Explorations of Second-Order Nonlinear Optical Materials in the Alkaline Earth Barbiturate System: Noncentrosymmetric Ca(H3C4N2O3)2·H2O and Centrosymmetric Sr(H5C8N4O5)2·4H2O

Two new alkaline earth barbiturates, Ca(H 3 C 4 N 2 O 3 ) 2 ·H 2 O and Sr(H 5 C 8 N 4 O 5 ) 2 ·4H 2 O, were synthesized via the mild hydrothermal technique. For Ca(H 3 C 4 N 2 O 3 ) 2 ·H 2 O, the stacking (H 3 C 4 N 2 O 3 ) − anions along the c axis are interconnected by CaO 7 polydedra forming a three-dimensional structure. Also, for Sr(H 5 C 8 N 4 O 5 ) 2 ·4H 2 O, it has two-dimensional layers composed by SrO 7 polyhedra and (H 5 C 8 N 4 O 5 ) − anions. The (H 5 C 8 N 4 O 5 ) − anions can be seen as two (H 3 C 4 N 2 O 3 ) − anions connected each other via the N–C bond. Powder second-harmonic generation (SHG) measurements revealed that Ca(H 3 C 4 N 2 O 3 ) 2 ·H 2 O is a phase-matchable material with a moderate SHG response (ca. 1.15× that of KDP). Furthermore, the birefringence values of the two crystals were measured as 0.49 and 0.475 at 546.1 nm, respectively. The theoretical calculations showed that the SHG response and large birefringence were primarily caused by the (H 3 C 4 N 2 O 3 ) − and (H 5 C 8 N 4 O 5 ) − groups.