Surface Optical Band Gap Research Articles

Using a supercell model and B3PW hybrid exchange-correlation functional in the framework of the density functional theory (DFT), as it is implemented in the CRYSTAL computer code, we performed ab initio calculations for the F-center located in the SrZrO3 bulk and on the ZrO2-terminated (001) surface. According to the results of performed relaxation of atoms around the defect, two nearest Zr and four Sr atoms are repulsed, but all oxygen atoms are attracted towards both, the bulk and (001) surface F-center. The displacement magnitudes of atoms surrounding the bulk F-center are smaller than around the (001) surface F-center. The B3PW calculated SrZrO3 bulk optical band gap (5.00 eV) is in a fair agreement with the experimental value of 5.6 eV. The ZrO2-terminated (001) surface optical band gap (4.63 eV) is smaller in regard to the bulk value. The bulk and (001) surface F-center bands in SrZrO3, calculated using 3 × 3 × 3 and 3 × 3 × 1 times expanded supercells, are located 1.12 and 0.93 eV under the conduction band bottom. The O vacancy in the SrZrO3 bulk attracts 1.25e, and even less, only 1.10e on the ZrO2-terminated (001) surface. Performed calculations reveal significant enhancement of the chemical bond covalency among the SrZrO3 bulk F-center and its nearest Zr atoms of 0.244e in comparison with the ideal SrZrO3 perovskite ZrO chemical bond covalency of 0.100e. Our detected defect formation energy for the F-center located on the ZrO2-terminated (001) surface 7.52 eV is somewhat smaller than in the SrZrO3 bulk 7.55 eV. Our calculated formation energy difference may trigger the segregation of the F-center from the SrZrO3 bulk towards the ZrO2-terminated (001) surface.