Tin(II)-Induced Large Birefringence Enhancement in Metal Phosphates.

Two alkali tin(II) phosphates, namely, Rb[SnF(HPO4)] and Rb(Sn3O)2(PO4)3, were synthesized through mild hydrothermal methods. They belong to the orthorhombic Pnma and Pbcn space groups, respectively. Rb[SnF(HPO4)] features a layered structure based on 1D [SnF(HPO4)]∞ chains interconnected by hydrogen bonds, with Rb+ cations located at the interlayer space. For Rb(Sn3O)2(PO4)3, each pair of [Sn3O]4+ clusters is bridged by a pair of [P(1)O4]3- tetrahedra to build a 1D [Sn-P-O]∞ chain. These 1D [Sn-P-O]∞ chains are further cross-linked though [P(2)O4]3- tetrahedra to construct a 3D network with 7- and 10-membered-ring channels. The tin(II) ions in Rb[SnF(HPO4)] and Rb(Sn3O)2(PO4)3 with stereochemically active lone pairs (SCALPs) significantly enhance the birefringences of metal phosphates: Δn = 0.147@1064 nm for Rb[SnF(HPO4)] and 0.082@1064 nm for Rb(Sn3O)2(PO4)3.