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(H3C4N2O3)2·H2O and Sr(H5C8N4O5)2·4H2O, were synthesized via the mild hydrothermal technique. For Ca(H3C4N2O3)2·H2O, the stacking (H3C4N2O3)- anions along the c axis are interconnected by CaO7 polydedra forming a three-dimensional structure. Also, for Sr(H5C8N4O5)2·4H2O, it has two-dimensional layers composed by SrO7 polyhedra and (H5C8N4O5)- anions. The (H5C8N4O5)- anions can be seen as two (H3C4N2O3)- anions connected each other via the N-C bond. Powder second-harmonic generation (SHG) measurements revealed that Ca(H3C4N2O3)2·H2O 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 (H3C4N2O3)- and (H5C8N4O5)- groups.