Γ for both KCaF3, NaCaF3 and from M to Γ for K0.5Na0.5CaF3. Furthermore we have analyzed the thermodynamic properties by using the quasi-harmonic Debye model. We have also, predicted the dependent-temperature behavior of heat capacities and the volume of unit cell in wide range temperatures for these compounds.

Structural, electronic and hyperfine characterization of pure and Ta-dopedZrSiO4

The electronic structure of pure and Ta-doped $\mathrm{ZrSi}{\mathrm{O}}_{4}$ in the tetragonal $I{4}_{1}/amd$ phase with and without defects has been studied using the ab initio full-potential linear augmented plane wave plus local orbitals method. From the determined charge densities, the electric field gradient tensor at native Zr sites and at Ta impurities localized on cation sites of $\mathrm{ZrSi}{\mathrm{O}}_{4}$ were derived and compared to experimental data obtained using hyperfine techniques. The effects of the Ta probe atom, including its different charge states on the lattice, are investigated. In addition, different types of defects, such as O or Si vacancies, Ta replacing Si, and Ta enclosed in microstructures of $\mathrm{Si}{\mathrm{O}}_{2}$ phases, are examined. The combination of experiments and theory enables us to identify the different interactions observed in Ta-doped $\mathrm{ZrSi}{\mathrm{O}}_{4}$ and to elucidate the role played by different defects.

Electronic structure and transport properties of CeNi9In2

We investigated CeNi9In2 compound, which has been considered as a mixed valence (MV) system. Electrical resistivity vs. temperature variation was analyzed in terms of the model proposed by Freimuth for systems with unstable 4f shell. At low temperature the resistivity dependence is consistent with a Fermi liquid state with a contribution characteristic of electron–phonon interaction. Ultraviolet photoemission spectroscopy (UPS) studies of the valence band did not reveal a Kondo peak down to 14K. A difference of the spectra obtained with photon energies of low and high photoionization cross sections for Ce 4f electrons indicated that 4f states are located mainly close to the Fermi energy. The peaks related to f5/21 and f7/21 final states cannot be resolved but form a plateau between −0.3eV and the Fermi energy. X-ray photoemission spectroscopy (XPS) studies were realized for the cerium 3d level. The analysis of XPS spectra within the Gunnarsson-Schönhammer theory yielded a hybridization parameter of 104meV and non-integer f level occupation, being close to 3. Calculations of partial densities of states were realized by a full potential local orbital (FPLO) method. They confirm that the valence band is dominated by Ni 3d states and are in general agreement with the experiment except for the behavior of f-electrons.

Temperature and pressure dependent structural, electronic and thermodynamic characterization of isotypic intermetallics: LaMgX (X = Ga, In, Tl)

The full-potential linear augmented plane wave plus local orbital method has been used to study the structural, electronic and thermodynamic properties of LaMgX (X = Ga, In, Tl). The equilibrium structural properties viz. lattice parameters (a 0 , c 0), bulk modulus (B 0) and its first order pressure derivative ( $$ B_{0}^{\prime } $$ ) have been calculated in the parent hexagonal P-62m phase. The electronic behavior has been shown in terms of band structure and density of states histograms, indicating dominant character of “5d” orbital electrons of Tl in electronic contribution to LaMgTl. Effects of temperature and pressure on unit cell volume (V), bulk modulus (B), Debye temperature (θ D ), Gruneisen parameter (γ), specific heat (C V ) and thermal expansion coefficient (α) have been investigated in wide temperature range and pressure range. The calculated parameters are in good agreement with available experimental/theoretical literature values.

Electronic structure of non-centrosymmetric superconductor LaPdSi3 and its reference compound LaPdGe3

Electronic structures of a superconductor without inversion symmetry, LaPdSi3, and its non-superconducting counterpart, LaPdGe3, have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused on analyses of densities of states at the Fermi level in comparison with previous experimental heat capacity data and an influence of the antisymmetric spin–orbit coupling on the band structures and Fermi surfaces (FSs) being very similar for both considered here compounds. Their FSs sheets originate from four bands and have a holelike character, but exhibiting pronounced nesting features only for superconducting LaPdSi3. It may explain a relatively strong electron–phonon coupling in the latter system and its lack in non-superconducting LaPdGe3.

Electronic structure of non-centrosymmetric superconductors Re24(Nb;Ti)5 by ab initio calculations

Electronic structures of superconducting Re24Nb5 and Re24Ti5 have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused on the influence of the antisymmetric spin–orbit coupling on band structures and Fermi surfaces of these non-centrosymmetric systems. The predicted here density of states at the Fermi level for Re24Ti5 is abnormally low with respect to that deduced from previous heat capacity measurements. This discrepancy suggests an intermediate coupled superconducting state in Re24Ti5. The differences between electronic properties of both compounds could explain more robust superconductivity in the Nb-based material.

Electronic structure of cage-like compound UB12 – Theory and XPS experiment

The electronic structure of cubic UB12 has been investigated by X-ray photoelectron spectroscopy (XPS) and first principles calculations employing the fully relativistic full-potential local-orbital (FPLO-9) method. The calculated densities of states (DOS) revealed unique fairly sharp five-peaks of the U 5f states with the lowest energy peak crossing the Fermi level. The calculated and experimental valence band XPS spectra are in very good accord, indicating a substantial delocalization of the U 5f electrons. On the other hand, our experimental U 4f core-level XPS spectrum exhibits both the 1-eV and 7-eV satellites, pointing to some localization effects of the U 5f electrons, which may exhibit a mixed-valence behavior. Our Fermi surfaces of UB12 calculated for ambient and such high pressure as 55GPa show only negligible differences, which can explain the earlier reported stability of the cubic crystal structure of the studied system under high pressure.

Half metallic properties of the quaternary CuFe2GaSe4 chalcogenide compound

The structural, electronic and magnetic properties of quaternary CuFe2GaSe4 in stannite structure (tetragonal structure, space group I4¯2m) were investigated using the full potential augmented plane wave plus local orbitals method based on density functional theory. For exchange–correlation potential, the local spin density approximation LSDA and LSDA+U schemes were adopted. It is shown that the application of the Coulomb interaction U increases the lattice parameters relative to the LSDA approach. The analysis of the densities of states shows that this compound is semiconductor and metallic in the majority and minority spin channels, respectively. This half metallicity of the majority spin exhibits a direct band gap of Eg=0.62eV, and a magnetic moment of 8μB. The magnetism comes from the d orbitals of Fe atoms and the p-Se, d-Fe coupling. These results may be of interest for spintronic applications.

Structural phase transition, mechanical and optoelectronic properties of the tetragonal NaZnP: Ab-initio study

Ab-initio full potential augmented plane wave plus local orbitals method has been used to investigate the structural phase transition, mechanical and optoelectronic properties of the Nowotny–Juza filled-tetrahedral compound NaZnP. The exchange-correlation potential was treated within the generalized gradient approximation of Perdew–Burke and Ernzerhof (GGA-PBE) and the modified Becke–Johnson potential (TB-mBJ) to improve the accuracy of the electronic band structure. Total-energy and geometry optimizations have been carried out for all structural phases of NaZnP. The following sequence of pressure-driven structural transitions has been found: Cu2Sb-type→β-phase→α-phase. The single-crystal elastic constants of NaZnP in the Cu2Sb-type structure have been calculated using total-energy versus strain method and their corresponding elastic moduli of polycrystalline aggregate, including Young’s modulus, shear modulus and Poisson’s ratio, have been derived. From the elastic parameters, it is inferred that this compound is brittle in nature. The elastic anisotropy was studied in detail using three different indexes; especially the 3D direction dependence of the Young’s modulus was visually described. Furthermore, calculated electronic band structure shows that NaZnP in the Cu2Sb-type phase has a direct energy band gap (Γ–Γ). The TB-mBJ approximation yields larger fundamental band gaps compared to those of PBE-GGA. The examined charge density distributions for the Cu2Sb-type structure show a covalent character for Zn–P bond and ionic nature for Na–P bond. Additionally, real and imaginary parts of the dielectric function, reflectivity and energy loss function spectra have been calculated for radiation up to 30.0eV with an incident radiation polarized parallel to both [100] and [001] crystalline directions.

First principles study on structural and magnetic properties of small and pure carbon clusters (C n , n = 2 - 12)

We have demonstrated the structural and magnetic properties of small carbon clusters (Cn, n = 2-12) in the framework of collinear approximation using density functional theory. The calculations were performed for amorphous, linear, and ring carbon clusters using full-potential local-orbital method. We have obtained stable structures, total energies, total magnetic moments, and HOMOLUMO energy gap of these clusters. We have found that the amorphous carbon clusters with minimum energy are not magnetic clusters, whereas their less-stable isomers with special configurations are ferromagnetic objects. Two robust magnetic moments were found for C5 carbon ring (6lB) and for pentagonal pyramid C6 structure (4lB).

Study of structural and electronic properties of NdSb employing local spin density approximation and Hubbard correction

Full-potential linear augmented plane wave plus local orbital method employing local spin-density approximation along with Hubbard-U corrections and spin–orbit coupling have been used to study electronic, structural and phase transition properties of NdSb. Under compression, they undergo first-order structural transition from B1 to BCT phase. Computed phase transition pressure is 15.1 GPa, this agrees well with their measured value of 15 GPa. Structural properties viz., equilibrium lattice constants, bulk modulus, its pressure derivative and volume collapse are also in closer agreement with experimental data. Present strategy shows significant impact on energy levels of occupied and unoccupied 4f states in electronic structure of NdSb and provides better description of crystal properties of present system.

Spin and orbital magnetism of coinage metal trimers (Cu3, Ag3, Au3): A relativistic density functional theory study

We have demonstrated electronic structure and magnetic properties of Cu3, Ag3 and Au3 trimers using a full potential local orbital method in the framework of relativistic density functional theory. We have also shown that the non-relativistic generalized gradient approximation for the exchange-correlation energy functional gives reliable magnetic properties in coinage metal trimers compared to experiment. In addition we have indicated that the spin-orbit coupling changes the structure and magnetic properties of gold trimer while the structure and magnetic properties of copper and silver trimers are marginally affected. A significant orbital moment of 0.21μB was found for most stable geometry of the gold trimer whereas orbital magnetism is almost quenched in the copper and silver trimers.

Prediction Study of the Mechanical and Thermodynamic Properties of the $$\hbox {RBRh}_{3}$$ RBRh 3 (R $$=$$ = Sm, Eu, Gd, and Tb) Compounds

The structural, elastic, and thermodynamic properties of the cubic anti-perovskite $$\hbox {RBRh}_{3}$$ (R = Sm, Eu, Gd, and Tb) compounds have been investigated using first principles full-potential augmented-plane wave plus local orbitals (FP-APW+lo) method with the generalized gradient approximation. The ground-state quantities such as the lattice parameter, bulk modulus, and its pressure derivative, as well as elastic constants are estimated. Computed equilibrium lattice constants agree well with the available experimental data. The full set of first-order elastic constants and their pressure dependence, which have not been calculated or measured yet, have been determined. The elastic moduli increase linearly with increasing pressure and satisfy the generalized elastic stability criteria for cubic crystals under hydrostatic pressure. The shear modulus, Young’s modulus, and Poisson’s ratio are calculated for ideal polycrystalline $$\hbox {RBRh}_{3}$$ aggregates. The Debye temperature is estimated from the average sound velocity. From the elastic parameter behavior, it is inferred that cubic anti-perovskites $$\hbox {RBRh}_{3}$$ are ductile in nature and that the bonding is predominantly of an ionic nature. Following the quasi-harmonic Debye model, the temperature effect on the lattice constant, bulk modulus, heat capacity, and Debye temperature is calculated reflecting the anharmonic phonon effects.

FP-LAPW+lo Study of Structural, Electronic, and Optical Properties of Mg1-xSrxTe Alloys

First principles total energy calculations have been performed using the full potential linear augmented plane wave plus local orbital (FP-LAPW+lo) method within density functional theory to investigate the effects of Sr-dopant on the structural, electronic, and optical properties of Mg1-xSrxTe alloys in the rock salt crystallographic phase. The generalized gradient approximation (GGA) parameterization scheme proposed by Wu and Cohen (WC) has been used for calculating the ground state structural properties, and their deviation from Vegard's law has been discussed. The Engel-Vosko (EV) GGA functional has also been employed for electronic and optical properties of the alloy systems under study, and the results of the two GGA functionals are compared. Our calculated optical bowing in the band gap for the ternary alloys is found to be mainly caused by the charge exchange mechanism. Dielectric functions, optical conductivity, and reflectivity calculations predict the use of these alloys for technologically important optical and optoelectronic devices.

First-Principles Calculations of the Structural, Electronic, Optical and Mechanical Properties of CdS, CdSe and CdTe

The structural, electronic, optical and elastic properties of zinc-blende compounds (CdX, X = S, Se and Te), were studied using full-potential augmented plane wave plus local orbitals method (FP-LAPW+ lo) within density functional theory, using generalized gradient approximation (GGA). Geometrical optimization of the unit cell (lattice constant, bulk modulus and its pressure derivative) is in good agreement with experimental data. Results for band structures, density of states, and elastic constants (C11, C12 and C44) are presented. We also report our results on optical properties like the complex dielectric functions and the refractive index (n) of these compounds. Our results are in reasonable agreement with the available theoretical and experimental data.

Electronic structure of cage-type ternaries ARu2Al10 – Theory and XPS experiment (A = Ce and U)

The electronic structure of the isomorphic, orthorhombic URu2Al10 and CeRu2Al10 aluminides have been studied by X-ray photoelectron spectroscopy (XPS) and ab initio calculations using the fully relativistic full-potential local-orbital (FPLO) method within the local density approximation (LDA). The calculated data of the former system revealed fairly sharp triple-peaks of the U 5f states around the Fermi level (EF) and a large broad contribution from the Ru 4d states expanded from EF to about 6.5eV of binding energy. Although the size and positions of the Ru 4d bands for the latter compound are quite similar to those of the U-based one, the double Ce 4f sharp peaks are placed almost completely above EF underlying their mostly localized character. We have also analyzed the Fermi surfaces (FSs) in these two aluminides. The calculated results of both ternaries were then compared with our experimental XPS data for URu2Al10 and with such data for CeRu2Al10 available in the literature. The results are in fairly good agreement between the theory and experiment. Especially, the fact that the spectrum weight of the Ce 4f electrons below EF turned out to be very much reduced, reflecting rather a small f–c hybridization of these electrons compared to considerably larger one in the U-based compound.