Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO 3
- PDF / 2,192,891 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 102 Downloads / 180 Views
37 Mat. Res. Soc. Symp. Proc. Vol. 586 @2000 Materials Research Society
*e.0
•
O
•o-
*Sr
@0
OTi
0
Figure 1.Z-contrast STEM image of a 15 [001] grain boundary, which shows both titanium and strontium dislocation cores.
EXPERIMENT The geometric structure of SrTiO 3 grain boundaries has been obtained from Z-contrast STEM images[7-91. The dark field image obtained from a high angle annular detector is incoherent, so the image is described as convolution of the atomic scattering cross section with the STEM probe profile, which allows direct location of atomic columns from the image without extra simulations. The intensity of each atomic column is directly related to the atomic number, so it is possible to deduce both structure and composition. The ability to distinguish Sr from Ti sites is especially useful in the case of interface or grain boundary structures. Figure 1 shows a Z-contrast image from a 15° tilt grain boundary in SrTiO 3 obtained with the VG Microscopes HB603U STEM which has a probe size of 1.3 A. It resolves the dislocation core structures comprising the boundary, showing the reconstructions which are typical of all grain boundaries so far observed in this material. As shown in the schematic, the two atomic columns in the centers of each dislocation core are spaced only 2.5 A apart. Ionic repulsion clearly excludes the possibility of both sites being simultaneously occupied in any single atomic layer normal to the tilt axis. Therefore, these two sites are alternate sites for cation occupation, and the core is reconstructed. While the high-angle scattered electrons form a dark field image, the small-angle scattered electrons are passed through a spectrometer to provide an electron energy loss spectrum (EELS). EELS provides complementary information for distinguishing atomic columns in addition to Zcontrast image, especially light atoms which are not detected in Z-contrast images. EELS is also sensitive to changes in electronic structure or valence induced by changes in local coordination [10].
38
00 00
*•
-"GB plane
Sr
coreobseGBe1d
i
S
a
• 0 9•
0ri half occupied Sr •
•
O• O
half occupied Ti
Figure 2. The relaxed supercell with two 53' symmetric {210} grain boundaries and dislocation cores observed in SrMi3 grain boundaries.
CALCULATIONS We performed self-consistent ab-initio density functional calculations using density functional theory, the local-density approximation for exchange-correlation, pseudopotential and plane waves. Supercells are set up containing two grain boundaries in order to allow periodic boundary condition. Ultrasoft pseudopotentials were used with a plane wave energy cutoff of 380 eV, and the Monkhorst-Pack scheme was used for Brillouin zone sampling[ 11]. In titanium, 3s and 3p states were treated as semicore states, and the strontium pseudopotential included 4s and 4p electrons. The all band mixing scheme is used for electronic minimization, and the ionic relaxation used the Broyden-Fletcher-Goldfarb-Shanno scheme[12]. The exchange-correlation potential
