Growth and Thermal and Spectral Properties of a New Nonlinear Optical Crystal TmAl3(BO3)4 Jia,†,‡
Tu,*,†,‡
Guohua Chaoyang Jianfu Yan Wang,† and Baichang Wu†
Li,†
Zhaojie
Zhu,†
Zhenyu
You,†
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 949-952
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China Received September 23, 2004;
Revised Manuscript Received November 30, 2004
ABSTRACT: Single crystals of the nonlinear optical (NLO) material TmAl3(BO3)4, space group R32, were grown successfully by the top seeded solution growth (TSSG) method for the first time in our laboratory. Morphology analysis reveals that the crystal is a rhombohedron with nine developed faces with the major forms (12 h 10), (21 h1 h 0), and (112 h 0) parallel to the c-axis. Grown crystals were characterized by X-ray powder diffraction (XRD), and second-harmonic generation (SHG) measurements. The two principal coefficients of thermal expansion along (001) and (100) were measured to be 5.33 × 10-6 and 2.46 × 10-6 K-1, respectively. In addition, the large difference of thermal expansion coefficients between the two crystallographic axes was discussed based on the structure of this crystal. Polarized absorption spectra were measured, and the results suggest that the optical properties of the TmAl3(BO3)4 crystal depend greatly on the polarization. This crystal is a promising candidate for NLO materials. Introduction In recent years, a myriad of efforts has been put forth in developing nonlinear optical (NLO) materials due to their excellent properties and important applications such as frequency shifting, optical modulating, and telecommunications and signal processing. Borate materials have been given particular interest owing to the unique structural characteristics of boron-oxygen groups.1,2 TmAl3(BO3)4 was first synthesized and grown in our laboratory. Its crystalline structure was analysis and refined. The compound crystallizes in the noncentrosymmetrical system, space group R32 (No. 155), with a ) 9.2741(13), c ) 7.218(3), R ) β ) 90°, γ ) 120°, v ) 537.7(2) Å3, and Dcal ) 4.494 g/cm3, and Z ) 3.3 In this paper, we discuss in greater detail the growth and crystal morphology of TmAl3(BO3)4. In addition, we present some of these characteristics, e.g., XRD, SHG, thermal expansion coefficients, and polarized spectra properties. Experimental Section TmAl3(BO3)4 was prepared by using solid-state reaction techniques. The initial mixture of Al2O3, H3BO3, MoO3, K2CO3 (of analytical grade), and Tm2O3 (of spectral grade) was finely ground in an agate mortar and then packed into a platinum crucible. The temperature was raised slowly to 600 °C to avoid ejection of raw materials from the crucible due to release of CO2, H2O, and the decomposition of H3BO3. After preheating the sample at 600 °C for 12 h, the products were heated to 1080 °C and kept at this point for 48 h to let the mixture become homogeneous. Crystals of TmAl3(BO3)4 were grown from K2Mo3O10-B2O3 as the flux by the top seeded solution growth (TSSG) method. * To whom correspondence should be addressed. Tel.: +86-591-83711368/2122; fax: +86-591-8371-4946; e-mail: tcy@ ms.fjirsm.ac.cn. † Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.
Single crystals were grown on small c-axis oriented seeds dipped in the solution. During the seeding process, the growth temperature was always found to be lower than the real saturation temperature point, causing large size, abnormity, and opacity of this crystal (shown in Figure 1). After the growth temperature was accurately determined and suitable ratios of the flux were taken into consideration, a transparent crystal with well-developed faces was obtained (Figure 3). XRD investigations were carried out with a CAD4 diffractometer equipped with CuKR radiation (λ ) 1.054056 Å). The data were collected using a Ni-filtered Cu-target tube at room temperature in the 2θ range of 5° to 85°. Each strong diffraction band was indexed. Figure 5 shows the XRD patterns of TmAl3(BO3)4 single crystal. We used the Kurtz method4 to measure the second harmonic response of the TmAl3(BO3)4 crystal. The fundamental wavelength is 1.06 µm generated by a Q-switched Nd:YAG laser. The SHG wavelength is about 0.53 µm, and microcrystalline KDP (KH2PO4) powder serves as the reference. Thermal expansions coefficients of TmAl3(BO3)4 along the crystallographic axes were measured by a diatometer 402 PC in the temperature range of 300-800 °C. The sample lengths along the (001) and (100) direction were 4.06 and 3.68 mm, respectively. The sample was kept in a fused silica sample holder and heated at a rate of 10 °C/min in air atmosphere. The figure of linear expansion versus temperature was shown in Figure 6. Room-temperature transmission and polarized absorption spectra of these crystals were recorded by a Perkin-Elmer UVVis-NIR spectrometer (Lambda-35). The transmission spectrum and polarized absorption spectra of TmAl3(BO3)4 crystal was shown in Figure 9 and Figure 10, respectively.
Results and Discussion TmAl3(BO3)4 crystals were obtained by the TSSG method. The flux growth is particularly preferable because it readily allows crystal growth at a temperature well below the melting point of the solute. In addition, crystals grown from flux have an enhedral habit and a reasonably lower degree of dislocation density.5 First, we used 72 wt % K2Mo3O10-8 wt % B2O3 as the flux. Small seeds were oriented and attached to the rod. The growing crystal rotated at a rate of 20 rpm
10.1021/cg049677e CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005
950
Crystal Growth & Design, Vol. 5, No. 3, 2005
Jia et al.
Figure 1. The first grown TmAl3(BO3)4 crystal. Figure 5. XRD pattern of TmAl3(BO3)4 crystal.
Figure 2. Temperature gradients of the platinum crucible. Figure 6. Thermal expansion of TmAl3(BO3)4 crystal along different directions.
Figure 3. The later grown TmAl3(BO3)4 crystal.
Figure 7. View of the helicoidal chains of AlO6 octahedra parallel to the c-axis in TmAl3(BO3)4.
Figure 4. Morphology of TmAl3(BO3)4
without pulling, and the temperature was decreased at a rate of 5 °C/day. After about 20 days, a relatively large, abnormal, and opaque crystal was obtained (Figure 1), with a weight of 40.2 g (average growth rate
2.01 g/day). The three dimensions of the crystal along the c-, a-, and b-axis directions are 21.0, 15.9, and 16.0 mm, respectively. In the following, several factors were taken into consideration in improving the qualities of this crystal. In accordance with the crystal growth procedure, suitable temperature gradients are very important. Figure 2 shows the temperature gradients of the platinum crucible. The crucible dimensions are
New Nonlinear Optical Crystal TmAl3(BO3)4
Figure 8. Chains of projection on the ab-plane of the TmAl3(BO3)4.
Figure 9. Transmission spectrum of the TmAl3(BO3)4 crystal.
Figure 10. Polarized absorption spectra of the TmAl3(BO3)4 crystal.
40 mm in diameter and 40 mm in height. The temperature from the crucible bottom gradually increases from about 980 °C to 1005 °C. The large temperature gap between the surface of the flux and the crucible bottom ensures that mass transportation can be successfully achieved. The weight ratio of TmAl3(BO3)4 was reduced to 18 wt %, and the flux was 73.8 wt % K2Mo3O10, 8.2 wt % B2O3. The saturation temperature was accurately determined by observing the growth/dissolving of a seed located at the center of the free surface of the solution. The temperature decrease rate was decreased to 2.5 °C/ day. As a result, a transparent crystal with welldeveloped faces was obtained after 30 days of growth (Figure 3). The dimensions of this crystal grown were 15.0 × 6.0 × 6.0 mm along the c-, a-, and b-axis directions, respectively, with a weight of 2.56 g (average growth rate 0.08 g/day). The usual morphology of TmAl3(BO3)4 obtained from K2Mo3O10-B2O3 is a thin rhombohedron with nine well-
Crystal Growth & Design, Vol. 5, No. 3, 2005 951
developed faces (Figure 5), and the surfaces of the crystal are very flat. There are three pair of faces parallel to c-axis indexed as (101 h 1), (01 h 11), and (1 h 101). Three faces indexed (12 h 10), (21 h1 h 0), and (112 h 0) compose the triangle cone. The dominance of the (12h 10) faces is thought to be attributed to the adsorption of a layer of molybdate ions on these faces, which is the most favorable for epitaxial adsorption of MoO3.5 This is the most prominent face, which dominates the crystal morphology. The compound crystallizes in a noncentrosymmetric space group, which is a basic precondition for a potential harmonic generation material. The experiment of secondharmonic generation measurement shows that the second-harmonic generation effect is about four times as large as that of KDP. Measurements of thermal expansion have greatly increased our knowledge of material properties such as lattice dynamics, electronic and magnetic interactions, thermal defects, and phase transitions.6 In addition, upon irradiation with a laser beam, the optical absorption of a NLO crystal causes thermal gradients that disturb phase matching.7 Figure 6 shows the temperature variation of the thermal expansion of the TmAl3(BO3)4 crystal along the two crystallographic directions. The average thermal expansion coefficients along the c-axis and a-axis were 5.33 × 10-6 and 2.46 × 10-6 K-1 respectively. It shows that the thermal expansion coefficients are positive as no thermal contraction occurs when the crystal is heated. In the uniaxial TmAl3(BO3)4 crystal, the thermal properties perpendicular to the crystallographic c-axis are theoretically equivalent. The value of thermal expansion along the c-axis is about two times larger than that of the a-axis. Thus, crystal growth along the c-axis can largely avoid internal pressure and fracture of this crystal. The large differences of thermal expansion coefficients between the c- and a- crystallographic axes can be explained based on the structure of this crystal. The structure of the TmAl3(BO3)4 can be viewed as formed by layers normal to the c-axis (Figure 7) in which there are TmO6 and AlO6 octahedra. TmO6 polyhedra are interconnected within the layers. Figure 8 shows the projection on the ac-plane of TmAl3(BO3)4. AlO6 octahedra form helicoidal chains, which run parallel to the c-axis. TmO6 trigonal prisms are isolated polyhedra, and each of them connects three helicoidal chains of AlO6 octahedra through common O oxygen atoms. This unique helical structure contributes to more internal extension along the c-axis when the crystal is heated. Consequentially, the relative expansion value in the c-axis direction is larger than that in the a-axis, which leads to a larger thermal expansion coefficient in the c-axis direction. We obtained the transmission spectrum of TmAl3(BO3)4 crystal with the light parallel to the (001) direction. It is transparent up to 2700 nm and its ultraviolet absorption edge is at 380 nm. This broad transmission range enables the study of rare earth ion transitions in the visible and infrared regions. The polarized absorption spectra were extensively studied. Figure 10 shows the polarized absorption spectra of the TmAl3(BO3)4 crystal. The spectra consist of four groups of resolved bands associated with the transitions from the 3H6 ground state to the 3F4, 3H5,
952
Crystal Growth & Design, Vol. 5, No. 3, 2005
3H , 4
and 3F3 excited states. There were significant differences between the two polarized spectra. Our detailed analysis was focused on the intensive absorption bands centered at about 800 nm, as absorption at a peak of about 800 nm corresponding to 3H6 f 3H4 transition is suitable for commercial laser diode (LD) GaAlAs pumping.8 The thulium concentration of this crystal was calculated to be 5.58 × 1021 ions/cm-3. The absorption cross section σa at 799 nm was determined using:
σa ) R/Nc here R is the absorption coefficient, R ) 2.303A/L, A is the absorbance, L is the thickness of the polished crystal, Nc is the Tm ion concentration. Here L ) 0.64 cm. For a π polarization spectrum, the most intense absorption at 796 nm has an absorption cross-section of 2.44 × 10-22 cm2 and an 11-nm full width at half maximum (fwhm), while for a σ polarization spectrum, the most intense absorption band is centered at 809 nm, which has the absorption cross-section of 4.20 × 10-22 cm2 and an 11-nm fwhm. The fwhm value of a σ absorption spectrum is comparable to that of a π absorption one; however, the absorption cross-section of the former σ spectrum is about two times larger than that of the π spectrum. Thus, the σ absorption spectrum is the preferable spectrum, and lights polarized in this direction are more important than those of other directions. Conclusion We have successfully grown TmAl3(BO3)4 crystals by the TSSG method. To improve crystal quality, several
Jia et al.
factors were considered in accordance with the growth procedure of this crystal. Relatively high flux ratios, lower annealing temperature rates, c-axis seeding, as well as suitable temperature gradients were crucial to the good quality and important in avoiding fracture of the crystal. Large differences between polarized absorption spectra suggest that this crystal has a large dependence on the polarization, indicating intensive anisotropic properties of the TmAl3(BO3)4 crystal. All of the foregoing results suggest that the TmAl3(BO3)4 crystal is a promising candidate for NLO materials. Acknowledgment. This project was supported by the Natural Science Foundation of the Fujian Province of China Grant Nos. 2002I016 and E0410028. References (1) Pan, S. L.; Wu, Y. C.; Fu, P. Z.; Zhang, G. C.; Li, Z. H.; Du, C. X.; Chen, C. T. Chem. Mater. 2003, 15, 2218-2221. (2) Chen, C. T.; Ye, N.; Lin, J.; Jiang, J.; Zeng, W. R.; Wu, B. C. Adv. Mater. 1999, 11, 1071-1078. (3) Jia, G. H.; Tu, C. Y.; Li, J. F.; Zhu, Z. J.; You, Z. Y.; Zhang, S. F.; Wei, M.; Wu, B. C., manuscript submitted to Chem. Mater. (4) Kurtz, S. W.; Perry, T. T. J. Appl. Phys. 1968, 39, 37983813. (5) Oishi, S.; Teshima, K.; Kondo, H. J. Am. Chem. Soc. 2004, 126, 4768-4769. (6) Choosuwan, H.; Guo, R.; Bhalla, A. S.; Balachandran, U. J. Appl. Phys. 2002, 91, 5051-5054. (7) Wang, X. Q.; Xu, D.; Lu, M. K.; Yuan, D. R.; Cheng, X. F.; Huang, J.; Wang, J. Y.; Yu, W. T.; Sun, H. Q.; Duan, X. L. Ren, Q.; Yang, H. L. Chem. Phys. Lett. 2003, 367, 230-237. (8) Ermeneux, F. S.; Goutaudier, C.; Moncorge, R.; Cohen-Adad, M. T.; Bettinelli, M.; Cavalli, E. Opt. Matter 8 1997, 8, 83-90.
CG049677E
