Polar polymorphism: α- and β-KCsWP2O9 nonlinear optical materials with a strong second harmonic generation response

Abstract

To design ultraviolet (UV) nonlinear optical (NLO) materials, we focused on the combination of second-order Jahn–Teller (SOJT) cations and PO₄. After systematic investigation, we discovered two polar polymorphs of KCsWP₂O₉, which were synthesized through a conventional high temperature method in an open system. These compounds exhibit a large second harmonic generation (SHG) response (up to 16.8 × KH₂PO₄, KDP) at 1064 nm. Meanwhile, high temperature phase β-KCsWP₂O₉ possesses a short UV cutoff edge (<270 nm) despite the presence of a d⁰ element W. First-principles electronic structure analyses suggest that the strong SHG response of α- and β-KCsWP₂O₉ is attributed to non-bonding O-2p orbitals (in the VB) and strong orbital interactions of W-5d orbitals and O-2p orbitals in WO₆ (in the CB). This work may offer new guidance for the subsequent synthesis of d⁰ transition metal containing NLO crystals with a large SHG response.