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Abstract
A colorless crystal of Ba2Mg(BO3)2 about 37 × 25 × 17 mm3 has been grown successfully by the Kyropoulos method. Thermal analyses results show that Ba2Mg(BO3)2 melts congruently. The phase purity of the raw materials and the grown crystals were confirmed by powder X-ray diffraction. Ba2Mg(BO3)2 exhibits a low UV cutoff of 187 nm and high transmittance below 3000 nm. The refractive indices of the crystal have been measured and the the least-squares fitting method was used to obtain the Sellmeier dispersion equations. The results indicate that Ba2Mg(BO3)2 is a potential birefringence crystal with large birefringence of 0.10422 at 546 nm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As an important functional material, birefringent crystals are widely used in fabricating the optical circulator, beam displacers, electro-optics Q switch and so on [1–4]. With the laser application fields extending to UV and DUV (λ < 200 nm) region, the demand for optical crystals with both large birefringence and high DUV transmission has increased dramatically. Nevertheless, few materials satisfied the requirements because of the low transparence in the DUV region. Commercial birefringent materials, such as calcite (CaCO3) [4,5], and YVO4 [6], show excellent performance in the visible range, but some defects impend their UV and DUV applications. Calcite crystal, an excellent birefringent material with transmission range of 300 - 2300 nm, is difficult to be grown by artificial. Furthermore, the cleavability, low transmission at 260 nm and various natural impurities also blocked its UV adhibition [7]. Another considerable birefringent material YVO4 is not transparent in UV and DUV region. High temperature phase BaB2O4 (α-BBO) possesses the merits of short cutoff edge located at 189 nm and a relatively large birefringence (0.119 at 546 nm), thus α-BBO is considered to be one of the important borate birefringent material used in UV range [1]. However, high quantity α-BBO crystal is difficult to be obtained for the drawbacks of phase transition at 925 °C [8,9]. Ca3(BO3)2, a crystal with relatively short UV transmission cutoff at 180 nm, has a large birefringence (0.0982 at 546 nm) [2]. Large Ca3(BO3)2 crytals can be grown by the Czochralski method. Nevertheless, for its high melting point at 1479 °C, the composition and optical quality of Ca3(BO3)2 crystal are encumbered by the component volatillization at high temperature during growth. Ba2M(C3N3O3)2 (M = Mg, Ca), a lately reported novel UV birefrigent materials, exhibit an extremely large birefrigence and short UV cutoff edge (230 nm) [10]. The calculated birefrigence reaches 0.35 at 800 nm, which is three times larger than that of Ca3(BO3)2 (0.0950 at 807.2 nm). Thus, Ba2M(C3N3O3)2 (M = Mg, Ca) is expect to be an excellent birefrigent material. Nonetheless, the attempts of growing large Ba2M(C3N3O3)2 (M = Mg, Ca) crystals used for birefringence experimentals are still in progress. Thus, it is necessary to find a large birefringence crystal which can be used in the UV especially in DUV spectra. Borates are expected to be the ideal UV and DUV candidates for its specifically short UV cutoff edge [11]. Hence, in this paper, borate was chosen as our priority research system for DUV applications.
The admirable optical properties of borates are proved to originate from the highly localized valence electrons of B-O bonds [12,13]. The electronegativity discrepancy between B and O ions is quite large, which is beneficial to improve the polarization. In borates, one boron atom is coordinated by three or four oxygen atoms forming two basic units: the BO3 planar triangular and the BO4 tetrahedral. By sharing corners or edges, these two basic groups can be further linked to construct other complicate structures, such as B2O5, B3O6, B3O7, etc [14]. Herein, our attentions are attracted by the borates with BO3 trigonal as its structure units because this units can produce large birefringence and low cutoff edge for its highly localized valence electrons and anisotropy polarizability [3,11,14,15]. Specifically, if the planar BO3 groups are arranged in coplanar alignment, the optical anisotropy along and vertical to the B-O planes will be further enhanced, leading to a reasonably large birefringence [12,13]. Alkaline earth metals are selected as the counterpart cations, mainly because the selected cations have no d-d or f-f electron transitions, which led to a high transparency in UV and DUV spectral range [16]. Based on the above analysis, employing the Ca3(BO3)2 crystal as a structure matrix, we successfully obtained Ba2Mg(BO3)2 crystal by substituting the multiple alkaline-earth metal Mg and Ba for alkaline-earth Ca.
Ba2Mg(BO3)2 crystal was first prepared by A. Akella et al. via flux method in 1995 [17]. Theoretical calculation results indicate Ba2Mg(BO3)2 is a potential negative birefringence material [12,13]. In this work, we performed the crystal growth by the Kyropoulos method. Meanwhile, the birefringence and other optical properties were investigated in detail. Results show that Ba2Mg(BO3)2 crystal exhibits a large birefringence and short cutoff edge.
2. Experimental details
2.1 Powder syntheses
The Ba2Mg(BO3)2 polycrystalline powder was prepared by high temperature solid state reaction. Firstly, stoichiometric BaCO3, (MgCO3)4·Mg(OH)2·5H2O and H3BO3 were mixed thoroughly in a blender mixer. Then, the mixture was transferred to a platinum crucible and heated at 500 °C for 4 h. After cooling to room temperature, the compounds were ground in an agate mortar and reheated at 950 °C for 24 h. The above processes were repeated until the products were reacted entirely. Finally, the resultant powder was ground thoroughly for characterization. All the raw materials are reagent grade and provided by Shanghai Aladdin Biochemical Technology Co., Ltd without further processing.
2.2 Thermal analyses
Thermal gravimetric and differential scanning calorimetry (TG-DSC) analysis for Ba2Mg(BO3)2 were performed on a SDT Q600 Simultaneous Thermal Analyzer instrument. About 10 mg Ba2Mg(BO3)2 powder sample was transferred into a small Pt crucible and then heated to 1400 °C with a heating rate of 10 °C/min in an argon atmosphere.
2.3 X-ray diffraction
The phase purities of the synthesized powder and the grown crystal were verified by X-ray diffraction (XRD). The data collection was performed on an APEX II CCD diffractometer (Cu Kα radiation λ = 1.5418 Å) under air at ambient temperature. The measurements were performed in the 2θ range from 10° to 80°. The scan step was set as 0.02° with a 1s/step fixed counting time.
2.4 Crystal growth
Based on the thermal analysis results, Ba2Mg(BO3)2 single crystals were prepared by Kyropoulos method. Polycrystalline samples of Ba2Mg(BO3)2 were synthesized as described above. The precalcined raw materials were thoroughly mixed and transferred into a Pt crucible. The temperature was raised to 1300 °C with a heating rate of 50 °C/h. Then, the reagents were sintered at this constant temperature for 10h. After melting homogenized, the mixture was cooled down to the crystallizing temperature. A c-oriented seed crystal was dipped into the melt with a cooling rate of 0.5 °C/day. The rotating rate was kept at 30 r/min during growth. Finally, the grown crystal was separated from the melt and cooled to room temperature at a rate of 10 °C/h. Figure 1 shows a typical colorless and transparent Ba2Mg(BO3)2 crystal with dimensions of 37 × 25 × 17 mm3.
2.5 Transmission spectrum
A Ba2Mg(BO3)2 crystal with 1 mm thick was double-sided polished for transmission spectrum measurement. The data record was carried out on a Lambda 900 UV/vis/NIR (PerkinElmer) spectrophotometer from 175 to 2500 nm at room temperature.
2.6 Refractive index measurement
The minimum deviation method was adopted for the refractive index measurements of Ba2Mg(BO3)2 crystal. To measure the refractive indices, a Ba2Mg(BO3)2 crystal was cut as a right-angle prism with an apex angle of 30.12°. The two right angle surfaces are (100) and (001), respectively. The data record was carried out utilizing HR SpectroMaster UV−vis-IR apparatus (Trioptics, Germany) at 13 spectral lines ranging from 253.7 nm to 1.5296 µm. The measurement accuracy is less than 1 × 10−5.
3. Results and discussion
3.1 Thermal properties
The TG-DSC thermal analysis results were plotted in Fig. 2. As shown for the TG curve, there is no conspicuous weight loss below 1250 °C for Ba2Mg(BO3)2, whereas one sharp endothermic peak located at about 1247 °C was observed in the DSC signal. Furthermore, the solidified melts were ground for powder X-ray diffraction analysis to confirm the melt behavior.
3.2 X-Ray diffraction analysis
The XRD pattern of the grown Ba2Mg(BO3)2 crystal is presented in Fig. 3, which is in agreement with the reported data (JCPDS 82–1883). The XRD results indicate that all of the diffraction peaks can be attributed to the Ba2Mg(BO3)2 phases. The powder XRD pattern after DSC experiment was also illustrated in Fig. 3. The XRD curve is identical to that of the grown crystal (Fig. 3), indicating that Ba2Mg(BO3)2 is a congruently melting compound. No phase transition was observed in the temperature of 20-1400 °C. Therefore, in principle, Ba2Mg(BO3)2 single crystal could be easily grown by the Kyropoulos method or Czochralski method.
Fig. 3. Experimental XRD patterns of Ba2Mg(BO3)2 crystal before and after DSC experiment compared with the reported one.
3.3 Transmission spectrum
The recorded transmission spectrum of Ba2Mg(BO3)2 are presented in Fig. 4. The obtained curve shows Ba2Mg(BO3)2 crystal possesses a high transmittance large than 80% in the spectral range of 300–3000 nm. The transmission curve below 220 nm is illustrated in the inset of Fig. 4. The UV cutoff edge is located at about 187 nm, while the corresponding value of α-BBO is 189 nm. This would indicate that the Ba2Mg(BO3)2 crystal is a potential DUV nonlinear optical material.
3.4 Birefringence
The minimum deviation method was employed to evaluate the refractive indices of the Ba2Mg(BO3)2 crystal. The extraordinary (ne) and ordinary (no) refractive indices measured in the different wavelengths are shown in Table 1. The least-squares fitting method was utilized to fit the experimental values of ne and no with Sellmeier’s equations [18] as follows:
in which λ represents the wavelength expressed with micrometer. The values of no are larger than ne at the same wavelengths, which means that Ba2Mg(BO3)2 is a typical·negative birefringence crystal. Ba2Mg(BO3)2 crystal has relatively high birefringence (Δn = 0.0834 - 0.1302) in the experimental wavelength range (2.3254 - 0.2537 µm). The measured refractive indices ne and no at specific wavelength together with the fitting curves are illustrated in Fig. 5. The birefringence (Δn = 0.10422) at 546 nm is superior to Ca3(BO3)2 (0.0982 at 546 nm) [2]. In short, Ba2Mg(BO3)2 is an ideal alternative to Ca3(BO3)2. Thus, Ba2Mg(BO3)2 crystal can be used to fabricate the birefringent modules including prism, beam displacers, and beam splitters, etc between DUV and near-infrared region.Fig. 5. The experimental refractive indices and the Sellmeier equations fitted curve for (a) Ca3(BO3)2 and (b) Ba2Mg(BO3)2.
Table 1. The measured refractive indices of the Ba2Mg(BO3)2 crystal
3.5 Comparison of the birefringence crystals with the similar structure
The crystal structure of Ba2Mg(BO3)2 is illustrated in Fig. 6. Ba2Mg(BO3)2 belongs to R${\bar{3}}$ m (no.166) space group. The fundamental planar BO3 trigonals are parallel to each other forming the layer distribution perpendicular to the c-axis. Two adjacent layers are connected by Ba atoms or Mg atoms [19]. For comparison, the lattice structure of a Ca3(BO3)2 crystal is shown in the Fig. 6. As illustrated, the Ca3(BO3)2 crystallizes in the R${\bar{3}}$ c (no.167) space group with parallel BO3 layers arrangement orthogonal to the c axis [2]. All BO3 groups in the same layer in both crystals are aligned in the same orientation. However, the BO3 trigonal units in Ca3(BO3)2 crystal are not strictly coplanar, but slightly deformed to form sub-layers, which is differing from coplanar arrangements in Ba2Mg(BO3)2 crystal.
Both Ba2Mg(BO3)2 and Ca3(BO3)2 are negative (no > ne) uniaxial borate crystal in which the higher number density BO3 units are distributed with parallel arrangement along c-axis. For negative birefringence borate crystals, the maximum refractive index no locates in the B-O planes while the minimum refractive index ne lies in the direction perpendicular to coplanar B-O units [19,21]. However, although Ca3(BO3)2 has a higher BO3 number density than Ba2Mg(BO3)2 (about 1.57 × 10−3 and 1.47 × 10−3, respectively), the birefringence of Ca3(BO3)2 (0.0974 at 589 nm) is lower than that of Ba2Mg(BO3)2 crystal (0.1034 at 588 nm). Why this discrepancy occurs can be explained as following:
- 1) The anionic group theory demonstrated that the parallel distribution of B-O anionic units will enlarge the optical anisotropy [22]. Thus, the parallel arrangement of BO3 groups in Ba2Mg(BO3)2 can enhance the optical anisotropy along the B-O planes, while the discordant layer arrangement of BO3 groups in Ca3(BO3)2 leads to a counteraction of anisotropy.
- 2) The empirical electronic polarizability of heavy elements Ba is higher than that of Ca [20]. In other words, the quantity of heavy elements will also influence the refractive indices. Thus, the addition of Ba by replacing the Ca atoms can significantly improve the refractive indices.
- 3) The Ba atom has a higher coordination number compared to the Ca atom. It can be found that the Ba atom of Ba2Mg(BO3)2 has six equatorial O atoms, which are differ from the cationic configuration of Ca3(BO3)2. This coordination plays a crucial role in forcing BO3 groups to arrange in the coplanar model which can improve the anisotropic polarization of the structure, resulting in a large birefringence.
4. Conclusion
A large Ba2Mg(BO3)2 crystal with a size of 37 × 25 × 17 mm3 has been obtained through the Kyropoulos method. It has a high transmission up between 300 and 3000 nm with a short UV absorption edges at 187 nm. Thermal analysis results elucidate that Ba2Mg(BO3)2 is a congruent material and no phase transitions occurs below melting point. The ordinary and extraordinary refractive indices measurements indicate that Ba2Mg(BO3)2 is an uniaxial optical crystal with a birefringence as large as 0.10422 at 546 nm. Therefore, the large birefringence combined with short UV cutoff edges makes Ba2Mg(BO3)2 to be a potential birefringent crystal for UV and deep-UV adhibition.
Funding
National Natural Science Foundation of China (51672164, 51772172); Major Scientific and Technological Innovation Project in Shandong (2017CXGC0414, 2018CXGC0412); Natural Science Foundation of Shandong Province (ZR2016EMM12, ZR2017MEM016, ZR2018BEM023); Youth Foundation of Shandong Academy of Sciences (2018QN0033).
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