2019 Volume 127 Issue 11 Pages 793-801
A first-principles energy band calculation is performed with respect to the V5+- and (Ca2+, V5+)-doped Y2Ti2O7 supercells to elucidate the effect of Ca2+ doping on the electronic structure and optical properties of a V5+-doped Y2Ti2O7 pigment in the present study. The structural optimization calculation reveals that the theoretical lattice constant of the Y2Ti2O7 unit cell slightly increases when compared with that in the experimental data. The forbidden gap at the Γ point is estimated to be 2.78 eV. On the basis of the density-of-states analysis, the valence band (VB) of Y2Ti2O7 mainly comprises the O 2p states and hybridizes with the Ti 3d and Y 4d states. The conduction band (CB) can be divided into two energy regions. The lower CB comprises the Ti 3d states and hybridizes with the O 2p states, whereas the upper CB comprises the Y 4d, Ti 3d, and O 2p states. When Y2Ti2O7 is doped with a V atom, the VB width and bandgap are observed to expand by 0.7 and 0.2 eV, respectively, with respect to the pristine Y2Ti2O7. Two strongly localized peaks, corresponding to the V 3d states, appear in the bandgap. Further, three strongly localized peaks appear in the bandgap when V5+-doped Y2Ti2O7 is doped with a Ca atom. In the dielectric function calculation of the V5+-doped Y2Ti2O7, there is broad absorption from the O 2p VB states to the V 3d gap states as well as a VB–CB optical transition in the host crystal. When the V5+-doped Y2Ti2O7 is doped with a Ca atom, the distortion of the VO6 octahedron becomes large, leading to an increment of O 2p state densities near the valence-band maximum of the host. Thus, it is considered the momentum matrix elements between occupied states (O 2p states) and unoccupied states (V 3d states) becomes large in comparison with the case before a Ca doping.