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Cations that possess stereochemically active lone-pair electrons often form a diverse range of unusual structures attributable to their variable coordination geometry. It is found that a much richer structural chemistry could be exhibited when lone-pair cations combined with other transition metal (TM) cations as well as halogen ions. In addition, the asymmetric coordination environment arising from the lone-pair could increase the possibility of forming noncentrosymmetric (NCS) materials that exhibit a number of functional properties such as ferroelectricity, pyroelectricity, piezoelectricity, and second-harmonic generation (SHG). Furthermore, the combination of lone-pair cations with TM cations as well as halogen ions into the same material have been proven to be a very effective synthetic route for new nonlinear optical (NLO) materials with high SHG intensity owing to the additive polarizations from both types of asymmetric groups. With this in mind, we systematically explored new compounds containing stereoactive lone-pairs cations in the following three systems: (1) d10 cations selenium-oxyhalides; (2) Lone-pair cations-selenium-oxyhalide; (3) d10 cations / lone-pair cationsmixed-valent selenium systems. Finally, we successfully synthesized a few new compounds in the above three systems. Three new compounds, Cd2(SeO3)F2, Cd5(SeO3)4Cl2(H2O) and Cd2(SeO3)(OH)Br were obtained in d10 cations-selenium-oxyhalides system. Among them, Cd2(SeO3)F2 andCd2(SeO3)(OH)Br are centrosymmetric (CS), whereas Cd5(SeO3)4Cl2(H2O) is noncentrosymmetric (NCS) and exhibits a SHG intensity of about 0.6 times of KH2PO4 (KDP). Four new compounds, Pb3(SeO3)(HSeO3)Br3, Pb3(SeO3)(OH)Br3, CdPb8(SeO3)4Cl4Br6 and RbBi(SeO3)F2 have also been successfully synthesized in the lone-pair cations-seleniumoxyhalide system. Although all four compounds are CS and exhibit three-dimensional (3D) crystal structures, there structures are vastly different. Three compounds, Pb2(SeO3)(SeO4), Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) were achieved in d10 cations / lone-pair cations mixed valent selenium system. Pb2(SeO3)(SeO4) is CS, whereas Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) are NCS and exhibit SHG intensities of approximately 0.6 and 0.8 times of KDP, respectively. For all of the compounds, infrared spectroscopy, UV-vis diffuse reflectance spectroscopy and thermogravimetric measurements were performed.



Oxyhalides, Oxide, Structures, Property


Portions of this document appear in: Shang, Meng, Lili Liu, and P. Shiv Halasyamani. "New cadmium-selenium-oxyhalides: Noncentrosymmetric Cd5 (SeO3) 4Cl2 (H2O) and centrosymmetric Cd2 (SeO3) F2, Cd2 (SeO3)(OH) Br." Journal of Solid State Chemistry 273 (2019): 106-111; and in: Shang, Meng, and P. Shiv Halasyamani. "Mixed lone-pair and mixed anion compounds: Pb3 (SeO3)(HSeO3) Br3, Pb3 (SeO3)(OH) Br3, CdPb8 (SeO3) 4Cl4Br6 and RbBi (SeO3) F2." Journal of Solid State Chemistry 282 (2020): 121121; and in: Shang, Meng, and P. Shiv Halasyamani. "Mixed-valent selenium compounds: Noncentrosymmetric Cd3 (SeO3) 2 (SeO4) and Hg3 (SeO3) 2 (SeO4) and centrosymmetric Pb2 (SeO3)(SeO4)." Journal of Solid State Chemistry 286 (2020): 121292.