Understanding and exploring the functional modules (FMs) consisting of local atomic groups can promote the development of the materials with functional performances. Oxygen-containing tetrahedral modules are popular in deep-ultraviolet (DUV) optical materials, but their weak optical anisotropy is adverse to birefringence. Here, the fluorooxosulfate group is proved as a new birefringence-enhanced FM for the first time. The birefringence of fluorooxosulfates can be 4.8-15.5 times that of sulfates with the same metal cations while maintaining a DUV band gap. The polarizing microscope measurement confirms the birefringence enhancement by using the millimeter crystals experimentally. The theoretical studies from micro and macro levels further reveal a novel universal strategy that the fluorine induced anisotropic electronic distribution in fluorooxo-tetrahedral group is responsible for the enhancement of birefringence. This study will guide the future discovery of DUV optical materials with enlarged birefringence.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202003594

Fluorine-Driven Enhancement of Birefringence in the Fluorooxosulfate: A Deep Evaluation from a Joint Experimental and Computational Study

Lone pair activity is an important factor to regulate the crystal structure symmetry and related optical response. Herein, a new type of hydrated bismuth fluoride, BiF3·H2O, was synthesized and characterized by a facile hydrothermal reaction. Obviously, the introduction of a water molecule significantly changed the coordination environment of Bi3+, thereby seriously changing the crystal structure of the compound BiF3. Compared with asymmetric BiFm fundamental building blocks in other compounds, BiF3·H2O possesses unprecedented near-isotropic BiF9 polyhedra and exhibits regulated birefringence values. Therefore, this work will contribute to the structural diversity of BimFn-based compounds and explore the development of new birefringent crystals.

From BiF3 to BiF3·H2O: diverse Bi(III) coordination for structural transformation and birefringence regulation

As a significant nonlinear optical compound, phosphate has been wildly used in all kinds of advanced optical systems. In this work, the electronic structures, and optical properties of a series of XPO 3 F and X 3 (PO 4 ) 2 (X = Sn, Pb, Sr, Ba) compounds were investigated using the first-principles method in both molecular and crystal levels. The results show that introducing F atoms can lead to enhanced HOMO-LUMO gap and enlarged anisotropy of polarizability. The anionic groups give main contribution to the total birefringence of alkaline earth phosphate and fluorophosphate. As for the post-transition metal phosphate and fluorophosphate, both anionic groups and cation-centric polyhedra play a critical role in determining birefringence. The X-p (X = Sn, and Pb) states nearby the Fermi level are beneficial to get enhanced birefringence, but they do not play a vital role in determining total birefringence. • Fluorophosphate and phosphate were investigated at molecular and crystal level. • Introducing fluorine atoms enhances HOMO-LUMO gap, polarizability anisotropy. • Both anionic groups and lone-pair cations give main contribution to birefringence. 

First-principles study of the influence of lone pair electrons and molecular groups on the birefringence of phosphate and fluorophosphate

Pb(OH)I: A lead(II)-based oxyhalide with a large birefringence

Rare-earth phosphates were thought to be good candidates as ultraviolet/deep ultraviolet optical materials due to their relatively large bandgap and optical properties. In this paper, the authors screened out a family of XPO4 (X = Sc, Y, La, and Lu) compounds with an enhanced bandgap (HSE06 bandgap ≥ 7.61 eV) and birefringence (0.0934-0.2003@1064 nm) using first-principles calculations. The origin of enhanced optical properties was investigated using projected density of states, distortion indices, and Born effective charges. The results show that the PO4 anionic groups and X-O polyhedra give the main contribution in determining the optical properties, and the PO4 anionic groups give more contribution than other functional basic units. The spin-orbit interaction was also investigated. Similar band structures were found after spin-orbit coupling (SOC) was considered, and slightly enhanced birefringence was found when SOC was applied to these rare-earth phosphates.

The enhanced bandgap and birefringence of rare-earth phosphates XPO4 (X = Sc, Y, La, and Lu): a first-principles investigation.

The 6s2 lone-pair electrons (Pb2+, Bi3+) and 5s2 lone-pair electrons (Sn2+, Sb3+) show different responses to birefringence than they do to second harmonic generation (SHG). In the current work, different mechanisms of birefringence and SHG responses induced by Pb2+/Sn2+ cations were investigated using the first-principles method on AB2O3F2 (A = Ba, Sn, Pb). The obtained birefringence and SHG coefficients were in good agreement with the experimental values. The anisotropic lone-pair electron distribution was found at the states near the Fermi level containing cation sp-oxygen p states. Our analyses showed the birefringences of the AB2O3F2 (A = Ba, Sn, Pb) compounds to be directly affected by the anisotropic lone-pair electron distribution, however, the SHG responses of these three compounds have a more complicated relationship with the hybrid cation-oxygen states. The expanded hybrid cation-oxygen states including the lone-pair states were concluded to endow the PbB2O3F2 compound with a stronger SHG response than those displayed by the other compounds.

Different mechanism of response of asymmetric lone pair electrons around ns2 cations to birefringence and second harmonic generation

Phosphates possess a relatively large UV/DUV cutoff edge, but these compounds usually have very small birefringence. Recently the Te2P2O9 crystal was synthesized and its birefringence was reported to be as large as 0.106 at 1013.98 nm. Herein, we investigated the electronic structure and optical properties of Te2P2O9 using the first-principles method. The obtained results are in good agreement with the experimental values. The Born effective charges and SHG density of Te2P2O9 show that the contribution to the birefringence and SHG response mainly originates from the TeO5 group. The electronic structures and optical response of Ba2TeO(PO4)2 and Te3O3(PO4)2 were also investigated for comparison. The results show that these two tellurium phosphates also possess a large birefringence similar to Te2P2O9. Also, the birefringence originates from the TeOx polyhedrons, which was confirmed by the real-space atom-cutting results and distortion indices.

/pdf/tellurium-oxygen-group-enhanced-birefringence-in-tellurium-39zkq4n335.pdf

Tellurium–oxygen group enhanced birefringence in tellurium phosphates: a first-principles investigation

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Different mechanism of response of asymmetric lone pair electrons around ns2 cations to birefringence and second harmonic generation

Tellurium–oxygen group enhanced birefringence in tellurium phosphates: a first-principles investigation