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Growth, structure and optical properties of nonlinear optical crystal BaZnBO3F
Journal of Solid State Chemistry 233 (2016) 58–61
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Growth, structure and optical properties of nonlinear optical crystal BaZnBO3F Mingjun Xia, R.K. Li n Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 9 October 2015 Accepted 9 October 2015
Nonlinear optical (NLO) crystal BaZnBO3F (BZBF) with the size of about 20 20 0.5 mm3 is obtained from BaF2–NaF flux, and single crystal X-ray diffraction reveals that it belongs to space group P6̅ with cell parameters of a¼ 5.1045(6) Å, c¼ 4.3116(10) Å and Z¼1. In the structure of BZBF, the BO3 planar triangles are interconnected through O atoms from ZnO3F2 trigonal bipyramid to form (Zn3B3O6F6) twelvemembered rings (12-MRs), then the layers which are built with condensation from 12-MRs at ab plane, are further linked by the apical F from ZnO3F2 to form three dimensional framework along the c direction. The title crystal exhibits high transmittance in the range of 300–3000 nm with a UV transmission cutoff at 223 nm according to transmission spectra. Powder SHG tests indicate that the effective NLO coefficient of BZBF crystal is about 2.8 times that of KH2PO4 (KDP) crystal due to perfect alignment of the BO3 groups. & 2015 Elsevier Inc. All rights reserved.
Keywords: BaZnBO3F Borate compound Nonlinear optical material Optical properties
1. Introduction Since the advent of β-BaB2O4 (BBO) crystal, borate crystals play an important role in the nonlinear optical (NLO) fields because of their excellent physical and chemical properties, e.g: wide transmission range, high damage threshold and rich chemistry types [1–3]. It was known that the elimination of the dangling bonds of B–O groups or partial oxygen substitution by fluorine atoms can push the UV cut-off blue shift, thus fluoroborate compounds have been intensively investigated due to their wide band gaps (short UV cut-off) [4]. Among them the famous KBe2BO3F2 (KBBF) family NLO crystals including KBBF, RbBe2BO3F2 (RBBF), and CsBe2BO3F2 (CBBF) were found [5–7]. Until now, KBBF and RBBF crystals are the only two NLO materials capable to produce deep UV (below 200 nm) laser output through a simple second harmonic generation (SHG) process. Important applications of those crystals included recent developed scientific instruments equipped with the deep UV laser sources, such as angle-resolved photoemission spectroscopy (ARPES) and photoemission electron microscopy (PEEM) [8,9]. Other related fluoroborate NLO crystals have been subsequently discovered, such as BaAlBO3F2 (BABF) [10], Ca5(BO3)3F [11], BaMBO3F (M ¼Ca, Mg) [12], M3B6O11F2 (M¼ Ba, Sr) [13], and so on. BaZnBO3F (BZBF) compound was firstly synthesized by our n
Corresponding author. E-mail address: [email protected] (R.K. Li).
http://dx.doi.org/10.1016/j.jssc.2015.10.015 0022-4596/& 2015 Elsevier Inc. All rights reserved.
group and its crystal structure was solved by powder X-ray diffraction method [12c]. It was very difficult to synthesize pure BZBF samples. BaF2 is always present as the main impurity. In this paper, single crystal of BZBF was successfully grown by high temperature solution method, and its single crystal structure was solved with more accurate crystallographic parameters. Besides, optical characterization of the crystalline samples including transmission spectra and SHG tests was investigated.
2. Experimental section 2.1. Crystal growth Analytical pure chemicals from Sinopharm Chemical Reagent Co., Ltd. of BaCO3 (157.87 g, 0.8 mol), ZnO (65.11 g, 0.8 mol), H3BO3 (49.46 g, 0.8 mol), BaF2 (70.13 g, 0.4 mol) and NaF (10.07 g, 0.24 mol) in molar ratios of 1:1:1:0.5:0.3 were mixed in an Φ 50 50 cm3 platinum crucible. Then the mixtures were heated to 850 °C and held for 12 h to homogenize the melt. After determining the saturation point, a crack-free seed which was chosen from spontaneous crystallization in several crystal growth runs was dipped into the melt, then the crucible temperature was decreased at a cooling rate of 0.2 °C/h. Finally, the crystal was pulled from the melt and cooled down to room temperature at a rate of 20 °C/h.
M. Xia, R.K. Li / Journal of Solid State Chemistry 233 (2016) 58–61
Table 1 Crystal data BaZnBO3F.
and
structure
Cell parameters (Å) Volume (Å3) Z Density (g/cm3) μ (mm 1) F(000) Crystal size (mm3) GOF on F2 R indices [I 42s(I)]a R indices (all data)a Flack factor Extinction coefficient
∑ FO
Table 3 The important bond distances (Å) for BaZnBO3F.
for
Ba–O 6 Ba–F 3 Zn–O 3
BaZnBO3F 280.52 0.71073 Hexagonal
Empirical formula Formula weight Wavelength (Å) Crystal system Space group
a R1 = ∑ FO − FC
refinement
; wR2={
P 6̅ a ¼5.1045(6) c ¼4.3116(10) 97.29(3) 1 4.788 16.102 124 0.08 0.03 0.03 1.230 R1 ¼ 0.0222, wR2 ¼ 0.0490 R1 ¼ 0.0222, wR2 ¼ 0.0490 0.00(9) 0.89(7)
2.7. SHG measurement
2.3. Powder X-ray diffraction X-ray diffraction (XRD) data were collected on a Bruker D8 Focus powder X-ray diffractometer using Cu Kα radiation at room temperature in the 2θ range of 7–70° with the step size of 0.02° and counting time of 0.1 s per step. The XRD patterns on ground crystals are well indexed and in good agreement with the calculated patterns from single crystal structure (Fig. S1). 2.4. Thermal analysis The differential scanning calorimetric (DSC) analysis was performed on a NETZSCH TG–DTA-MS apparatus under N2 flow with a sample heating rate of 15 °C per minute from room temperature to 960 °C. 2.5. Transmission spectroscopy The transmittance spectrum of BZBF crystal with the thickness about 0.15 mm was performed using a Perkin-Elmer Lambda 900 Table 2 Atomic coordinates and equivalent isotropic displacement parameters for BaZnBO3F. Ueq is defined as one third of the trace of the orthogonalized Uij tensor. z 0.5 0 0 0.5 0
2.1558(5) 1.381(4)
Single crystal of BZBF was chosen to perform Raman spectra on a Renishaw inVia-Reflex micro-zone Raman spectrometer. The room Raman spectrum was recorded in the range from 50 to 1700 cm 1 under 532 nm excitation.
∑[w (F 2 − F 2 )2] 1 O C }2 ∑[w (F 2 )2] O
y 0.6667 0.3333 0.7406(10) 0.3333 0
Zn–F 2 B–O 3
2.6. Raman spectroscopy
The crystal data were collected on a Bruker Smart APEX II diffractometer equipped with Mo Kα radiation (λ ¼0.71073 Å) at room temperature. The crystal structure was solved by SHELXS-97 and refined by full matrix least squares on F2 by SHELXL-97 programs [14]. The structure was checked for missing symmetry elements using the PLATON program [15]. The detailed crystallographic data for BZBF are given in Table 1. Atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. The selected bond lengths are presented in Table 3.
x 0.3333 0.6667 0.7195(12) 0.6667 0
2.816(4) 2.9471(4) 1.958(5)
UV–vis-NIR spectrometer in the range of 185–3000 nm. The interference pattern of BZBF crystal along the c axis was observed on an Olympus BX51TRF microscopy.
2.2. Single crystal X-ray diffraction
Atom Ba Zn O F B
59
Ueq 0.0058(4) 0.0046(4) 0.0126(10) 0.0189(12) 0.0169(13)
The polycrystalline BZBF samples ground by using as-grown crystal were sieved into six different particle sizes ( o20, 20–50, 50–75, 75–105, 105–150, and 150–200 μm). The sieved KH2PO4 (KDP) crystals in the same size with BZBF were referred as the reference. The intensity of the second harmonic at 532 nm emitted from the samples illuminated with a Q-switched Nd:YAG 1064 nm laser was recorded by the photomultiplier tube.
3. Results and discussion BZBF crystal with the dimensions of about 20 20 0.5 mm3 was obtained for about twenty days growth period (Fig. S2). It exhibits typical layer habit, which resembles that of KBBF family crystals. The uniaxial character of the crystal is obviously observed, confirming the single crystal structure. 3.1. Crystal structure The crystallographic parameters of BZBF are in good agreement with that obtained by powder X-ray diffraction [12c]. In the structure, B atom coordinates to three O atoms in an ideal D3h symmetry with bond length of 1.381(4) Å, which is slightly longer than that of the powder one (1.372(7) Å) but reasonable in agreement considering the standard deviation. Zn atom is connected with three O atoms and two F atoms to form ZnO3F2 trigonal bipyramid (Fig. 1). The bond distances of Zn–O (1.958(5) Å) and Zn–F (2.1558(5) Å) are also in good agreement with the previous values (d(Zn–O) ¼ 1.940(11) Å and d(Zn–F) ¼2.140 Å). The (ZnBO3F)2 layers at ab plane are formed by (Zn3B3O12F6) twelvemembered rings (12-MRs) which are built from the BO3 and ZnO3F2 groups with interconnection via corner O atoms. The (Zn3B3O12F6) 12-MRs, which resemble (Al3B3O12F6) and in BABF and (Al3B3O15) in K2Al2B2O7 (KABO) [16], respectively, were also found in two centrosymmetric zinc-containing fluoroborates KMZn2(BO3)2F (M ¼Ca, Cd) [17]. Then the (ZnBO3F)2 layers are further connected through the apical F atoms of ZnO3F2 bipyramids to form a three dimensional framework. The Ba atom, which is in coordination to six O atoms and three F atoms, occupies among the layers and maintains charge balance. 3.2. Thermal analysis DSC curve exhibits one single endothermic peak in the heating curve and two exothermic peaks in the cooling curve, indicating that BZBF melts incongruently (Fig. S3). Powder XRD on residual
60
M. Xia, R.K. Li / Journal of Solid State Chemistry 233 (2016) 58–61
Fig. 1. (a) The crystal structure viewed along the c axis, and (b) the layer at ab plane. (Ba, blue; Zn, purple; O, red; F, yellow; B, green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
after DSC measurement demonstrates that BZBF decomposes to BaF2 and ZnO after melting, further confirming that BZBF is an incongruent melt compound (Fig. S1). 3.3. Optical properties As presented in Fig. 2, the transmission spectra of BZBF crystal possess high transmittance (80%) in the range from 300 to 3000 nm, with the UV absorption edge at about 223 nm. Comparing to β-Zn3BPO7 crystal, the UV cut-off of BZBF is blue shifted about 27 nm possibly resulting from the two Zn–F bonds substitution of one Zn–O bond while Zn atoms are only coordinate to four O atoms in β-Zn3BPO7[18]. 3.4. Raman spectra The peaks located at 1231 cm 1, 936 cm 1 and 651 cm 1 can be assigned to the B–O asymmetric stretching (ν4), symmetric stretching (ν1) and out-of-plane bending (ν3) of the BO3 group, which are in good accordance with the structure solution of only one single BO3 group in ideal D3h symmetry in the unit cell [19]. The strongest peak at 387 cm 1 and a weak peak at 311 cm 1 may be attributed to the symmetric stretching modes (ν1, ν2) of the ZnO3F2 unit also in D3h symmetry [20,21]. And 181 cm 1 and 121 cm 1 may be from the vibrations of BaO6F3 group or lattice vibrations. The remaining smaller peaks around 1484 cm 1 might be due to muti-phonon process of Zn–O and BO3 groups (Fig. 3).
Fig. 3. Room temperature Raman spectra of BZBF.
Fig. 4. SHG intensities of BZBF versus particle sizes. (KDP as a reference).
3.5. SHG tests
Fig. 2. The transmittance spectrum of BZBF crystal. (Inset is in the range of 185– 500 nm).
The powder SHG measurement at 1064 nm shows that BZBF crystal is a phase matchable material according to the Kurtz–Perry rule (Fig. 4) [22]. It is well known that the powder SHG signal intensity is proportional to squares of effective NLO coefficient. Thus, the effective NLO coefficient of BZBF crystal is estimated to
M. Xia, R.K. Li / Journal of Solid State Chemistry 233 (2016) 58–61
be about 2.8 times that of KH2PO4 (KDP) reference crystal in the particle size range of 150–200 μm according to the square roots of the ratios of their corresponding SHG signal intensities. The obtained NLO coefficient is also in accordance with the reported calculated and experimental values as reported in reference [12c].
4. Conclusions NLO crystal BZBF with the dimensions of 20 20 0.5 mm3 has been grown by high temperature solution method in the BaF2–NaF flux. In the structure of BZBF, the (Zn3B3O12F6) 12-MRs are formed by sharing equatorial oxygen atoms from BO3 planar triangle and ZnO3F2 trigonal bipyramidal groups. Those 12-MRs are inter-connected to form two dimensional (ZnBO3F)2 layers. The BO3 groups in the whole structure are in the perfect alignment manner, which are the main contributor to the macroscopic SHG coefficients and result in an observed large NLO effective coefficient (2.8 deff(KDP)). The title crystal also possesses chemical stability and high transmittance in the wavelength range of 300–3000 nm with UV cut-off 223 nm. Due to superior optical properties in the visible to UV range, it is necessary to exploit several methods or flux to overcome strong anisotropic growth habit for growth of large size crystals in the future.
Acknowdgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 51502307) and National Instrumentation Program (No. 2012YQ120048).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2015.10.015.
References [1] C.T. Chen, T. Sasaki, R.K. Li, Y.C. Wu, Z.S. Lin, Y. Mori, Z.G. Hu, J.Y. Wang, G. Aka, M. Yoshimura, Y. Kaneda, Nonlinear Optical Borate Crystals, Weily-VCH, Germany, 2012. [2] D.N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Growth, structure and optical properties of nonlinear optical crystal BaZnBO3F Mingjun Xia, R.K. Li n Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 9 October 2015 Accepted 9 October 2015
Nonlinear optical (NLO) crystal BaZnBO3F (BZBF) with the size of about 20 20 0.5 mm3 is obtained from BaF2–NaF flux, and single crystal X-ray diffraction reveals that it belongs to space group P6̅ with cell parameters of a¼ 5.1045(6) Å, c¼ 4.3116(10) Å and Z¼1. In the structure of BZBF, the BO3 planar triangles are interconnected through O atoms from ZnO3F2 trigonal bipyramid to form (Zn3B3O6F6) twelvemembered rings (12-MRs), then the layers which are built with condensation from 12-MRs at ab plane, are further linked by the apical F from ZnO3F2 to form three dimensional framework along the c direction. The title crystal exhibits high transmittance in the range of 300–3000 nm with a UV transmission cutoff at 223 nm according to transmission spectra. Powder SHG tests indicate that the effective NLO coefficient of BZBF crystal is about 2.8 times that of KH2PO4 (KDP) crystal due to perfect alignment of the BO3 groups. & 2015 Elsevier Inc. All rights reserved.
Keywords: BaZnBO3F Borate compound Nonlinear optical material Optical properties
1. Introduction Since the advent of β-BaB2O4 (BBO) crystal, borate crystals play an important role in the nonlinear optical (NLO) fields because of their excellent physical and chemical properties, e.g: wide transmission range, high damage threshold and rich chemistry types [1–3]. It was known that the elimination of the dangling bonds of B–O groups or partial oxygen substitution by fluorine atoms can push the UV cut-off blue shift, thus fluoroborate compounds have been intensively investigated due to their wide band gaps (short UV cut-off) [4]. Among them the famous KBe2BO3F2 (KBBF) family NLO crystals including KBBF, RbBe2BO3F2 (RBBF), and CsBe2BO3F2 (CBBF) were found [5–7]. Until now, KBBF and RBBF crystals are the only two NLO materials capable to produce deep UV (below 200 nm) laser output through a simple second harmonic generation (SHG) process. Important applications of those crystals included recent developed scientific instruments equipped with the deep UV laser sources, such as angle-resolved photoemission spectroscopy (ARPES) and photoemission electron microscopy (PEEM) [8,9]. Other related fluoroborate NLO crystals have been subsequently discovered, such as BaAlBO3F2 (BABF) [10], Ca5(BO3)3F [11], BaMBO3F (M ¼Ca, Mg) [12], M3B6O11F2 (M¼ Ba, Sr) [13], and so on. BaZnBO3F (BZBF) compound was firstly synthesized by our n
Corresponding author. E-mail address: [email protected] (R.K. Li).
http://dx.doi.org/10.1016/j.jssc.2015.10.015 0022-4596/& 2015 Elsevier Inc. All rights reserved.
group and its crystal structure was solved by powder X-ray diffraction method [12c]. It was very difficult to synthesize pure BZBF samples. BaF2 is always present as the main impurity. In this paper, single crystal of BZBF was successfully grown by high temperature solution method, and its single crystal structure was solved with more accurate crystallographic parameters. Besides, optical characterization of the crystalline samples including transmission spectra and SHG tests was investigated.
2. Experimental section 2.1. Crystal growth Analytical pure chemicals from Sinopharm Chemical Reagent Co., Ltd. of BaCO3 (157.87 g, 0.8 mol), ZnO (65.11 g, 0.8 mol), H3BO3 (49.46 g, 0.8 mol), BaF2 (70.13 g, 0.4 mol) and NaF (10.07 g, 0.24 mol) in molar ratios of 1:1:1:0.5:0.3 were mixed in an Φ 50 50 cm3 platinum crucible. Then the mixtures were heated to 850 °C and held for 12 h to homogenize the melt. After determining the saturation point, a crack-free seed which was chosen from spontaneous crystallization in several crystal growth runs was dipped into the melt, then the crucible temperature was decreased at a cooling rate of 0.2 °C/h. Finally, the crystal was pulled from the melt and cooled down to room temperature at a rate of 20 °C/h.
M. Xia, R.K. Li / Journal of Solid State Chemistry 233 (2016) 58–61
Table 1 Crystal data BaZnBO3F.
and
structure
Cell parameters (Å) Volume (Å3) Z Density (g/cm3) μ (mm 1) F(000) Crystal size (mm3) GOF on F2 R indices [I 42s(I)]a R indices (all data)a Flack factor Extinction coefficient
∑ FO
Table 3 The important bond distances (Å) for BaZnBO3F.
for
Ba–O 6 Ba–F 3 Zn–O 3
BaZnBO3F 280.52 0.71073 Hexagonal
Empirical formula Formula weight Wavelength (Å) Crystal system Space group
a R1 = ∑ FO − FC
refinement
; wR2={
P 6̅ a ¼5.1045(6) c ¼4.3116(10) 97.29(3) 1 4.788 16.102 124 0.08 0.03 0.03 1.230 R1 ¼ 0.0222, wR2 ¼ 0.0490 R1 ¼ 0.0222, wR2 ¼ 0.0490 0.00(9) 0.89(7)
2.7. SHG measurement
2.3. Powder X-ray diffraction X-ray diffraction (XRD) data were collected on a Bruker D8 Focus powder X-ray diffractometer using Cu Kα radiation at room temperature in the 2θ range of 7–70° with the step size of 0.02° and counting time of 0.1 s per step. The XRD patterns on ground crystals are well indexed and in good agreement with the calculated patterns from single crystal structure (Fig. S1). 2.4. Thermal analysis The differential scanning calorimetric (DSC) analysis was performed on a NETZSCH TG–DTA-MS apparatus under N2 flow with a sample heating rate of 15 °C per minute from room temperature to 960 °C. 2.5. Transmission spectroscopy The transmittance spectrum of BZBF crystal with the thickness about 0.15 mm was performed using a Perkin-Elmer Lambda 900 Table 2 Atomic coordinates and equivalent isotropic displacement parameters for BaZnBO3F. Ueq is defined as one third of the trace of the orthogonalized Uij tensor. z 0.5 0 0 0.5 0
2.1558(5) 1.381(4)
Single crystal of BZBF was chosen to perform Raman spectra on a Renishaw inVia-Reflex micro-zone Raman spectrometer. The room Raman spectrum was recorded in the range from 50 to 1700 cm 1 under 532 nm excitation.
∑[w (F 2 − F 2 )2] 1 O C }2 ∑[w (F 2 )2] O
y 0.6667 0.3333 0.7406(10) 0.3333 0
Zn–F 2 B–O 3
2.6. Raman spectroscopy
The crystal data were collected on a Bruker Smart APEX II diffractometer equipped with Mo Kα radiation (λ ¼0.71073 Å) at room temperature. The crystal structure was solved by SHELXS-97 and refined by full matrix least squares on F2 by SHELXL-97 programs [14]. The structure was checked for missing symmetry elements using the PLATON program [15]. The detailed crystallographic data for BZBF are given in Table 1. Atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. The selected bond lengths are presented in Table 3.
x 0.3333 0.6667 0.7195(12) 0.6667 0
2.816(4) 2.9471(4) 1.958(5)
UV–vis-NIR spectrometer in the range of 185–3000 nm. The interference pattern of BZBF crystal along the c axis was observed on an Olympus BX51TRF microscopy.
2.2. Single crystal X-ray diffraction
Atom Ba Zn O F B
59
Ueq 0.0058(4) 0.0046(4) 0.0126(10) 0.0189(12) 0.0169(13)
The polycrystalline BZBF samples ground by using as-grown crystal were sieved into six different particle sizes ( o20, 20–50, 50–75, 75–105, 105–150, and 150–200 μm). The sieved KH2PO4 (KDP) crystals in the same size with BZBF were referred as the reference. The intensity of the second harmonic at 532 nm emitted from the samples illuminated with a Q-switched Nd:YAG 1064 nm laser was recorded by the photomultiplier tube.
3. Results and discussion BZBF crystal with the dimensions of about 20 20 0.5 mm3 was obtained for about twenty days growth period (Fig. S2). It exhibits typical layer habit, which resembles that of KBBF family crystals. The uniaxial character of the crystal is obviously observed, confirming the single crystal structure. 3.1. Crystal structure The crystallographic parameters of BZBF are in good agreement with that obtained by powder X-ray diffraction [12c]. In the structure, B atom coordinates to three O atoms in an ideal D3h symmetry with bond length of 1.381(4) Å, which is slightly longer than that of the powder one (1.372(7) Å) but reasonable in agreement considering the standard deviation. Zn atom is connected with three O atoms and two F atoms to form ZnO3F2 trigonal bipyramid (Fig. 1). The bond distances of Zn–O (1.958(5) Å) and Zn–F (2.1558(5) Å) are also in good agreement with the previous values (d(Zn–O) ¼ 1.940(11) Å and d(Zn–F) ¼2.140 Å). The (ZnBO3F)2 layers at ab plane are formed by (Zn3B3O12F6) twelvemembered rings (12-MRs) which are built from the BO3 and ZnO3F2 groups with interconnection via corner O atoms. The (Zn3B3O12F6) 12-MRs, which resemble (Al3B3O12F6) and in BABF and (Al3B3O15) in K2Al2B2O7 (KABO) [16], respectively, were also found in two centrosymmetric zinc-containing fluoroborates KMZn2(BO3)2F (M ¼Ca, Cd) [17]. Then the (ZnBO3F)2 layers are further connected through the apical F atoms of ZnO3F2 bipyramids to form a three dimensional framework. The Ba atom, which is in coordination to six O atoms and three F atoms, occupies among the layers and maintains charge balance. 3.2. Thermal analysis DSC curve exhibits one single endothermic peak in the heating curve and two exothermic peaks in the cooling curve, indicating that BZBF melts incongruently (Fig. S3). Powder XRD on residual
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Fig. 1. (a) The crystal structure viewed along the c axis, and (b) the layer at ab plane. (Ba, blue; Zn, purple; O, red; F, yellow; B, green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
after DSC measurement demonstrates that BZBF decomposes to BaF2 and ZnO after melting, further confirming that BZBF is an incongruent melt compound (Fig. S1). 3.3. Optical properties As presented in Fig. 2, the transmission spectra of BZBF crystal possess high transmittance (80%) in the range from 300 to 3000 nm, with the UV absorption edge at about 223 nm. Comparing to β-Zn3BPO7 crystal, the UV cut-off of BZBF is blue shifted about 27 nm possibly resulting from the two Zn–F bonds substitution of one Zn–O bond while Zn atoms are only coordinate to four O atoms in β-Zn3BPO7[18]. 3.4. Raman spectra The peaks located at 1231 cm 1, 936 cm 1 and 651 cm 1 can be assigned to the B–O asymmetric stretching (ν4), symmetric stretching (ν1) and out-of-plane bending (ν3) of the BO3 group, which are in good accordance with the structure solution of only one single BO3 group in ideal D3h symmetry in the unit cell [19]. The strongest peak at 387 cm 1 and a weak peak at 311 cm 1 may be attributed to the symmetric stretching modes (ν1, ν2) of the ZnO3F2 unit also in D3h symmetry [20,21]. And 181 cm 1 and 121 cm 1 may be from the vibrations of BaO6F3 group or lattice vibrations. The remaining smaller peaks around 1484 cm 1 might be due to muti-phonon process of Zn–O and BO3 groups (Fig. 3).
Fig. 3. Room temperature Raman spectra of BZBF.
Fig. 4. SHG intensities of BZBF versus particle sizes. (KDP as a reference).
3.5. SHG tests
Fig. 2. The transmittance spectrum of BZBF crystal. (Inset is in the range of 185– 500 nm).
The powder SHG measurement at 1064 nm shows that BZBF crystal is a phase matchable material according to the Kurtz–Perry rule (Fig. 4) [22]. It is well known that the powder SHG signal intensity is proportional to squares of effective NLO coefficient. Thus, the effective NLO coefficient of BZBF crystal is estimated to
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be about 2.8 times that of KH2PO4 (KDP) reference crystal in the particle size range of 150–200 μm according to the square roots of the ratios of their corresponding SHG signal intensities. The obtained NLO coefficient is also in accordance with the reported calculated and experimental values as reported in reference [12c].
4. Conclusions NLO crystal BZBF with the dimensions of 20 20 0.5 mm3 has been grown by high temperature solution method in the BaF2–NaF flux. In the structure of BZBF, the (Zn3B3O12F6) 12-MRs are formed by sharing equatorial oxygen atoms from BO3 planar triangle and ZnO3F2 trigonal bipyramidal groups. Those 12-MRs are inter-connected to form two dimensional (ZnBO3F)2 layers. The BO3 groups in the whole structure are in the perfect alignment manner, which are the main contributor to the macroscopic SHG coefficients and result in an observed large NLO effective coefficient (2.8 deff(KDP)). The title crystal also possesses chemical stability and high transmittance in the wavelength range of 300–3000 nm with UV cut-off 223 nm. Due to superior optical properties in the visible to UV range, it is necessary to exploit several methods or flux to overcome strong anisotropic growth habit for growth of large size crystals in the future.
Acknowdgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 51502307) and National Instrumentation Program (No. 2012YQ120048).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2015.10.015.
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