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)center dot H2O and Sr(H5C8N4O5)(2)center dot 4H(2)O, were synthesized via the mild hydrothermal technique. For Ca(H3C4N2O3)(2)center dot H2O, the stacking (H3C4N2O3)(-) anions along the c axis are interconnected by CaO7 polydedra forming a three-dimensional structure. Also, for Sr(H5C8N4O5)(2)center dot 4H(2)O, 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)center dot H2O is a phase-matchable material with a moderate SHG response (ca. 1.15x 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.