New Sulfates Deep-ultraviolet Nonlinear Optical Crystalline Materials

Nonlinear optical?NLO?materials are of great current interests to modern laser science and techniques.In the past decades,great efforts have been paid on the search for new NLO materials that can work from deep-ultraviolet?UV?to even far-infrared regions.In particular,deep-UV NLO materials,which have significant applications in a variety of bran-new scientific instruments used in a series of frontier fields,become a hotspot in recent years.Traditionally,the search for these materials was mainly limited to ?-conjugated systems.Recently,scientists became aware of the possibility of non-p-conjugated systems as deep-UV NLO materials,such as phosphates.Analogous to phosphates,sulfates belong to non-p-conjugated systems.However,sulfates have long been ignored as deep-UV NLO materials.A major cause may be that sulfates are prone to decomposing and releasing SO₃ gas at high temperature so that their bulk crystals are difficult to grow by the popular high-temperature molten techniques.Therefore,in this paper,we synthesized some new sulfates deep-UV NLO materials,and investigated their optical properties in detail.Through this work,we introduced sulfates into deepUV NLO materials,and provided an innovative source for deep-UV NLO materials.The main contents are as following.1.Because sulfates tend to decompose and release SO₃ gas at high temperature,it is difficult to use the popular high-temperature molten techniques to explore sulfates NLO materials.In the first work,we used the facile water solution method to synthesize a new asymmetric sulfate Li₈NaRb₃?SO₄?₆?2H₂O.The single-crystal structure of Li₈NaRb₃?SO₄?₆·2H₂O features a three-dimensional framework composed of SO₄ and LiO₄ tetrahedra.Powder second harmonic generation?SHG?tests show that Li₈NaRb₃?SO₄?₆·2H₂O is phase-matchable at ? = 1064 nm with a SHG response of about 0.5 × KDP.UV-Vis-NIR diffuse reflectance spectra show that it has a wide transparent region down to ? < 190 nm.Theoretical calculations reveal that the optical properties of the Li₈NaRb₃?SO₄?₆·2H₂O crystal are mainly attributed to S-O groups.We believe that the discoveries in this work will attract scientists' attention to sulfate systems for their potential as deep-UV NLO materials.2.The compound in the first work appears in hydrate forms that are easy to lose the crystalline water under the heat aroused by the illumination of high-power lasers.To overcome this problem,we introduced both alkaline elements and ammonium into sulfate systems,and synthesized two asymmetric anhydrous sulfates,which are composed of non-p-conjugated [SO₄]^2-anions.Compared with the compound in the first work,these anhydrous sulfates have higher thermal stability.Relevant optical measurements reveal that both sulfates are deep-UV transparent and phase-matchable.However,these two sulfates exhibit distinct SHG responses of 1.1 and 0.5 times that of KDP,respectively.The first-principles calculations reveal that the S-O groups play a key role in optical properties of both sulfates,whereas the contribution from Li⁺,Na⁺ and NH₄⁺ cations are negligibly small.In-depth first-principles analyses suggest that the SHG gap between both sulfates are mainly aroused by the different orientations of nonbonding O 2p orbitals in the crystallographically independent S1O₄ groups in both sulfates.Giving the highly “NLO-active” [SO₄]^2-anions are only about one third that of the overall ones in NH₄NaLi₂?SO₄?₂,it is expected that there is still much room to boost the SHG responses for sulfates.This work provides an innovative non-p-conjugated source that is distinct from the traditional p-conjugated ones for deep-UV NLO materials.