Tight-binding Linear Muffin-tin Orbital Research Articles

AB INITIO APPROACH TO STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF B-, C- AND N-DOPED ANATASE

The effect of doping with boron, carbon and nitrogen on crystal lattice parameters, electronic band structure and optical absorption of anatase has been studied by means of an ab initio density functional theory approach. The investigations included optimization of crystal structure based on the pseudo-potential plane-wave approach. The spin-polarized calculations of the band structure with the account of on-site Coulomb correlations (LSDA+U) employing the tight-binding linear muffin-tin orbitals method (TB-LMTO) have also been performed. The evaluations of optical absorption were based on the calculations of dielectric function with local field effects taken into account. We find that the crystal lattice relaxation around the doping atoms produces noticeable changes in the band structure, magnetic state and optical properties of the doped compounds. The most considerable effects are the collapse of magnetic moment on carbon atom and an essential reduction of the optical absorption in the region of the impurity band — impurity band transitions. Comparing optical absorption for different kinds of doping and taking into account the intensity distribution of the solar light we come to the conclusion than the doping with boron is the most promising kind of doping for photocatalytic applications of the doped anatase.

Electronic and thermal properties of B 2-type AlRE intermetallic compounds: A first principles study

The ground state electronic structure and thermal properties of B 2-type intermetallic compounds AlRE (RE: Pm, Sm, Eu, Tb, Gd and Dy) have been studied using a self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) method at ambient as well as at high pressure. These compounds show metallic behavior under ambient condition. The band structure, total energy, density of states and ground state properties like lattice parameter, bulk modulus are calculated in the present work. The Debye–Grüneisen model is used to calculate the Debye temperature and the Grüneisen constant. The calculated results are in good agreement with the reported experimental and other theoretical results. The variation in the Debye temperature with pressure has also been reported. We present a detailed analysis of the role of f electrons of RE in the AlRE system.

Investigations on cobalt doped GaN for spintronic applications

(Ga1−xCox)N was synthesized at 1223K using pure gallium and cobalt metal as precursors in a reactor specially designed for this purpose with ammonia as the nitrogen source. The structural confirmation of wurtzite phase of GaN is done using powder X-ray diffractometer. Photoluminescence shows red shift in band edge emission as the cobalt concentration increases from 1 to 5 atomic percentage together with nitrogen related vacancy emission at 3.24eV and cobalt related emission at 3.13eV. The magnetic properties were studied at 10K using superconducting quantum interface device (SQUID). The obtained maximum magnetic moments for 1%, 3% and 5% cobalt doped GaN are 1.63, 0.21 and 0.14μB, respectively. The decrease in magnetic moment with increase in dopant concentration may be due to the formation of secondary phases or cobalt cluster, which contributes to anti-ferromagnetism. From temperature dependent magnetization measurements it is observed that there is no drastic change in the magnetic moment upto room temperature for 1% cobalt doped GaN. Theoretical calculations based on the density functional theory within Tight Binding Linear Muffin-tin Orbital (TBLMTO) method has been carried out to study the magnetic properties of cobalt doped GaN. The obtained magnetic moment values per Co atom are 2.56μB for 1.56% and 2.77% and 1.50μB for 4.20%. This is found to be higher when compared with experimental values and this difference is explained based on structural defects and percentage of dopant used during synthesis. The half metallic behavior is observed for 1.56% and 2.77% Co dopants, but for higher percentage of dopant the system becomes metallic. Hence, it is concluded that only lower percentage of cobalt doped GaN is suitable for spintronic applications.

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.

Theoretical study of electronic, magnetic and structural properties of Mo and W based group V (N, P, As, Sb and Bi) compounds

Magnetic and electronic structure calculations were carried out for WC-, MnP-, NaCl- and zinc blende (ZB)-type Mo and W based group V compounds, TMX (TM = Mo and W; X = N, P, As, Sb and Bi), using the tight-binding linear muffin-tin orbital (TB-LMTO) method. For these compounds in these four structures, the total energy has been calculated as a function of volume in spin-polarization and non-spin-polarization calculations. From the total energy calculations, it is observed that these compounds favour the structures for which there is octahedral co-ordination for the anion rather than tetrahedral co-ordination. For the large volume expansion, these compounds prefer tetrahedral co-ordination compared to octahedral co-ordination. Moreover, these compounds in the ZB-type structure are found to exhibit half-metallic property with a magnetic moment of 3.00 μ B per formula unit under expansion of volume. The equilibrium lattice constant, bulk modulus, heat of formation, magnetic moment, spin-flip-gap and minority spin bandgap are calculated and compared with available results. The band structure and density of states are presented.

Pressure-induced structural phase transition and electronic properties of actinide nitrides: Ab initio calculations

We report ab initio calculations of pressure-induced structural phase transition and electronic properties of the NaCl-type actinide nitrides namely AnN (An=Np and Cm). The total energy as a function of volume is obtained using the self-consistent tight-binding linear muffin tin orbital (TB-LMTO) method by performing spin and non-spin-polarized calculations. Magnetically the AnN compounds are stable in ferromagnetic (FM) state at ambient condition. From the present study, we predict a structural phase transition from ferromagnetic (FM) NaCl-type (B 1 phase) structure to ferromagnetic CsCl-type (B 2 phase) structure for these in the pressure range of 80.0–50.0 GPa. The pressure-induced transition is found to be first order. The band structure and density of states (DOS) are reported in both FM-B 1 phase and FM-B 2 phase. The ground-state properties such as magnetic moment, lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.

Pressure-induced phase transition and electronic structure of curium pnictides: Ab initio calculations

We report an ab initio calculation of pressure-induced structural phase transition and electronic properties of the NaCl-type Cm-based pnictides namely CmX (X = N, P, As, Sb and Bi). The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin tin orbital (TB-LMTO) method by performing spin and non-spin polarized calculation. Magnetically the CmX compounds are stable in ferromagnetic (FM) state and their crystal structure is NaCl-type at ambient condition. From the present study, we predict a structural phase transition from ferromagnetic (FM) NaCl-type ( B 1 phase) structure to ferromagnetic CsCl-type ( B 2 phase) structure for curium pnictides in the pressure range of 51.0–10.0 GPa (CmN to CmBi). The pressure-induced transition is found to be first order. The band structure and density of states (DOS) are plotted in FM- B 1 phase and FM- B 2 phase. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.

Study of electronic and structural properties of half metallic rare earth mononitrides

In the present work we investigated theoretically the electronic, magnetic and structural properties of two rare-earth nitrides (REN: RE = Sm, Eu) by using self- consistent tight-binding linear muffin tin orbital (TBLMTO) method. Magnetically, both the rare earth nitrides (RENs) are stable in ferromagnetic (FM) state, while its ambient structure is found to be stable in NaCl-type (B1) structure. From the present study we predict the pressure induced structural phase transition in both RENs from the relatively open NaCl-type structure into more dense CsCl-type structure at 8.6 GPa and 14.6 GPa respectively. They form a new class of half-metallic magnets with high magnetic moments and are strong candidates for applications in spintronics and spinfiltering devices. We have therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for REN compounds in both B1 and B2 phases.

Structural and electronic properties of Er-monopnictides under high pressure

Abstract We present the results of theoretical calculations on the structural, magnetic and electronic properties of Er-monopnictides using self-consistent first principles tight-binding linear-muffin-tin-orbital (TB-LMTO) method within the atomic-sphere approximation (ASA). Both spin-polarized and non-spin-polarized calculations are performed to check the magnetic stability of these compounds. We find that ErN, ErP and ErAs are metallic in ferromagnetic (FM) phase in both the spin channels and stable in NaCl-type (B1) structure at ambient pressure. We predict NaCl-type (B1) to CsCl-type (B2) structural phase transition in ErN, ErP and ErAs at pressures of 146.1, 60.2 and 53.2 GPa, respectively and remain metallic ferromagnetic at high pressure. We calculate equilibrium lattice constants (a), bulk modulus (B), magnetic moments (μB) and electronic properties of these compounds in B1 and B2 phases and compare with available experimental and theoretical results.

Bandgaps and band bowing in semiconductor alloys

The bandgap and band bowing parameter of semiconductor alloys are calculated with a fast and realistic approach. The method is a dielectric scaling approximation that is based on a scissor approximation. It adds an energy shift to the bandgap provided by the local density approximation (LDA) of the density functional theory (DFT). The energy shift consists of a material-independent constant weighted by the inverse of the high-frequency dielectric constant. The salient feature of the approach is the fast calculation of the dielectric constant of alloys via the Green function (GF) of the TB-LMTOs (tight-binding linear muffin-tin orbitals) in the atomic sphere approximation (ASA). When it is applied to highly mismatched semiconductor alloys (HMAs) like Zn Te x Se 1− x , this method provides a band bowing parameter that is different from the band bowing parameter calculated with the LDA due to the bowing exhibited also by the high-frequency dielectric constant.

Electronic structure and the optical and photocatalytic properties of anatase doped with vanadium and carbon

The electronic structures of undoped anatase and anatase doped with carbon and vanadium have been calculated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the LSDA + U approximation. It has been shown that the doping of TiO2 leads to the formation of narrow bands of the C and Vimpurity states in the band gap. The calculations of the imaginary part of the dielectric function have made it possible to estimate the intensity of the optical absorption. It has been established that the doping with vanadium and carbon leads to optical absorption in the visible range and to an increase in the absorption in the ultraviolet range up to 4 eV. This should result in an increase in the photocatalytic activity on the surface of the doped anatase. The experimental determination of the photocatalytic activity of whiskers of the anatase doped with carbon and vanadium in the reaction of hydroquinone oxidation has confirmed the increase in the activity of the doped materials under exposure to ultraviolet, visible, and blue light. The phenomenon of dark catalysis in the anatase doped with carbon and vanadium has been interpreted within the concept of low-energy electronic excitatio ns between the impurity levels of carbon.

The effect of oxygen non-stoichiometry and doping with vanadium on the nature of magnetism in titanium dioxide with the anatase structure

Abstract The effect of oxygen non-stoichiometry and doping with vanadium on the electronic structure and magnetic properties of the titanium dioxide with the anatase structure have been studied by employing the tight-binding linear muffin-tin orbitals (TB-LMTO) method in the local spin-density approximation with the account of on-site Coulomb correlations (LSDA+U). It has been shown that the appearance of ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds with 0 ≤ x ≤ 1 / 16 , 0 ≤ y ≤ 1 / 16 is favored by the oxygen non-stoichiometry and location of the exchange-coupled vanadium atoms in the ac and bc planes of the crystal structure. At the vanadium concentration of 0.125 the super-exchange mechanism of the ferromagnetism is essential, with the Curie temperature (340 K) being in good relation to experimental data. The calculations demonstrate that the probability of formation of magnetic polarons is high at the vanadium concentration of 0.0625, but low at the concentration of 0.125. The reduction of magnetic moment with the rise of vanadium concentration corresponds to the general trend revealed for dilute magnetic semiconductors. It was established that the mechanisms of the high-temperature ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds are different and depend on oxygen non-stoichiometry and vanadium concentrations.

FIRST-PRINCIPLES STUDY OF ELECTRONIC STRUCTURE AND GROUND-STATE PROPERTIES OF ALKALI-METAL SELENIDES AND TELLURIDES (M2A)[M: Li, Na, K; A: Se, Te

First-principles calculations have been performed to investigate the electronic structure and ground-state properties of alkali-metal Selenides (M2Se) and Tellurides (M2Te) [ M: Li, Na, K ] using the Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) method. The exchange correlation energy is described within the local density approximation (LDA) using the von Barth and Hedin parameterization scheme. At ambient conditions, these compounds are found to crystallize in the face center cubic antifluorite (anti- CaF 2-type) structure. Ground-state properties such as total energy, equilibrium lattice parameter, and bulk modulus are calculated for these compounds. The calculated equilibrium lattice parameter is in agreement with experimental result. From the electronic structure calculations, we find that Li 2 Se , Li 2 Te , K 2 Se , and K 2 Te are indirect bandgap semiconductors, whereas Na 2 Se and Na 2 Te are direct bandgap semiconductors. The present results are compared with the earlier results of series of alkali-metal sulfides (M2S) and alkali-metal oxides ( M 2 O ), allowing us to make predictions about the total energy, bulk modulus, valence-band width, and bandgap behavior of the rest of the alkali-chalcogenide crystals.

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ B for 6.25% of Ni doping and 1.32 μ B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy ( V Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy ( V N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy ( V Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V Ga and formed a strong Ni–N bond. In this Ni– V Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ B.

Theoretical Interpretation of the Structural Variations along the Eu(Zn1−xGex)2 (0 ≤ x ≤ 1) Series

The electronic structures of EuZn(2), Eu(Zn(0.75)Ge(0.25))(2), Eu(Zn(0.5)Ge(0.5))(2), Eu(Zn(0.25)Ge(0.75))(2), and EuGe(2) have been investigated using tight-binding, linear muffin-tin orbital (TB-LMTO) and pseudopotential methods to understand the structural preferences influenced by valence electron counts and to explain the observed homogeneity range of the AlB(2)-type phases as reported in the companion article. A crystal orbital Hamilton population (COHP) analysis for Zn-Zn contacts in EuZn(2) suggests a possible homogeneity width for the KHg(2)-type phase, which is indicated from analysis of X-ray powder diffraction patterns. Total electronic energy comparisons, as well as density of states (DOS) and COHP analysis for a hypothetical Zn-rich compound, Eu(Zn(0.75)Ge(0.25))(2), indicate that two distinct phases, KHg(2)-type EuZn(2) and AlB(2)-type Eu(Zn(1-x)Ge(x))(2) (0.5 < or = x < or = 0.70), are more favorable than a single Zn-rich composition adopting the AlB(2)-type phase. Among 10 structural models of Eu(Zn(0.5)Ge(0.5))(2), the one with heteroatomic Zn-Ge interactions both within and perpendicular to the 6(3) nets is energetically the most favorable structure. The experimentally observed Zn-Ge bond distance is attributed to the contribution of both sigma- and pi-bond interactions. Zn-Ge, Eu-Zn, and Eu-Ge COHP curves of the minimum energy form of Eu(Zn(0.5)Ge(0.5))(2) show bonding character above the Fermi level and explain the observed wide homogeneity width of the AlB(2)-type phase. In the Ge-rich case, Eu(Zn(0.25)Ge(0.75))(2), the planar hexagonal nets are not energetically favorable due to the significant antibonding character of Ge-Ge bonding at the Fermi level. Structural relaxation using pseudopotentials also revealed that the hexagonal nets tend to pucker rather than being planar, in agreement with the observed incommensurately modulated superstructure. An electron localization function analysis for Eu(Zn(0.5)Ge(0.5))(2) reveals that there exists no two-center, two-electron bond or multicentered interactions between interlayer Zn...Ge contacts.

Complex formation study of thermodynamical, structural properties and density of states of Al–Mg binary alloy

Within the framework of complex formation model the thermodynamical, structural and surface properties of Al–Mg alloy have been studied presuming the formation of Al 3 Mg 2 complexes. The Gibb's free energy of mixing ( G M ), Heat of formation ( H M ) and entropy of mixing ( S M ) are compute for the system assuming it to be a pseudo-binary mixture. The alloy appears to be weakly interacting in nature and the thermodynamical properties are almost symmetrical about the equiatomic composition. We have also performed the first-principles calculation of the electronic band structure of Al–Mg alloy employing full-potential linearized augmented plane wave (FLAPW) as well tight-binding linear muffin-tin orbital (TBLMTO) methods. Total energy minimization enables us to estimate the equilibrium volume, bulk modulus and its pressure derivative. We have also described the total density of states (DOS) and the partial DOS (PDOS) around the Fermi energy to explain the variation of resistivity of the alloy with carrier concentration.

Theoretical investigation on structural, magnetic and electronic properties of ferromagnetic GdN under pressure

The tight-binding linear muffin tin orbital (TB-LMTO) method within the local density approximation is used to calculate structural, electronic and magnetic properties of GdN under pressure. Both nonmagnetic (NM) and magnetic calculations are performed. The structural and magnetic stabilities are determined from the total energy calculations. The magnetic to ferromagnetic (FM) transition is not calculated. Magnetically, GdN is stable in the FM state, while its ambient structure is found to be stable in the NaCl-type (B 1) structure. We predict NaCl-type to CsCl-type structure phase transition in GdN at a pressure of 30.4 GPa. In a complete spin of FM GdN the electronic band picture of one spin shows metallic, while the other spin shows its semiconducting behavior, resulting in half-metallic behavior at both ambient and high pressures. We have, therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for GdN in the B 1 and B 2 phases. The magnetic moment, equilibrium lattice parameter and bulk modulus is calculated to be 6.99 μ B, 4.935 Å and 192.13 GPa, respectively, which are in good agreement with the experimental results.

Pressure-induced electronic and structural phase transformation properties in half-metallic PmN: A first-principles approach

Abstract The electronic, structural and magnetic properties of half-metallic promethium nitride (PmN) have been studied using first-principles tight-binding linear muffin-tin orbital (TB-LMTO) method within the local-density approximation (LDA). We calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic PmN at ambient as well as at high pressure. Both spin-polarised and non-spin-polarised electronic calculations are performed to check the magnetic stability of PmN. It is found that PmN is ferromagnetically stable. From the total energy calculations it is also found that PmN is stable in the NaCl-type structure under ambient conditions. The structural stability of PmN changes under the application of pressure. We predict a structural phase transformation from NaCl- to CsCl-type structure at the pressure of 3.4 GPa. The high-pressure phase of PmN remains half-metallic and ferromagnetic. The calculated equilibrium lattice parameter and bulk modulus are in good agreement with the experimental and theoretical work.

Quantum-chemical modeling of the electronic structure and magnetic properties of Sn1 − x − y M x Sb y O2, M = Cr, Mn, Co, or Ni (x = 0.25; y = 0, 0.25)

The electronic and magnetic structures of the Sn0.75 M 0.25O2 and Sn0.5 M 0.25Sb0.25O2 (M = Cr, Mn, Co, Ni) compounds with a structure that is derivative of the rutile structure are modeled using the ab initio spin-polarized tight-binding linear muffin-tin orbital (TB-LMTO) method. The magnetic moments of the transition metal atoms are calculated. The data obtained are used to analyze the influence of the composition of Sn1 − x − y M x Sb y O2 phases on their electronic spectra and the magnetic and transport characteristics.

Investigation on some of the salient features of II–IV–V2 pnictides using band structure calculations as a tool

The first principle investigations on electronic structure of ABC2; A=Cd; B=Si, Ge, Sn; C=P, As pnictides using the tight binding linear muffin tin orbital (TBLMTO) method with in local density approximation (LDA) and some results analyzed using the projector-augmented-wave method (PAW) with generalized gradient approximation (GGA) are reported. The band gaps (Eg) range from 0.09eV to 1.20eV using TBLMTO and 0.12–1.38eV using PAW methods. The equilibrium lattice parameters, structural parameters, bond lengths, anion alternation parameter (u) and band gap (Eg) values are compared with experimental and other theoretical values.