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Crystal growth, structure and phase transition of the nonlinear optical crystal BaCaBO3F
Journal of Crystal Growth 382 (2013) 47–51
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth, structure and phase transition of the nonlinear optical crystal BaCaBO3F R.K. Li n, Q.D. Zeng 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 10 May 2013 Received in revised form 22 July 2013 Accepted 1 August 2013 Communicated by V. Fratello Available online 13 August 2013
The nonlinear optical (NLO) crystal BaCaBO3F (BCBF) was reported about 20 years ago and was shown to have favorable lasing and self frequency doubling properties when doped with ytterbium. Detailed studies on its structure and thermal physical properties are still lacking. We grew BCBF crystals with dimensions up to 32 33 14 mm3 in an optimized solution below 1000 1C with NaF as an additive. A reversible phase transition at 242 1C was revealed by specific heat, differential thermal analysis, thermal expansion and variable temperature powder X-ray diffraction measurements. It is found that BCBF crystallizes in the reported hexagonal space group P-62m at high temperature (e.g. at 400 1C, a ¼9.09470(9) Å, c ¼4.37147(4) Å and Z¼3). It undergoes a phase transition when the temperature drops below 242 1C to a trigonal space group R3 with a large unit cell of a ¼27.1478(5) Å, c ¼12.9835(3) Å and Z¼ 81 at room temperature (RT), which is related to the high temperature structure by tripling in every three dimensions. The main difference of the two structures concerns one third of the BO3 groups in the unit cell: in the RT phase they (27 of 81 in total) are long range ordered whereas above the phase transition temperature they lose long range correlation and collapse to only one BO3 group in a disordered position at the corner of the small hexagonal unit cell. The newly found structures of BCBF may help to understand its mechanisms for laser and NLO performance. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A1. X-ray diffraction A2. Growth from melt B1. Borates B2. Nonlinear optic materials
1. Introduction Since the discoveries of β-BaB2O4(BBO) and LiB3O5(LBO) as efficient nonlinear optical (NLO) crystals [1,2], borates have been focused on for they often possess large nonlinear coefficients, high laser damage threshold, wide transparency in the ultraviolet (UV) range, high optical quality and good chemical stability [3]. Besides, boron can coordinate to three or four oxygen atoms forming isolated or connected nets of BO3 planar and BO4 tetrahedral groups that guarantee complexity of the structure types and leave room to find or design noncentrosymmetric (NCS) crystals. Based on the understanding of UV absorption of the borate crystals or glasses [4], KBe2BO3F2(KBBF) was proposed and successfully fabricated as a deep UV NLO crystal that is highly transparent down to 150 nm [5]. Efficient sixth harmonic of a Nd:YAG laser at 177 nm and UV laser output with wavelength as short as 153 nm have been realized with KBBF crystals as frequency converters [6,7]. In light with these findings, several related borate fluoride compounds were also found to possess NLO properties, e.g. BaCaBO3F (BCBF), BaAlBO3F2, BaZnBO3F, BaMgBO3F and Ba4B11O20F [8–11].
n
Corresponding author. Tel.: +86 10 82543711; fax: +86 10 82543706. E-mail address: [email protected] (R.K. Li).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.08.001
Among them, BCBF was first reported as an NLO crystal by Keszler in 1994 [8]. Then, a single crystal of Yb3+: BaCaBO3F was grown by the Czochralski method by Schaffers et al. who studied its spectroscopic properties [12]. Broad absorption and emission bands and large absorption and emission cross sections were found for BCBF, some of which even surpass the well-known laser crystal Yb:YAG. It was pointed out that the spectral features in BCBF were broader than those of most crystalline hosts and were assigned to Yb–O vibration assisted broadening [12], which might be problematic since the Yb–O vibrations should be present in all the Yb doped oxides. Laser performance with a slope efficiency as high as 38% was obtained with a Ti:Al2O3 pump laser. Because of the long energy storage time of the lasing Yb3+ ion and absence of absorption at the second harmonic wavelength in Yb:BCBF, it was proposed as a promising self-frequency doubling crystal. Later growth of relatively good quality crystals of BCBF was reported by Zhang et al. from this lab and Xu et al. and the linear and nonlinear optical properties of BCBF were studied in more detail [13–15]. Spectroscopic studies on other rare earth dopants in BCBF crystals were also reported more recently [16,17]. Though detailed studies on its properties and large single crystals were obtained by several research groups, we find that previous structural studies of BCBF are ambiguous and unreliable. BCBF was originally reported to belong to the space group P-62m
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with unit cell parameters of a ¼9.049 Å, c ¼4.326 Å by Keszler in 1994 [8], but structure details such as atomic coordinates were not given. Only much more recently, in 2009, Xu et al. reported the BCBF structure with basically the same space group and cell volume from a Rietveld refinement of powder X-ray diffraction (XRD) pattern although growth of a large single crystal was described in the same paper [15]. This unusual situation and our recent study of thermal properties of BCBF revealing that a peak at 242 1C appeared in the specific heat curve prompted us to do a more thorough study of its structure at room and high temperatures. Surprisingly, single crystal XRD at room temperature demonstrated that BCBF actually crystallizes with a very large unit cell with a ¼27.1478(5) Å and c¼12.9835(3) Å, tripling in every dimension in comparison to the unit cell reported so far. An obvious phase transition was proven to take place at 242 1C by specific heat, variable temperature XRD and thermal expansion measurements. Above the phase transition temperature, Rietveld refinements of the XRD patterns can be done with the reported P-62m structure.
step¼0.031/2θ and scan speed¼2 s/step). The General Structure Analysis System (GSAS) package [19] was applied to refine the structures of α-BCBF and high temperature BCBF (β-form, 300 1C and above) from the variable powder XRD data based on the models obtained from the single crystal data above and that of literature. Differential thermal analysis (DTA) was originally recorded with a Labsys™ TG-DTA16 (SETARAM) thermal analyzer with a 10.6 mg powdered BCBF crystal in the temperature range from 25 1C to 1100 1C. The measurement was performed again with a large quantity of 116 mg powdered BCBF crystal to increase the signal to noise ratio in the temperature range from 25 1C to 350 1C. The heating rates for both runs were the same at 10 1C/min. The specific heat was measured by a differential scanning calorimeter (DSC) in the temperature range of 25–300 1C at a heating rate of 5 1C/min (diamond DSC, Perkin-Elmer) with oriented crystal samples of 0.94 4 4 mm3(a c c) and 4 4 0.94 mm3(a a c), individually. The thermal expansions in both a and c directions were measured in the temperature range of 25–500 1C in air at a heating rate of 5 1C/min using a diamond thermal mechanical analyzer (Perkin-Elmer) with oriented crystal samples of 4.87 4 4 mm3 (a c c) and 4 4 4.87 mm3 (a a c).
2. Experimental BCBF single crystals have been grown from a melt with analytically pure starting materials of BaCO3 (197.3 g, 1 mole), CaCO3 (300.3 g, 3 moles), BaF2 (350.6 g, 2 moles), H3BO3 (186.7 g, 3 moles). NaF (typically 10–50 g) serves as an additive to lower the growth temperature to below 1000 1C. The starting materials were thoroughly mixed and melted at 1030 1C in a Pt crucible with diameter of 70 mm. The crucible with the melt was then transferred to a resistive furnace and heated above 1050 1C for 72 h with stirring to homogenize. Typical growth runs with the saturation temperature of 990 1C as determined by growth tests on a seed crystal. First a seed oriented along [001] direction was slowly dipped into the solution at a temperature 3 1C above the saturation temperature. Then the solution was kept at a constant temperature for 30 min to dissolve the outer surface of the seed and followed by decrease the temperature at 0.01–0.1 1C/day with a rotation and pulling rates of 20–30 rpm and 0.1–1 mm/day respectively. Single crystals with well developed morphology were obtained with sizes up to 32 33 14 mm3 (30.13 g) as shown in Fig. 1. Single crystal XRD data were collected with a BruckerAXS Smart APEX II diffractometer at room temperature (RT). The structure of room temperature BCBF (α-form) was solved and refined with the Shelx package [18]. Powder XRD patterns were recorded at seven different temperatures between RT and 600 1C at a 100 1C intervals with a Bruker D8 Discover X-ray diffractometer (CuKα radiation, scan
3. Results and discussion Though BCBF was found to melt congruently at around 1100 1C (Fig. 2), stoichiometric BaCaBO3F cannot be directly synthesized by the solid state reaction because BaF2 can be oxidized to BaO2 at elevated temperatures, which also causes corrosion of the crucible and introduces impurity to the grown crystal. Previous crystal growth efforts of BCBF all took the two step approach by preparing BaCa2(BO3)2 first and then reacting it with BaF2. We found that with a larger molar excess (2 vs. 1.5 moles) of BaF2 in the starting materials, BaCaBO3F can be formed in one pot and no corrosion to the Pt crucible was observed. Highly transparent and well developed single crystals with sizes up to 32 33 14 mm3 can be repeatedly obtained. Preliminary treatment of the single crystal diffraction data revealed that trigonal space groups of P3 and P31 were compatible with the observed diffraction reflection conditions. However, stable structural parameters could not be obtained from subsequent ab initio structure search and refinement. Then trials with other space groups with absence violations were also tested and a rhombohedral lattice with space group R3 or R32 was found leading to acceptable results. Refinement with the space group R32 resulted in too many split positions of F and O atoms, which actually gave an average structure and was discarded in the
Fig. 1. Typical grown crystal and its morphology. Only simple indices of (100), (110) and (001) were found for the observed surfaces.
R.K. Li, Q.D. Zeng / Journal of Crystal Growth 382 (2013) 47–51
Fig. 2. Differential thermal analysis of a BCBF crystal.
Table 1 Crystallographic and thermo-physical data for BaCaBO3F. Parameters
α-BCBF
β-BCBF
Chemical formula Temperature Space group a (Å) c (Å) Z Crystal sizes (mm3) Radiation Method Refined parameters/restraints wR2 (Rwp) R1 (Rp) Residual electron density (e/Å3) Thermal Expansion αa Coefficients (10 6/K) αc Phase transition temperature Tp Enthalpy change ΔH
BaCaBO3F 293 K R3 27.1478(5) 12.9835(3) 81 0.23 0.17 0.16 MoKα Shelx 578/355 0.1377 (all 5988 reflections) 0.0532 (all 5988 reflections) 3.0/ 2.31 9.52(2) 24.27(4) 615 K 355 J/mol
BaCaBO3F 673 K P-62m 9.09470(9) 4.37147(4) 3 Powder CuKα GSAS 52 0.0680 0.0531 11.36(1) 21.71(4)
following discussion. Starting with the space group R3 with cell parameters of a ¼27.1478(5) Å and c ¼12.9835(3) Å, the heavy Ba and Ca atoms were first found by the direct method using the program Shelxs and all other atoms were located with differential Fourier syntheses during subsequent structure refinement rounds with program Shelxl [18]. We were able to refine all atoms anisotropically, though constraints had to be applied on the lighter elements. Some of the F atoms (F1, F4, and F5) were found occupying split positions. Altogether 578 parameters were refined against 5988 reflections and final agreement indices of wR2 ¼0.1377 and R1 ¼0.0532 were reached at the last round of refinement. Crystallographic data and atomic parameters are listed in Table 1 and Supplemental Table S1, respectively. At this stage we can compare the present result with those reported structures. The major interesting feature in the present structure is that in the unit cell 27 (B1, B2 and B3) of the total 81 BO3 groups are inclined to the c direction and the other remaining 54 BO3 are all perpendicular to the c axis. Fig. 3c shows the arrangement of those inclined BO3 groups in the unit cell relating to that of the reported structure. In the structure model reported by Xu et al. [15], the BO3 group at the corner of the unit cell is disordered, which actually corresponds to the 27 inclined BO3 groups in our structure model. In our model, those 27 BO3 groups are ordered in a fashion that along c direction the inclination of the BO3 groups in a column rotates 1201 in successive layers while
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along a or b direction two of the BO3 groups point up or down while the other one points to the opposite direction (Fig. 3d). Therefore, it is the long range ordering of those BO3 groups that causes tripling of the unit cell in all the 3 directions. It is now well accepted that the NCS borate groups are the major contributors to the nonlinear optical effect in borate compounds [20,21]. Based on a procedure proposed by one of the present authors and others [22], the NLO coefficients of BCBF can be calculated on present and previously reported structures. The calculation shows d12 ¼ d11 ¼0.437 pm/V and d21 ¼ d22 ¼0.050 pm/V, d31 ¼d32 ¼ -0.008 pm/V, d33 ¼0.016 pm/V for the present structure and d11 ¼ d12 ¼ 0.387 pm/V for that reported by Xu et al. and d11 ¼ d12 ¼0.302 pm/V by Keszler et al. All the results show the major contribution to the NLO effect is d11 (d12 is a symmetric equivalent) and it agrees well with the experimental findings that BCBF possesses about the same level of NLO effect as that of KDP (d36 ¼0.39 pm/V) [23]. One thing that needs to be pointed out is that the smallness of the calculated d31, d32, d33 coefficients with the present structure is in accordance with the pseudo R32 space group. There are 11 different boron atoms in the unit cell, the bond valence sums (BVS, in the last column of Table S1) [24] show that the inclined ones are over-bonded whereas the parallel ones are slightly under-bonded but all fall in the usual range. For the 9 different Ba2+ and Ca2+ ions, it shows that Ba ions are always under-bonded with calculated BVS ranging from 1.628 to 1.765 whereas Ca2+ ions are over-bonded with BVS from 2.278 to 2.524. These severely under-bonded Ba2+ and over-bonded Ca2+ mean that larger monovalent and smaller trivalent cations are more favorable to substitute on the Ba and Ca sites respectively. Looking in more detail into the Ca coordination, similar to the previous reported models, Ca coordinates to 5 oxygen atoms in the equatorial plane and two F atoms in the apical positions. Among the 5 equatorial oxygen atoms, 4 of them contributed by two BO3 groups form 4 longer chelating bonds. These two BO3 groups are coplanar and point approximately to the same direction and give additive contributions to the dominant NLO coefficient d11. The fifth oxygen atom comes from the inclined BO3 (B1, B2 or B3) groups forming quite short single Ca–O bonds. These inclined BO3 groups point in the opposite direction to the coplanar groups in the ab plane and cancel about half of the contributions from the coplanar BO3 groups to d11 [10]. When inclined to make an angle to the ab plane, the detrimental effect of those inclined BO3 groups decreases, which is the reason that the calculated NLO coefficients increase from the fully coplanar model of Keszler et al. to the disordered model of Xu et al. and the present structure with long range ordered inclined BO3 groups. Furthermore, the large variation of BVS for Ca2+ means that when doped with laser active ions like Yb3+ and Nd3+, the dopants will spread over 9 sites with very different coordination environments, which may be the real reason that Yb:BCBF possesses exceptionally broad absorption and emission bands [12]. Therefore, the original explanation of phonon assisted broadening assuming only one Ca position may need revising. It would also be interesting to see whether heavy doping (430%) of the Ca sites by rare earth elements can be achieved by simultaneous monovalent charge compensation on the Ba sites given that most of the BVSs for Ca are over 2.3. During the study of thermal properties of properly oriented, cut and polished pieces of BCBF crystal, a clear peak showed up at 242 1C in the specific heat curve (Fig. 4). This λ shaped peak is well known to be the indication of a second order phase transition [25]. When we revisited our previously obtained DTA curve (Fig. 1, inset), an endothermic peak was also present at 244 1C but submerged by the large endothermic and exothermic events during melting and recrystallization. We did the measurement again on a sample with a much large quantity (over 10 times, 116 mg) of powdered BCBF crystal; peaks with much improved
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Fig. 3. Crystal structure of BaCaBO3F at room temperature: (a). projection onto the ab plane, (b) the arrangement of triangular BO3 groups, (c) structure relation to those reported, (d) the structure along the a or b direction. Cations and F are removed for clarity in (b) and (d).
Fig. 4. Specific heat and differential thermal analysis of BCBF.
signal to noise ratios appeared both in the warming up and cooling down processes. The peak positions showed only 3 1C lagging on average in comparison to the specific heat peak, indicating that the phase transition is fast and reversible. Furthermore, from the specific heat data an enthalpy change of the phase transition can R be estimated to be: ΔHp ¼ CpdT ¼0.355 kJ/mol, which is only one thousandth that of the Ca–O bond energy (402.1 kJ/mol) and less than one tenth of the hydrogen bond movements during ice melting (6.008 kJ/mol) [25]. Therefore, it can be assumed that the phase transition does not involve any bond breaking and reconstruction processes, but rather should be accompanied by only small movements of the ions and BO3 groups. In order to understand the relation of the structure to the phase transition and the high temperature structure of BCBF, variable temperature powder XRD was performed with powdered BCBF single crystal. Lattice constants (large dots) deduced from those XRD patterns with Rietveld refinement agree very well with those measured by thermal expansion (Fig. 5, solid lines). It is found that the lattice constant a varies smoothly with temperature change,
Fig. 5. Cell parameters (dots) and their thermal expansions (lines) of BCBF.
whereas there is a slope change at around 250 1C in the c parameters, which is also an indication of the phase transition. Careful structure refinements of the XRD patterns (Fig. 6) taken at RT and 400 1C were performed with the GSAS program [19]. The structure model at RT was directly imported from the single crystal XRD results and for 400 1C we took the disordered BO3 model from reference [15]. The refinements lead to good agreement indices [19] of Rwp ¼0.0690, Rp ¼0.0526 for RT and Rwp ¼ 0.0680, Rp ¼0.0531 for 400 °C (Rwp ¼(∑w(Io Ic)2/∑wIo2)1/2, Rp ¼ ∑| Io Ic|/∑Io). From the inset in Fig. 3, the single crystal structure can
R.K. Li, Q.D. Zeng / Journal of Crystal Growth 382 (2013) 47–51
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structures, two thirds of the BO3 groups are coplanar with the ab plane while one third of them incline to the c direction. The tripling of the unit cell when cooling down from the high temperature phase is mainly due to the ordering of the one third inclined BO3 groups in the unit cell. In the RT phase the inclined 27 of 81 of the BO3 groups are long range ordered; whereas above the phase transition temperature they lose long range correlation and collapse to only one BO3 group in a disordered position at the corner of the small hexagonal unit cell. This order–disorder phase transition model is in good accordance with the λ type Cp behavior and small enthalpy change, which are both indicative of a second order phase transition. Because of the phase transition which occurs at a relative low temperature, care must be taken when BCBF is applied as a laser material or during cooling after growth to avoid cracking.
Acknowledgment Fig. 6. Rietveld refinement of the power XRD at 4001C and its comparison to room temperature refinement (inset). Lines are calculated and difference patterns. Diffraction peak positions are marked by the vertical bars and some indices in parentheses. Asterisk signs indicate features originating from sample holder or the furnace materials.
We thank the National Science Foundation of China (Nos. 90922036 and 51032004/E0201) for financial support.
Appendix A. Supporting information account for all the minor peaks as labeled there. If looking at the XRD patterns in detail in reference [15] especially in the difference plot, some of these minor peaks are also present but were neglected [15] (e.g. peaks at 421 and 451 with a different CoKα radiation source there, corresponding to peaks of (205), (215) and (704) here). Therefore, a phase transition must also have happened in their grown crystals when cooling down from growth temperature. When temperature increases to above the transition temperature, those minor peaks disappeared except one bump at 13–141 and one peak around 261 indicated by the asterisk signs in Fig. 6 inset, which are proven to come from sample holder or furnace materials as they also appeared in other compounds in the same conditions. At 400 1C, the refined structure does not show much difference from that reported by Xu et al. [15], so they actually obtained the high temperature structure from refinement of XRD patterns at 293 K. It is now clear that BCBF shows a reversible phase transition at 242 1C: at RT the inclined BO3 groups are long range ordered; after the phase transition at elevated temperatures they lose the long range correlation and become disordered to form a small unit cell. This order–disorder phase transition with only loss of long range correlation of the alignment for one third of the BO3 groups is in good accordance with the small enthalpy change found above. The finding of this structural transition may help to design a suitable cooling procedure after growth to avoid cracking of grown crystals and it will also help to understand the mechanisms of laser and NLO performance. 4. Conclusions We found that large size and good optical quality BCBF crystals can be grown in a melt containing a large excess of BaF2 below 1000 1C with NaF as an additive. A phase transition at 242 1C was originally observed in the specific heat measurement and confirmed by DTA, thermal expansion and variable temperature XRD experiments. At temperatures below the phase transition, BCBF actually crystallizes in a trigonal space group R3 with a large unit cell of a ¼27.1478(5) Å, c ¼12.9835(3) Å and Z¼ 81; whereas above the phase transition temperature, BCBF belongs to the hexagonal space group P-62m reported previously [15]. In both of the
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jcrysgro.2013. 08.001. References [1] C.T. Chen, B.C. Wu, A.D. Jiang, G.M. You, Scientia Sinica B 28 (1985) 235–243. [2] C.T. Chen, Y.C. Wu, A.D. Jiang, B.C. Wu, G.M. You, R.K Li, S.J. Lin, Journal of the Optical Society of America B 6 (1989) 616–621. [3] C.T. Chen, T. Sasaki, R.K. Li, Y. Wu, Z. Lin, Y. Mori, Z. Hu, J. Wang, S. Uda, M. Yoshimura, Y. Kaneda, Nonlinear Optical Borate Crystals, Wiley-VCH, Weinheim, 2012. [4] R.K. Li, Journal of Non-Crystalline Solids 111 (1989) 199–204. [5] C. Chen, Y. Wang, Y. Xia, B. Wu, D. Tang, K. Wu, W. Zeng, L. Yu, L. Mei, Journal of Applied Physics 77 (1994) 2268–2272. [6] C. Chen, G. Wang, X. Wang, Z. Xu, Applied Physics B 97 (2009) 9–25. [7] Y. Nomura, Y. Ito, A. Ozawa, X.Y. Wang, C.T. Chen, S. Shin, S. Watanabe, Y. Kobayashi, Optics Letters 36 (2011) 1758–1760. [8] D.A. Keszler, A. Akella, K.I. Schaffers, T. Alekel, Materials Research Society Symposium Proceedings 329 (1994) 15–22. [9] Z.G. Hu, M. Yoshimura, Y. Mori, T. Sasaki, Journal of Crystal Growth 260 (2004) 287–290. [10] R.K. Li, P. Chen, Inorganic Chemistry 49 (2010) 1561–1565. [11] H. Wu, H. Yu, Z. Yang, X. Hou, X. Su, S. Pan, K.R. Poeppelmeier, J.M. Rondinelli, Journal of the American Chemical Society 135 (2013) 4215–4218. [12] K.I. Schaffers, L.D. Deloach, S.A. Payne, IEEE Journal of Quantum Electronics 32 (1996) 741–748. [13] G.C. Zhang, H.J. Liu, X.A. Wang, F.D. Fan, P.Z. Fu, Journal of Crystal Growth 289 (2006) 188–191. [14] X. Wang, G.C. Zhang, Y. Zhao, F.D. Fan, H.J. Liu, P.Z. Fu, Optical Materials 29 (2007) 1658–1661. [15] K. Xu, P. Loiseau, G. Aka, Journal of Crystal Growth 311 (2009) 2508–2512. [16] W. Zhao, W. Zhou, M. Song, G. Wang, J. Du, H. Yu, J. Chen, Optical Materials 33 (2011) 647–654. [17] W. Zhao, W. Zhou, M. Song, G. Wang, J. Du, H. Yu, J. Chen, Optoelectronics and Advanced Materials—Rapid Communications 5 (2011) 49–53. [18] G.M. Sheldrick, Acta Crystallographica A 64 (2008) 112–122. [19] A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86-748; Los Alamos, NM, 2004. [20] C.T. Chen, Y.C. Wu, R.K. Li, International Reviews in Physical Chemistry 8 (1989) 65–91. [21] C.T. Chen, Y.C. Wu, R.K. Li, Journal of Crystal Growth 99 (1990) 790–798. [22] R.K. Li, C.T. Chen, Acta Physica Sinica 34 (1985) 823–827. (in Chinese). [23] V.G. Dimitriev, G.G. Gurzadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, 3rd ed., Springer, Heidelberg, 1999. [24] I.D. Brown, D. Altermatt, Acta Crystallographica B 41 (1985) 244–247. [25] P.W. Atkins, Physical Chemistry, 4th ed., W.H. Freeman and Company, New York, 1990.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth, structure and phase transition of the nonlinear optical crystal BaCaBO3F R.K. Li n, Q.D. Zeng 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 10 May 2013 Received in revised form 22 July 2013 Accepted 1 August 2013 Communicated by V. Fratello Available online 13 August 2013
The nonlinear optical (NLO) crystal BaCaBO3F (BCBF) was reported about 20 years ago and was shown to have favorable lasing and self frequency doubling properties when doped with ytterbium. Detailed studies on its structure and thermal physical properties are still lacking. We grew BCBF crystals with dimensions up to 32 33 14 mm3 in an optimized solution below 1000 1C with NaF as an additive. A reversible phase transition at 242 1C was revealed by specific heat, differential thermal analysis, thermal expansion and variable temperature powder X-ray diffraction measurements. It is found that BCBF crystallizes in the reported hexagonal space group P-62m at high temperature (e.g. at 400 1C, a ¼9.09470(9) Å, c ¼4.37147(4) Å and Z¼3). It undergoes a phase transition when the temperature drops below 242 1C to a trigonal space group R3 with a large unit cell of a ¼27.1478(5) Å, c ¼12.9835(3) Å and Z¼ 81 at room temperature (RT), which is related to the high temperature structure by tripling in every three dimensions. The main difference of the two structures concerns one third of the BO3 groups in the unit cell: in the RT phase they (27 of 81 in total) are long range ordered whereas above the phase transition temperature they lose long range correlation and collapse to only one BO3 group in a disordered position at the corner of the small hexagonal unit cell. The newly found structures of BCBF may help to understand its mechanisms for laser and NLO performance. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A1. X-ray diffraction A2. Growth from melt B1. Borates B2. Nonlinear optic materials
1. Introduction Since the discoveries of β-BaB2O4(BBO) and LiB3O5(LBO) as efficient nonlinear optical (NLO) crystals [1,2], borates have been focused on for they often possess large nonlinear coefficients, high laser damage threshold, wide transparency in the ultraviolet (UV) range, high optical quality and good chemical stability [3]. Besides, boron can coordinate to three or four oxygen atoms forming isolated or connected nets of BO3 planar and BO4 tetrahedral groups that guarantee complexity of the structure types and leave room to find or design noncentrosymmetric (NCS) crystals. Based on the understanding of UV absorption of the borate crystals or glasses [4], KBe2BO3F2(KBBF) was proposed and successfully fabricated as a deep UV NLO crystal that is highly transparent down to 150 nm [5]. Efficient sixth harmonic of a Nd:YAG laser at 177 nm and UV laser output with wavelength as short as 153 nm have been realized with KBBF crystals as frequency converters [6,7]. In light with these findings, several related borate fluoride compounds were also found to possess NLO properties, e.g. BaCaBO3F (BCBF), BaAlBO3F2, BaZnBO3F, BaMgBO3F and Ba4B11O20F [8–11].
n
Corresponding author. Tel.: +86 10 82543711; fax: +86 10 82543706. E-mail address: [email protected] (R.K. Li).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.08.001
Among them, BCBF was first reported as an NLO crystal by Keszler in 1994 [8]. Then, a single crystal of Yb3+: BaCaBO3F was grown by the Czochralski method by Schaffers et al. who studied its spectroscopic properties [12]. Broad absorption and emission bands and large absorption and emission cross sections were found for BCBF, some of which even surpass the well-known laser crystal Yb:YAG. It was pointed out that the spectral features in BCBF were broader than those of most crystalline hosts and were assigned to Yb–O vibration assisted broadening [12], which might be problematic since the Yb–O vibrations should be present in all the Yb doped oxides. Laser performance with a slope efficiency as high as 38% was obtained with a Ti:Al2O3 pump laser. Because of the long energy storage time of the lasing Yb3+ ion and absence of absorption at the second harmonic wavelength in Yb:BCBF, it was proposed as a promising self-frequency doubling crystal. Later growth of relatively good quality crystals of BCBF was reported by Zhang et al. from this lab and Xu et al. and the linear and nonlinear optical properties of BCBF were studied in more detail [13–15]. Spectroscopic studies on other rare earth dopants in BCBF crystals were also reported more recently [16,17]. Though detailed studies on its properties and large single crystals were obtained by several research groups, we find that previous structural studies of BCBF are ambiguous and unreliable. BCBF was originally reported to belong to the space group P-62m
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with unit cell parameters of a ¼9.049 Å, c ¼4.326 Å by Keszler in 1994 [8], but structure details such as atomic coordinates were not given. Only much more recently, in 2009, Xu et al. reported the BCBF structure with basically the same space group and cell volume from a Rietveld refinement of powder X-ray diffraction (XRD) pattern although growth of a large single crystal was described in the same paper [15]. This unusual situation and our recent study of thermal properties of BCBF revealing that a peak at 242 1C appeared in the specific heat curve prompted us to do a more thorough study of its structure at room and high temperatures. Surprisingly, single crystal XRD at room temperature demonstrated that BCBF actually crystallizes with a very large unit cell with a ¼27.1478(5) Å and c¼12.9835(3) Å, tripling in every dimension in comparison to the unit cell reported so far. An obvious phase transition was proven to take place at 242 1C by specific heat, variable temperature XRD and thermal expansion measurements. Above the phase transition temperature, Rietveld refinements of the XRD patterns can be done with the reported P-62m structure.
step¼0.031/2θ and scan speed¼2 s/step). The General Structure Analysis System (GSAS) package [19] was applied to refine the structures of α-BCBF and high temperature BCBF (β-form, 300 1C and above) from the variable powder XRD data based on the models obtained from the single crystal data above and that of literature. Differential thermal analysis (DTA) was originally recorded with a Labsys™ TG-DTA16 (SETARAM) thermal analyzer with a 10.6 mg powdered BCBF crystal in the temperature range from 25 1C to 1100 1C. The measurement was performed again with a large quantity of 116 mg powdered BCBF crystal to increase the signal to noise ratio in the temperature range from 25 1C to 350 1C. The heating rates for both runs were the same at 10 1C/min. The specific heat was measured by a differential scanning calorimeter (DSC) in the temperature range of 25–300 1C at a heating rate of 5 1C/min (diamond DSC, Perkin-Elmer) with oriented crystal samples of 0.94 4 4 mm3(a c c) and 4 4 0.94 mm3(a a c), individually. The thermal expansions in both a and c directions were measured in the temperature range of 25–500 1C in air at a heating rate of 5 1C/min using a diamond thermal mechanical analyzer (Perkin-Elmer) with oriented crystal samples of 4.87 4 4 mm3 (a c c) and 4 4 4.87 mm3 (a a c).
2. Experimental BCBF single crystals have been grown from a melt with analytically pure starting materials of BaCO3 (197.3 g, 1 mole), CaCO3 (300.3 g, 3 moles), BaF2 (350.6 g, 2 moles), H3BO3 (186.7 g, 3 moles). NaF (typically 10–50 g) serves as an additive to lower the growth temperature to below 1000 1C. The starting materials were thoroughly mixed and melted at 1030 1C in a Pt crucible with diameter of 70 mm. The crucible with the melt was then transferred to a resistive furnace and heated above 1050 1C for 72 h with stirring to homogenize. Typical growth runs with the saturation temperature of 990 1C as determined by growth tests on a seed crystal. First a seed oriented along [001] direction was slowly dipped into the solution at a temperature 3 1C above the saturation temperature. Then the solution was kept at a constant temperature for 30 min to dissolve the outer surface of the seed and followed by decrease the temperature at 0.01–0.1 1C/day with a rotation and pulling rates of 20–30 rpm and 0.1–1 mm/day respectively. Single crystals with well developed morphology were obtained with sizes up to 32 33 14 mm3 (30.13 g) as shown in Fig. 1. Single crystal XRD data were collected with a BruckerAXS Smart APEX II diffractometer at room temperature (RT). The structure of room temperature BCBF (α-form) was solved and refined with the Shelx package [18]. Powder XRD patterns were recorded at seven different temperatures between RT and 600 1C at a 100 1C intervals with a Bruker D8 Discover X-ray diffractometer (CuKα radiation, scan
3. Results and discussion Though BCBF was found to melt congruently at around 1100 1C (Fig. 2), stoichiometric BaCaBO3F cannot be directly synthesized by the solid state reaction because BaF2 can be oxidized to BaO2 at elevated temperatures, which also causes corrosion of the crucible and introduces impurity to the grown crystal. Previous crystal growth efforts of BCBF all took the two step approach by preparing BaCa2(BO3)2 first and then reacting it with BaF2. We found that with a larger molar excess (2 vs. 1.5 moles) of BaF2 in the starting materials, BaCaBO3F can be formed in one pot and no corrosion to the Pt crucible was observed. Highly transparent and well developed single crystals with sizes up to 32 33 14 mm3 can be repeatedly obtained. Preliminary treatment of the single crystal diffraction data revealed that trigonal space groups of P3 and P31 were compatible with the observed diffraction reflection conditions. However, stable structural parameters could not be obtained from subsequent ab initio structure search and refinement. Then trials with other space groups with absence violations were also tested and a rhombohedral lattice with space group R3 or R32 was found leading to acceptable results. Refinement with the space group R32 resulted in too many split positions of F and O atoms, which actually gave an average structure and was discarded in the
Fig. 1. Typical grown crystal and its morphology. Only simple indices of (100), (110) and (001) were found for the observed surfaces.
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Fig. 2. Differential thermal analysis of a BCBF crystal.
Table 1 Crystallographic and thermo-physical data for BaCaBO3F. Parameters
α-BCBF
β-BCBF
Chemical formula Temperature Space group a (Å) c (Å) Z Crystal sizes (mm3) Radiation Method Refined parameters/restraints wR2 (Rwp) R1 (Rp) Residual electron density (e/Å3) Thermal Expansion αa Coefficients (10 6/K) αc Phase transition temperature Tp Enthalpy change ΔH
BaCaBO3F 293 K R3 27.1478(5) 12.9835(3) 81 0.23 0.17 0.16 MoKα Shelx 578/355 0.1377 (all 5988 reflections) 0.0532 (all 5988 reflections) 3.0/ 2.31 9.52(2) 24.27(4) 615 K 355 J/mol
BaCaBO3F 673 K P-62m 9.09470(9) 4.37147(4) 3 Powder CuKα GSAS 52 0.0680 0.0531 11.36(1) 21.71(4)
following discussion. Starting with the space group R3 with cell parameters of a ¼27.1478(5) Å and c ¼12.9835(3) Å, the heavy Ba and Ca atoms were first found by the direct method using the program Shelxs and all other atoms were located with differential Fourier syntheses during subsequent structure refinement rounds with program Shelxl [18]. We were able to refine all atoms anisotropically, though constraints had to be applied on the lighter elements. Some of the F atoms (F1, F4, and F5) were found occupying split positions. Altogether 578 parameters were refined against 5988 reflections and final agreement indices of wR2 ¼0.1377 and R1 ¼0.0532 were reached at the last round of refinement. Crystallographic data and atomic parameters are listed in Table 1 and Supplemental Table S1, respectively. At this stage we can compare the present result with those reported structures. The major interesting feature in the present structure is that in the unit cell 27 (B1, B2 and B3) of the total 81 BO3 groups are inclined to the c direction and the other remaining 54 BO3 are all perpendicular to the c axis. Fig. 3c shows the arrangement of those inclined BO3 groups in the unit cell relating to that of the reported structure. In the structure model reported by Xu et al. [15], the BO3 group at the corner of the unit cell is disordered, which actually corresponds to the 27 inclined BO3 groups in our structure model. In our model, those 27 BO3 groups are ordered in a fashion that along c direction the inclination of the BO3 groups in a column rotates 1201 in successive layers while
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along a or b direction two of the BO3 groups point up or down while the other one points to the opposite direction (Fig. 3d). Therefore, it is the long range ordering of those BO3 groups that causes tripling of the unit cell in all the 3 directions. It is now well accepted that the NCS borate groups are the major contributors to the nonlinear optical effect in borate compounds [20,21]. Based on a procedure proposed by one of the present authors and others [22], the NLO coefficients of BCBF can be calculated on present and previously reported structures. The calculation shows d12 ¼ d11 ¼0.437 pm/V and d21 ¼ d22 ¼0.050 pm/V, d31 ¼d32 ¼ -0.008 pm/V, d33 ¼0.016 pm/V for the present structure and d11 ¼ d12 ¼ 0.387 pm/V for that reported by Xu et al. and d11 ¼ d12 ¼0.302 pm/V by Keszler et al. All the results show the major contribution to the NLO effect is d11 (d12 is a symmetric equivalent) and it agrees well with the experimental findings that BCBF possesses about the same level of NLO effect as that of KDP (d36 ¼0.39 pm/V) [23]. One thing that needs to be pointed out is that the smallness of the calculated d31, d32, d33 coefficients with the present structure is in accordance with the pseudo R32 space group. There are 11 different boron atoms in the unit cell, the bond valence sums (BVS, in the last column of Table S1) [24] show that the inclined ones are over-bonded whereas the parallel ones are slightly under-bonded but all fall in the usual range. For the 9 different Ba2+ and Ca2+ ions, it shows that Ba ions are always under-bonded with calculated BVS ranging from 1.628 to 1.765 whereas Ca2+ ions are over-bonded with BVS from 2.278 to 2.524. These severely under-bonded Ba2+ and over-bonded Ca2+ mean that larger monovalent and smaller trivalent cations are more favorable to substitute on the Ba and Ca sites respectively. Looking in more detail into the Ca coordination, similar to the previous reported models, Ca coordinates to 5 oxygen atoms in the equatorial plane and two F atoms in the apical positions. Among the 5 equatorial oxygen atoms, 4 of them contributed by two BO3 groups form 4 longer chelating bonds. These two BO3 groups are coplanar and point approximately to the same direction and give additive contributions to the dominant NLO coefficient d11. The fifth oxygen atom comes from the inclined BO3 (B1, B2 or B3) groups forming quite short single Ca–O bonds. These inclined BO3 groups point in the opposite direction to the coplanar groups in the ab plane and cancel about half of the contributions from the coplanar BO3 groups to d11 [10]. When inclined to make an angle to the ab plane, the detrimental effect of those inclined BO3 groups decreases, which is the reason that the calculated NLO coefficients increase from the fully coplanar model of Keszler et al. to the disordered model of Xu et al. and the present structure with long range ordered inclined BO3 groups. Furthermore, the large variation of BVS for Ca2+ means that when doped with laser active ions like Yb3+ and Nd3+, the dopants will spread over 9 sites with very different coordination environments, which may be the real reason that Yb:BCBF possesses exceptionally broad absorption and emission bands [12]. Therefore, the original explanation of phonon assisted broadening assuming only one Ca position may need revising. It would also be interesting to see whether heavy doping (430%) of the Ca sites by rare earth elements can be achieved by simultaneous monovalent charge compensation on the Ba sites given that most of the BVSs for Ca are over 2.3. During the study of thermal properties of properly oriented, cut and polished pieces of BCBF crystal, a clear peak showed up at 242 1C in the specific heat curve (Fig. 4). This λ shaped peak is well known to be the indication of a second order phase transition [25]. When we revisited our previously obtained DTA curve (Fig. 1, inset), an endothermic peak was also present at 244 1C but submerged by the large endothermic and exothermic events during melting and recrystallization. We did the measurement again on a sample with a much large quantity (over 10 times, 116 mg) of powdered BCBF crystal; peaks with much improved
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Fig. 3. Crystal structure of BaCaBO3F at room temperature: (a). projection onto the ab plane, (b) the arrangement of triangular BO3 groups, (c) structure relation to those reported, (d) the structure along the a or b direction. Cations and F are removed for clarity in (b) and (d).
Fig. 4. Specific heat and differential thermal analysis of BCBF.
signal to noise ratios appeared both in the warming up and cooling down processes. The peak positions showed only 3 1C lagging on average in comparison to the specific heat peak, indicating that the phase transition is fast and reversible. Furthermore, from the specific heat data an enthalpy change of the phase transition can R be estimated to be: ΔHp ¼ CpdT ¼0.355 kJ/mol, which is only one thousandth that of the Ca–O bond energy (402.1 kJ/mol) and less than one tenth of the hydrogen bond movements during ice melting (6.008 kJ/mol) [25]. Therefore, it can be assumed that the phase transition does not involve any bond breaking and reconstruction processes, but rather should be accompanied by only small movements of the ions and BO3 groups. In order to understand the relation of the structure to the phase transition and the high temperature structure of BCBF, variable temperature powder XRD was performed with powdered BCBF single crystal. Lattice constants (large dots) deduced from those XRD patterns with Rietveld refinement agree very well with those measured by thermal expansion (Fig. 5, solid lines). It is found that the lattice constant a varies smoothly with temperature change,
Fig. 5. Cell parameters (dots) and their thermal expansions (lines) of BCBF.
whereas there is a slope change at around 250 1C in the c parameters, which is also an indication of the phase transition. Careful structure refinements of the XRD patterns (Fig. 6) taken at RT and 400 1C were performed with the GSAS program [19]. The structure model at RT was directly imported from the single crystal XRD results and for 400 1C we took the disordered BO3 model from reference [15]. The refinements lead to good agreement indices [19] of Rwp ¼0.0690, Rp ¼0.0526 for RT and Rwp ¼ 0.0680, Rp ¼0.0531 for 400 °C (Rwp ¼(∑w(Io Ic)2/∑wIo2)1/2, Rp ¼ ∑| Io Ic|/∑Io). From the inset in Fig. 3, the single crystal structure can
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structures, two thirds of the BO3 groups are coplanar with the ab plane while one third of them incline to the c direction. The tripling of the unit cell when cooling down from the high temperature phase is mainly due to the ordering of the one third inclined BO3 groups in the unit cell. In the RT phase the inclined 27 of 81 of the BO3 groups are long range ordered; whereas above the phase transition temperature they lose long range correlation and collapse to only one BO3 group in a disordered position at the corner of the small hexagonal unit cell. This order–disorder phase transition model is in good accordance with the λ type Cp behavior and small enthalpy change, which are both indicative of a second order phase transition. Because of the phase transition which occurs at a relative low temperature, care must be taken when BCBF is applied as a laser material or during cooling after growth to avoid cracking.
Acknowledgment Fig. 6. Rietveld refinement of the power XRD at 4001C and its comparison to room temperature refinement (inset). Lines are calculated and difference patterns. Diffraction peak positions are marked by the vertical bars and some indices in parentheses. Asterisk signs indicate features originating from sample holder or the furnace materials.
We thank the National Science Foundation of China (Nos. 90922036 and 51032004/E0201) for financial support.
Appendix A. Supporting information account for all the minor peaks as labeled there. If looking at the XRD patterns in detail in reference [15] especially in the difference plot, some of these minor peaks are also present but were neglected [15] (e.g. peaks at 421 and 451 with a different CoKα radiation source there, corresponding to peaks of (205), (215) and (704) here). Therefore, a phase transition must also have happened in their grown crystals when cooling down from growth temperature. When temperature increases to above the transition temperature, those minor peaks disappeared except one bump at 13–141 and one peak around 261 indicated by the asterisk signs in Fig. 6 inset, which are proven to come from sample holder or furnace materials as they also appeared in other compounds in the same conditions. At 400 1C, the refined structure does not show much difference from that reported by Xu et al. [15], so they actually obtained the high temperature structure from refinement of XRD patterns at 293 K. It is now clear that BCBF shows a reversible phase transition at 242 1C: at RT the inclined BO3 groups are long range ordered; after the phase transition at elevated temperatures they lose the long range correlation and become disordered to form a small unit cell. This order–disorder phase transition with only loss of long range correlation of the alignment for one third of the BO3 groups is in good accordance with the small enthalpy change found above. The finding of this structural transition may help to design a suitable cooling procedure after growth to avoid cracking of grown crystals and it will also help to understand the mechanisms of laser and NLO performance. 4. Conclusions We found that large size and good optical quality BCBF crystals can be grown in a melt containing a large excess of BaF2 below 1000 1C with NaF as an additive. A phase transition at 242 1C was originally observed in the specific heat measurement and confirmed by DTA, thermal expansion and variable temperature XRD experiments. At temperatures below the phase transition, BCBF actually crystallizes in a trigonal space group R3 with a large unit cell of a ¼27.1478(5) Å, c ¼12.9835(3) Å and Z¼ 81; whereas above the phase transition temperature, BCBF belongs to the hexagonal space group P-62m reported previously [15]. In both of the
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