Exploration of New Birefringent Crystals in Bismuth d0 Transition Metal Selenites.

The first examples of bismuth fluoride selenites with d0 -TM/TeVI polyhedrons, namely, Bi4 TiO2 F4 (SeO3 )4 (1), Bi4 NbO3 F3 (SeO3 )4 (2), Bi4 TeO4 F2 (TeO3 )2 (SeO3 )2 (3), Bi2 F2 (MoO4 )(SeO3 ) (4) and Bi2 ZrO2 F2 (SeO3 )2 (5) have been successfully synthesized under hydrothermal reactions by aliovalent substitution. The five new compounds feature three different types of structures. Compounds 1-3, containing TiIV , NbV and TeVI respectively, are isostructural, exhibiting a new 3D framework composed of a 3D bismuth oxyfluoride architecture, with intersecting tunnels occupied by d0 -TM/TeVI octahedrons and selenite/tellurite groups. Interestingly, compound Bi4 TeO4 F2 (TeO3 )2 (SeO3 )2 (3) is the first structure containing SeIV and mixed-valent TeIV /TeVI cations simultaneously. Compound 4 features a new 3D structure formed by a 3D bismuth oxyfluoride network with MoO4 tetrahedrons and selenites groups imbedded in the 1D tunnels. Compound 5 displays a novel pillar-layered 3D open framework, consisting of 2D bismuth oxide layers bridged by the [ZrO2 F2 (SeO3 )2 ]6- polyanions. Theoretical calculations revealed that the five compounds displayed very strong birefringence. The birefringence values of compounds 1-3, especially, are above 0.19 at 1064 nm, which are larger than the mineral calcite. Based on the structure and property analysis, it was found that the asymmetric SeO3 groups (and TeO3 in compound 3) displayed the largest anisotropy, compared with the bismuth cations and the d0 -TM/Te polyhedra, which is beneficial to the birefringence.