Pb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub>: A Lead\nVanadate Tellurate with Wide Mid-IR Transparency and Large Birefringence\nInduced by Multiple Birefringence-Active Groups

Birefringent materials as the key materials in laser\nscience and\ntechnology have attracted continuous attention due to their ability\nto modulate polarized light. Herein, a new lead vanadate tellurate,\nPb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub>, has been synthesized\nthrough the rational integration of different kinds of birefringence-active\nfunctional units. Pb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub> features\na unique two-dimensional (2D) [TeV<sub>2</sub>O<sub>10</sub>]<sub>∞</sub> layered structure consisting of [VO<sub>6</sub>]<sup>7–</sup> and [TeO<sub>6</sub>]<sup>6–</sup> octahedra,\nand Pb<sup>2+</sup> cations reside between the [TeV<sub>2</sub>O<sub>10</sub>]<sub>∞</sub> layers. In addition, the rare edge-sharing\nmode of [VO<sub>6</sub>]<sup>7–</sup> and [TeO<sub>6</sub>]<sup>6–</sup> octahedra was found in this structure. Attributed\nto the high polarizability and appropriate arrangement of PbO<sub>8</sub>, VO<sub>6</sub>, and TeO<sub>6</sub> units, Pb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub> possesses a great theoretical birefringence\nof 0.275 at 532 nm, which is the largest among the vanadate tellurate\nfamily. The spectral tests also prove that Pb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub> showcases a broad transparency window (439 nm–10\nμm), covering an important mid-infrared (IR) atmospheric window\n(3–5 μm). In addition, in order to improve the transparency,\nalkali and alkaline earth metal cations were introduced by the substitution\nstrategy, and then the compound K<sub>2</sub>Sr<sub>2</sub>Te<sub>2</sub>O<sub>9</sub> was synthesized. It owns a shorter ultraviolet\n(UV) cutoff edge of 234 nm and a wider transparency window (234 nm–13.8\nμm). The findings of Pb<sub>2</sub>TeV<sub>2</sub>O<sub>10</sub> and K<sub>2</sub>Sr<sub>2</sub>Te<sub>2</sub>O<sub>9</sub> enrich\nthe structure chemistry of the tellurate family and provide new insights\nfor designing new compounds with large optical anisotropy and wide\nspectral transparency.