The Large Second-Harmonic Generation of LiCs₂ PO₄ is caused by the Metal-Cation-Centered Groups

We evaluated the individual atom contributions to the second harmonic generation (SHG) coefficients of LiCs₂ PO₄ (LCPO) by introducing the partial response functionals on the basis of first principles calculations. The SHG response of LCPO is dominated by the metal-cation-centered groups CsO₆ and LiO₄ , not by the nonmetal-cation-centered groups PO₄ expected from the existing models and theories. The SHG coefficients of LCPO are determined mainly by the occupied orbitals O 2p and Cs 5p as well as by the unoccupied orbitals Cs 5d and Li 2p. For the SHG response of a material, the polarizable atomic orbitals of the occupied and the unoccupied states are both important.