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Exploring of Potential Beryllium-free Deep-Ultraviolet Optical Crystals in the Rare Earth Fluoride- Carbonate-Water System qian Wang, Fangfang He, Ling Huang, Daojiang Gao, Jian Bi, and Guohong Zou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00431 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018
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Crystal Growth & Design
Exploring of Potential Beryllium-free Deep-Ultraviolet Optical
Crystals
in
the
Rare
Earth
Fluoride-
Carbonate-Water System Qian Wang, † Fangfang He, † Ling Huang,* † Daojiang Gao, † Jian Bi, † and Guohong Zou*‡ †
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu，
610068, P. R. China. ‡
College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
E-mail:
[email protected];
[email protected] ACS Paragon Plus Environment
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Abstract: Rare earth fluoride-carbonates have been proved to be potential deep-ultraviolet optical materials, but lacked the systematic study on phase stability domains to guide synthesis. In this work, detailed investigations in the Na2CO3–GdF3–H2O system have been done in sub-critical hydrothermal conditions. A series of sodium-rare earth carbonates crystals NaGdCO3F2 (1), Na2GdCO3F3 (2), Na3Gd(CO3)3 (3), and Na3Gd(CO3)3·6H2O (4) have been synthesized. NaGdCO3F2 (1) is composed of 3-D [GdCO3F2]- framework with Na+ as the charge balance cation. Na2GdCO3F3 (2) showed 2-D [GdCO3F3]2- layers along the c-axis and further bridged by Na+ to form 3-D framework. Na3Gd(CO3)3 (3) is made up of alternately stacked layers [Na(CO3)2]∞ along the a-axis and [Na2Gd(CO3)2]∞ in the bc plane, forming a three-dimensional framework.
Na3Gd(CO3)3·6H2O (4) exhibits an intricate three-dimensional structure composed of [Gd(CO3)3(H2O)3]3- complex and bridging Na+ ions. The results from the UV-vis diffuse reflectance spectroscopy study indicated that the short-wavelength absorption edge of all the four compounds were below 200 nm, suggesting that they are potential deep-ultraviolet optical materials. Powder second harmonic generation measurements indicated that the compounds 3 and 4 are phase-matchable (type I), and they exhibit relatively strong SHG responses of approximately 3 and 2 times that of KH2PO4 (KDP).
Keywords: rare earth fluoride-carbonate, deep-UV, nonlinear optical crystals, phase stability regions
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Crystal Growth & Design
Introduction Deep-ultraviolet (DUV) nonlinear optical (NLO) materials, the key components of solid-state lasers producing DUV coherent light through a cascaded frequency conversion, have gained increasing attention owing to their versatile scientific and technological applications such as angle-resolved
photoemission
spectroscopy,
laser-cooling,
super-high-resolution
and
semiconductor photolithography.1-10 Even though a few DUV NLO crystals have been reported in recent years, it still presents a particularly difficult challenge in searching for suitable NLO materials with wide UV transparency especially in the DUV region, large NLO coefficients, appropriate birefringence and stable physical and chemical properties.11-15 In the past decades, chemists and materials scientists have paid many efforts on understanding the relationship between NLO properties and crystals structures which is critical in guiding the exploration of new NLO materials. The anionic group theory, the macroscopic nonlinearity of crystal is the geometrical superposition of the microscopic second-order susceptibility of the NLO-active anionic groups, which was proposed by Chen et al. made a spectacular success in developing UV and DUV NLO crystals in borates, such as LiB3O5 (LBO),16 β-BaB2O4 (BBO),17 CsB3O5 (CBO)18 and CsLiB6O10 (CLBO),19 and SrBe2BO7 (SBBO).20 However, commercially available DUV NLO materials are still lacking. Currently, KBe2BO3F2 (KBBF)21 is still the only material to generate coherent light at wavelengths below 200 nm by direct SHG response. But two fatal weaknesses of KBBF, strong layer habit with respect to crystal growth and the use of highly toxic BeO, hinder its extensive application. Thus, it is in urgent demand to search for new DUV NLO materials that could overcome these demerits. In recent years, several beryllium-free chemical systems, such as fluorooxoborates,9,
22, 23
phosphates,24 carbonates,25 etc., have been developed as the alternative systems for searching for DUV NLO materials. Rare earth fluoride carbonates26-28 have an enormous potential for DUV optical applications for three reasons: (1) analogous to the [BO3]3- group, the planar [CO3]2- can also generate large second-order susceptibility and appropriate birefringence; (2) the inclusion of fluoride could make the absorption edge blue-shifted; (3) the compounds contain rare earth atoms usually exhibit excellent physical and chemical properties and optical properties, for example, YAl3(BO3)4 and LuAl3(BO3)4.29-30 Two alkaline rare earth fluoride carbonates Na8Lu2(CO3)6F2 and Na3Lu(CO3)2F2,28 exhibiting short UV cutoff edges (
