Tight-binding Linear Muffin-tin Orbital Research Articles

Magnetization Dynamics in FexCo1-x in Presence of Chemical Disorder

In this paper, we present a theoretical formulation of magnetization dynamics in disordered binary alloys, based on the Kubo linear response theory, interfaced with a seamless combination of three approaches: density functional-based tight-binding linear muffin-tin orbitals, generalized recursion and augmented space formalism. We applied this method to study the magnetization dynamics in chemically disordered FexCo1−x (x = 0.2, 0.5, 0.8) alloys. We found that the magnon energies decreased with an increase in Co concentration. Significant magnon softening was observed in Fe20Co80 at the Brillouin zone boundary. Magnon–electron scattering increased with increasing Co content, which in turn modified the hybridization between the Fe and Co atoms. This reduced the exchange energy between the atoms and softened down the magnon energy. The lowest magnon lifetime was found in Fe50Co50, where disorder was at a maximum. This clearly indicated that the damping of magnon energies in FexCo1−x was governed by hybridization between Fe and Co, whereas the magnon lifetime was controlled by disorder configuration. Our atomistic spin dynamics simulations show reasonable agreement with our theoretical approach in magnon dispersion for different alloy compositions.

New ternary phosphides RE5Pd9P7 (RE=Tm, Lu): synthesis, crystal and electronic structure

AbstractThe crystal structure of the new ternary phosphideTm5Pd9P7 was determined from powder X‐ray diffraction data: space group P , a=9.9971(2), c=3.94371(8) Å, RI=0.0518, Rp=0.0667, Rp=0.0667, wRp=0.1065. The crystal structure of the phosphide Tm5Pd9P7 is closely related with the Zr2Fe12P7‐type, and is formed by combination of two types of structural units with the compositions [TmPd6P3] and [TmTm3Pd3P3] mutually displaced by 1/2 along the c axis. Connection of the last units forms the trigonal prisms [Tm6] around P3 atoms in the crystallographic position 1a (0 0 0). The isotypic ternary phosphide Lu5Pd9P7 was synthesized (a=9.9934(7), c=3.9207(4) Å). The electronic and bonding properties of Tm5Pd9P7 are obtained from tight binding linear muffin‐tin orbital (TB‐LMTO) calculations and a crystal orbital Hamiltonian population (COHP) bonding analysis. These analyses show strong covalent bonding within (Pd9P7)15− nanotubes which are ionically bound via the Tm atoms to three dimensional network.

Experimental and Theoretical Study on the Nonmagnetic Li/Mg Cosubstitution in the Gd5-x(Li/Mg)xGe4 (x = 1.04, 1.17, 1.53) System.

Three Li- and Mg-cosubstituted compounds in the Gd5-x(Li/Mg)xGe4 (x = 1.04(2), 1.17(2), 1.53(2)) system have been successfully prepared by conventional high-temperature reactions. According to powder and single-crystal X-ray diffraction analyses, all three compounds adopt a Gd5Si4-type phase with the orthorhombic Pnma space group (Pearson code oP16, Z = 4) and six crystallographically independent atomic sites. The crystal structure can be described as a combination of two-dimensional Mo2FeB2-type ∞2[Gd2(Li/Mg)Ge2] layers and [Ge2] dimers. Interestingly, as 64% of Li and 26% of Gd at the RE3 and RE2 sites, respectively, were exclusively substituted by Mg in Gd3.47(1)Li0.36(2)Mg1.17(3)Ge4, the lattice parameter b was selectively shortened as a result of the RE3-Ge1 bond shrinkage in comparison to that in Gd4LiGe4, while lattice parameters a and c remained nearly intact. A series of theoretical calculations using the tight-binding linear muffin-tin orbital (TB-LMTO) method indicated that the reduction of the particular RE3-Ge1 bond distance in the title compounds could also be explained by an optimization of bonding based on the corresponding RE3-Ge1 crystal orbital Hamilton population (COHP) curve. Moreover, the specific site preference of Mg for the RE3 site was supported by both size-factor as well as electronic-factor criteria on the basis of the smallest atomic size and the highest electronegativity of Mg among the three cations. Therefore, the overall electronic structure was further interrogated by a density of states (DOS) analysis. The influence of nonmagnetic Li/Mg cosubstitution for the magnetic Gd atoms in the title Gd5-x(Li/Mg)xGe4 system on the magnetic characteristics was also thoroughly studied by isofield magnetization at 100 Oe and 10 kOe and isothermal magnetization measurements at 4 K using two of the title compounds: Gd3.83(1)Li0.48Mg0.69(3)Ge4 and Gd3.47(1)Li0.36(2)Mg1.17(3)Ge4.

Investigation in the ternary Ta-Ni-P system: Solid state phase equilibria at T=1070 K, crystal and electronic structures of new ternary phosphides

The solid state phase diagram for the ternary Ta-Ni-P system was established at T=1070 K in the region of up to 67 at% of P by means of X-ray powder diffraction methods (PXRD). Six ternary compounds, namely Ta4NiP (Nb4CoSi-type), Ta1.10–0.82Ni0.90–1.18P (TiNiSi-type), TaNiP2 (NbNiP2-type), Ta5.00–4.81Ni4.00–4.19P4 (Nb5Cu4Si4-type), TaNi2P (own structure type) and Ta1–0.08(1)Ni0.08(1)P2 (OsGe2-type) have been confirmed to exist. Rather minor Ta/Ni homogeneity ranges have been found for α-Ta3-xNixP (x=0.2) (Ті3Р-type), Ta1-xNixP (x=0.18) (NbAs-type), Ni3-xTaxP (x=0.2) (Ni3P-type) and Ni2-xTaxP (x=0.25) (Fe2P-type). The crystal structure of Ta4.811(9)Ni4.189(9)P4 has been refined from single crystal X-ray diffraction data (Nb5Cu4Si4-type, space group I4/m,a =9.8474(17), c=3.5182(7) Å, R1=0.0283, wR2=0.0470), while that of the new TaNi2P compound was determined by means of PXRD. This phosphide crystalizes in its own structure type (space group Pnma, a=8.3588(3), b=3.5208(1), c=6.7051(3) Å, RІ=0.044, RР=0.161). A new isostructural Fe-compound, TaFe2P (a=8.358(2), b=3.5194(7), c=6.703(1) Å), was also synthetized. The electronic structures of Ta5Ni4P4 and TaNi2P were analyzed using the tight-binding linear muffin-tin orbital (TB-LMTO) and extended Hückel methods.

ATiO3 (A = Ca, Sr, Ba & BP) Pervoskites in Cubic and Tetragonal Phase using TB-LMTO-ASA Method

Ground state properties of ATiO3 (A = Ca, Sr, Ba & Pb) pervoskite structures in cubic and tetragonal phase were studied by tight binding linear muffin-tin orbital (TB-LMTO) method in the framework of density functional theory (DFT) with the atomic-sphere approximation (ASA). The total energy of all the compounds come under the above said structures have shown that the cubic phase is the stable structure in the ambient condition. Among these pervoskites maximum bulk modulus was obtained for BaTiO3 . Direct (cubic) and indirect (tetragonal) band gap was observed from the band structure calculations and the values fall within the range of 1.5 – 1.7 eV. Electron distribution of each element in the valence and conduction bands was clearly obtained from the density of states (DOS) and partial density of states (PDOS) for all the compounds. The magnetization values were found in the range of 0.4 – 0.56 x 10-5µB. The‘d’ orbital position of Ti was observed for all the ABO3 compounds and shifted away from the Fermi level except for Ti in BaTiO3 . The refractive indices of the pervoskites were calculated from the energy band gap and the value is above 3 for all the compounds.

Structural and magnetic properties of the RAl0.2Ge2 compounds (R = Tb, Dy, Ho, Er, Tm)

The structural and magnetic properties of the RAl0.2Ge2 (R = Tb-Tm) germanides were studied by X-ray and neutron diffraction, scanning electron microscopy and magnetometric techniques. The compounds crystallize in the CeNiSi2-type crystal structure (space group Cmcm). Magnetic measurements reveal an antiferromagnetic ordering below TN equal to 38 K (R = Tb), 23 K (Dy), 9 K (Ho) and 6.5 K (Er), and no magnetic ordering in the Tm-compound down to 1.9 K. Based on the neutron diffraction data the magnetic structures can be described by the propagation vector k = (0.4714(2), 0, 0.3128(2)) for TbAl0.2Ge2 and k = (0.4858(5), 0, 0.3214(6)) for DyAl0.2Ge2 while in HoAl0.2Ge2 and ErAl0.2Ge2 a simple collinear antiferromagnetic ordering of the G-type was observed. The electronic structures of the title compounds were calculated using the tight-binding linear muffin-tin orbital (TB-LMTO) method.

Band structure calculation and rietveld refinement of nanoscale GdFeO3 with affirmation of Jahn Teller's distortion on electric and magnetic properties

We report the meticulous study of freshly synthesized rare earth element substituted R-FeO3 (R-Gd) orthoferrite via co-precipitation route and subsequent sintering. The R-FeO3 powder sample is characterized by X-ray Diffractometer (XRD) for the investigation of crystal structure and is confirmed as orthorhombic with space group Pbnm structure. Nanoparticle dimension is captured by Scanning Electron microscope (SEM). Magnetic character of the sample is examined using vibrating sample magnetometer (VSM), it discloses a nearly linear magnetisation behaviour (paramagnetic behaviour). Dielectric studies and electric Polarization (PE loop) are investigated. A shift is observed in the dielectric constant εr from 250 °C to 300 °C. The Rietveld refinement of the crystal structure from the powder XRD confirms GdFeO3 is distorted at octahedral site of FeO6. This distortion is ascertained as weak Jahn -Teller's distortion leads to the unsaturated electric polarization. The electronic properties, total and partial density of states of GdFeO3 are studied and calculated using Tight - Binding Linear Muffin-Tin Orbital (TBLMTO) method with the Atomic Sphere Approximation (ASA). The band gap of the GdFeO3 is found to be 0.32 eV.

Influence ofp-Type Double-Doping on the Crystals and Electronic Structures of Two Polar Intermetallics: La4.57(1)Li0.43Ge3.80(3)In0.20and Nd4.32(1)Li0.68Ge3.87(3)In0.13

Two quaternary polar intermetallic compounds La4.57(1)Li0.43Ge3.80(3)In0.20 and Nd4.32(1)Li0.68Ge3.87(3)In0.13 were synthesized using a conventional high temperature synthetic method as we attempted to introduce the p-type double-doping of Li and In for RE and Ge in the RE5-xLixGe4-y (RE = rare-earth metals) system, and their crystal structures were characterized by single crystal X-ray diffraction experiments. The two title compounds crystallize in the orthorhombic space group Pnma (Pearson code oP16, Z = 4) with six crystallographically independent asymmetric atomic sites and adopt the Gd5Si4-type structure. Overall crystal structures of two isotypic title compounds can be described as a 1:1 assembly of the hypothetical 2-dimensional (2D) RE2(RE/Li)(In/Ge)2 layered structure adopting the Mo2FeB2-type structure and the dumbbell-shaped inter-slab (In/Ge)2 dimers bridging two such neighboring 2D layers along the crystallographic b-axis direction. The observed “direction selective” structural transformation from the Sm5Ge4-type to the Gd5Si4-type structure can be understood as a result of the simultaneous double-doping by the relatively smaller amount of Li substitution for La at the RE3 site than that in the La4LiGe4 and the partial In substitution for Ge at both of the M1 and M3 sites. The site-preference of In for two particular anionic sites were thoroughly studied using four hypothetical La4LiGe3In models having different atomic arrangements by the tight-binding linear muffin-tin orbital (TB-LMTO) method. The overall electronic structure and individual chemical bonding influenced by the given double-doping were also discussed on the basis of the density of states (DOS) and crystal orbital Hamilton population (COHP) curves analyses.

Gd(Co1-xGax)2: Synthesis, crystal structures, and investigation of structural transformations and magnetic properties

Gd(Co1-xGax)2 (x = 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1) phases were synthesized by arc melting the constituent elements and subsequent annealing. The samples were characterized by powder and single crystal X-ray diffraction, magnetic measurements, and electronic structure calculations. An interesting structural sequence was obtained: cubic MgCu2-type structure for x = 0 and 1/6; MgZn2-type structure for x = 1/3; orthorhombic SrMgSi-type structure for x = ½; orthorhombic CeCu2-type structure for x = 2/3; hexagonal AlB2-type structure for x = 5/6 and 1. Tight-binding linear-muffin-tin orbital (TB-LMTO) calculations were performed on GdCo2, GdCoGa, and GdGa2 to trace the origin of their structural transformations, which appear to be driven by the changes in the valence electron count (VEC). In addition, some conclusions of these fully stoichiometric compounds were obtained from the rigid band model. GdGa2 (x = 1) is antiferromagnetic, while the other ones are either ferrimagnetic or ferromagnetic. TC of Gd(Co1-xGax)2 decreases monotonically with the increasing Ga content, suggesting that promising room temperature (RT) magnetocaloric materials could be obtained between x = 1/6 and x = 1/3.

Structure and Chemical Bonding of the Li-Doped Polar Intermetallic RE₂In1-xLixGe₂ (RE = La, Nd, Sm, Gd; x = 0.13, 0.28, 0.43, 0.53) System.

Four polar intermetallic compounds belonging to the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system have been synthesized by the traditional solid-state reaction method, and their crystal structures have been characterized by single-crystal X-ray diffraction (SXRD) analyses. The isotypic crystal structures of four title compounds adopt the Mo2FeB2-type structure having the tetragonal space group P4/mbm (Z = 2, Pearson code tP40) with three crystallographically independent atomic sites and can be simply described as a pile of the identical 2-dimensioanl (2D) RE2In1-xLixGe2 slabs stacked along the c-axis direction. The substituting Li atom shows a particular site preference for replacing In at the Wyckoff 2a site rather than Ge at the Wyckoff 4g in this crystal structure. As the size of a used rare-earth metal decreases from La3+ to Gd3+ throughout the title system, the Ge-Ge and Ge-In/Li bond distances, both of which consist of the 2D anionic Ge2(In/Li) layer, gradually decrease resulting in the reduction of a unit cell volume. A series of theoretical investigations has been performed using a hypothetical structure model Gd2In0.5Li0.5Ge2 by tight-binding linear muffin-tin orbital (TB-LMTO) method. The resultant densities of states (DOS) value at the Fermi level (EF) suggests a metallic conductivity for this particular composition, and this calculation result is in a good agreement with the formal charge distribution assigning two extra valence electrons for a metal-metal bond in the conduction band. The thorough analyses of six crystal orbital Hamilton population (COHP) curves representing various interatomic interactions and an electron localization function (ELF) diagram indicating the locations of paired-electron densities are also provided in this article.

Effect of disorder on the optical response of NiPt and Ni3Pt alloys

In this communication we present a detailed study of the effect of chemical disorder on the optical response of Ni1−xPtx (0.1⩽x⩽0.75) and Ni3(1−x)/3Ptx (0.1⩽x⩽0.3). We shall propose a formalism which will combine a Kubo-Greenwood approach with a DFT based tight-binding linear muffin-tin orbitals (TB-LMTO) basis and augmented space recursion (ASR) technique to explicitly incorporate the effect of disorder. We show that chemical disorder has a large impact on optical response of Ni-Pt systems. In ordered Ni-Pt alloys, the optical conductivity peaks are sharp. But as we switch on chemical disorder, the UV peak becomes broadened and its position as a function of composition and disorder carries the signature of a phase transition from NiPt to Ni3Pt with decreasing Pt concentration. Quantitatively this agrees well with Massalski’s Ni-Pt phase diagram (Massalski, 1990). Both ordered NiPt and Ni3Pt have an optical conductivity transition at 4.12eV. But disordered NiPt has an optical conductivity transition at 3.93eV. If we decrease the Pt content, it results a chemical phase transition from NiPt to Ni3Pt and shifts the peak position by 1.67eV to the ultraviolet range at 5.6eV. There is a significant broadening of UV peak with increasing Pt content due to enhancement of 3d(Ni)-5d(Pt) bonding. Chemical disorder enhances the optical response of NiPt alloys nearly one order of magnitude. Our study also shows the fragile magnetic effect on optical response of disordered Ni1−xPtx (0.4<x<0.6) binary alloys. Our theoretical predictions agree reasonably well with both previous experimental findings as well as theoretical investigations.

Band-gap tuning and optical response of two-dimensionalSixC1−x: A first-principles real-space study of disordered two-dimensional materials

We present a real-space formulation for calculating the electronic structure and optical conductivity of such random alloys based on the Kubo-Greenwood formalism interfaced with the augmented space recursion (ASR) [A. Mookerjee, J. Phys. C: Solid State Phys. {\bf 6}, 1340 (1973)] formulated with the Tight-binding Linear Muffin-tin Orbitals (TB-LMTO) basis with van Leeuwen-Baerends corrected exchange (vLB) [Singh et al, Phys. Rev B {\bf 93}, 085204, (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of 2D honeycomb siliphene Si$_{x}$C$_{1-x}$ beyond the usual Dirac-cone approximation. We predicted the quantitative effect of disorder on both the electronic-structure and optical response over a wide energy range, and the results discussed in the light of the available experimental and other theoretical data. Our proposed formalism may open up a facile way for planned band gap engineering in opto-electronic applications.

STRUCTURAL AND ELECTRONIC PROPERTIES OF TRANSITION METAL DI-CHALCOGENIDES (MX2) M=(Mo, W) AND X=(S, Se) IN BULK STATE: A FIRST-PRINCIPLES STUDY

Transition metal di-chalcogenides (MX2) M=(Mo, W) and X=(S, Se) in bulk state are of great interest due to their diverse applications in the field of modern technology as well as to understand the fundamental aspect of Physics. We performed structural and electronic properties of selected systems using density functional theory implemented in Tight Binding Linear Muffin- tin Orbital (TBLMTO) approach with subsequent refinement. The structural optimization is performed through energy minimization process and lattice parameters of optimized structures for MoS2, MoSe2, WS2 and WSe2 are found to be 3.20Å, 3.34Å, 3.27Å and 3.34Å respectively, which are within the error bar less than 5% with experimental values. The band gaps for all TMDCs are found to be of indirect types with semiconducting behaviours. The values of band gap of MoS2, MoSe2, WS2 and WSe2 in bulk state are found to be 1.16eV, 108eV, 1.50eV and 1.29eV respectively which are comparable with experimental and previously calculated data. Due to the symmetric nature of up spin and down spin channels of Density of States (DOS) all the systems selected are found to be non magnetic. However it fully supports the results obtained from band structure calculations. The potential and charge distributions plots support the results. The charge density plots reveals the covalent nature of bond in (100) plane. However (110) plane shows mixed types of bonding.Journal of Institute of Science and TechnologyVolume 22, Issue 1, July 2017, page: 41-50

Effect of random vacancies on the electronic properties of graphene and T graphene: a theoretical approach

In this communication we present together four distinct techniques for the study of electronic structure of solids : the tight-binding linear muffin-tin orbitals (TB-LMTO), the real space and augmented space recursions and the modified exchange-correlation. Using this we investigate the effect of random vacancies on the electronic properties of the carbon hexagonal allotrope, graphene, and the non-hexagonal allotrope, planar T graphene. We have inserted random vacancies at different concentrations, to simulate disorder in pristine graphene and planar T graphene sheets. The resulting disorder, both on-site (diagonal disorder) as well as in the hopping integrals (off-diagonal disorder), introduces sharp peaks in the vicinity of the Dirac point built up from localized states for both hexagonal and non-hexagonal structures. These peaks become resonances with increasing vacancy concentration. We find that in presence of vacancies, graphene-like linear dispersion appears in planar T graphene and the cross points form a loop in the first Brillouin zone similar to buckled T graphene that originates from $\pi$ and $\pi$* bands without regular hexagonal symmetry. We also calculate the single-particle relaxation time, $\tau(\vec{q})$ of $\vec{q}$ labeled quantum electronic states which originates from scattering due to presence of vacancies, causing quantum level broadening.

Structural and Electronic Properties of Thulium Compounds

The tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA) is used to study the electronic structure and high pressure behaviour of thulium compounds TmX (X= P, As, S, and Se). We also predict a structural phase transition from NaCl to CsCl-type structure. The transition pressures were found to be 40.0, 31.0, 58.0 and 49.0 GPa, for TmP, TmAs, TmS and TmSe respectively. Apart from this, the lattice parameter (a0), bulk modulus (B0), band structure and density of states are calculated. From energy band diagram, it is observed that these compounds exhibit weak metallic character. The calculated values of lattice parameters and bulk modulus are of reasonable agreement with available data.

First-principles evaluation of intrinsic, side-jump, and skew-scattering parts of anomalous Hall conductivities in disordered alloys

We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $\sigma_{xy}$. This method is based on the different microscopic conductive processes of each origin of $\sigma_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value of each origin of $\sigma_{xy}$ exhibits reasonable dependencies on the electron scattering in these disordered alloys.

Ab Initio Theory of the Gilbert Damping in Random Ferromagnetic Alloys

We present an ab initio theory of the Gilbert damping in ferromagnetic alloys with substitutional disorder. The theory is based on nonlocal torques that are represented by nonrandom, site-off-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The formalism is developed for the relativistic tight-binding linear muffin-tin orbital (TB-LMTO) method and the coherent potential approximation (CPA). The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on random local torques. The theory is illustrated by calculations for various random transition metal alloys: FeNi, FeCo, Heusler alloys, permalloy with impurities, Fe with vacancies, and stoichiometric FePt and CoPt alloys with a varying degree of L1 0 atomic long-range order. Results are in a reasonable agreement with other calculations and accessible experimental data.

Ab Initio Study of Structural Phase Transition and Electronic Properties in Samarium (Sm) Compounds

The electronic structure and high pressure structural phase transition of SmTe and SmPo have been studied by using tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). The total energy as a function of volume is obtained and it is found that these compounds are stable in NaCl-type (B1-phase) structure and transform to CsCl-type (B2-type) structure. The transition pressure of SmTe and SmPo are found to be 6.6 GPa and 8.6 GPa respectively. We have also calculated lattice parameter (a0), bulk modulus (B0), band structure (BS) and density of states (DOS). From energy band diagram, we observed that these compounds exhibit weakly metallic behaviour. The calculated values of lattice parameter and bulk modulus agree well with the available data.

Electronic and magnetic properties at rough alloyed interfaces of Fe/Co on Au substrates: An augmented space study**Project supported by the INSPIRE Program Division, Department of Science and Technology, India.

We studied the interface electronic and magnetic properties of Fe/Co deposited on Au substrate and researched the effects of roughness at the interfaces within augmented space formalism (ASF). The full calculation is carried out by recursion and tight-binding linear muffin tin orbital (TB-LMTO) methods. The amount of roughness is different at different atomic layers. The formalism is also applied to sharp interface, when interdiffusion of atoms is negligible. Our results of one monolayer transition metal agree with other reported results. A realistic rough interface is also modeled with three and four monolayers of transition metals, deposited on Au substrates.

Electronic and structural properties of rare earth pnictides

Abstract In the present paper structural and electronic properties of rare earth pnictides have been presented. The present calculation has been performed using self-consistent tight binding linear muffin tin orbital (TB-LMTO) method within the local density approximation (LDA). The studied compounds undergo a structural phase transition from NaCl-type structure to CsCl-type structure. The electronic band structure and density of states of the pnictides have been reported. The equilibrium lattice parameter a (Å), bulk modulus B (GPa), number of f-states at the Fermi level Nf (states/Ry cell) and volume collapse of AmBi and CmBi have also been reported. The calculated equilibrium structural parameters are in good agreement with the available experimental results.