Alignment of Polar Moieties Leading to Strong Second Harmonic Response in KCsMoP₂O₉

The combination of the MoO₆ octahedra and PO₄ tetrahedra generates a new nonlinear optical crystal, KCsMoP₂O₉. To the best of our acknowledge, it exhibits the strongest powder second harmonic generation (SHG) response among the reported molybdates, phosphates, and molybdophosphates, 27 times that of KDP at 1064 nm. Meanwhile, it possesses a shorter UV cutoff edge (about 260 nm) than other molybdates. First-principles electronic structure analyses suggest that the strong SHG response originates from constructive alignment of O-2p orbitals in the occupied states and vibrant interaction of Mo–O orbitals in the unoccupied states.