Co-doping of (Bi(0.5)Na(0.5))TiO(3): secondary phase formation and lattice site preference of Co.

Bismuth sodium titanate (Bi(0.5)Na(0.5))TiO(3) (BNT) is considered to be one of the most promising lead-free alternatives to piezoelectric lead zirconate titanate (PZT). However, the effect of dopants on the material has so far received little attention from an atomic point of view. In this study we investigated the effects of cobalt-doping on the formation of additional phases and determined the preferred lattice site of cobalt in BNT. The latter was achieved by comparing the measured x-ray absorption near-edge structure (XANES) spectra to numerically calculated spectra of cobalt on various lattice sites in BNT. (Bi(0.5)Na(0.5))TiO(3) + x mol% Co (x = 0.0, 0.5, 1.0, 2.6) was synthesized via solid state reaction. As revealed by SEM backscattering images, a secondary phase formed in all doped specimens. Using both XRD and SEM-EDX, it was identified as Co(2)TiO(4) for dopant levels >0.5 mol%. In addition, a certain amount of cobalt was incorporated into BNT, as shown by electron probe microanalysis. This amount increased with increasing dopant levels, suggesting that an equilibrium forms together with the secondary phase. The XANES experiments revealed that cobalt occupies the octahedral B-site in the BNT perovskite lattice, substituting Ti and promoting the formation of oxygen vacancies in the material.

Agglomeration of As antisites in As-rich low-temperature GaAs: nucleation without a critical nucleus size.

To investigate the early stages of nucleation and growth of As precipitates in GaAs grown at low substrate temperature, we make use of a self-consistent-charge density-functional based tight-binding method. Since a pair of As antisites already shows a significant binding energy which increases when more As antisites are attached, there is no critical nucleus size. Provided that all excess As has precipitated, the clusters may grow in size since the binding energies increase with increasing agglomeration size. These findings close the gap between experimental investigation of point defects and the detection of nanometer-size precipitates in transmission electron microscopy.

Formation of thermal vacancies on the Si sublattice of the intermetallic compound MoSi2.

For a detailed understanding of high-temperature processes in complex solids the identification of the sublattice on which thermal defects are formed is of basic interest. Theoretical studies in intermetallic compounds favor a particular sublattice for thermal vacancy formation. In the present study we detect in ordered MoSi2 thermal vacancies with a low formation enthalpy of H(F)(V)=(1.6+/-0.1) eV, and we succeed in showing by experimental and theoretical efforts that they are preferentially formed on the Si sublattice. By these data self-diffusion in MoSi2 can be understood.