Full-potential Linear Muffin-tin Orbital Method Research Articles

Total Energy Differences Between Silicon Carbide Polytypes and their Implications for Crystal Growth

ABSTRACTThe energy differences between various SiC polytypes are calculated using the full-potential linear muffin-tin orbital method and analyzed in terms of the anisotropie next nearest neighbor interaction (ANNNI) model. The fact that J1 + 2J2 < 0 with J1 > 0 implies that twin boundaries in otherwise cubic material are favorable unless twins occur as nearest neighbor layers. Contrary to some other recent calculations we find J1 > |J2|. We discuss the consequences of this for stabilization of cubic SiC in epitaxial growth, including considerations of the island size effects.

Characterization Of Bulk, Polycrystalline Indium Nitride Grown At Sub-Atmospheric Pressures

AbstractPolycrystalline, wurtzitic indium nitride was synthesized by saturating indium metal with atomic nitrogen from a microwave plasma source. Plasma synthesis avoids the high equilibrium pressures required when molecular nitrogen is used as the nitrogen source. Two types of growth were observed: 1) small amounts of indium nitride crystallized from the melt during cooling and 2) hexagonal platelets formed adjacent to the In metal source on the crucible sides. The mechanism of this latter growth is not established, but may involve transport of indium as a liquid film. The crystals were characterized by electron diffraction, X-ray diffraction, elemental analysis, scanning electron microscopy, and Raman spectroscopy. Lattice parameter and Raman active phonon modes are reported and compared with calculations based on the full-potential linear muffin-tin orbital method (FP-LMTO).

Local Density Band Theory Calculations of Optical and Magneto-Optical Properties of CeS

Optical and magneto-optical properties of cerium compound CeS are studied using the spin-and-orbitally polarized fullpotential linear muffin-tin orbital (LMTO) method. The diagonal and off-diagonal conductivity tensor elements are calculated from self consistent electronic band structure theory over the full frequency range studied experimentall. We have compared the optical part of the conductivity with available experimental data and previous investigations of other compounds. We have found that while overall agreement ofthe band theory is quite good, it fails to place f-electrons correctly within the spectrum because of its inability to include correlation effects.

Full-potential linear muffin-tin orbital calculation of atomic forces and optimization of atomic structure of fullerence epoxide derivatives C60O and C60O2

Atomic force calculations and the optimization of atomic structure in the ground state have been made in the cluster version of the full-potential linear muffin-tin orbital method. Relaxation along atomic forces in the ground state enabled us to estimate the main parameters of the atomic structure and total energy in the ground state of fullerene epoxide derivatives C60O and C60O2, obtained recently in the reaction of C60 with ozone in toluene. We found the C60O isomer, with the oxygen atom bridging the 6.6 carbon bond, to be more stable than that with the oxygen atom bridging the 6,5 bond. The most polar isomer of C60O2 with the oxygen atoms placed at the same 6,6 ring, is found to have the lowest energy compared with other bridgings of oxygen atoms, in agreement with experimental data.

Total-energy study of electronic structure and mechanical behavior of C15 Laves phase compounds: NbCr2 and HfV2.

First-principles electronic structure calculations based on a full-potential linear muffin-tin orbital method have been used to study the electronic and mechanical properties of the early transition metal C15 Laves phase compounds ${\mathrm{NbCr}}_{2}$ and ${\mathrm{HfV}}_{2}$. For both compounds, total energies are computed and compared for the two Laves phase crystal structures C15 and C14. The lower total-energy structure for ${\mathrm{NbCr}}_{2}$ is found to be the C15 structure as opposed to the C14 structure for ${\mathrm{HfV}}_{2}$. We have calculated the equilibrium unit cell volumes, bulk moduli, cohesive energies, and heats of formation. We have obtained the density of states and charge density contour plots. Based on these results, we discuss the elastic properties, the stability of the C15 phase, and the bonding and deformation mechanisms in Laves phases. \textcopyright{} 1996 The American Physical Society.

Optical properties of the group-IVB refractory metal compounds.

We have calculated ab initio the direct interband electric dipole transitions of the carbides, nitrides, and oxides of Ti, Zr, and Hf in the rocksalt structure using the full-potential linear muffin-tin orbital method. The dipole matrix elements are calculated explicitly. Our results are in extraordinarily good agreement with experiment. The optical spectra are analyzed and we explain the origin of the different structures in the spectra in terms of the calculated electronic structure. We also discuss the trends in the optical properties as the metal and nonmetal atom types are changed and how these trends relate to general concepts such as band filling, nuclear charge, and bandwidth. \textcopyright{} 1996 The American Physical Society.

Ab-initio Band Structure Calculations and 61 Ni-Mössbauer Studies of BaNiO2 , BaNiO3 and CaNiN

The electron density distribution of the nickel compounds BaNiO2 , BaNiO3 and CaNiN has been investigated experimentally by 61Ni Mössbauer spectroscopy and theoretically by band structure calculations using the FP-LMTO (Full Potential Linear Muffin-Tin Orbital) method. For all compounds good agreement is found between the experimental and theoretical values of the electric field gradient qexp. and qtheor. at the nickel site. BaNiO2 contains nickel in a square-planar coordination forming puckered chains of edge-sharing NiO4 squares. |q| at nickel is large: qexp = - 15.7(1.5) • 1021 Vm-2 and qtheor. = - 15.59 • 1021 Vm-2 . The principal axis z is perpendicular to the NiO4 squares. The crystal structure of BaNiO3 contains face-sharing chains of NiO6 octahedra. In BaNiO3 q(Ni) is small: qexp. = ± 3.6(2.0) • 1021 Vm-2 and qtheor. = - 1.86 • 1021 Vm-2 . Because of the small broadening of the Mössbauer resonance line the sign of q could not be determined experimentally. The nitridoniccolate CaNiN contains infinite linear chains 1 ∞[NiN2/2 ] which run perpendicular to the c axis. Unexpectedly, |q(Ni)| in CaNiN is small: qexp. = 0.0 (2.0) • 1021 Vm-2 and qtheor. = - 3.05 • 1021 Vm-2. Again the sign of q(Ni) could not be determined experimentally. It is found theoretically that the small value of q(Ni) is caused by severe cancellation between σ and π contributions.

Full-potential photoemission calculations for KTaO 3

We have calculated the electronic structure of cubic KTaO 3 using the self-consistent tight-binding linear muffin-tin orbital method (TB-LMTO) as well as the full-potential linear muffin-tin orbital method (FP-LMTO). Due to the generalization of the one-step model of photoemission to the case of several atoms per unit cell and the development of the full-potential photoemission theory, a detailed study of angle-resolved ultraviolet photoemission (ARUPS) intensities is possible now. Therefore, we present calculated photoemission spectra from the (100) surface of a KTaO 3 crystal in the cubic phase by excitation with unpolarized Ne I and He I radiation. The results include a comparison of theoretical data calculated from TB-LMTO and FP-LMTO potentials with recently measured experimental spectra. The overall agreement between theory and experiment turns out to be very satisfactory.

Dynamics of the perovskite interphase boundary under pressure

The study focuses on the investigation of the hydrostatic pressure effect on the phase transition kinetics in ferroelectric KNbO 3. We discuss the interfacial kinetics within an exactly soluble mean-field model which is the time-dependent Ginzburg-Landau theory. We propose here to use this model for studying the pressure-induced motion of the interphase boundary in KNbO 3. The calculations are done on the basis of the experimental data, with some important missing parameters being calculated it ab initio in the framework of the full-potential linear muffin-tin orbital method.

Calculated spin and orbital moments in the surfaces of the 3d metals Fe, Co, and Ni and their overlayers on Cu(001).

The spin and orbital moments of the surfaces of Fe, Co, and Ni as well as of overlayers of these metals deposited on Cu have been calculated using a full-potential linear muffin-tin orbital method in a slab geometry. With one exception, Ni on Cu(001), the calculated spin moments are considerably enhanced at the surface, which is in agreement with previous theoretical and experimental work for some of the presently studied systems. We argue that the Ni-$d$---Cu-$d$ hybridization is responsible for the reduced Ni spin moment in the case of a Ni monolayer on a Cu(001) substrate. The orbital moment is enhanced at the surface for all of the studied cases, sometimes by as much as a factor of 2 relative to the value found in the bulk. Based on our theoretical calculations we propose that an experimental confirmation of an enhanced orbital magnetic moment on a surface is most likely to be found for a monolayer of Co on a Cu(001) substrate or for the Fe(001) surface layer. Experimental work on Co on Cu(001) confirm this behavior.

Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio bandstructure theory

Using a spin-polarized full-potential linear muffin-tin orbital method we present calculations of the nonlinear magneto-optical Kerr effect for thin bcc Fe films within a slab geometry. Film layer dependent contributions to the Kerr spectrum are determined. Thus, we calculate the magneto-optical Kerr spectra for the linear and nonlinear case. Our results show clearly that the Kerr spectra of thin films are characteristicly different from those at surfaces of bulk materials. In the case of linear Kerr spectra of Au/Fe(bcc)/Au(001) films our theoretical results are in good agreement with observed frequency- and thickness-dependent spectra.

First-principles calculations of the vibrational properties of H centers in KCl crystals.

The atomic and electronic structure of the radiation-induced interstitial halide atoms in KCl crystals (called H centers) is calculated using 16-atom supercells and the first-principles full-potential linear muffin-tin orbital method. The vibrational properties of the H centers are studied for the two orientations of the molecular axis of the H centers known to exist in alkali halide crystals --- along the (110) and (111) axis. Only in the former case (which is known to be energetically more favorable in KCl), the local vibrational frequency is close to that experimentally observed by means of Raman spectroscopy. A strong coupling is found between the stretching molecular mode of the H center and the breathing motion of its six nearest ${\mathrm{K}}^{+}$ cations. This is predicted to result in another local mode around 257 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. \textcopyright{} 1996 The American Physical Society.

Theoretical Study of Group-III-Nitride Alloys

ABSTRACTBand gap bowing, structural relaxations, and energies of formation were calculated for the three pseudobinary nitride zincblende alloy systems Al-Ga, In-Ga and In-Al using the full-potential linearized muffin-tin orbital method. The cluster expansion and Connolly-Williams approaches were used to relate calculated band structures and energies of formation of ordered compounds to the behavior of disordered alloys. Effects of bond length and volume variation on those properties are discussed. An interpolation formula for the gap of the full pseudoternary AlxGayInzN system is proposed and tested by separate calculations. Extension of the results to the wurtzite alloys is discussed.

Local Density Band Theory Calculations of Magnetic and Magneto- Optical Properties of Use

A study has been conducted to investigate magnetic and magneto-optical properties of USe using spin and orbitally polarized full-potential linear muffin-tin orbital (LMTO) method. USe belongs to a class of uranium compounds known as correlated electron systems. We have calculated diagonal and off-diagonal conductivity tensor elements from electronic band structure and compared them with experimental results. Band theory calculations reproduced diagonal conductivity (related to optical transitions) quite well while off-diagonal conductivity (magneto-optical transitions) agrees poorly with experiment. We mention briefly a possible improvement of band theory.

Stacking fault energy of the NbCr2laves phase

Abstract A total energy study has been performed on the NbCr2 Laves phase using first-principles electronic structure calculations based on the full-potential linear muffin-tin orbital method. For the two Laves phase structures, C15 and C14, cohesive energies and heats of formation were obtained. A method was developed to calculate the stacking fault energy, γ, in C15 Laves phases, using only two quantities: the cohesive energy difference between C15 and C14 Laves phases, and the C15 lattice constant. For C15 NbCr2, the calculated stacking fault energy is 90mJ m−2. The calculated result is in good agreement with an experimental result deduced from an extended dislocation node.

Elastic constants of hexagonal transition metals: Theory.

The five different elastic constants of all the hexagonal 4d transition metals (Y, Zr, Tc, and Ru) and the 5d transition metals Re and Os have been calculated by means of first-principles electronic-structure calculations using the full-potential linear muffin-tin orbital method. The calculated data agree with the experimental values within \ensuremath{\sim}30%. We demonstrate, using experimental data, that the hexagonal transition metals obey the Cauchy relations much better than the cubic ones. This is due to the fact that the shape of the density of states for the hexagonal materials retains its form to a larger extent, for all types of shears, than it does for the cubic metals. We introduce normalized elastic constants ${\mathrm{C}}_{\mathrm{ij}}^{\ensuremath{'}}$=${\mathrm{C}}_{\mathrm{ij}}$/B, where B is the bulk modulus, which show a regular behavior for the hexagonal transition metals, in contrast to the cubic transition metals, where large irregularities are observed. These regular as well as irregular behaviors are well reproduced by the full-potential calculations.

Electronic structure of superconductors without apical oxygen: Sr2CuO2F2, Sr2CuO2Cl2, and Ca2CuO2Cl2.

The electronic structures of Sr[sub 2]CuO[sub 2]F[sub 2], Sr[sub 2]CuO[sub 2]Cl[sub 2], and Ca[sub 2]CuO[sub 2]Cl[sub 2] are investigated by means of the full-potential linear muffin-tin orbital method. As in other high-[ital T][sub [ital c]] cuprates, the electronic structures of these compounds display strong two-dimensional features including a low density of states at [ital E][sub [ital F]] and a simple Fermi surface of the form of a rounded square. A major van Hove saddle-point singularity exists near the Fermi level. We predict the optimum superconductivity properties in these materials to occur when doped by 0.35--0.38 holes per unit cell.

Elastic Constants and Related Properties of the Group III-Nitrides

ABSTRACTThe elastic constants of the Group-III nitrides, c-BN, AlN and GaN were calculated from first-principles using the full-potential linear muffin-tin orbital method and local density approximation. The relation between the elatic constants in zincblende and wurtzite is studied by means of a tensor coordinate transformation approach. The latter combined with a correction for the internal displacement of the rotated tetrahedra is found to provide good results for the Ch11Ch12 and Ch44 but not for Ch13 and Ch33. These two require explicit calculations involving distortions along the c-axis. The calculations also provide information on the transverse optical phonons. By deriving Keating model parameters we show that BN is much stiffer against bond-angle distortions than the other nitrides.

Fermi surface of alkali metals using the full-potential linear muffin-tin orbital method and the generalized gradient approximation.

The Fermi surface (FS) of the alkali metals (Li, Na, K, Rb, and Cs) has been studied using a full-potential linear muffin-tin orbital (FPLMTO) method together with both the local-density approximation (LDA) and the generalized gradient approximation of Perdew and Wang (PW91). We demonstrate that FPLMTO calculations based on the LDA improve the FS of K, Rb, and Cs compared to LMTO calculations within the atomic-sphere approximation (ASA). Most importantly, our FPLMTO-LDA calculations reproduce all essential features of the FS of Cs, whereas the previous LMTO-ASA LDA calculations erroneously resulted in a FS that is open at the N point. The gradient-corrected FPLMTO calculations further improve the FS for K, Rb, and Cs and give results in good agreement with experiment. For Li and Na the FPLMTO calculations slightly worsen the FS, both when using the LDA and PW91.

X-ray spectra and electronic structure of Li x NbO2 superconductor and other niobium oxide compounds

X-ray emission Nb\(L_{\beta _{2,15} } \) and OKα spectra and O1s total photoelectron yield spectra for superconductor Li0.71NbO2 and other niobium oxide compounds NbO, BaNb4O6, Ba2Nb5O9, BaNb5O8. LiNbO2, NbO2, LiNbO3 were investigated. It was found that in stoichiometric LiNbO2, niobium oxidation degree is 3+. Under the lithium deintercalation down to Li0.71NbO2 composition, the niobium ions oxidation degree increases up to 3.3+. As more electrons are supplied by Nb to fill the O2p band, the degree of O2p-Nb4d hybridization is increased. Using the full-potential linear muffin-tin orbital method, the self-consistent band structure calculations of Li x NbO2 (x=1.0; 0.7; 0.5) compounds were carried out. Agreement between partial Nb4d and O2p densities of states and corresponding X-ray spectra was observed. For thex=0.7; 0.5 the Fermi level is placed on the slope of sharp density of states peak formed mostly by the Nb4d states. With the decrease ofx thep-d hybridization becomes stronger, although remains well below the values typical for the high-temperature superconductors.