Exploration of New Birefringent Crystals in Bismuth d 0 Transition Metal Selenites.

The first examples of bismuth fluoride selenites with d(0)-TM/Te-VI polyhedrons, namely, Bi4TiO2F4(SeO3)(4) (1), Bi4NbO3F3(SeO3)(4) (2), Bi4TeO4F2(TeO3)(2)(SeO3)(2) (3), Bi2F2(MoO4)(SeO3) (4) and Bi2ZrO2F2(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 Ti-IV, Nb-V and Te-VI respectively, are isostructural, exhibiting a new 3D framework composed of a 3D bismuth oxyfluoride architecture, with intersecting tunnels occupied by d(0)-TM/Te-VI octahedrons and selenite/tellurite groups. Interestingly, compound Bi4TeO4F2(TeO3)(2)(SeO3)(2) (3) is the first structure containing Se-IV and mixed-valent Te-IV/Te-VI 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 [ZrO2F2(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 d(0)-TM/Te polyhedra, which is beneficial to the birefringence.