M I M II P 3 O 9 (M I = Rb, M II = Cd, Mg, Ca; M I = Cs, M II = Pb, Sr; M I = K, M II = Mg): Cation Substitution Application in Cyclophosphate Family and Nonlinear Optical Properties.

By the application of cation substitution, five new members of cyclophosphates, RbCdP3 O9 , CsPbP3 O9 , CsSrP3 O9 , RbMgP3 O9 , and RbCaP3 O9 , were obtained by a high-temperature melt method and structurally analyzed. The five compounds have identical stoichiometries, and all of them feature a three-dimensional network, which consists of MII O6 (MII = Cd, Mg, Ca, Pb, Sr) octahedra and cyclic P3 O9 units, while alkali-metal atoms are located within the network. However, RbCdP3 O9 and CsPbP3 O9 belong to the asymmetric space groups P6̅ c2 and Pna21 , respectively, and the other three compounds belong to orthorhombic space group Pnma. Detailed structure comparisons in the MI -MII -P-O and (MI MII P3 O9 ) n ( n = 1, 2, 6) systems are discussed. Remarkably, KMgP3 O9 as one of the cyclophosphate members exhibits a second harmonic generation (SHG) intensity about 0.2 times that of KH2 PO4 (KDP), RbCdP3 O9 , and CsPbP3 O9 also show SHG responses 0.1 times that of KDP. In addition, the related optical properties are discussed. Furthermore, theoretical calculations and dipole moments were calculated to explain the relation of structure and optical properties.