Communication pubs.acs.org/IC
Na4MgM2Se6 (M = Si, Ge): The First Noncentrosymmetric Compounds with Special Ethane-like [M2Se6]6− Units Exhibiting Large LaserDamage Thresholds Kui Wu, Zhihua Yang, and Shilie Pan* Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China S Supporting Information *
chosen as the representative. In its structure (Figure 1a), the Si atoms are not bonded with Se atoms to form typical SiSe4
ABSTRACT: Two new noncentrosymmetric compounds, Na4MgM2Se6 (I, M = Si; II, M = Ge), that contain special ethane-like [M2Se6]6− units were reported for the first time. Remarkably, they exhibit high laser-damage thresholds [9 (I) and 7 (II) × benchmark AgGaS2] and moderate second-harmonic-generation responses with type I phase matching.
R
ecently, nonlinear-optical (NLO) materials, as critical frequency-shifting devices, have played increasingly important roles in the frequency−conversion domain.1 As for the UV−visible domain, numerous famous NLO materials were discovered, and several have been applied in the commercial operation.2−5 However, although a few of the NLO crystals [AgGaQ2 (Q = S, Se) and ZnGeP2] have shown efficient application in the IR field, there still exist some of self-defects among them, hindering their development, such as low laserinduced damage thresholds (LIDTs) and strong two-photon absorption (TPA).6 Therefore, searching for high-performance (especially for high-LIDT) IR NLO materials is still valuable and urgent work. As is known to all, the incorporation of electropositive elements including alkaline or alkaline-earth elements into the crystal structures is conducive to enlarging the band gaps of the products and then generating the high LIDT and avoiding the effect of TPA.7 Several related examples with large band gaps and high LIDTs include BaGa4Q7 (Q = S, Se), LiCdMS4 (M = Ge, Sn), Li2Ga2GeS6, Na2Ge2Se5, Ba2Ga8MS16 (M = Si, Ge), and LiMS2 (M = Ga, In), and Ba4CuGa5Q12 (Q = S, Se).8−11 Inspired by the above strategy, we have focused our main attention on the A−AE−M−Se system (A = alkali metal; AE = alkaline-earth metal; M = Si, Ge) and finally discovered two new isostructural NCS compounds: Na4MgSi2Se6 (I) and Na4MgGe2Se6 (II), which have special ethane-like [Si2Se6]6− or [Ge2Se6]6− dimers in their structures. Interestingly, some materials with Si−Si or Ge−Ge bonds were also found, but none of them possess NCS structures.12,13 Up to now, to our best knowledge, compounds I and II are the synthesized NCS materials containing ethane-like [M2Se6]6− dimers for the first time. Remarkably, they exhibit high LIDTs of about 9 (I) and 7 (II) times that of benchmark AgGaS2 (AGS) and moderate second-harmonic-generation (SHG) responses, respectively. Compounds I and II are isostructural and crystallize in NCS space group C2 of a monoclinic system. Herein, compound I is © 2015 American Chemical Society
Figure 1. (a) View of the structure of Na4MgSi2Se6 along the c axis (all of the Na−Se bonds are removed). (b) In compound I, the Si−Si bonds of the [Si2Se6]6− units are parallel to the plane of the layer (along the a axis); (c) Crystal structure of III. (d) In compound III, the Si−Si bonds are almost perpendicular to the layer (along the c axis).
tetrahedra, but they can form Si−Si bonds and be linked with three Se atoms to form ethane-like [Si2Se6]6− units (Figure 1b). The Mg atoms are connected with six Se atoms to form [MgSe6]10− octahedra and further bonded with isolated ethanelike [Si2Se6]6− to form a [Mg2(Si2Se6)2]8− layer structure, which is located at the ab plane. It is interesting to note that the Si−Si bonds are parallel to the plane of the layer and the [Si2Se6]6− dimers show special staggered formations in the layer. The Na atoms are also connected with six Se atoms to form [NaSe6]11− octahedra and filled in the interlayer spaces and channels to form a 3D framework (Figure 1a). In comparison with the structure of the previously reported Na8Pb2(Si2Se6)2 (III; Figure 1c), although it has stoichiometric proportion and ethane-like [Si2Se6]6− units similar to those of the title compounds, it crystallizes in the centrosymmetric space group C2/m.13b The structural differences of compounds III and I (C2/m vs C2) are as follows: (i) the asymmetric unit and Z (number of molecules in a unit cell) of compound III have six crystallographically distinct sites and Z = 1, which are different from those of the title compounds (nine and Z = 2); (ii) for compound III, one site is occupied by constitutionally disordered Na and Pb atoms, which is also different from the total occupation of atoms on each site Received: August 14, 2015 Published: October 20, 2015 10108
DOI: 10.1021/acs.inorgchem.5b01859 Inorg. Chem. 2015, 54, 10108−10110
Communication
Inorganic Chemistry
For IR crystals, the band gap is a key characteristic parameter because a high LIDT generally corresponds to a large-energy band gap.7c In this work, experimental band gaps of 2.85 eV (I) and 2.53 eV (II) are given in Figure 3a and are much larger than
for compounds I and II; (iii) the most critical structural difference is that the Si−Si bonds in compound III are almost perpendicular to the layer (along the c-axis direction; Figure 1d), which are also different from those of compounds I and II, in which their Si−Si or Ge−Ge bonds are located in opposite directions (parallel to the plane of the layer; Figure 1b). The reason for the structural changes may be induced by the cation size effect. Similar phenomena were also found in other related compounds, such as Cs2Hg3M2S8 (P1)̅ and Rb2Hg3M2S8 (P21/ c), where M = Ge and Sn.14 In compounds I and II, the bond lengths of Na−Se are about 2.915−3.151 and 2.943−3.143 Å, respectively (Table S3 in the Supporting Information, SI). The bond distances of Mg−Se range from 2.764−2.816 and 2.769−2.833 Å, respectively. d(Si− Si) and d(Ge−Ge) are 2.305 and 2.400 Å, respectively, which are similar to those of related compounds, such as Na8Pb2(Ge2S6)2 [d(Ge−Ge) = 2.395 Å], Na8Sn2(Ge2S6)2 [d(Ge−Ge) = 2.393 Å], and Na8Pb2(Si2Se6)2 [d(Si−Si) = 2.344 Å].13b Because the title compounds belong to a NCS space group (C2), we have also investigated their SHG response. Through investigation by a Q-switch laser (2.09 μm, 3 Hz, and 50 ns), compound II is found to be phase-matchable (PM) and exhibits good SHG efficiency of about 1.3 times that of the benchmark AGS at 150−200 μm particle size (Figure 2). In addition, because
Figure 3. (a) Experimental band gaps of compounds I and II. (b) LIDTs of the title compounds versus AGS.
those of commercial crystals (AgGaSe2 and ZnGeP2), which indicates that the title compounds can be expected to avoid TPA of common laser pumping (1.06 or 1.55 μm) and have high LIDTs for future practical application. Moreover, to explore the structure−performance relationship, theoretical band gaps are calculated as 2.78 eV (I) and 2.35 eV (II) (Figure S4 in the SI) and are smaller than the test values, which is a common phenomenon for generalized gradient approximation calculation.18 The calculated total and partial densities of states (TDOS and PDOS) plots (Figure S5 in the SI) also illuminate that the optical absorptions of the title compounds are determined by the [Si2Se6] or [Ge2Se6] units, respectively. To assess LIDTs, with a pulse laser (1.06 μm, 10 Hz, and 10 ns), the results of commercial AGS (as a reference) and the title compounds were obtained. The LIDTs of the title compounds [45.8 MW/cm2 (I) and 35.8 MW/cm2 (II)] are obtained and are about 9 (I) and 7 (II) times that of the benchmark AGS (5.1 MW/cm2; Figure 3b and Table S3 in the SI). Such high LIDTs show that the title compounds have potential application prospects in the high-power IR field. IR spectra show that transmission ranges of the title compounds can extend to about 20 μm (Figure S2 in the SI), thus ensuring that they have good application prospects for two well-known atmospheric windows (3−5 and 8−12 μm). Raman spectra (Figure S3 in the SI) display strong absorption peaks at 278 cm−1 for compound I or 258 and 244 cm−1 for compound II attributed to the characteristic absorptions of the Si−Si or Ge− Ge modes, respectively, which are also similar to the reported results of known compounds Na8M2(Ge2S6)2 (M = Sn, Pb).13b In conclusion, we have reported two new NCS materials with ethane-like [M2Se6]6− units, Na4MgM2Se6 (I, M = Si; II, M = Ge). Remarkably, they show high powder LIDTs [9 (I) and 7 (II) × AGS], moderate SHG responses [0.5 (I) and 1.3 (II) × AGS], and wide IR transmission ranges (0.45−20 μm), which may effectively eliminate the critical defects (low LIDTs and TPA) of the commercially IR NLO materials. Thus, we believe that the title compounds can be applied in the IR domain as potential NLO materials.
Figure 2. (a) Phase-matching curves for compound II and AGS (i.e., SHG response vs particle size). (b) Oscilloscope traces of the SHG signals of the title compounds and benchmark AGS at the particle size (150−200 μm).
of the limited sample amount for compound I, only its SHG response at the particle size (150−200 μm) was measured to be about half that of AGS (Figure 2b). Therefore, the appreciable frequency conversion abilities of the title compounds indicate that they have good potential to be applied in the IR NLO field. Moreover, the SHG coefficients and birefringences Δn of the title compounds are calculated by a scissors-corrected method.15 The calculated maximum SHG coefficients are d22= 7.6 pm/V for I and d22= −24.9 pm/V for II, which agree well with the test values, and the SHG coefficient of II is slightly larger than that of AGS (11 pm/V).16 Note that calculated Δn values (Figure S6 in the SI) are about 0.10 and 0.09 for compounds I and II at the wavelength ∼1 μm, respectively, and much larger than that of AGS (0.039),17 which also indicates that I is PM for the SHG in the IR region. We have also studied the origin of the SHG response (static d22) with the dependence of cutoff energy, and the results are shown in Figure S7 in the SI. It can be obviously found that the VB-1 and CB-3 regions offer the dominating effect to the d22 coefficient from this figure. Among them, the Se 4p state and a small amount of the Si 3p or Ge 4p state provide the biggest contribution for VB-1, and CB-3 is mainly dominated by the Se 4p and Si 3p or Ge 4p states with a small mixture of Mg 2p. In conclusion, the electronic transitions from Se 4p, Si 3p, or Ge 4p and Mg 2p play the main role for the SHG efficient.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01859. 10109
DOI: 10.1021/acs.inorgchem.5b01859 Inorg. Chem. 2015, 54, 10108−10110
Communication
Inorganic Chemistry
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Synthesis, experimental details, structural refinement and crystal data, powder XRD, IR and Raman spectra, electronic structures, TDOS and PDOS plots, and cutoff-energy-depending static SHG coefficients (PDF) CIF files (ZIP)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Grant 2014731029), the Western Light Foundation of CAS (Grant XBBS201318), and the National Natural Science Foundation of China (Grants 51402352 and 51425206).
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DOI: 10.1021/acs.inorgchem.5b01859 Inorg. Chem. 2015, 54, 10108−10110
