Ba3Sb2(PO4)4 and Cd3Sb2(PO4)4(H2O)2: Two New Antimonous Phosphates with Distinct [Sb(PO4)2] Structure Types and Enhanced Birefringence.

Two new antimonous phosphates, namely Ba3Sb2(PO4)4 and Cd3Sb2(PO4)4(H2O)2, have been successfully prepared through mild hydrothermal reactions. Ba3Sb2(PO4)4 features a 1D [Sb(PO4)2]3- chain structure separated by Ba2+ cations while Cd3Sb2(PO4)4(H2O)2 presents a 2D [Sb(PO4)2]3- layered structure with Cd2+ located at the interlayer space. The [Sb(PO4)2]3- chain in Ba3Sb2(PO4)4 is the first example of 1D antimonous phosphate structure, and Cd3Sb2(PO4)4(H2O)2 represents the first d10 transition metal antimonous phosphate. Based on UV-vis-NIR spectra, both Ba3Sb2(PO4)4 and Cd3Sb2(PO4)4(H2O)2 can display large optical band gaps (4.30 and 4.36 eV, respectively). But their transparent ranges are quite different because of the coordination water of Cd3Sb2(PO4)4(H2O)2 (500-2000 and 500-1300 nm for Ba and Cd compounds). The anhydrous Ba3Sb2(PO4)4 shows high thermal stability in the nitrogen atmosphere (900 °C). Because of the incorporation of the lone pair cation of Sb(III), the birefringence of Ba3Sb2(PO4)4 and Cd3Sb2(PO4)4(H2O)2 has been enhanced to 0.086 and 0.053 at 532 nm, respectively.