Impurity Formation Energy Research Articles

Light impurity effects on the electronic structure in TiAl

By using first-principles DMol and the discrete-variational method (DVM) based on densityfunctional theory, we investigated the effect of some light impurities, H, B, C, N and O, onthe electronic structure of their corresponding different impurity-doped systems inγ-TiAl. The impurity formation energy, Mulliken occupation, bond order and charge densitydifference have been calculated to study the impurity-induced changes in the energetics andelectronic structure. According to the impurity formation energy, it is found that theimpurities energetically prefer to occupy the Ti-rich octahedron interstitial sites in the orderH<B<O<N<C. Charge transfer suggests that the effect of the impurities is localized and the bond orderas well as charge density difference results show that B, C and N are beneficial for theductility of TiAl, while H and O are not.

Simultaneously B- and P-doped silicon nanoclusters: Formation energies and electronic properties

The effects of B and P codoping on the impurity formation energies and electronic properties of Si nanocrystals (Si-nc) are calculated by a first-principles method. We show that, if carriers in the Si-nc are perfectly compensated by simultaneous doping with n- and p-type impurities, the Si-nc undergo a minor structural distortion around the impurities and that the formation energies are always smaller than those for the corresponding single-doped cases. The band gap of the codoped Si-nc is strongly reduced with respect to the gap of the pure ones showing the possibility of an impurity based engineering of the photoluminescence properties of Si-nc.

Manganese Impurity in Boron Nitride and Gallium Nitride

We carried out a theoretical investigation on the properties of manganese impurity centers in cubic boron and gallium nitrides. The calculations were performed using the all electron spin-polarized full-potential linearized augmented plane wave methodology. Our results indicate that manganese in boron nitride, in a neutral charge state, is energetically more favorable in a divacancy site as compared to a substitutional cation site. We present the results on stability, spin states, impurity magnetic moment, hyperfine parameters, and formation and transition energies of manganese at the divacancy site in several charge states.

Electronic structure of edge dislocation of core-doped Ti in Fe*

The electronic structure of an edge dislocation doped Ti lying in the (001) plane with Burgers Vector along [ 100] direction in body-centered cubic iron is investigated using the first principles discrete variational method (DVM) based on the density-functional theory. The binding energy, impurity formation energy, interatomic energy, Mulliken orbital populations and charge density difference are presented in this paper. By calculating the binding energy of the clean dislocation system and the Ti-doped system, it is found that the binding energy of Ti-doped dislocation system is lower than that of the clean dislocation system, which implies that the Ti-doped dislocation system is more stable than the clean dislocation system.. The calculated result of the impurity formation energy predicts the trapping effect of dislocation core for Ti, which shows that Ti atom prefers to occupy the place at the dislocation core. The calculated results of the interatomic energy and the difference charge density of dislocation doped Ti system indicate that the stronger bonding formed between the Ti impurity and its neighbor Fe atoms will affect the mechanical property of edge dislocation. Considering the influence of Ti on the electronic structure and the energies, we can predict that the trace Ti in transition metal Fe with dislocation defect can give a significant contribution to the solid solution hardening effects and will influence the mechanical property of materials.

The preferred-site tendency of alloying element Nb in Fe ? phase and its effect on grain boundary cohesion

The discrete variational (DV) and DMol methods within the framework of density functional theory are used to study the effect of alloying element Nb on Fe γ phase. The impurity formation energy in bulk and segregation energies at grain boundary and free surface are calculated. The results show that Nb prefers to segregate at grain boundary. The difference in segregation energies between the grain boundary and the corresponding free surface is −0.39 eV for solute Nb. According to Rice-Wang model, it can be predicted that Nb can enhance grain boundary cohesion. The calculated results of interatomic energy and charge density show that charge would be redistributed, and the bonds across grain boundary are strengthened by the substitution of Nb for Fe. As a result, it is difficult for the grain boundary to move. Thus the dragging effect of Nb is explained electronically.

First-principles investigation of hydrogen embrittlement in polycrystallineNi3Al

The discrete-variational method within the framework of density-functional theory is used to study the effect of hydrogen on the embrittlement of polycrystalline ${\mathrm{Ni}}_{3}\mathrm{Al}$. The calculated results of impurity formation energy show that hydrogen atoms prefer to segregate at the grain boundaries, and occupy the Ni-rich interstitial holes in the polycrystalline ${\mathrm{Ni}}_{3}\mathrm{Al}$. The equilibrium positions of H atoms are near a certain atom instead of the center of these holes. The calculated results of interatomic energies reveal that the bonding strength of host atoms, which are near hydrogen atoms, are reduced evidently by the presence of hydrogen. The overall effect of H is to decrease the cohesive strength of ${\mathrm{Ni}}_{3}\mathrm{Al}$ grain boundaries and make intergranular fracture easier.

Electronic effects of oxygen and vanadium impurities in TiAl

The electronic mechanism of oxygen embrittlement of TiAl and the effect of vanadium on the ductility of TiAl have been investigated by the first-principles LDF DV embedded cluster method. According to the impurity formation energy, oxygen energetically prefers to occupy the Ti-rich octahedral interstitial site in TiAl compared with hydrogen. When O impurity is added to pure TiAl, a strong Ti - O bond forms due to Ti d/O p hybridization while no Al - O bond can be observed. The bonding character between Ti atoms changes from type to type. In V-doped TiAl, V greatly enhanced the intraplanar and interplanar bonds. The local environmental total bond order (LTBO) provides a reasonable description of the cohesive properties of the local impurity environment. The O impurity reduces the LTBO and partial LTBO of the pure TiAl, while V improves them greatly. Therefore, O should be regarded as an embrittler, and V as a ductilizer.

Formation energy and electronic structure of silicon impurities in diamond

Formation energy and electronic structure of silicon impurities in diamond

First-Principles Investigations of Acceptors in ZnSe

ABSTRACTWe present a theoretical formalism to investigate doping efficiencies of impurities in semiconductors, and show results for various acceptor impurities (Li, Na and N) in ZnSe. These results, obtained from first-principles calculations, yield important insights in the mechanisms that govern impurity formation energies and solubilities.

Total-energy and pressure calculations for random substitutional alloys.

We present the details and the derivation of density-functional-based expressions for the total energy and pressure for random substitutional alloys (RSA) using the Korringa-Kohn-Rostoker Green's-function approach in combination with the coherent-potential approximation (CPA) to treat the configurational averaging. This includes algebraic cancellation of various electronic core contributions to the total energy and pressure, as in ordered-solid muffin-tin-potential calculations. Thus, within the CPA, total-energy and pressure calculations for RSA have the same foundation and have been found to have the same accuracy as those obtained in similar calculations for ordered solids. Results of our calculations for the impurity formation energy, and for the bulk moduli, the lattice parameters, and the energy of mixing as a function of concentration in fcc Cu{sub {ital c}}Zn{sub 1{minus}{ital c}} alloys show that this generalized density-functional theory will be useful in studying alloy phase stability.