Crystal Structure, Second Harmonic Generation, and Vibrational Spectroscopy of K2Mg2(SCN)6·3H2O

Single crystals of K2Mg2(SCN)6·3H2O were obtained by the reaction of the binary compounds MgCl2·xH2O and KSCN at 220 using the Bridgman technique. The compound crystallizes hexagonally with the noncentrosymmetric space group P63cm (Z = 2, a = 885.2(1) pm, c = 1732.7 (3) pm, R1all = 0.1035). In the crystal structure, the Mg2+ ions are coordinated by oxygen and nitrogen atoms while the K+ ions are exclusively surrounded by sulfur atoms. The two crystallographically different Mg2+ ions are linked via common nitrogen atoms to dimers, [Mg2(SCN)6(H2O)3]2-. The K+ ions are connected with each other by two crystallographically different sulfur atoms to hexagonal rings which form layers in the (001) plane. The dimers are connected to these layers in equal orientation leading to the absence of inversion symmetry. Frequency doubling measurements on K2Mg2(SCN)6·3H2O as well as on KDP powder samples were carried out. The second harmonic generation intensity of the rhodanide is about 40% of that of KDP. Furthermore, IR and Raman measurements were performed, and the results were compared to other thiocyanates. The energies of the vibrations of the molecular SCNions demonstrate their bridging function in the crystal structure, and the energies and the shapes of the H2O bands indicate hydrogen bonds