Modulation of Framework and Centricity: Cation Size Effect in New Quaternary Selenites, ASc(SeO3)2 (A = Na, K, Rb, and Cs)

Four new stoichiometrically equivalent quaternary scandium selenites, ASc­(SeO3)2 (A = Na, K, Rb, and Cs) have been hydrothermally synthesized using A2CO3, Sc­(NO3)3·xH2O (or ScO3), and SeO2 as starting materials. All four materials share similar bond networks that are composed of corner-shared distorted ScO6 octahedra and SeO3 trigonal pyramids. However, each material reveals different frameworks and centricities. Detailed structural analysis suggests that the structural variation is attributed to the difference in size and subsequent coordination number for the alkali metal cations. Powder second-harmonic generation (SHG) measurements on noncentrosymmetric (NCS) RbSc­(SeO3)2 show that the compound has an SHG efficiency similar to that of (NH4)­H2PO4. The observed SHG efficiency is due to the remaining net polarization after cancellation of oppositely aligned moments for SeO3 and ScO6 groups. Thorough characterizations such as spectroscopic, thermal, and elemental analyses for the new materials are presented as are dipole moment and out-of-center distortion calculations.本研究以A₂CO₃、Sc(NO₃)₃·xH₂O（或ScO₃）与SeO₂为起始原料，通过水热法合成了四种化学计量等价的四元亚硒酸钪化合物，其通式为ASc(SeO₃)₂（A = Na、K、Rb、Cs）。该系列化合物均具有由共角畸变ScO₆八面体与SeO₃三角锥构成的相似键合网络，但各自的骨架结构与中心对称性均存在差异。详细的结构分析表明，这种结构变异源于碱金属阳离子的尺寸差异及其配位数的变化。对非中心对称（noncentrosymmetric, NCS）的RbSc(SeO₃)₂开展粉末二次谐波发生（second-harmonic generation, SHG）测试，结果显示该化合物的SHG效率与磷酸二氢铵（(NH₄)H₂PO₄）相当。该化合物所呈现的SHG效率，源于SeO₃与ScO₆基团的反向偶极矩相互抵消后剩余的净极化强度。本研究同时对该系列新型化合物开展了全面表征，包括光谱分析、热分析与元素分析，并完成了偶极矩与偏心畸变（out-of-center distortion）的理论计算。