First-principles study of crystal structure, electronic structure, and second-harmonic generation in a polar double perovskite Bi2ZnTiO6

Within the density functional theory with the generalized gradient approximation, we present a systematic ab initio investigation of crystal structure, electronic structure, and linear and nonlinear optical responses in a polar double perovskite Bi2ZnTiO6. The effect of B-site ordering is explored by comparing three possible configurations: A-type with alternative Zn and Ti layers stacking along the c axis; C-type with Zn and Ti c axis chains; and G-type with every Zn(Ti) atoms is surrounded by its nearby six Ti(Zn) atoms. It is found that the system with G-type B-site ordering is energetically favorable, which is lower in the total energies of 0.055 and 0.133 eV/formula unit than C-type and A-type, respectively. Optical calculations indicate that all the three configurations show large second-harmonic generation (SHG) coefficients, and the largest static SHG observed in the C-type system reaches 123 (10(-9) esu), the value of which is much larger than ever known polar oxides, e.g., 72 (10(-9) esu) in LiNbO3. The predicted significant nonlinear optical properties are consistent with the calculated high tetragonality as well as the large off-center displacement of Zn, Ti, and Bi atoms. In particular, a large off-center displacement greater than 0.5 A in Zn atoms is revealed for the first time. A further microscopic picture is presented via the successful connection of the prominent feature of SHG in Bi2ZnTiO6 with that of the linear optical dielectric function in terms of single-photon and double-photon resonances. Our calculations demonstrate the promising application of Bi2ZnTiO6 in optoelectronics.