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

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.