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Crystal Growth, Optical Properties Measurement, and Theoretical Calculation of BPO4 Zhihua Li,*,†,‡ Zheshuai Lin,† Yicheng Wu,† Peizhen Fu,† Zhizhong Wang,† and Chuangtian Chen† Beijing Center for Crystal R&D, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, P.O. Box 2711, Beijing 100080, China, and College of Chemistry, Chemical and Materials Science, Shandong Normal University, Jinan, Shandong 250014, China Received January 6, 2004. Revised Manuscript Received March 31, 2004
Optically perfect single crystals of BPO4 (BPO) have been grown by the top-seeding flux method. The refractive indices are measured with the prism minimum-deviation method for the first time, on the basis of which the Sellmeier equation is obtained. The measured powder nonlinear optical second-harmonic generation effect is about twice that of KH2PO4 (KDP). Electronic structure, the linear refractive indices, and second-harmonic generation (SHG) coefficients of BPO are calculated on the basis of the plane-wave pseudopotential ab initio method. The calculation values are in line with the experimental data. The crystal is nonhygroscopic, chemically stable, and has a hardness of 6.5 Mohs.
1. Introduction With the development of technology and science, there is a need for improved frequency-conversion materials for UV and vacuum ultraviolet (VUV) generation. At present, available frequency conversion materials such as β-BaB2O4 (BBO)1 and LiB3O5 (LBO)2 play an indispensable role in obtaining a tunable source in the deep ultraviolet region. However, their further applications, for example in photolithography and high-resolution photoemission spectroscopy, are limited because of the long UV-cutoff edge (189 nm) for BBO and the small birefringence index for LBO. Although the discovery of the nonlinear optical crystal KBBF (KBe2BO3F2) can lead to the second harmonic generation at 172.5 nm and sum-frequency mixing at 163.3 nm,3 we cannot affirm its application prospects owing to the difficulty of its crystal growth. Therefore, the search for the NLO materials, which can apply in deep ultraviolet region, is also an important task. It is well-known that phosphate and borate are characterized by a large nonlinear coefficient, a broad transparency range, and high damage threshold. It is possible for compounds consisting of P, B, and O to have excellent properties. Therefore, as a potential NLO material BPO crystal has been investigated since 1991.4-6 However, many properties * To whom correspondence should be addressed. E-mail: zhlil68@ hotmail.com † Beijing Center for Crystal R&D. ‡ Shandong Normal University. (1) Chen, C.; Wu, B.; Jiang, A.; You, G. Sci. Sin., Ser. B 1985, 28, 235-243. (2) Chen, C.; Wu, Y.; Jiang, A.; Wu, B.; You, G.; Li, R.; Lin, S. J. Opt. Soc. Am. B 1989, 6, 616-621. (3) Togashi, T.; Kanai, T.; Sekikawa, T.; Watanabe, S.;Chen, C.; et al. Opt. Lett. 2003, 2 (15), 254-256. (4) Wei, L.; Guiqin, D.; Qingzhen, H.; Jingui, L.; Qinjie, G.J. Synth. Cryst. 1991, 20 (3-4), 309. (5) Yao, J.; Shen, W.; Ye, N.; et al. J. Synth. Cryst. 2000, 29 (5), 137 (in Chinese).
of BPO crystal could not be tested because of the difficulties of BPO crystal growth. In this paper we report in three sections our major work. First, using Li2O-Li4P2O7 as flux system, goodquality BPO crystals have been grown by the topseeding solution growth (TSSG) method. Second, the refractive indices of BPO have been measured with the prism minimum-deviation method for the first time. Third, the electronic band structure of BPO crystal was calculated on the basis of local-density approximation with the CASTEP package.7 The linear refractive indices and SHG coefficients were calculated from their band structure. 2. Experimental Section Crystal Growth. The BPO compound was prepared by solid-state reaction, with analytical purity B2O3 and NH4H2PO4 in stoichiometric proportions. The mixture was heated at 900 °C for 48 h, and then it was confirmed that the compound was a single phase by the X-ray diffraction pattern. The topseeding solution growth (TSSG) method was applied because BPO melts incongruently. Li4P2O7 (LPO) was adopted as flux according to the LPO-BPO phase diagram.8 During several growth runs, we found that good-quality crystals cannot be obtained in the LPO-BPO system. Many defects, such as inclusions, mosaic structure, and veil, can be found in the grown crystals owing to the large viscosity of the melt. To decrease the viscosity of the melt and to grow highquality crystals the flux LPO/Li2O mixture was adopted.9 Here, we use the weight of Li2CO3 to replace that of Li2O. The weight ratio of starting materials was WBPO/WLPO/WLi2CO3 ) 6:4-6:24. Our experiments show that the weight ratio WBPO/WLPO/ (6) Shen, W.; Hu, X.; Fu, P.; Yao, J.; Zeng, W.; Chen, C.; Tian, Y.; Jiang, J. J. Synth. Cryst. 2001, 30 (2) (in Chinese). (7) Parr, R. G.; Yang, W. T. Density Functional Theory of AtomMolecules, Oxford University Press: Oxford, 1989. (8) Tien, T. Y.; Hummel, F. A. J. Am. Ceram. Soc. 1961, 44 (8), 393. (9) Elwell, D.; Scheel, H. J.; Crystal Growth from High-Temperature Solutions; Academic Press: London, 1975.
10.1021/cm040121o CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004
Growth and Properties of BPO4 Crystal
Chem. Mater., Vol. 16, No. 15, 2004 2907
no2 ) 2.52049 +
Figure 1. Grown crystal of BPO4.
Figure 2. Transmission spectrum of BPO. Table 1. Calculated and Experimental Values of Refractive Indices of BPO Crystal no
ne
wavelength λ (µm)
exp.
calc.
exp.
calc.
0.4358 0.4916 0.5461 0.5780 0.5893 0.6561 0.6943
1.6027 1.5983 1.5952 1.5937 1.5932 1.5908 1.5897
1.5963 1.5939 1.5922 1.5914 1.5912 1.5900 1.5896
1.6084 1.6040 1.6008 1.5993 1.5987 1.5963 1.5951
1.6038 1.6014 1.5996 1.5989 1.5987 1.5975 1.5970
WLi2CO3 ) 6:4:3 is suitable to grow BPO crystals. Crystal growth was performed in air in a vertical cylindrical electric furnace by means of the TSSG method. The platinum crucible (φ 40 × 40 mm) containing the mixture was transported to the growth furnace and heated gradually to a temperature above 850 °C. After the starting materials had melted, a test-seeded technique was employed to measure the saturation point of the solution (about 800 °C). A formal seed was then introduced at a few degrees higher than the saturation point. Then the temperature was slowly reduced at a rate of 0.2-0.5 °C/day and the seed-rotation rate was 9 rpm. Figure 1 shows the grown crystal wascrack free and mostly transparent.
3. Results Transmission Spectrum of BPO. Figure 2 shows the transmission curve of BPO (the sample length is 1 mm) exhibiting a high and constant transmittance in the measured wavelength region. The ultraviolet cutoff edge is about 130 nm. Linear Optical Properties. The refractive indices at 7 wavelengths in the visible region were measured with the minimum deviation method (Table 1), showing that BPO is a negative uniaxial optical crystal. The Sellmeier equations, which are fitted with the abovemeasured refractive indices, are as follows:
ne2 ) 2.50739 +
0.01212 - 0.01128 λ2 (λ2 - 0.00242)
0.01408 - 0.00901 λ2 (λ - 0.01663) 2
Here λ is the wavelength expressed in micrometers. Second Harmonic Generation of Nd:YAG Laser. The BPO powder sample was employed to investigate the optical second harmonic generation (SHG) effect of BPO because 532-nm lasers could not be observed from the grown crystals. Fundamental 1064-nm light was generated with a Q-switched Nd:YAG laser. Microcrystalline KDP (KH2PO4) served as the standard. The SHG effect was about twice that of KDP, which agreed with the calculated values of SHG coefficients. During the experiment, we found that the bigger the grains, the smaller the SHG effect was. This indicated that BPO crystal could not bear phase-matching of SHG in the visual region. Theoretical Calculation. CASTEP,10 a plane-wave psuedopotentiol total energy package, is used for solving the electronic and band structure as well as linear and nonlinear optical properties of BPO crystal. The theoretical basis of CASTEP is the density functional theory (DFT) in the local density approximation (LDA)11 or gradient-corrected LDA developed by Perdew and Wang.12 Our group has used CASTEP to calculate the electronic band structures and optical properties of many borate crystals, for example, BBO,13 LBO, CBO and CLBO,14 and KBBF15 and obtained reasonable calculated results. The basic structure feature of BPO crystal is shown in Figure 3. The geometrical parameters of BPO are as follows: space group I4 h , a ) b ) 4.332(6) Å, c ) 6.640(8) Å, R ) β ) γ ) 90°, z ) 2.16 In the BPO crystal, both B3+ and P5+ are tetrahedral coordinated. Figure 4 gives the total density of states (DOS) and partial DOS projected on the constitutional atoms of BPO crystal. It shows that the balance band (VB) of BPO is divided into two subgroups separated by a gap of about 3 eV, which are a little complicated and not flat. The upper one is the conduction band (CB) with a width of about 5 eV. The CB of BPO crystal contains some of 3p orbitals of phosphorus atom, which have higher energy than those of 2p orbitals of B and O, and it widens the gap between the VB and CB. As a result, the BPO has a short cutoff wavelength of about 140 nm. In BPO crystal the B3+ and P5+ are tetrahedral coordinated and in sp3 hybridization. In general, tetrahedral coordination crystals have strong chemical bonds and wider energy gaps. In addition, for BPO crystal some of the 3p orbitals of phosphorus atom in the conduction band raise its levels, and it also results in a short cutoff wavelength of about 140 nm, which is one of the shortest cutoff wavelengths in all borate NLO materials. According to the computational formula given in ref 13, the theoretical optical coefficients have been calcu(10) CASTEP3.5 Program developed by Molecular Simulation Inc., 1997. (11) Kohn, W.; Sham, L. J. Phys. Rev. 1965, A140, 113. (12) Perdew, P.; Wang, Y. Phys. Rev. 1992, B45, 2310. (13) Lin, J.; Lee, M. H.; Liu, Z. P.; Chen, C. T.; Pickard, C. J. Phys. Rev. 1999, B60, 13380. (14) Lin, Z. S.; Lin, J.; Wang, Z. Z.; Chen, C. T. Phys. Rev. B 2000, 62, 1757. (15) Lin, Z. S.; Wang, Z. Z.; Chen, C. T.; Chen, S. K.; Lee, M. H. Chem. Phys. Lett. 2003, 367, 523. (16) Schulzer, G. E. R. Z. Phys. Chem., Abteil. B 1934, 24, 215.
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1:4) for 3 h, which indicates that the BPO was chemically stable, free of moisture, and nonhygroscopic. The hardness of BPO crystal was about 6.5 Mohs, close to that of LBO. The chemical stability and the good mechanical properties make it easy to cut, polish, and coat by normal processing. 4. Discussion
Figure 3. Structure of BPO.
Though the BPO-LPO-Li2O system can decrease the viscosity of the melt, the melt volatility increased. In the growth process, a white volatile substance was collected from the seeding rod and the cover of the furnace. This was confirmed to be BPO by using X-ray powder diffraction (Bruker D8 ADVANCE Analytical X-ray Systems). The measured volatility (the losing weight of the melt per day) was about 0.53%. Although the solution of BPO- LPO- Li2O flux system can grow BPO4 crystals, a high volatility of the melt makes the component of the melt change and infects the crystal growth. These leads to the formation of defects such as inclusions and mosaic structure and veil of the solution in the growing crystals. So the new complex flux and suitable dopants should be tested and tried in a further investigation. The morphological faces of the grown crystals are always the form {101} or {011} [the (101) and (011) plane are equivalent faces], the phenomena are associated with the crystallization environment (the component variety of the melt such as anions or ions). The calculated cutoff edge of BPO is about 140 nm, which is in line with the experimental value (130 nm). Table 1 shows that the calculated values and experimental refractive indices agree very well. The measured birefringence index was about 0.005, which is too small to achieve SHG phase-matching. However, the cutoff wavelength of 130 nm of BPO intrigues us to learn more about its phase-matching conditions in the DUV region. Some phase-matching applications of BPO crystal are in progress. 5. Conclusion
Figure 4. Density of states and partial density of states (DOS and PDOS) figure of BPO.
lated. Table 1 lists the calculated refractive indices for several wavelengths, based on DFT, which shows that the calculated and experimental refractive indices agree very well. The values of SHG coefficients d14 and d15 of BPO are calculated as 0.93 and 0.28 pm/v, respectively. Miscellaneous Characteristics of BPO. The weight of BPO crystal was not changed when we soaked a 10-g sample in water for one week. Moreover, this crystal did not dissolve when it was dipped into boiled phosphorus acid or muriatic acid or nitric acid (volume ratio
We have grown the BPO4 crystal with dimensions of 15 × 10 × 12 mm by the top-seeded solution growth technique using Li2O- Li4P2P7 as fluxes, and measured its refractive indices with the prism minimum-deviation method for the first time. Using the first-principle method, the electronic band structures, and linear and nonlinear optical coefficients are calculated for BPO crystal. The calculated refractive indexes and SHG coefficients are in good agreement with experimental values. BPO crystal is nonhygroscopic, chemically stable, and has a hardness of 6.5 Mohs. Acknowledgment. This work was supported by the Chinese National Basic Research Project. Z.S.L. gratefully acknowledges the support of the K. C. Wang Education Foundation. CM040121O
