K 2 SrP 4 O 12 : a deep-UV transparent cyclophosphate as a nonlinear optical crystal

Benefiting from high transmittance of PO4 tetrahedra in the deep-ultraviolet (deep-UV) region, phosphates have been investigated as deep-UV nonlinear optical (NLO) candidates for years. So far, reported deep-UV NLO phosphates are orthophosphates and polyphosphates while cyclophosphates, important members of the phosphates family, have usually been neglected for use as deep-UV NLO crystals. Here, we first report a novel cyclotetraphosphate K2SrP4O12 (KSPO) as a deep-UV transparent NLO crystal. In its structure, the fundamental building groups are ring [P4O12]4- groups. KSPO exhibits a moderate second-harmonic generation (SHG) response that is 0.5 times that of KH2PO4 (KDP) and a short deep-ultraviolet absorption edge smaller than 200 nm.