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

By the application of cation substitution, five new members of cyclophosphates, RbCdP<sub>3</sub>O<sub>9</sub>, CsPbP<sub>3</sub>O<sub>9</sub>, CsSrP<sub>3</sub>O<sub>9</sub>, RbMgP<sub>3</sub>O<sub>9</sub>, and RbCaP<sub>3</sub>O<sub>9</sub>, 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 M<sup>II</sup>O<sub>6</sub> (M<sup>II</sup> = Cd, Mg, Ca, Pb, Sr) octahedra and cyclic P<sub>3</sub>O<sub>9</sub> units, while alkali-metal atoms are located within the network. However, RbCdP<sub>3</sub>O<sub>9</sub> and CsPbP<sub>3</sub>O<sub>9</sub> belong to the asymmetric space groups <i>P</i>6̅<i>c</i>2 and <i>Pna</i>2<sub>1</sub>, respectively, and the other three compounds belong to orthorhombic space group <i>Pnma</i>. Detailed structure comparisons in the M<sup>I</sup>-M<sup>II</sup>-P-O and (M<sup>I</sup>M<sup>II</sup>P<sub>3</sub>O<sub>9</sub>)<sub><i>n</i></sub> (<i>n</i> = 1, 2, 6) systems are discussed. Remarkably, KMgP<sub>3</sub>O<sub>9</sub> as one of the cyclophosphate members exhibits a second harmonic generation (SHG) intensity about 0.2 times that of KH<sub>2</sub>PO<sub>4</sub> (KDP), RbCdP<sub>3</sub>O<sub>9</sub>, and CsPbP<sub>3</sub>O<sub>9</sub> 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.