crystals Article
Crystal Growth and Associated Properties of a Nonlinear Optical Crystal—Ba2Zn(BO3)2 Weiguo Zhang 1 , Hongwei Yu 1 , Hongping Wu 1,2 and P. Shiv Halasyamani 1, * 1 2
*
Department of Chemistry, University of Houston, 112 Fleming Building, Houston, TX 77204-5003, USA;
[email protected] (W.Z.);
[email protected] (H.Y.);
[email protected] (H.W.) Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China Correspondence:
[email protected]; Tel.: +1-713-743-7716
Academic Editors: Helmut Cölfen and Ning Ye Received: 25 May 2016; Accepted: 10 June 2016; Published: 15 June 2016
Abstract: Crystals of Ba2 Zn(BO3 )2 were grown by the top-seeded solution growth (TSSG) method. The optimum flux system for growing Ba2 Zn(BO3 )2 crystals was 2BaF2 :2.5B2 O3 . The transmission spectra of a (100)-orientated crystal indicated an absorption edge of 230 nm. Powder second-harmonic generation measurement revealed that Ba2 Zn(BO3 )2 can achieve type-I phase matching behavior at the fundamental wavelengths of 1064 and 532 nm respectively. The second-harmonic generating efficiency is around 0.85 and 0.58 times that of β-BaB2 O4 when radiated with 1064 and 532 nm lasers. Keywords: crystal growth; top-seeded solution growth method; borates; Ba2 Zn(BO3 )2 ; second-harmonic generation
1. Introduction Nonlinear optical (NLO) materials play a very important role in laser applications owing to their ability to produce coherent light and thus expand the spectral ranges of solid state lasers from ultraviolet (UV) to infrared (IR) [1–4]. Among these NLO materials, borate NLO materials have predominance in UV applications owing to their high UV transmittance and large laser damage threshold (LDT) [2,5]. The first report of NLO phenomena in borates was on KB5 O8 ¨ 4H2 O (KB5) [6]. The intense research on the NLO phenomena of borates was triggered by the second-harmonic generation (SHG) reported on β-BaB2 O4 (β-BBO) and LiB3 O5 (LBO) in the late 1980s [7,8]. Thereafter, a variety of NLO borate materials have been discovered, such as BaZn2 (BO3 )2 [9], CsB3 O5 (CBO) [10], Ba2 Zn(BO3 )2 [11], KBe2 BO3 F2 (KBBF) [12], Sr2 Be2 B2 O7 (SBBO) [13], CsLiB6 O10 (CLBO) [14], K2 Al2 B2 O7 (KABO) [15], BaAl2 B2 O7 (BABO) [15], LiAB4 O7 (A = K, Rb) [16], M2 B5 O9 X (M = Pb, Ca, Sr, Ba; X = Cl, Br) [17], MBi2 B2 O7 (M = Ca, Sr) [18], Ca5 (BO3 )3 F [19], BiAlGa2 (BO3 )4 [20], Bi2 ZnOB2 O6 [21], Ba3 Sr4 (BO3 )3 F5 [22], BaMBO3 F (M = Zn, Mg) [23], M3 B6 O11 F2 (M = Sr, Ba) [24], K3 B6 O10 Cl [25], Li3 Cs2 B5 O10 [26], Li4 Cs3 B7 O14 [27], K2 SrVB5 O12 [28], Pb4 O(BO3 )2 [29], KSr4 B3 O9 [30], Ba4 B11 O20 F [31], Cs3 Zn6 B9 O21 [32], Li2 Sr4 B12 O23 [33], Li4 Sr(BO3 )2 [34] and K3 Ba3 Li2 Al4 B6 O20 F [35]. Despite these new borate-based NLO materials, β-BBO, LBO and CLBO are still the most frequently used NLO crystals in UV region, which is attributable to the difficulties of growing high quality crystals or the low transmittance in the UV region. In order to explain the relationship between the microstructure and NLO properties of a crystal, Chen et al. proposed an ‘anionic group theory’ which considered the anionic group mainly responsible for the bulk NLO coefficient [36]. According to the anionic group theory, only 10 kinds of (Bx Oy )n ´ units would be of practical interest to borate NLO materials, i.e., (BO3 )3´ , (BO4 )5´ , (B2 O5 )4´ , (B2 O7 )8´ , (B3 O6 )3´ , (B3 O7 )5´ , (B3 O8 )7´ , (B3 O9 )9´ , (B4 O9 )6´ , and (B5 O10 )5´ [36]. As some large units can be taken as linked small units, such as (B3 O9 )9´ formed by three end-to-end corner-shared BO4 tetrahedra, microscopic second-order susceptibilities (χ2 ) and Crystals 2016, 6, 68; doi:10.3390/cryst6060068
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band gaps (∆Eg ) of four primary borate anionic units were calculated and the relative orders are 68 O )5´ > χ2 (BO )3´ >> χ2 (BO )5´ and ∆E (BO )5´ > ∆E (BO )3´ « ∆E (B 2 of 7 5´ « χ6,2 (B χ2 (B3Crystals O6 )3´2016, g g g 3 O7 ) 3 7 3 4 4 3 > ∆Eg (B3 O6 )3´ , respectively [1]. Even though the (B3 O6 )3´ group has the largest second-order corner-shared BO4 tetrahedra, microscopic second-order susceptibilities (χ2) and band gaps (ΔEg) of 3´ group seems to be the optimum choice for borate NLO materials based on susceptibility, the (BO ) 3 anionic units were calculated and the relative orders are χ2(B3O6)3− ≈ χ2(B3O7)5− > four primary borate overall considerations of nonlinearity, birefringence, UV cut-off and laser damage threshold [1,37]. χ2(BO 3)3− >> χ2(BO4)5− and ΔEg(BO4)5− > ΔEg(BO3)3− ≈ ΔEg(B3O7)5− > ΔEg(B3O6)3−, respectively [1]. Even Among the(Baforementioned newly discovered borates, Ba2 Zn(BO )2 crystalizes in thetoacentric though the 3O6)3− group has the largest second-order susceptibility, the3(BO 3)3− group seems be spacethe group, Pca2choice , and its structure is composed of ZnO tetrahedra and BO triangles [11]. optimum for borate NLO materials based on overall considerations of nonlinearity, 1 4 3 ˝ birefringence, UV cut-off and laser damage threshold [1,37]. In addition, thermal analysis indicated that Ba2 Zn(BO3 )2 may congruently melt around 984 C which thecrystal aforementioned newly borates, Ba2Zn(BO 3)2 crystalizes the acentric provides aAmong piece of grown from meltdiscovered for structure refinement [11]. CalculatedinSHG coefficients space group, Pca2 1, and its structure is composed of ZnO4 tetrahedra and BO3 triangles [11]. In were also reported, i.e., d24 = 0.61 pm/V, around one third of d11 of β-BBO [11]. Other than the crystal addition, thermal analysis indicated that Ba2Zn(BO3)2 may congruently melt around 984 °C which structure, thermal properties and calculated SHG coefficients, neither large size crystals nor other provides a piece of crystal grown from melt for structure refinement [11]. Calculated SHG physical properties were reported. Considering that Ba2 Zn(BO3 )2 melts congruently and (BO3 )3´ unit coefficients were also reported, i.e., d24 = 0.61 pm/V, around one third of d11 of β-BBO [11]. Other than resides its structure, wethermal suggest that Ba2 and Zn(BO be coefficients, easily grown as large crystals and show 3 )2 could theincrystal structure, properties calculated SHG neither large size crystals a higher than calculated. In this Considering work, we report on2Zn(BO crystal growth as well as physical nor SHG other efficiency physical properties were reported. that Ba 3)2 melts congruently and 3− properties of Ba Zn(BO ) . (BO3) unit2 resides 3in2 its structure, we suggest that Ba2Zn(BO3)2 could be easily grown as large crystals and show a higher SHG efficiency than calculated. In this work, we report on crystal growth
2. Results Discussion as welland as physical properties of Ba2Zn(BO3)2.
2.1. Polycrystalline Synthesis and Characterization 2. Results and Powder Discussion Polycrystalline Ba2 Zn(BO3 )2 was synthesized through standard solid-state techniques (see 2.1. Polycrystalline Powder Synthesis and Characterization Section 3.1). The powder X-ray diffraction (XRD) pattern of the polycrystalline material is in good Ba2Zn(BO 3)2 was synthesized through standard solid-state techniques (see agreementPolycrystalline with the calculated pattern from the single crystal data (see Figure 1) [11]. As mentioned Section 3.1). The powder X-ray diffraction (XRD)˝pattern of the polycrystalline material is in good earlier [11], Ba2 Zn(BO3 )2 melts congruently at 984 C and Ba2 Zn(BO3 )2 crystals for structure analysis agreement with the calculated pattern from the single crystal data (see Figure 1) [11]. As mentioned were grown from melt at 1050 ˝ C. 1 g of polycrystalline Ba2 Zn(BO3 )2 , placed in a platinum crucible earlier [11], Ba2Zn(BO 3)2 melts congruently at 984 °C and Ba2Zn(BO3)2 crystals for structure analysis ˝ to determine if a homogenous melt could be obtained. However, a transparent and heated to 1100 were grown fromCmelt at 1050 °C. 1 g of polycrystalline Ba2Zn(BO3)2 , placed in a platinum crucible ˝ C for 20 h. homogenous melt was°Cnot obtainedifeven after maintaining temperature at 1100 and heated to 1100 to determine a homogenous melt could the be obtained. However, a transparent ˝ After homogenous slow cooling (50wasC/h) to roomeven temperature, Ba2 Zn(BO melt not obtained after maintaining the temperature at 1100 °Cdecomposed for 20 h. Aftereven 3 )2 was partially though the main(50 phase not change according to powder XRD (see Figure 1). The extrathe peaks slow cooling °C/h)did to room temperature, Ba2Zn(BO 3)2 was partially decomposed even though main by phase didarrows not change according to powder XRD Figure 1). 79-0206). The extra peaks indicated by of indicated black in the figure are assigned to(see ZnO (PDF# Therefore, crystals black 3arrows in the figure assigned ZnO (PDF# 79-0206). Therefore, crystals of Ba2Zn(BO3)2 Ba2 Zn(BO )2 cannot simply beare grown fromtoits stoichiometric melt. cannot simply be grown from its stoichiometric melt.
Figure1.1. Calculated experimental XRD of Ba2Zn(BO . The black arrows refer to ZnO (PDF# 79-0206). Figure Calculatedand and experimental XRD of Ba32)2Zn(BO 3 )2 . The black arrows refer to ZnO (PDF# 79-0206). 2.2. Crystal Growth
Our experiments indicate that Ba2Zn(BO3)2 does not melt congruently. Therefore, a modified 2.2. Crystal Growth flux method, top-seeded solution growth (TSSG), was employed to grow Ba2Zn(BO3)2 crystals. We
Our experiments indicate that Baas not melt congruently. Therefore, a modified 2 Zn(BO 3 )2 does attempted many flux systems, such B2O3, BaO-B 2O3, and BaF2-B2O3. When solely taking B2O3 as flux method, top-seeded solution growth (TSSG), was employed to grow Ba2 Zn(BO3 )2 crystals.
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We attempted many flux systems, such as B2 O3 , BaO–B2 O3 , and BaF2 –B2 O3 . When solely taking 2016,we 6, 68can obtain Ba2 Zn(BO3 )2 crystals from the system, but the viscosity is very 3 of 7 high B2 O3Crystals as flux, that prevents growing large and high quality crystals. A BaO–B2 O3 flux also works for growing flux, we can obtain Ba2Zn(BO3)2 crystals from the system, but the viscosity is very high that prevents Ba2 Zn(BO3 )2 crystals. However, adding BaO to the system raised the crystallization temperature and growing large and high quality crystals. A BaO-B2O3 flux also works for growing Ba2Zn(BO3)2 narrowed the growth temperature range. After many attempts, a BaF2 –B2 O3 flux was determined to crystals. However, adding BaO to the system raised the crystallization temperature and narrowed be optimum to grow Ba2 Zn(BO The attempts, proper molar ratio for growing Ba2 Zn(BO3 to )2 crystals 3 )2 crystals. the growth temperature range. After many a BaF 2–B2O3 flux was determined be is Ba2optimum Zn(BO3 )to :B O :BaF = 15:2.5:2. Figure 2 shows the as-grown crystals and the powder 2 grow 2 3 Ba2Zn(BO 2 3)2 crystals. The proper molar ratio for growing Ba2Zn(BO3)2 crystals is XRD patterns confirmed phase (see red pattern in Figure 3). From Figure it is clear that Ba2Zn(BO 3)2:B2Oits 3:BaF 2 = 15:2.5:2. Figure 2 shows the as-grown crystals and2,the powder XRDBa patterns 2 Zn(BO3 )2 confirmed its phase (seegrowth. red pattern Figure face 3). From 2, ittoisbe clear that Ba2Zn(BO 3)2 crystals crystals layer during their The in layered was Figure indexed (100) plane (see blue pattern in layer their growth. face wasofindexed to be (100) plane (see blue pattern in Figure 3). during As discussed earlierThe [11],layered the structure Ba2 Zn(BO ) crystal contains two dimensional 3 2 Figure 3). As discussed earlier [11], the structure of Ba 2Zn(BO3)2 crystal contains two dimensional layers which are perpendicular to the (100) direction. This may explain the layered growth tendency which are perpendicular to the (100) direction. This may explain the layered growth tendency of Ba2layers Zn(BO 3 )2 crystals. of Ba2Zn(BO3)2 crystals.
Figure 2. Photo of as-grown BaBa Theminimum minimum scale on the is 1 mm. Figure 2. Photo of as-grown 2Zn(BO crystals. The scale on the rulerruler is 1 mm. 2 Zn(BO 3 3))22 crystals.
Figure 3. Calculated and experimental patternsofofBaBa Figure 3. Calculated and experimental XRD XRD patterns 2Zn(BO 3)2 crystals. 2 Zn(BO 3 )2 crystals.
2.3. UV-Visible-Near Infrared TransmissionSpectra Spectra 2.3. UV-Visible-Near Infrared Transmission The UV-visible-Near infrared (UV-vis-NIR) transmission spectra of an unpolished (100) plate of
The UV-visible-Near infrared (UV-vis-NIR) transmission spectra of an unpolished (100) plate Ba2Zn(BO3)2 with a size of 6 × 4 × 0.2 mm3 3indicated that the UV transmission starts to rapidly of Badecrease )2around with a245 sizenm ofand 6 ˆthe 4 ˆUV 0.2cut-off mm edge indicated that the UV transmission starts to rapidly 2 Zn(BO3at for Ba2Zn(BO3)2 is around 230 nm (see Figure 4). decrease at around 245 nm and the UV cut-off edge for Ba around 230 nmwell (seebelow Figure 4). 2 Zn(BO 3 )2 is could According to Chen’s calculation, the borates containing (BO 3)3− group transmit 3 ´ According Chen’s calculation, the borates containing group could wellare below 200 nmto[1]. This result is consistent with KBBF and KABO(BO crystals where the UV transmit cut-off edges 3) 200 nm [1]. This result is consistent with KBBF and KABO crystals where the UV cut-off edges are
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155 and 180 nm respectively [38]. However, Chen’s calculation did not consider the contribution of 155 and2016, 1806,nm of7 Crystals 68 respectively [38]. However, Chen’s calculation did not consider the contribution of transition metals residing in borates to the UV cut-off edge. As the band gap of ZnO is 3.3 4eV [39], transition metals residing in borates to the UV cut-off edge. As the band gap of ZnO is 3.3 eV [39], adding Zn cations to borates may red-shift theChen’s UV cut-off edge, did e.g.,not BaZnBO a UV cut-offofedge 3 F has 155 andZn 180cations nm respectively consider adding to borates [38]. mayHowever, red-shift the UV calculation cut-off edge, e.g., BaZnBO 3Fthe hascontribution a UV cut-off of 223 nm, whereas BaMgBO F has a UV cut-off edge of 190 nm [40,41]. 3 borates to the UV cut-off edge. As the band gap of ZnO is 3.3 eV [39], transition metals residing in edge of 223 nm, whereas BaMgBO3F has a UV cut-off edge of 190 nm [40,41]. adding Zn cations to borates may red-shift the UV cut-off edge, e.g., BaZnBO3F has a UV cut-off edge of 223 nm, whereas BaMgBO3F has a UV cut-off edge of 190 nm [40,41].
Figure 4. UV-vis-NIR transmissionspectra spectraof ofBa Ba2Zn(BO 3)2 (100) plate. Insert is enlarged UV range. Figure 4. UV-vis-NIR transmission 2 Zn(BO3 )2 (100) plate. Insert is enlarged UV range.
2.4. Powder Measurements FigureSHG 4. UV-vis-NIR transmission spectra of Ba2Zn(BO3)2 (100) plate. Insert is enlarged UV range.
2.4. Powder SHG Measurements
Powder SHG efficiency versus particle size (20–125 um) (see Figure 5) revealed that Ba2Zn(BO3)2 2.4. Powder SHGefficiency Measurements Powder SHG versus particle (see Figure 5) revealed that Ba2 Zn(BO has an SHG efficiency approximately 0.85size and (20–125 1.5 timesum) of β-BaB 2O4 and KDP when radiated with 3 )2 has 1064 an SHG approximately 0.85 and timesum) ofofβ-BaB when radiated Powder SHG efficiency versus size1.5 (20–125 (see Figure 5) that Ba 2times Zn(BOof 3)with 2 2 O4 3and nm efficiency lasers respectively. Whileparticle the SHG efficiency Ba2Zn(BO )2 revealed is KDP around 0.58 has SHG efficiency approximately 0.85 and 1.5 times of β-BaB 2 O 4 and KDP when radiated with 1064β-BaB nman lasers respectively. While the SHG efficiency of Ba Zn(BO ) is around 0.58 times of β-BaB 2O4 under the fundamental wavelength of 532 2 nm. The 3 2 measurements also indicate 2 O4 1064 nm lasers respectively. While the SHG efficiency of Ba 2 Zn(BO 3also )2 is around 0.58 times of under the fundamental wavelength ofat532 nm. The measurements indicate Ba2 Zn(BO Ba2Zn(BO 3)2 is type-I phase-matchable both wavelengths, and falls in the class A category of SHG 3 )2 is β-BaB 2 O 4 under the fundamental wavelength of 532 nm. The measurements also indicate materials [42]. The decreased SHG efficiency at falls 532 nm is class probably attributable to the slightly [42]. type-I phase-matchable at both wavelengths, and in the A category of SHG materials Ba 2Zn(BO3transmission )2SHG is type-I phase-matchable at probably both wavelengths, andto falls the nm) class A category of SHG at the generating wavelength (266 (see Figuretransmission 4). The decreased decreased efficiency atsecond 532 nmharmonic is attributable theinslightly decreased materials [42]. The decreased SHG efficiency at 532 nm is probably attributable to the slightly at the second harmonic generating wavelength (266 nm) (see Figure 4). decreased transmission at the second harmonic generating wavelength (266 nm) (see Figure 4).
Figure 5. Powder SHG measurement of polycrystalline Ba2Zn(BO3)2 at the fundamental wavelengths of (a) 1064 nm (b) 532 nm. Note the curves are drawn to guide the eye and are not a fit to the data. Figure 5. Powder SHG measurementofofpolycrystalline polycrystalline Ba Ba2Zn(BO 3)2 at the fundamental wavelengths Figure 5. Powder SHG measurement 2 Zn(BO3 )2 at the fundamental wavelengths 3.ofMaterials and Methods (a) 1064 nm (b) nm. Note thecurves curvesare aredrawn drawn to to guide not a fit to the data. (a)of1064 nm (b) 532532 nm. Note the guidethe theeye eyeand andare are not a fit to the data.
3. Materials Methods 3.1. Materials and and Polycrystalline Powder Synthesis
3. Materials and Methods
Polycrystalline Ba2Zn(BO3)2 was synthesized by solid-state reaction techniques. Stoichiometric 3.1. Materials and Polycrystalline Powder Synthesis
3.1. amounts Materialsofand Polycrystalline Powder Synthesis BaCO 3 (Alfa Aesar, Ward Hill, MA, USA, 99%), ZnO (Alfa Aesar, Ward Hill, MA, USA,
Polycrystalline Ba2Zn(BO3)2 was synthesized by solid-state reaction techniques. Stoichiometric Polycrystalline was synthesized by solid-state reaction techniques. 2 Zn(BO 3 )2 Ward amounts of BaCOBa 3 (Alfa Aesar, Hill, MA, USA, 99%), ZnO (Alfa Aesar, Ward Hill,Stoichiometric MA, USA,
amounts of BaCO3 (Alfa Aesar, Ward Hill, MA, USA, 99%), ZnO (Alfa Aesar, Ward Hill, MA, USA,
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98%) and H3 BO3 (Alfa Aesar, Ward Hill, MA, USA, 98%) were ground, placed into a platinum crucible, and heated in air to 600 ˝ C for 24 h and 900 ˝ C for 40 h with intermittent re-grindings. 3.2. Powder X-Ray Diffraction Powder X-ray diffraction measurements were carried out on a PANalytical X’Pert Pro diffractometer (Almelo, The Netherlands) equipped with Cu Kα radiation (λ = 1.54056 Å) in the 2θ range from 10˝ to 70˝ . 3.3. Single Crystal Growth of Ba2 Zn(BO3 )2 Polycrystalline Ba2 Zn(BO3 )2 , H3 BO3 , and BaF2 (Alfa Aesar, 99%) with a molar ratio of 15:5:2 were placed in a platinum crucible, and heated to 950 ˝ C for 24 h in order to form a homogenous melt. Heating BaCO3 , ZnO, H3 BO3 and BaF2 with a molar ratio 30:15:35:2, at a rate of 50 ˝ C/h to 950 ˝ C and holding for 24 h will also form a homogenous melt. Either may be used for the crystal growth. Crystals of Ba2 Zn(BO3 )2 were first grown by spontaneous crystallization, i.e., crystals formed on a platinum wire dipped into the melt. Seed crystals were selected from these spontaneously grown crystals. A crystallization temperature of 937 ˝ C was determined by observing the growth or dissolution of the seed crystals when dipped into the melt. A piece of seed was introduced into the melt at a rotation rate of 10 rpm at 2 ˝ C higher than the crystallization temperature of 937 ˝ C in order to reduce surface defects. The temperature was decreased to the crystallization temperature at a cooling rate of 0.5 ˝ C/min. From the crystallization temperature, the melt was cooled at a rate of 0.5 ˝ C per day to 935 ˝ C. When the crystal growth process was done, i.e., by observing the gap between the growing crystal and the crucible wall (when the crystal edge is very close to the crucible wall the crystal growth is considered complete), the crystal is pulled from the melt and cooled (20 ˝ C/h) to room temperature. 3.4. Optical Spectra Measurement UV-vis-NIR transmission data were collected on a Varian Cary 5000 scan UV-vis-NIR spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA) over the 200–2000 nm spectral range at room temperature. An unpolished piece of (100) crystal plate with a thickness of 0.2 mm was used to perform the measurement. 3.5. Powder Second Harmonic Generation Measurement Powder SHG measurements were performed on a modified Kurtz-NLO system [42] using a pulse Nd:YAG laser (Quantel Laser, Ultra 50, Bozeman, MT, USA) with wavelengths of 1064 and 532 nm. A detailed description of the equipment and methodology has been published elsewhere [43]. As the powder SHG efficiency has been shown to depend strongly on particle size, [42] polycrystalline Ba2 Zn(BO3 )2 was ground and sieved into distinct particle size ranges (
