First-principles Study Of Native Defects Research Articles

First-principles study of native defects in bulk Sm2CuO4 and its (001) surface structure

Using the first-principles calculations based on the density functional theory, we have studied the bulk defect formation and surface structures of Sm2CuO4. To ensure the accuracy of calculations, the spin order of Cu atoms is rechecked and it is the well-known nearest-neighbor antiferromagnetic ground state, which can be attributed to the hole-mediated superexchange through the strong pdσ hybridization interaction between Cu dx2-y2 electron and the neighboring oxygen px (or py) electron. Under each present experimental condition, the Sm vacancy has a very high formation energy and is unlikely to be stable. The Cu vacancy is a shallow acceptor, which is preferred under O-rich conditions, whereas the O vacancy is a donor and energetically favorable under O-poor conditions. To construct its (001) surface structure, CuOO, CuO, and Cu terminated surfaces are found to be most favorable under different experimental conditions. The stable surface structures are always accompanied by significant surface atomic reconstructions and electron charge redistribution, which are intimately correlated to each other.

First-principles study of native defects in LiTi2O4

In this study, we report on a fundamental first-principles investigation of native point defects in spinel LiTi2O4, an alternative electrode material for lithium ion batteries (LIBs). Under O-rich conditions, Ti vacancy (VTi) and Li antisite (LiTi) are energetically favorable, but Ti interstitial (Tii) has highest formation energy and is hence unstable. While Li interstitial (Lii) is the most stable configuration in O-poor conditions, and O vacancy (VO), Ti interstitial (Tii), Li antisite (LiTi) and Ti antisite (TiLi) have almost the same formation energies, suggesting spontaneous formation of defect complexes. According to the calculated electronic structures of defects, no localized states are found in the gap between the O 2p and Ti t2g states, which is considerable related to the charge compensation process between O and Ti ions. Besides, Li vacancy (VLi) and Li interstitial (Lii) are the most stable defect types with negligible structural changes, while other defects will induce large octahedral disorders and suppress Li ions mobility. These results not only reveal the original defect information of LiTi2O4, but also provide practical guidance to the development of this material.

First-principles study of native defects in TlBr: Carrier trapping, compensation, and polarization phemomenon

First-principles calculations are carried out to study the native defect properties in TlBr. Three important results emerge: (1) the native defects are benign in terms of electron trapping because the low-energy defects do not induce electron traps; (2) the dominant defects in nearly stoichiometric TlBr are Schottky defects that pin the Fermi level near the midgap, leading to high resistivity; and (3) the calculated low diffusion barriers for several native defects show that ionic conductivity can occur at room temperature. The important impacts of these material properties on the room-temperature radiation detection using TlBr are discussed.

First-principles study of the formation and migration of native defects inNaAlH4

We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $V_{\rm{Na}}^-$ and $V_{\rm{H}}^+$, may be responsible for diffusion necessary for desorption. We discuss the possible role of $V_{\rm{H}}^+$-H$_i^-$ complex formation. The values we find are in the range of activation energies reported for catalyzed desorption.

First-principles study of native defects inCdGeAs2

First-principles results are presented for various native defects in ${\text{CdGeAs}}_{2}$ as function of the relevant elements' chemical potentials. The defect formation energies were calculated using fully relaxed 64 atom supercells by means of the full-potential linearized muffin-tin orbital implementation of the density-functional theory in the local-density approximation (LDA). The LDA band gap is adjusted using the $\text{LDA}+U$ approach by introducing a semiempirical orbital dependent $U$ shift to the $s$ orbitals of Cd and Ge and the $d$ orbitals of Cd. The transition energies of the vacancies ${V}_{\text{Cd}}$, ${V}_{\text{Ge}}$, and ${V}_{\text{As}}$, and antisites ${\text{Ge}}_{\text{Cd}}$, ${\text{Cd}}_{\text{Ge}}$, ${\text{Ge}}_{\text{As}}$, and ${\text{As}}_{\text{Ge}}$ are calculated. Defect levels are interpreted in a simple-molecular orbital theory picture and the relation between Kohn-Sham band structures and transition levels is discussed. The vacancies are generally found to have higher energy of formation than the antisites. In particular, the somewhat deeper acceptor ${V}_{\text{Ge}}$ is found to have the highest energy of formation among the defects studied. Among the three shallow acceptors $({V}_{\text{Cd}},\text{ }{\text{Cd}}_{\text{Ge}},\text{ }{\text{Ge}}_{\text{As}})$, the lowest energy of formation is found for ${\text{Cd}}_{\text{Ge}}$, but only the ${\text{Ge}}_{\text{As}}$ antisite is expected to be active in electron paramagnetic resonance (EPR). This is consistent with experimental data, establishing a link between the EPR-active center and the shallow acceptor responsible for optical absorption. Both ${\text{Ge}}_{\text{Cd}}$ and ${\text{As}}_{\text{Ge}}$ are found to be deep donors.

First-principles study of native defects in rutile TiO2

Native point defects in the rutile TiO2 are studied via first-principles pseudopotential calculations. Except for the two antisite defects, all the native point defects have low formation energies. Under the Ti-rich growth condition, high concentrations of titanium interstitials and oxygen vacancies would form spontaneously in p-type samples; whereas high concentrations of titanium vacancies would form spontaneously in n-type samples regardless of the oxygen partial pressure.