CRYSTAL GROWTH & DESIGN
Growth and Characterization of BaGa4S7: A New Crystal for Mid-IR Nonlinear Optics Xinsong Lin, Ge Zhang, and Ning Ye* Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China
2009 VOL. 9, NO. 2 1186–1189
ReceiVed September 21, 2008; ReVised Manuscript ReceiVed NoVember 3, 2008
ABSTRACT: The new nonlinear optical (NLO) crystal BaGa4S7 for the mid-infrared (IR) has been grown by a Bridgman-Stockbarger technique. Polycrystalline materials with stoichiometric composition were synthesized from BaS, Ga, and S as the initial materials by solid-state reactions. The ultraviolet (UV) and IR transmittance of the crystal was determined with polished crystal pieces. The UV and IR optical absorption edges were found to be at 350 nm and 13.7 µm, respectively. From optical measurements of second harmonic generation on powders, the NLO coefficient d33 was determined to be 12.6 pm/V. The laser damage threshold of a single crystal reached about 1.2 J/cm2 at 1.064 µm. The Vickers-hardness value of the crystal is 327.5 HV5, which is equivalent to Mohs’ hardness of about 5. Introduction Over the past few decades, with the increasing demands of military and other civil applications, the generation of highpower tunable lasers in the range of 3-20 µm, especially in band II (3-5 µm) and band III (8-14 µm) of three atmospheric transparent windows, has become the research focus of infrared (IR) laser technology.1-4 It is a particularly difficult challenge to have suitable nonlinear optical (NLO) materials working in those two bands, which possess high NLO coefficients and high laser damage threshold simultaneously. AIIBIVCV2 and AIBIIICVI2 chalcopyrite semiconductors with high NLO coefficients, such as AgGaS2 (AGS) and ZnGeP2 (ZGP), are well-known to be the commercially available materials for mid-IR nonlinear optics. The major shortcoming, however, is their lower laser-induced damage threshold due to their narrow band gap, which limited their applications in optical parametric oscillator (OPO) and high power laser output.5,6 Recently, a new family of ternary chalcogenides containing Li element, such as LiGaS2 (LGS) and LiInS2 (LIS), has been discovered.7,8 Owing to their wider band gap, these types of materials have a higher laser-induced damage threshold than that of chalcopyrite semiconductor. The theoretical studies also indicated that the replacement of lighter cations can effectively make the band gap of the crystal wider, which improves its optical damage threshold.9 These results motivated us to investigate new NLO materials within the series of chalcogenide compounds containing alkaline or alkaline-earth elements. Hence, a noncentrosymmetric compound BaGa4S7 (BGS) was selected for this work. BGS was formerly synthesized by Eisenmann et al., and its single-crystal structure was reported as belonging to Pmn21 in 1983.10 Owing to its similar structure to those of Li-contained chalcogenides, BGS is expected to have a wide band gap and large NLO coefficients. Recently, the phase diagram of the BaS-Ga2S3 binary system has been investigated by Hidaka et al.11 In this contribution, we report for the first time the growth with the BridgmanStockbarger technique and the optical properties of BGS crystal. Experimental Section Crystal Growth. Polycrystalline samples of BGS were synthesized by solid-state reaction techniques. A stoichiometric mixture of BaS * Corresponding author. Phone: +86-591-83776076. Fax: +86-591-83750713. E-mail:
[email protected].
Figure 1. Axial temperature gradient of furnace. compound (analytical grade), Ga (analytical grade), and S (analytical grade) was evacuated and sealed in a quartz ampule, which was slowly heated to 700 °C, held for about 10 h, and then sintered at 900 °C for 48 h in a muffle furnace. The chemical reaction taking place in the sealed tube was
BaS + 4Ga + 6S f BaGa4S7
(1)
BGS single crystals were grown by a Bridgman-Stockbarger technique through spontaneous nucleation using BGS polycrystalline materials in a vertical furnace with an axial temperature gradient as shown in Figure 1. The heating area was divided into two zones by a baffle, and the desired temperature gradient was realized by carefully adjusting the baffle position in the furnace. The ampule used in the growth was specially designed using a Φ10 × 100 mm quartz tube so as to have a small cone-shaped tip at its bottom. The ampule was first heated to 1130-1140 °C in the high temperature zone, and held for 10-20 h until BGS polycrystalline was fully melted. Then growth process was carried out by lowering the ampule at a rate of 0.5-2 mm/h. The initial nucleation started from the tip at the bottom when it passed through the hole at the center of the baffle, where the temperature is close to BGS’s melting point. The crystal continued to grow with the help of the seed in the tip until the whole ampule was moved into the low temperature zone. After the growth, the furnace was slowly cooled to room temperature at a speed of 10-30 °C /h.
10.1021/cg8010579 CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
Growth and Characterization of BaGa4S7
Crystal Growth & Design, Vol. 9, No. 2, 2009 1187
Table 1. Crystal Data Structure Refinement for BGS formula formula mass temperature wavelength space group, Z a b c V density (calculated) absorption coefficient crystal dimensions θ range for data collection refinement method goodness-of-fit on F2 final R (I > 2σ (I)) R/wR (all data) largest diff peak and hole Flack parameter
BaGa4S7 640.64 amu 293(2) K 0.71073 Å Pmn21, 2 14.775(5) Å 6.228(2) Å 5.929(2) Å 544.8(3) Å3 3.905 g/cm3 14.601 mm-1 0.1 × 0.05 × 0.04 mm 3.47-29.91° full-matrix least-squares on F2 0.947 0.0185 0.0201/0.0390 +0.98 and -0.90 e/Å- 3 0.032(13)
Figure 2. (a) As-grown BGS crystal; (b) polished piece of BGS crystal.
Table 2. Atomic Positions and Isotropic Displacement Factors for BGS atom
Wyckoff position
x
Ba1 Ga1 Ga2 S1 S2 S3 S4
2a 4b 4b 2a 4b 4b 4b
0 0.24839(3) 0.11964(3) 0 0.11401(7) 0.11546(7) 0.24608(7)
y
Z
Ueq (Å)
0.66909(6) 0.05687(7) 0.02126(14) 0.68799(6) 0.64255(9) 0.00983(13) 1.17289(7) -0.34069(8) 0.00933(16) 1.3657(2) -0.4588(3) 0.0113(3) 0.82612(15) 0.5507(2) 0.0114(2) 0.19048(16) 0.0417(2) 0.0117(2) 0.67115(14) 0.0365(2) 0.0088(2)
Crystal Characterization. X-ray powder diffraction data were obtained on a Rigaku DMAX2500 diffractometer by using Cu KR radiation. Single-crystal X-ray diffraction data were collected at room temperature on a Rigaku Mercury CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.71073 Å). A transparent crystal with dimensions of 0.14 × 0.1 × 0.06 mm was mounted on a glass fiber with epoxy for structure determination. The structure was solved by direct methods and then refined by a full-matrix least-squares refinement on F2 with the computer program SHELXL-97 as programmed in the software suite WinGX v 1.64.03.12 Crystal data and refinement summaries are given in Table 1. Final atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. The Vickers-hardness measurements were carried out with the grown crystals and a 401 MVA Vickers hardness tester. The UV and IR transmittance spectra were recorded at room temperature by using a 0.70-mm-thick BGS slab polished on both sides and a Lamda900 spectrophotometer with a range of 190-3300 nm and a Spectrumone IR spectrophotometer with a range of 2.5-25 µm, respectively. Second harmonic generation (SHG) was measured by using the Kurtz and Perry method with a 2.05 µm Q-switch laser.13 The samples were ground and sieved by using a series of mesh sizes in the range of 25-210 µm. A sample of LGS was prepared as a reference material in an identical fashion. Laser damage testing was performed using a repetitively Q-switched Nd:YAG 1.064 µm laser operating with 15 ns pulse widths at a 1 Hz pulse rate frequency as the damage-inducing source. A lens with a focal length of 50 mm focused the beam 20 mm before the polished surface of a crystal sample, where the 1/e2 beam diameter was 0.4 mm, and M2 ) 1.05. The beam was periodically recharacterized during the period of damage testing to ensure constant beam quality. Surface damage was detected audibly under a microscope.
Results and Discussion BGS is a congruently melted compound according to the BaSGa2S3 pseudobinary phase diagram;11 therefore, it can be grown with a Bridgman-Stockbarger technique. Light yellowish and transparent BGS crystals as grown with dimensions up to Φ10 × 20 mm and a polished crystal slab are shown in Figure 2. The X-ray powder diffraction patterns of BGS single crystals and sintered sample are shown in Figure 3, which match the
Figure 3. X-ray powder diffraction patterns of (a) sintered sample, (b) crystal sample, and (c) simulation results.
theoretical simulation one from a single crystal structure very well. The Vickers-hardness value of the BGS crystal is 327.5 HV5, which is equivalent to Mohs’ hardness of about 5. The result shows that the mechanical property of BGS is well situated for cutting and polishing. The structure of BGS is illustrated in Figure 4, in comparison to that of LGS. The tetrahedral GaS4 groups are aligned parallel along the c axis and connected to each other by sharing their vertexes. The arrangement of the Ga-S framework of BGS is as same as that of LGS as shown in Figure 4; besides, the volume density of GaS4 group of the former (1.468 × 10-2 Å-3) is larger than that of the latter (1.380 × 10-2 Å-3). According to the theoretical study on the NLO properties of the LGS family by using the ab initio method,9 GaS4 groups provide the main contribution to the overall NLO coefficients. So the nonlinearities of LGS and BGS will be proportional to their geological factors and volume densities of GaS4 groups, on the basis of Anionic Group Theory.14 Since BGS has a similar structure configuration to that of LGS, and has a larger GaS4 group density, it is predicted to have about 1.1 times larger overall NLO coefficients than those of LGS. Figure 5gives the curves of the SHG signal as a function of particle size from the measurements made on ground crystals for BGS and LGS, respectively, which are consistent with phasematching behavior according to the rule proposed by Kurtz and Perry.13 The second-harmonic signal produced by BGS powders with a 2.05 µm fundamental corresponds about to 1.4 × LGS. These signals are proportional to the squares of the nonlinear d33 coefficients, assuming that the coherent lengths for the two materials
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Figure 6. Transmittance curves of the BGS crystal in the UV and IR. Table 3. Comparison of Important Parameters of Selected Crystals
Figure 4. Crystal structure of BGS and LGS.
crystal
point group
UV absorption edge (nm)
LiInS23 AgGaS215 ZnGeP215 BaGa4S7
C2V D2d D2d C2V
410 500 740 350
IR absorption edge (µm)
NLO coefficient (pm/V)
laser damage threshold at 1.064 µm (J/cm2)
12 13.0 12.0 13.7
16 (d33) 13 (d14) 75.4 (d14) 12.6 (d33)
1 0.4 0.03 1.2
a high laser damage threshold, which indicates that it may be used in high power laser output for an optical parametric oscillator. Conclusions
Figure 5. Particle-size dependence of second-harmonic intensity for BGS and LGS powders.
are the same. As a result, the ratio d33 (BGS)/d33 (LGS) ) 1.18, which is of general agreement to that of the theoretical prediction as described above. Since the reported d33 coefficient for LGS is 10.7 pm/V,7 the derived result for BGS is 12.6 pm/V. As shown in Figure 6, the crystals exhibit high transparency in a broad spectral range from 350 nm to 13.7 µm, which covers the important band ranges of 3-5 µm and 8-14 µm of atmospheric transparent windows. In our measurements, the short-wavelength transmission cutoff extends to approximately 350 nm, which corresponds to band gap energy about 3.54 eV. In comparison to the values of LGS (3.87 eV), AGS (2.64 eV), and ZGP (1.75 eV), BGS crystal has a relatively large band gap energy among these materials, which suggests that it may have high laser damage threshold and can be used for mid-IR optics. The laser energy reached about 1.5 mJ until the surface damage occurred under the current experiment setup. Therefore, the damage threshold of BGS single-crystal may be derived from following equation
Ithreshold ) E/(πr2) ) 1.2J/cm2
(2)
where E is the laser energy of single pulse, and r is the 1/e2 beam diameter. The experimental results showed that BGS had
We have discovered a new NLO crystal BaGa4S7 for mid-IR nonlinear optics. Crystal boules up to Φ10 × 20 mm have been grown by a Bridgman-Stockbarger technique. Linear and NLO properties were measured, indicating a wide optical transparent region (350 nm to 13.7 µm), high second-order susceptibility coefficients (d33 ) 12.6 pm/V), and high laser damage threshold (1.2 J/cm2 at 1.064 µm and a 15 ns pulse width). Table 3 summarizes these properties in comparison to those of other mature mid-IR NLO crystals, which indicates that BGS would be a promising material for practical applications to high power frequency conversion in the mid-IR region. Acknowledgment. This material is based upon work supported by Science and Technology Innovation Founding of Chinese Academy of Sciences (Grant No. CXJJ-140). The authors thank Prof. Wenxiong Lin for the laser damage measurements. Supporting Information Available: X-ray crystallographic file in CIF format for the BGS single crystal. This material is available free of charge via the Internet at http://pubs.acs.org.
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