First-principles study of transition metal impurities in Si

By using ab initio electronic structure calculations within density functional theory, we study the structural, electronic, and magnetic properties of Si doped with a transition metal impurity. We consider the transition metals of the 3d series V, Cr, Mn, Fe, Co, and Ni. To get insight into the level filling mechanism and the magnetization saturation, we first investigate the transition metal-Si alloys in the zinc-blende structure. Next, we investigate the doping of bulk Si with a transition metal atom, in which it occupies the substitutional site, the interstitial site with tetrahedral symmetry, and the interstitial site with hexagonal symmetry. It is found that all of these transition metal impurities prefer an interstitial position in Si. Furthermore, we show that it is possible to interpret the electronic and magnetic properties by using a simple level filling picture and a comparison is made to Ge doped with the same transition metal atoms. In order to get insight into the effect of a strained environment, we calculate the formation energy as a function of an applied homogeneous pressure and we show that an applied pressure can stabilize the substitutional position of transition metal impurities in Si. Finally, the energies of the ferromagnetic states are compared to those of the antiferromagnetic states. It is shown that the interstitial site of the Mn dopant helps us to stabilize the nearest neighbor substitutional site to realize the ferromagnetic state. For doping of Si with Cr, a ferrimagnetic behavior is predicted.

Density-functional study of lanthanum, ytterbium, and lutetium dimers

La-2, Yb-2, and Lu-2 have been studied by use of the density-functional methods B3LYP, BLYP, B3PW91, BHLYP, BP86, B3P86, MPW1PW91, and PBE1PBE. In these density-functional methods, the exchange functional is from either Becke's three-parameter HF-DFT hybrid exchange functional (B3), pure DFT exchange functional of 1988 (B), a modification of the half-and-half HF/DFT hybrid method (BH), Perdew-Wang 1991 (PW91), or Barone's modified PW91 (MPW1), while the correlation functional is from either Lee, Yang, and Parr (LYP), Perdew-Wang 1991 (PW91), or Perdew 86 (P86). PBE1PBE is the generalized-gradient-approximation exchange-correlation functional of Perdew, Burke, and Ernzerhof. For La-2, the calculated bond distance is in reasonable agreement with the experiment, but the calculated vibrational frequency is underestimated significantly compared with the experiment. Only BP86 and B3P86 have the best performance in reproducing the experimental dissociation energy for La-2. For the van der Waals dimer Yb-2, three functionals, B3LYP, BLYP, and BHLYP have excellent performance in reproducing the spectroscopic constants compared with both the experiment and previous theoretical studies.

Ab initio study on the physical properties of CoN3 and RhN3 with skutterudite structure

The elastic and electronic properties of hypothetical CoN3 and RhN3 with cubic skutterudite structure were studied by first principles calculations based on density functional theory. By choosing different initial geometries, two local minima or modifications were located on the potential energy surface, termed as modifications I and II. Both compounds are mechanically stable. For each compound, modification I is lower in energy than II. Thermodynamically stable phases can be achieved by applying pressures. Modification II is lower in energy than I at above 50 GPa for both compounds.

Ab initio study on the electronic and mechanical properties of ReB and ReC

The structural, electronic, and mechanical properties of ReB and ReC have been studied by use of the density functional theory. For each compound, six structures are considered, i.e., hexagonal WC, NiAs, wurtzite, cubic NaCl, CsCl, and zinc-blende type structures. The results indicate that for ReB and ReC, WC type structure is energetically the most stable among the considered structures, followed by NiAs type structure. ReB-WC (i.e., ReB in WC type structure) and ReB-NiAs are both thermodynamically and mechanically stable. ReC-WC and ReC-NiAs are mechanically stable and becomes thermodynamically stable above 35 and 55 GPa, respectively. The estimated hardness from shear modulus is 34 GPa for ReB-WC, 28GPa for ReB-NiAs, 35GPa for ReC-WC and 37GPa for ReC-NiAs, indicating that they are potential candidates to be ultra-incompressible and hard materials.

Ab Initio Study Of Zno-Based Gas-Sensing Mechanisms: Surface Reconstruction And Charge Transfer

Density functional theory (DFT) calculations were employed to explore the gas-sensing mechanisms of zinc oxide (ZnO) with surface reconstruction taken into consideration. Mix-terminated (10 (1) over bar0) ZnO surfaces were examined. By simulating the adsorption process of various gases, i.e., H-2, NH3, CO, and ethanol (C2H5OH) gases, on the ZnO (10 (1) over bar0) surface, the changes of configuration and electronic structure were compared. Based on these calculations, two gas-sensing mechanisms were proposed and revealed that both surface reconstruction and charge transfer result in a change of electronic conductance of ZnO. Also, the calculations were compared with existing experiments.

Ab initio calculations for the neutral and charged O vacancy in sapphire

The energetics, lattice relaxation, and the defect-induced states of st single O vacancy in alpha-Al2O3 are studied by means of supercell total-energy calculations using a first-principles method based on density-functional theory. The supercell model with 120 atoms in a hexagonal lattice is sufficiently large to give realistic results for an isolated single vacancy (square). Self-consistent calculations are performed for each assumed configuration of lattice relaxation involving the nearest-neighbor Al atoms and the next-nearest-neighbor O atoms of the vacancy site. Total-energy data thus accumulated are used to construct an energy hypersurface. A theoretical zero-temperature vacancy formation energy of 5.83 eV is obtained. Our results show a large relaxation of Al (O) atoms away from the vacancy site by about 16% (8%) of the original Al-square (O-square) distances. The relaxation of the neighboring Al atoms has a much weaker energy dependence than the O atoms. The O vacancy introduces a deep and doubly occupied defect level, or an F center in the gap, and three unoccupied defect levels near the conduction band edge, the positions of the latter are sensitive to the degree of relaxation. The defect state wave functions are found to be not so localized, but extend up to the boundary of the supercell. Defect-induced levels are also found in the valence-band region below the O 2s and the O 2p bands. Also investigated is the case of a singly occupied defect level (an F+ center). This is done by reducing both the total number of electrons in the supercell and the background positive charge by one electron in the self-consistent electronic structure calculations. The optical transitions between the occupied and excited states of the: F and F+ centers are also investigated and found to be anisotropic in agreement with optical data.

Ab initio study on the electronic, magnetic, and mechanical properties of CaCu3V4O12

The electronic, magnetic, and mechanical properties of CaCu3V4O12 are investigated by use of the density functional theory method. The calculated results indicate that CaCu3V4O12 is a half-metallic and ferrimagnetic compound. The magnetic coupling for Cu-V is antiferromagnetic, while those for Cu-Cu and V-V are ferromagnetic. The obtained elastic constants suggest that the compound is mechanically stable. The calculated oxidation states and density of states reveal the existence of a mixed valence for Cu and V. This supports the experimental observation of the mixed valence in Ca2+Cu2+Cu2+(V25+V24+)O-12.

Trends in elasticity and electronic structure of 5d transition metal diborides: first-principles calculations

We investigate the cohesive energy, heat of formation, elastic constant and electronic band structure of transition metal diborides TMB2 (TM = Hf, Ta, W, Re, Os and Ir, Pt) in the Pmmn space group using the ab initio pseudopotential total energy method. Our calculations indicate that there is a relationship between elastic constant and valence electron concentration (VEC): the bulk modulus and shear modulus achieve their maximum when the VEC is in the range of 6.8-7.2. In addition, trends in the elastic constant are well explained in terms of electronic band structure analysis, e.g., occupation of valence electrons in states near the Fermi level, which determines the cohesive energy and elastic properties. The maximum in bulk modulus and shear modulus is attributed to the nearly complete filling of TM d-B p bonding states without filling the antibonding states. On the basis of the observed relationship, we predict that alloying W and Re in the orthorhombic structure OsB2 might be harder than alloying the Ir element. Indeed, the further calculations confirmed this expectation.

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally. (c) 2005 American Institute of Physics.

Ab initio investigation of the elasticity and stability of aluminium

On the basis of the pseudopotential plane-wave (PP-PW) method in combination with the local density functional theory (LDFT), complete stress-strain curves for the uniaxial loading and uniaxial deformation along the [001] and [111] directions, and the biaxial proportional extension along [010] and [001] for aluminium are obtained. During the uniaxial loading, certain general behaviours of the energy versus the stretch and the load versus the stretch are confirmed; in each case, there exist three special unstressed structures: f.c.c., b.c.c., and f.c.t. for [001]; f.c.c., s.c., and b.c.c. for [111]. Using stability criteria, we find that all of these states are unstable, and always occur together with shear instability, except the natural f.c.c. structure. A Pain transformation from the stable f.c.c. structure to the stable b.c.c. configuration cannot be obtained by uniaxial compression along any equivalent [001] and [111] direction. The tensile strengths are similar for the two directions. For the higher energy barrier of the [111] direction, the compressive strength is greater than that for the [001] direction. With increase in the ratio of the biaxial proportional extension, the stress and tensile strength increase; however, the critical strain does not change significantly. Our results add to the existing ab initio database for use in fitting and testing interatomic potentials.