Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations Yangwu Guo,†,‡,⊥ Fei Liang,†,‡,⊥ Jiyong Yao,*,† Zheshuai Lin,*,† Wenlong Yin,*,§ Yicheng Wu,†,∥ and Chuangtian Chen† †
Center for Crystal Research and Development, Key Lab Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China ∥ Institute of Functional Crystal Materials, Tianjin University of Technology, Tianjin 300384, People’s Republic of China S Supporting Information *
simultaneously exhibiting a strong SHG response and wide phase-match region, is still valuable and urgent work. Generally, the optical response of a crystal is determined by two key parameters: spatial alignment of functional groups and distribution of valence electrons. Metal chalcogenides show rich types of microscopic noncentrosymmetric (NCS) groups including triangle-planar groups (BS3, HgSe3), tetrahedral units (GaS4, GeSe4), second-order Jahn−Teller (SOJT) effect octahedral groups, and stereochemically active lone pairs groups.24 In particular, closed packing arrangements of tetrahedral units in diamond-like (DL) structure result in additive superposition of the microscopic second-order nonlinear susceptibility, consequently leading to a strong SHG effect more than AgGaS2 (d36 = 13 pm/V).25 For instance, Li2HgGeS4 and Li2HgSnS4 have been reported to possess strong SHG responses of 3 and 4 times that of AgGaS2.26 On the other hand, polar oriented nonbonding electrons in [AsS3] groups also result in enhanced SHG response in LiAsS2.27 In this Communication, we propose that special attention should be paid to the defect DL structures, which possess perfect alignment of functional units and nonbonding electron states owing to the deficiency of unoccupied cation sites. We systematically investigated the optical properties of ternary Hg-containing defect DL selenide Hg2GeSe428 for the first time. Experiment results show that Hg2GeSe4 exhibits a strong NLO effect about 2.1 times that of the benchmark AgGaSe2 (d36 = 33 pm/V) in the particle size range 150−200 μm and is found to be phase-matchable. More importantly, first-principles calculations were performed to demonstrate the crucial role of nonbonding electrons in improving the SHG effect. Hg2GeSe4 crystallizes in a noncentrosymmetric tetragonal space group I4̅ (no.82) and exhibits a typical defect DL structure. The asymmetric unit contains two unique Hg atoms, one Ge atom, and one Se atom. It can be thought of as a derivative from the classical compound AgGaSe2 (Figure 1 and Supporting Information): Half of the Ag+ cations in the unit are substituted by Ge4+ cations, leaving the other half of the sites as vacancies, and all the Ga3+’s in AgGaSe2 are replaced by the bivalent Hg2+ cations. The Hg−Se distances range from 2.646
ABSTRACT: A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 (d36 = 33 pm/V) at the particle size of 150−200 μm, as well as good phasematchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δn = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.
I
nfrared (IR) coherent sources in the spectral range of 3−20 μm have been applied in the fields of civil and military applications, including medical treatment, environmental monitoring, industrial manufacturing, laser guidance, and infrared remote sensing.1,2 Parametric frequency conversion technology using IR nonlinear optical (NLO) crystals is an efficient way to convert existing laser sources to mid-IR wavelengths. To date, although the traditional chalcopyrite crystals (AgGaS2,3 AgGaSe2,4 ZnGeP25) are commercially available for IR NLO applications, there still exist some inherent drawbacks among them, which restrict their development and applications. As a result of more than 20 years’ exploratory synthesis of new IR NLO materials, hundreds of metal chalcogenides are proposed to exhibit promising IR NLO properties, 6−10 including K 2 P 2 Se 6 , 11 Ba 4 CuGa 5 Q 12 , 12 PbGa2MSe6 (M = Si, Ge),13 KCd4Ga5Q12 (Q = S, Se),14,15 Ba5CdGa6Se15,16 AgGa2PS6,17 and so on. However, only a few new materials (BaGa4Q7,18,19 BaGa2GeQ6 (Q = S, Se),20 and LiGaGe2Se621) have been grown into bulk single crystals for detailed study, and even fewer can be applied in laser generation (BaGa4Q7).22,23 Most of the new compounds are limited by a weak SHG effect (
