High-temperature Raman spectroscopy of the Cs2O ... - IUCr Journals

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ISSN 1600-5767

High-temperature Raman spectroscopy of the Cs2O–B2O3–MoO3 system for CsB3O5 crystal growth Shanshan Liu,a,b Guochun Zhang,a* Songming Wan,c Jinglin You,d Mohamed-Ramzi Ammar,e Aure´lien Canizare`s,e Patrick Simone and Yicheng Wua

Received 11 June 2015 Accepted 19 January 2016

Edited by G. Kostorz, ETH Zurich, Switzerland Keywords: CsB3O5 crystals; high-temperature Raman spectroscopy; crystal–solution boundaries; microstructure.

a

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, People’s Republic of China, b University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China, cAnhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031, People’s Republic of China, dShanghai Enhanced Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai, 200072, People’s Republic of China, and eConditions Extreˆmes et Mate´riaux: Haute Tempe´rature et Irradiation UPR3079, CNRS, Orle´ans, 45071, France. *Correspondence e-mail: [email protected]

Raman spectroscopy at high temperature has been applied to study in situ the microstructure of the solution in a Cs2O–B2O3–MoO3 growth system. A crystal– solution interface was observed. The BO groups consist of spiral chains based on B3O4Ø2 rings in the solution (Ø is a bridging O atom). The Raman spectrum of the solution indicates that MoO4 tetrahedra existed in the growth system. The nonbridging O atoms of the chains combined with the MoO4 groups, which decreased the viscosity. The Raman spectra near the interface show that at the boundary an isomerization reaction from three- to four-coordinated boron occurred. The formation of B3Ø7 triborate groups occurred. The morphology of the CsB3O5 crystal resulting from spontaneous crystallization was observed to correspond to our expectations.

1. Introduction

# 2016 International Union of Crystallography

J. Appl. Cryst. (2016). 49, 479–484

The CsB3O5 (CBO) crystal is an excellent nonlinear optical crystal. It possesses a relatively large effective nonlinear optical coefficient, a high laser damage threshold (26 GW cm 2 for 1.0 ns pulses at 1053 nm) and a wide optical transparency range (167–3400 nm) (Wu et al., 1993, 1997). For the generation of a high-power 355 nm laser by thirdharmonic generation of Nd:YAG (neodymium-doped yttrium aluminium garnet) laser radiation, the CBO crystal is expected to be more efficient than the commercially available LiB3O5 crystal (Wu et al., 2005; Wang et al., 2006; Rajesh et al., 2008). CBO melts congruently at 1108 K and many growth techniques can be used to grow CBO single crystals. In 1993, transparent single crystals of CBO of centimetre size were successfully grown from a stoichiometric melt for the first time (Wu et al., 1993). Between 1997 and 1999, Kyropoulos and Czochralski methods were applied in growing CBO crystals (Fu et al., 1999; Wu, 1997). It is very difficult to obtain large CBO crystals with high quality, and much effort has been devoted to growing these crystals (Kagebayashi et al., 1999; Kitano et al., 2003; Saji et al., 2005; Chang et al., 2005). However, the scattering centers still exist in some way. As Cs2O is more volatile than B2O3 during the growth process, especially in the case of the large temperature gradient required for the Czochralski method, the composition of melts deviates from the stoichiometry of CBO. Excess Cs2O evaporation gives rise to an undesirable supersaturated part near the solution surface. At the same time, the high viscosity http://dx.doi.org/10.1107/S160057671600114X

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research papers limits the mixing and mass transport in the solution. These problems lead to unstable growth (spontaneous nucleation, inclusions, hopper growth etc.) of the interface so that it is difficult to grow high-quality and large size CBO crystals from a stoichiometric melt. In the crystal growth process, the composition of the solution is very significant for the crystal growth. It is especially important to investigate a suitable flux for the CBO crystal. Recently, large CBO crystals without scattering centers have been grown from the Cs2O–B2O3–MoO3 system by a topseeded solution growth method (Liu et al., 2012). It is crucial to study the growth mechanism of the CBO crystal in this system, because it is helpful for the selection of more optimum flux and growth conditions. Many previous studies have proved that temperaturedependent Raman spectroscopy is a suitable method for research into both the crystal and its growth system (Voronko et al., 1993; Ney et al., 1998; Wan et al., 2008; Hou et al., 2011; Wang et al., 2013). Compared with other analysis techniques, high-temperature Raman spectroscopy has many advantages, such as in situ observation, microscale analysis and hightemperature measurement (Simon et al., 2003; Wan et al., 2008; Voronko et al., 1993; Ney et al., 1998; You et al., 2002). Therefore, it is widely used to investigate the microstructure of crystals or melts at different temperatures. The triborate group ([B3Ø7], Ø = bridging oxygen atom), consisting of two three-coordinated BØ3 units and one four-coordinated BØ4 unit, is the basic structural group forming the CBO crystal structure (Wu et al., 1993). The vibrational Raman and infrared active modes of CBO crystals have been determined at room temperature (Wang et al., 2008). In 2011, Hou and coworkers recorded Raman spectra of a CBO crystal at different temperatures. In order to study the mechanism of CBO crystal growth in a certain system, it is important to study the structure of the melt for CBO crystal growth (Hou et al., 2011). In 2008, Wan et al. (2008) applied high-temperature Raman spectroscopy to investigate the melt structure near the CBO crystal–melt boundary layer in a stoichiometric melt and explained some growth habits of the CBO crystal. To our knowledge, there are still no reports about the Cs2O–B2O3– MoO3 system for CBO crystal growth. In this paper, the structures of the Cs2O–B2O3–MoO3 glass and solution for CBO crystal growth were investigated in situ using Raman spectroscopy. On the basis of the microstructure of hightemperature solutions, the CBO crystal growth habits in solution are discussed.

2. Experiments Unpolarized Raman spectra were obtained using a Jobin Y’von LABRAM-HR laser Raman spectrometer coupled with an Olympus BH-2 confocal microscope. The 514.5 nm line of a continuous Ar+ laser was used as the excitation source. The laser power was fixed at 30 mW in order to avoid the laser-induced heat damaging solution structures. The backscattering light was recorded using a CCD detector. During the scattering integration time of 10 s, Raman spectra over a

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wide frequency range of 100–1800 cm 1 were obtained with spectral resolution better than 2 cm 1. The spatial resolution was better than 2 mm. A high-temperature solution with the composition Cs2O– B2O3–MoO3 was used to grow the CBO crystal. High-purity reagents Cs2CO3, H3BO3 and MoO3 were accurately weighed in the molar ratio of 1.8:4.5:2, mixed homogeneously in an agate mortar, and then transferred to a Pt crucible. The mixture was slowly heated to 323 K above the melting point at a rate of 30 K h 1 in order to avoid ejection of powdered raw material from the crucible due to vigorous evolution of carbonate, then held at that temperature for 10 h to ensure that the sample melted homogeneously, and finally the melt was rapidly quenched. As a result, a sample of Cs2O–B2O3– MoO3 glass for crystal growth was obtained. For studying the microstructure near the CBO crystal growth interface in high-temperature solution, a special electronic microfurnace was applied to couple with the spectrometer. A platinum boat with size of 12 7 2 mm was filled with the sample. A CBO single crystal was cut into slices which could be exactly positioned into the platinum boat. One such slice was then placed at the cool side of the platinum boat, while the powder of growth material prepared before was placed at the heated side. The heating system provided a horizontal temperature gradient in the boat. Firstly, the spectrum of the growth material glass was recorded at room temperature. After slow heating at a rate of 10 K min 1, the growth material powder melted completely at 1003 K. The Raman spectrum of the growth material solution was recorded. Subsequently, the temperature was slowly decreased to allow the new CBO crystal to grow gradually from the crystal surface in order to observe a stable crystal–solution interface at about 963 K. The sample was kept for 5 min at every target temperature before the spectrum was recorded. The experimental parameters are consistent with the roomtemperature measurement. However, owing to blackbody radiation at high temperature, the highest measured temperature was limited to under 750 K. For all spectra, the intensities were corrected for background and the Bose– Einstein population reduced factor, and then fitted using a Gaussian-type profile by the least-squares method to obtain the peak elements, such as peak position, shape and area.

3. Result and discussion To understand the structure of the corresponding solution for CBO crystal growth, the Raman spectra of the glass at room temperature and the solution (the melted glass) at about 963 K were recorded, as shown in Fig. 1. The Raman spectrum of the glass contained primary bands at 205, 331, 765, 882 and 927 cm 1. When the glass melted completely, the Raman spectrum of the solution was also recorded. In the Raman spectrum of the solution, all the bands shift to low frequency and broaden slightly compared with those of the glass. The band located at 765 cm 1 was assigned to the symmetric breathing vibration of six-membered rings with one four-coordinated boron atom (Bril & Eindhoven,

High-temperature Raman spectroscopy of Cs2O–B2O3–MoO3

J. Appl. Cryst. (2016). 49, 479–484

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Figure 1 Raman spectra of the CBO growth system at room temperature and 963 K.

1976; Meera & Ramakrishna, 1993; Feller et al., 1982). Hou et al. (2011) studied the structure of the molten CBO crystal. Their results indicated that the structure of the CBO liquid consists of spiral chains based on B3O4Ø2 rings which were formed from (B3O6)3 groups. Also, according to their Raman spectrum of the melt and the Raman spectra of the glass in Fig. 1, it can be considered that spiral chains based on B3O4Ø2 rings exist in our solution. The strong vibration bands located at 875, 915 and 328 cm 1 are the predominant features of MoO groups. The bands at 875 and 915 cm 1 were assigned to the stretching vibration of Mo—O bonds in (MoOn) species, while the band at 328 cm 1 was assigned to the bending vibration of the Mo—O bonds. Caurant et al. (2010) proposed that the wide and intense band observed in the 898–913 cm 1 range of the Raman spectra for all borosilicate glasses containing MoO3 is due to the symmetric stretching vibration of Mo—O bonds of molybdate tetrahedra. Hanuza et al. (2010) considered the bands centered at 872 and 912 cm 1 to be assigned to the symmetric stretching vibration and the asymmetric stretching vibration, respectively, of MoO4 tetrahedra. The band at 328 cm 1 results from the bending vibration of MoO4 tetrahedra. In more detail, in MoO3 derivative

compounds (e.g. K2Mo4O13) consisting of MoO6 octahedral units, terminal bonds having a double-bond character, Mo O, are stabilized and the stretching vibrations due to Mo O bonds give the Raman bands at 960–1000 cm 1 (Sekiya et al., 1995; Seguin et al., 1995). On the other hand, it is known that the symmetric and the asymmetric stretching vibrations of MoO4 tetrahedral units show Raman bands at 895–950 and 810–880 cm 1 (Hanuza et al., 2010,1999; Seguin et al., 1995; Basiev et al., 2000; Saraiva et al., 2008; Luz-Lima et al., 2010). From the Raman spectra of both the glass and the solution, there are no bands at 960–1000 cm 1. Consequently, the MoO groups are mainly composed of MoO4 tetrahedra rather than MoO6 octahedra in the Cs2O–B2O3–MoO3 system. Voronko et al. (2014a,b) studied in detail the structure of molten alkali-metal molybdates using Raman scattering spectroscopy. According to their results, a transformation of molybdenum coordination from octahedral to tetrahedral occurred. ortho-[MoO4]2 and pyromolybdate anions [Mo2O7]2 made up of two linked [MoO4] units may exist in our solution (Wang et al., 2013; Voronko et al., 2014a,b). On the other hand, B3O4Ø2 chains contain a number of nonbridging O atoms (NBOs) which can bring about a relatively intense band near 1500 cm 1. However, this band was very weak in the melts. The reason was probably that the NBOs combined with the [MoO4] groups, as shown in Fig. 2. Fig. 1 shows that the intensity of the 875 cm 1 band involving [MoO4]2 decreased compared with the 915 cm 1 band relevant to [Mo2O7]2 with increasing temperature. Generally, a temperature increase enhances depolymerization, resulting in the increase of monomer [MoO4]2 (Voronko et al., 2014a,b), which is not shown in Fig. 1. This might indicate that the terminations of B—ONB are mostly connected with [MoO4]2 rather than [Mo2O7]2 , which could explain the low viscosity of Cs2O–B2O3–MoO3 shown in our previous work (Liu et al., 2012). In order to understand the mechanism of CBO crystal growth in the Cs2O–B2O3–MoO3 system, especially how the microstructure changes during the crystal growth process, we have focused on the boundary region near the crystal–solution interface, as shown in Fig. 3. The morphology and crystal growth steps of the new CBO crystal can be clearly seen. We recorded the Raman spectra in the solution at different positions (A, B and C are at 50, 20 and 0 mm, respectively,

Figure 2 The microstructure and the isomerization reaction in the bulk solution. J. Appl. Cryst. (2016). 49, 479–484

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research papers from the crystal growth interface). The spectra are shown in Fig. 4. The Raman spectra in the range of 250–450 cm 1 and 600–1100 cm 1 at positions A, B and C are emphasized. The deconvolution spectra are shown in Figs. 4(b)–4(g). As the measurement position moves toward the crystal–solution interface (from position A to C), the Raman spectra of the solution change gradually. For position A, the spectrum is similar to that of the bulk solution other than the relative intensity of the bands. In the range 250–450 cm 1 of the spectrum, as the measuring point comes closer to the crystal, the intensity of the peak located at 328 cm 1 decreases and that of the peak at 368 cm 1 increases gradually. The band at 368 cm 1 results from the bending vibration of BØ4 (Wang et al., 2008). On the other hand, the range 600–1000 cm 1 varies more obviously at the different points in the system. As the measuring point approaches the CBO crystal, the bands located at 655, 694, 747, 833 and 956 cm 1 involving the characteristic modes of the CBO crystal become stronger, whereas those at 875 and 916 cm 1 are weakened. The band in the range of 400–500 cm 1 is assigned to the symmetric or the asymmetric stretching vibration of BØ4 tetrahedra in the triborate groups (Kamitsos & Karakasside, 1989). This band (near 470 cm 1) is associated with the symmetric stretching vibration of the oxygen atom that connects two B3O4Ø2 rings, namely triborate rings with one BØ4 tetrahedron (a nonbridging oxygen) (Galeener & Thorpe, 1983). The band around 540 cm 1 is attributed to the symmetric stretching vibration of BØ4. As the measured points shift towards the crystal, the intensity of the bands at 470 and 540 cm 1 becomes stronger. This indicates that B—O—B bridges between two B3O4Ø2 rings or BØ4 formed gradually. With the precipitation of CBO from solution, the growth of CBO from this solution assisted the diffusion process. That is the reason why no composition trends such as MoO3-rich solution can be found in Fig. 4. The thickness of the transitional region in the solution is about 50 mm, which is smaller than that of the growth system reported by Wan et al. (2008). According to these results, the derivation of CBO crystal growth habits in the Cs2O–B2O3–MoO3 system can be observed. In the growth solution, the B—O group mainly

existed in B3O4Ø2 rings. In the boundary region, the B—O—B bridges between two B3O4Ø2 chains formed. In other words, a reaction from three- to four-coordinated boron occurred. The triborate group [B3Ø7], consisting of two three-coordinated BØ3 units and one four-coordinated BØ4 unit, is the basic structural group forming the CBO crystal structure. The asgrown CBO crystal always presents as a long column and the predominant faces are invariably (011) and (101) (Chang et al., 2005). The slowest growing and, hence, the largest faces are those with the weakest strength of bonding (BerkovitchYellin, 1985). From the CBO crystal structure, the (011) and (101) faces are bonded along a B—O—B bridge bond between two triborate groups. Therefore, it can be said that the weakest bond is the B—O—B bridge bond between two triborate

Figure 4 Figure 3 The boundary of the crystal solution in the Cs2O–B2O3–MoO3 solution.

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(a) Raman spectra of different positions recorded near the CBO crystal– solution interface. (b)–(d) Deconvolution spectra located at 250– 450 cm 1. (e)–(g) Deconvolution spectra located at 600–1100 cm 1.


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Figure 6 The morphology of as-grown CBO crystal in the solution.

projection along the a and b axes of the CBO crystal in Fig. 5, the growth directions of the faces (011) and (101) are dominated by the weak B—O—B bonds. Therefore, these faces grow slower and should be the larger developed faces. The morphology of the as-grown CBO crystal is shown in Fig. 6. The result is in good agreement with our prediction.

4. Conclusions The crystallization of a CBO crystal in a high-temperature solution of Cs2O–B2O3–MoO3 occurred in the boundary region. From the Raman spectra of Cs2O–B2O3–MoO3 glass at room temperature and solution at high temperature, the crystal growth solution is mainly composed of [MoO4] tetrahedra ([MoO4]2 , [Mo2O7]2 ) and B3O4Ø2 chains with BØ4 tetrahedra. The CBO crystal–solution boundary was observed. The Raman results of the crystal growth boundary region suggest the progressive transformation of boron coordination number from three to four near the CBO crystal–solution interface. As a useful flux to decrease the viscosity of the solution, the MoO groups are mainly composed of MoO4 tetrahedra rather than MoO6 octahedra in the Cs2O–B2O3– MoO3 system. The BO groups consist of spiral chains based on B3O4Ø2 rings in the solution. The NBOs of the chains combined with the MoO4 groups, which decreased the viscosity. The microstructure evolution of CBO in high-temperature solution should provide helpful information about the growth of the crystal.

Figure 5 The projection of the CBO crystal structure along (a) the a axis and (b) the b axis.

groups. According to the Hartman–Perdok theory (Hartman & Perdok, 1955a,b,c; Hartman, 1987), the morphology of a crystal is governed by interrupted and periodic chains of strong bonds running through the structure. The growth rate is relative to the strength of bonds in the chains. The directions through the chains with strong bonds possess a high growth rate (Hartman & Perdok, 1955a,b,c). Generally, the crystal morphology depends on the relative growth rates of various crystal faces. The face that grows slower appears to be the larger developed face (Hartman, 1987). According to the J. Appl. Cryst. (2016). 49, 479–484

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant Nos. 50932005 and 51132008).

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