Hg₄(Te₂O₅)(SO₄): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Abstract

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO 4 group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO 2 (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg 4 (Te 2 O 5 )(SO 4 ) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO 4 (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg 4 (Te 2 O 5 )(SO 4 ) features a novel two-dimensional layered structure composed of [Hg 4 (Te 2 O 5 )] 2+ layers separated by isolated (SO 4 ) 2− tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO 4 tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg 2 O 2 ) 2− , (Te 2 O 5 ) 2− , and (SO 4 ) 2− units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.

Pb2(SeO3)(SiF6): the first selenite fluorosilicate with a wide bandgap and large birefringence achieved through perfluorinated group modification

“…Currently, we are pursuing two approaches to simultaneously improve the bandgap and birefringence of selenites. The first approach involves introducing SO 4 tetrahedral groups with a wide transmittance window, 31,32 exemplified by compounds like Hg 2 (SeO 3 )(SO 4 ) (0.133 @ 532 nm, 3.58 eV) 33 and Hg 3 (SeO 3 ) 2 (SO 4 ) (0.118 @ 546 nm, 4.70 eV). 34 Another approach involves the use of partially fluorinated metal oxide polyhedrons.…”

New ultraviolet transparent rare-earth borates with enhanced birefringence induced by cation chemical substitution

“…31–34 The strategy has proven effective by reported oxides with large birefringence, such as the K 3 Nb 3 Ge 2 O 13 (0.196@546 nm), 35 Sn 2 B 5 O 9 Cl (0.196@546 nm), 36 and Hg 4 (Te 2 O 5 )(SO 4 ) (0.542@546 nm). 37 However, an unwanted red shift of the cutoff edge is always caused by these cations. 38–40 As a result, the strategy is not suitable to enhance the birefringence of UV, especially DUV optical materials.…”

Designing promising ultraviolet (UV) birefringent crystals with different hydrogen-bonded phosphate frameworks

“…Birefringent crystals are key materials used in the production of optical devices, including polarizers, optical isolators, circulators, and phase retarders. 1–7 Inorganic phosphate crystals, known for their short ultraviolet cutoff edges and good crystal growth habits, represent a classic optoelectronic material. 8–13 KDP (KH 2 PO 4 ) and KTP (KTiPO 4 ) are quintessential examples of phosphates, boasting significant application as nonlinear optical (NLO) materials.…”