Exploration of New Birefringent Crystals in Bismuth d⁰ Transition Metal Selenites

The first examples of bismuth fluoride selenites with d 0 ‐TM/Te VI polyhedrons, namely, Bi 4 TiO 2 F 4 (SeO 3 ) 4 ( 1 ), Bi 4 NbO 3 F 3 (SeO 3 ) 4 ( 2 ), Bi 4 TeO 4 F 2 (TeO 3 ) 2 (SeO 3 ) 2 ( 3 ), Bi 2 F 2 (MoO 4 )(SeO 3 ) ( 4 ) and Bi 2 ZrO 2 F 2 (SeO 3 ) 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 Bi 4 TeO 4 F 2 (TeO 3 ) 2 (SeO 3 ) 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 MoO 4 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 [ZrO 2 F 2 (SeO 3 ) 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 SeO 3 groups (and TeO 3 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.