Self-consistent Total-energy Calculations Research Articles

Ab Initio Construction of Magnetic Phase Diagrams in Alloys: The Case of Fe(1-x)Mn(x)Pt.

A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. Application to the Fe(1-x)Mn(x)Pt "magnetic chameleon" system yields the sequence of magnetic phases at T=0 and the c-T magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of the magnetic phase diagram is demonstrated.

Electronic structure of a thermoelectric material: CsBi 4Te 6

We have calculated the electronic structure of CsBi 4Te 6 by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. From our calculated electronic structure we have calculated the frequency dependent dielectric function. Our calculations shows that CsBi 4Te 6 a semiconductor with a band gap of 0.3 eV. The calculated dielectric function is very anisotropic. Our calculated density of state support the recent experiment of Chung et al. [Science 287 (2000) 1024] that CsBi 4Te 6 is a high performance thermoelectric material for low temperature applications.

First-principles calculations of the electronic band structure of In4Sn3O12 and In5SnSbO12

The electronic band structures of M7O12 rhombohedral In4Sn3O12 and In5SnSbO12 are studied using the projector-augmented-wave method based upon the density-functional theory within the generalized gradient approximation. The cation ordering in both materials, as determined by means of ab initio self-consistent total-energy calculations, confirms the results of previous structural studies. It is found that the valence-band maximum states of these two materials are mainly formed by O 2p states hybridized with In 5d characters whereas the bottoms of conduction bands are due primarily to Sn 5s electrons at M2 sites. The present study shows that the introduction of antimony into In4Sn3O12 broadens the splitting of the conduction band, which is presumably related to the substantial change in electrical conductivity observed in In5SnSbO12.

Ab initio study of the stability of X ( X=Cr, Nb, Ag) ultra-thin layers on Cu(001)

Full potential Linearized Plane Waves method within the framework of the generalized gradient approximation is applied to study X/Cu(001) ( X=Cr, Nb, Ag) layered structures. Self-consistent total energy calculations are carried out to determine the energetics and growth modes of different X/Cu(001) systems: interdiffusion, monolayer, bilayer, trilayer, and island formations. Ideal works of separation and the resulting interfacial energies are estimated for the Cr/Cu, Nb/Cu and Ag/Cu layered structures. The obtained results, which are compared with previously published experimental and theoretical data, show that Ag and Nb elements have quite different behavior than Cr.

A cluster study of aluminum adsorption on Ga-rich GaAs(100)(2 × 1) and β (4 × 2) surfaces

Ab initio self-consistent total energy calculations using second order Moller-Plesset perturbation theory and Hay-Wadt effective core potentials with associated basis sets (HWECP’s) for gallium and arsenic have been used to investigate the chemisorption properties of atomic aluminum on the Ga-rich GaAs(100)-(2 × 1) and β(4 × 2) surfaces. Finite sized hydrogen saturated clusters with the experimental zinc-blende lattice constant of 5.654 A and the energy optimized Ga dimer bond length of 2.758 A have been used to model the semiconductor surface. To investigate the effects of the core electrons of aluminum in the adsorption process, we have represented the Al adatom with both HWECP’s and an all electron 6-311++G** basis set. Detailed energetics of chemisorption on the (100) surface, including chemisorption energies, nearest surface neighbor bond lengths, and Mulliken population analysis, have been reported for all considered sites of chemisorption.

Pressure induced phase transitions in AmCm alloy

AbstractWe have studied the crystal structure of the AmCm binary alloy under high pressure by means of first-principles self-consistent total-energy calculations using the generalized gradient approximation (GGA) for the density functional theory (DFT). The virtual crystal approximation (VCA) is used for the description of the alloy system. In the present study, we investigated the double hexagonal (P63/mmc) structure, the face centered cubic (Fm3m) structure, the face-centered orthorhombic (Fddd) structure and the primitive orthorhombic (Pnma) structure for the AmCm alloy. Antiferromagnetic calculations have been compared with ferromagnetic calculations for all these phases. Our results are in general good agreement with recent experiment performed by Lindbaum et al. [J. Phys.: Condens. Matter. 15, S2297 (2003)].

Initial growth of GaN and InN over GaAs (110) substrates

We present, a systematic theoretical study of the adsorption of Ga, In and N over GaAs (110) surfaces based on parameter-free, self-consistent total energy and force calculations using the density functional theory. We analyzed the changes in the bond-lengths and in the bond-angles before and after deposition, as well as the total energy behaviour with the adsorbate chemical potential variation. Our results show that the GaN growth over GaAs (110) is energetically more favorable than the InN growth. However, the resulting InN film can be cubic only if we apply small strains on the substrate, while the GaN one is hexagonal.

Pressure-induced phase transitions of KNbO3

We studied crystal structures of potassium niobate (KNbO3) at high pressure bymeans of first-principles self-consistent total-energy calculations within the localdensity approximation using the full-potential linear muffin-tin orbital method.For the first time, we have calculated the atomic equilibrium volume, bulkmodulus, total energy, and transition pressures for KNbO3, covering the fullpressure range for which the above-mentioned experiments have been done. Twopressure-induced transitions are derived theoretically, namely one fromorthorhombic (I) to cubic structure at around 13.2 GPa and a secondfrom cubic to orthorhombic (II) structure at a pressure of 39.7 GPa. Thisfully confirms the recent experiments by Kobayashi et al(Kobayashi Y,Endo S, Ashida T, Ming L C and Kikegawa T 2000 Phys. Rev. 61 5819)

Modeling of compound semiconductors: Analytical bond-order potential for Ga, As, and GaAs

An analytical bond-order potential for GaAs is presented, that allows one to model a wide range of properties of GaAs compound structures, as well as the pure phases of gallium and arsenide, including nonequilibrium configurations. The functional form is based on the bond-order scheme as devised by Abell-Tersoff and Brenner, while a systematic fitting scheme starting from the Pauling relation is used for determining all adjustable parameters. Reference data were taken from experiments if available, or computed by self-consistent total-energy calculations within the local density-functional theory otherwise. For fitting the parameters, only structural data of the metallic phases of gallium and arsenide as well as those of different GaAs phases were used. A number of tests on point defect properties, surface properties, and melting behavior have been performed afterward in order to validate the accuracy and transferability of the potential model, but were not part of the fitting procedure. While point defect properties and surfaces with low As content are found to be in good agreement with literature data, the description of As-rich surface reconstructions is not satisfactory. In the case of molten GaAs we find support for a structural model based on experiment that indicates a polymerized arsenic phase in the melt.

High Pressure Theoretical Studies of Actinide Dioxides

Actinide dioxides (ThO 2 , UO 2 , Pu 2 etc.) compounds have the CaF 2 -type structure at ambient pressure and temperature. Under high pressure, they exist in the PbCl 2 -type structure, belonging to space group Pnma [1]. We have studied crystal structures under high pressure in actinide dioxides by means of first-principles self-consistent total-energy calculations with the non-local Perdew, Burke and Ernzerhof (PBE) exchange correlation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. The atomic equilibrium volume, bulk modulus and transition pressure for actinide dioxides were calculated, covering the full pressure range for which the mentioned experiments have been done [2].

Theoretical prediction of a phase transition in gold

We performed theoretical investigations on gold under high pressure through first principles self-consistent total-energy calculations within the local-density approximation as well as generalized gradient approximation using full-potential muffin-tin-orbital method and found a phase transition from fcc-type to hcp-type of structure at 241 GPa. The stability of this phase has been explained through electronic density of state.

Theoretical prediction of the Cmca phase in Ge under high pressure

We have performed first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method to investigate crystal structures at high pressure in Ge. We predict a crystallographic phase transition from the simple hexagonal structure to the Cmca phase, which is similar to the Si–VI phase which has recently been observed by Hanfland et al. [Phys. Rev. Lett. 82, 1197 (1999)]. Our calculated pressure for this phase transition is around 94 GPa in Ge. Further, at the very high pressure of about 150 GPa, the Cmca phase is predicted to transform into the hexagonal-close-packed phase.

Theoretical high-pressure studies of Cs metal

We have studied the crystal structures of cesium metal under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. Our results confirm the recent high pressure experimental observations of crystallographic phase transformations in Cs [Schwarz et al., Phys. Rev. Lett. 81, 2711 (1998)]. The calculated transition pressures agree well with the measured data. Also the computed axial $c/a$ ratio for Cs-IV and Cs-VI is found to be in good agreement with experiment. Similarly the calculated a/b, c/b ratios as well as the three internal parameters for the Cs-V phase agrees very well with measurement. The structural stability of different phases in Cs is also supported by canonical d-band model calculations.

First-principles investigation of structural and electronic properties of solid cubane and its doped derivatives

The electronic and structural properties of molecular and solid cubane have been studied by first-principles, self-consistent field total energy calculations. Calculated molecular properties such as equilibrium geometry and electronic and vibrational spectra are found to be in good agreement with experimental data. Structural parameters and the energetics of both the low-temperature, orientationally ordered and high-temperature, orientationally disordered or plastic phases of solid cubane are determined. The valence band of solid cubane is derived from the molecular states; the energy gap between the lowest unoccupied and highest occupied molecular orbital bands is rather large due to the saturated carbon atoms. The effect of alkali-metal-atom doping on the electronic energy bands is investigated. It is found that the metallic band of doped cubane is derived from the undoped solid cubane's lowest conduction band with a significant contribution from the alkali-metal atom.

High pressure studies of sodium and silver halides

We have investigated the structural phase transitions in sodium and silver halides theoretically under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. Our results confirm the recent high pressure experimental observations of crystallographic phase transformations in sodium halides (Leger et al. (1998) J. Phys.: Condens. Matter, 10, 4201) and silver halides (Hull and Keen (1999) Phys. Rev., B59, 750. The calculated transition pressures agree with the experimental data.

Electronic structure of Ti3SiC2

We have investigated the electronic structure of Ti3SiC2 by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. Our calculated volume is in good agreement (within 4%) with experiment. Also, the calculated values for c/a and positional parameters are in excellent agreement with the experimental data. We have compared the electronic structure of Ti3SiC2 with TiC. Our calculations show that Ti3SiC2 has a chemical bonding similar to the one of TiC, which is predominantly covalent.

Quasiab initiomolecular dynamic study of Cu melting

We have investigated the melting of Cu theoretically by means of a molecular dynamic method employing the Sutton-Chen model for the interatomic interaction. This interaction has been fitted to reproduce results from first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method for the bcc, fcc, hcp, and liquid configurations. No experimental data were used to tune the potential. A large number of properties including equation of state, melting temperature, high-pressure melting curve, change of volume and entropy at melting, liquid structure, diffusion coefficient in liquid, and vacancy formation energy are all in good agreement with experimental data. Inclusion of the full potential energy of a liquid configuration in the fitting procedure is critical for obtaining good agreement with experiment. Different ways to calculate the melting transition are shown to produce very different results. The use of a large number of particles in combination with the solid-liquid interface as an initial configuration in the simulation is essential in order to obtain the correct melting temperatures.

A Comparative Study of the Influence of the Local Density Approximation and the Generalized Gradient Approximation on the Calculated Properties of the III-Nitride (110) Surfaces

ABSTRACTWe present a systematic theoretical study of several III-nitride (110) surfaces based on accurate parameter-free, self-consistent total energy and force calculations using the density functional theory, the local density approximation (LDA), as parametrized by Perdew and Zunger, and the generalized gradient approximation (GGA), as proposed by Perdew, Burke, and Ernzerhof, for the exchange-correlation term; we use the Full Potential Linear Augmented Plane Wave (FPLAPW) approach (WIEN-97 code) associated with the slab supercell model to simulate the (110) surface. We studied BN, AlN, GaN, InN and compared the theoretical results as related to the use of the LDA and the GGA. We conclude that although the results for both approximations are similar, differences in structural parameters may be as large as 10%.

Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces

We present a systematic theoretical study of several III-nitride (110) surfaces based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the generalized gradient approximation(GGA) for the exchange-correlation term, and the Full Potential Linear Augmented Plane Wave (FPLAPW) approach associated with the slab supercell model. We studied AlN, BN, GaN and InN and analyzed the theoretical trends for the equilibrium atomic structures and surface band structures. We used supercells built up of 7 atomic layers and a vacuum region equivalent of 5 atomic layers. For the Ga and In species, the 3d and 4d electrons were treated properly as valence electrons. In connection with the atomic structures, we are concerned with the LEED parameters delta1<FONT FACE=Symbol>^</FONT>, delta1x, delta2<FONT FACE=Symbol>^</FONT> , d12<FONT FACE=Symbol>^</FONT>, d12x and omega for the (110) surface. We analyzed the changes in the bond-lengths and in the bond-angles at the anion and cation sites. We conclude that similarly to the III-arsenide (110) and III-phosphide (110) surfaces, the III-nitride (110) surfaces relax such that the cation-surface atom moves inward and the N-surface atom moves outward. The large Coulomb energy of the III-nitrides as compared with the other III-V compounds re ects in the smaller value of the tilt angle omega and in the small value of delta1<FONT FACE=Symbol>^</FONT>. To our knowledge, this is the rst time the FPLAPW method is used for such a systematic study of III-nitrides, and we compare our results with recent results obtained with other approaches as reported in the literature.

Theoretical high-pressure studies of silicon VI

We have investigated Si theoretically under high compressions by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. Our results confirm the recent high-pressure experimental observations of crystallographic phase transformations in Si [Hanfland et al., Phys. Rev. Lett. 82, 1197 (1999)]. The calculated transition pressure for the occurrence of the silicon-VI crystal structure agrees very well with the experimental data.