Stability Of Defect Clusters Research Articles

Defect complexes in congruentLiNbO3and their optical signatures

The structure and stability of defect clusters in ${\mathrm{LiNbO}}_{3}$, as well as their influence on the linear and nonlinear optical susceptibilities, are calculated within density functional theory (DFT) using semilocal and hybrid exchange-correlation functionals. In particular, the complexes modeling the Li shortage during the crystal growth, the Li-vacancy model and the Nb-vacancy model, are examined in detail. It is found that clustering significantly decreases the formation energies of all considered defects with respect to the dilute limit. The Li-vacancy model is energetically preferred with respect to the total formation energy, while the Nb-vacancy model has the lowest formation energy per single point defect. The independent-particle approximation based on the hybrid DFT electronic structure describes the ${\mathrm{LiNbO}}_{3}$ optical response much better than semilocal DFT. A further improvement between the calculated optical absorption and second-harmonic generation spectra with experiment is achieved if the calculations take defect complexes into account. Nb antisite polarons give rise to optical absorption within the band gap.

Nucleation and stability of defect clusters: New energetic model in the case of substituted wustites

Abstract For wustite Fe1−z O (z < 0.08) an energetic model accounts for the stability of cubic defect clusters (m/n) which are partly ordered in the crystal. The Gibbs energy GT (N) associated with clusters, including their distorted envelope, is expressed as a sum of a volume term in N 3 and of surface terms in N 4; N is the number of bonds characteristic of the cluster size. In the case of a (10/4) type cluster, this energy is negative and minimum for Nm ranging between 4 and 5, when the volume and surface energies range between specific values. Using simple assumptions, a volume energy −0.80 eV per vacancy is found in accordance with the value of stabilization energy calculated by theorists for the (10/4) cluster. The substitution of Fe2+ by Ca2+ should lead to a decrease of cluster size; this has been recently suggested by neutron diffraction studies.