Introduction of the Cl–anion in the borate systems generates a new perovskite-like phase, K3B6O10Cl, which exhibits a large second harmonic response, about four times that of KH2PO4 (KDP), and is transparent from the deep UV (180 nm) to middle-IR region. K3B6O10Cl crystallizes in the noncentrosymmetric and rhombohedral space group R 3 m. The structure consists of the A-site hexaborate [B6O10] groups and the BX3 Cl-centered ...