Songming
Wan
*a,
Bing
Teng
*b,
Xia
Zhang
a,
Jinglin
You
c,
Wenping
Zhou
a,
Qingli
Zhang
a and
Shaotang
Yin
a
aAnhui Institute of Optics and Fine Mechanics, 350 Kexueyuan Road, Hefei, 230031, China. E-mail: smwan@aiofm.ac.cn; Fax: +86 0551 5591039; Tel: +86 0551 5591039
bCollege of Physics Science, Qingdao University, Qingdao, 266071, China
cSchool of Material Science and Engineering, Shanghai University, Shanghai, 200072, China
First published on 2nd September 2009
High-temperature Raman spectroscopy has been applied to investigate the melt structure near the BiB3O6 (BIBO) crystal–melt interface. Based on the experimental results, the crystal growth mechanism was proposed. (BOØ)n (Ø = bridging oxygen atom) chains and free Bi3+ cations present in the BIBO bulk melt act as the crystal growth units. Bi-Ø bonds and [BØ4]-tetrahedra were found near the BIBO crystal–melt interface. Two neighboring oxygen atoms in a (BOØ)n chain or in two different (BOØ)n chains are connected by the Bi3+ cations to form the structural prototype of BIBO crystal. Two adjacent (BOØ)n chains are further connected to each other through the [BØ4] tetrahedra to form the BIBO crystal structure. The predicted growth habit of the BIBO crystal is consistent with the observed one based on the mechanism.
Dynamic processes near a solid–liquid interface are of key importance across broad areas of science and technology,12–16 which are also a fundamental problem of crystal growth. Crystal growth mechanism, which gives an insight into the microdynamic processes near a crystal–melt interface, can help us understand the formation of various crystal defects at a molecular level and then to develop more perfect growth techniques to improve crystal quality. Since crystal growth occurs at a crystal–melt interface, the melt structure near the interface plays a crucial role in the study of the crystal growth mechanism. However, experimental investigation on the melt structure in this region is very difficult due to the lack of a suitable high-temperature probing technique, especially in high-temperature crystal growth systems.17 The solution to this problem will greatly facilitate the fundamental research of crystal growth. In comparison with other probing techniques, high-temperature Raman spectroscopy possesses at least three features:18 (1) it can be used to study the systems at high temperature; (2) it can collect the information of the molecular structure in a tiny region; (3) it is an in situ probing technique and can be used without disturbing crystal growth process. These features make high-temperature Raman spectroscopy a promising tool for investigating crystal growth mechanism.19,20 In this paper, high-temperature Raman spectroscopy was used to study the melt structure near the BIBO crystal–melt interface for the first time. A mechanism for BIBO crystal growth was proposed to explain the crystal growth habit.
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Fig. 1 Raman spectrum of the BIBO crystal at room temperature. |
Clear assignments of the BIBO crystal vibrational modes are essential for us to more deeply understand the bonding properties of the BIBO crystal. Usually, the displacements of the Bi atoms are negligible in these assignments since the Bi atom is much heavier than boron and oxygen atoms. the Raman vibrational modes are empirically assigned with reference to the corresponding vibrational modes of BO3 triangle and BO4 tetrahedron reported in other borate crystals or glasses.22–24 Detailed assignments of the BIBO crystal vibrational modes have been offered by Hu et al. The strongest peak located at 576 cm−1 is important for the following discussion on the BIBO crystal growth mechanism, it has been assigned to the symmetric stretching vibration of [BØ4] tetrahedron (Ø = bridging oxygen atom).22
The high-temperature Raman spectra of the BIBO crystal were recorded at 200, 400 and 600 °C, as shown in Fig. 2. With increasing temperature, all of the peaks broaden and decrease in frequency, two adjacent peaks overlap each other, which is ascribed to the wider distribution of the bond angles and the increase in interatomic distances in the BIBO crystal structure when the crystal is heated.25
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Fig. 2 Raman spectra of the BIBO crystal at different temperatures: (a) 200 °C, (b) 400 °C, and (c) 600 °C. |
When the temperature increased above the melting point of the BIBO crystal, the sample started to melt at the hot side. By slow cooling the temperature, the crystal gradually grew to the hot side. Finally, a stable crystal–melt interface was obtained, as shown in Fig. 3. The melt structure near the crystal–melt interface was studied by high-temperature Raman spectroscopy. The measurement positions are also shown in Fig. 3, their corresponding Raman spectra are presented in Fig. 4.
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Fig. 3 A typical BIBO crystal–melt interface with the measurement positions. (Position a: in the crystal, 5 µm from the interface. Positions b, c and d: in the melt, 2, 10 and 50 µm from the interface, respectively.) |
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Fig. 4 Raman spectra recorded from different positions near the BIBO crystal–melt interface. (Position a: in the crystal, 5 µm from the interface; Positions b, c and d: in the melt, 2, 10 and 50 µm from the interface, respectively.) |
The Raman spectrum recorded from position d (in the BIBO bulk melt) has three vibrational bands located at around 350, 630, 1300 cm−1. Similar Raman spectra have been observed in several B2O3–Bi2O3 glasses with a composition around Bi2O3·3B2O3,26–28 it also resembles the Raman spectra of LiBO2 and CaB2O4 crystals which are known to contain simple (BOØ)n chains.29 Based on the results reported by Rulmont et al., the vibrational band around 1300 cm−1 can be assigned to the B–Ø stretching vibration of (BOØ)n chain; the vibrational band around 630 cm−1 can be assigned to the chain deformation modes; the vibrational band below 400 cm−1 can be assigned to the external vibrational modes which originate from the weak coupling between the (BOØ)n chain and the rest of the BIBO melt.29 The Raman spectrum suggests that the (BOØ)n chain and free Bi3+ cations are the primary structural units existing in the BIBO bulk melt.
From position d to b, the vibrational band located at around 350 cm−1 increases in intensity and divides to some smaller peaks (from 300 to 500 cm−1). Considering that the symmetric stretching vibrational frequency of the bridging oxygen atom in an angularly constrained Bi–Ø–Bi configuration is in the range of 300∼600 cm−1,30,31 the broad and strong bands should be associated with the vibration of the Bi–Ø (i.e., Bi–Ø–B) bond. The presence of the Bi–Ø bond indicates the free Bi3+ cations connect with the (BOØ)n chain near the crystal–melt interface. When the measuring point moved from the BIBO melt to crystal (from position b to a), a band located at around 575 cm−1 appears. This peak was assigned to the symmetric stretching vibration of [BØ4] tetrahedron by Hu et al.,22 which implies that an isomerization process, as shown in Scheme 1, occurs accompanying the crystalline process. The isomerization process has also been found in numerous borate systems,20,32–34 which leads to the conversion of some 3-coordinated boron atoms to 4-coordinated boron atoms. The (BOØ)n chains are connected by the isomerization process near the BIBO crystal–melt interface and gradually form a boron-oxygen network.
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Scheme 1 Isomerization process near the BIBO crystal–melt interface. |
The BIBO crystal structure is a continuous three-dimensional network, its basic unit is the Bi2B3Ø8 group (see Fig. 5), which contains two [BØ3]-triangles, one [BØ4]-tetrahedron, and two Bi3+ cations, forming two six-membered rings.21 From the above results, we know that the free Bi3+ cations and (BOØ)n chains play the roles of crystal growth units in the BIBO crystal growth process. On the basis of the BIBO crystal structure, we further propose a mechanism for the crystal growth, as shown in Scheme 2. First, two neighboring oxygen atoms in a (BOØ)n chain are connected by Bi3+ cations to form the BiB2Ø3 six-membered rings, and meanwhile two adjacent (BOØ)n chains are connected by Bi3+ cations through Ø–Bi–Ø bonds. As a result, the structural prototype of the BIBO crystal is present near the crystal–melt interface. Then, the boron atoms in a (BOØ)n chain (chain 2) connect with the oxygen atoms in the adjacent (BOØ)n chain (chain 1) to form the BIBO crystal structure. The (BOØ)n long chains, Bi-Ø bonds and the BØ4 groups near the crystal–melt interface lead to a very high viscosity, as reported in previous papers.9,10 It not only inhibits the initial nucleation of the crystal, but also limits the mixing and mass transport in the melt.
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Scheme 2 A mechanism for BIBO crystal growth. |
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Fig. 5 The basic unit in the BIBO crystal structure. Dark, medium and light grey balls denote oxygen, bismuth and boron atoms, respectively. |
The growth habit of the BIBO crystal can be explained based on the above results. According to crystal growth theories, the crystal habit is dominated by the slow-growing faces.35 The fast-growing faces will not be present in the final crystal morphology. In the attachment energy model for crystal habit, the relative growth rates of various faces are assumed to be proportional to their attachment energy (Ratt, defined as the energy released when one additional growth slice of thickness dhkl is attached to the crystal face identified by the Miller indices hkl.). The attachment energy is determined primarily by the binding strength between the growth units and the crystal face.36 By comparing the structure of the BIBO crystal to the BIBO melt, we conclude that the weak bonds in the BIBO crystal structure are the Bi–Ø and B4–Ø bonds (the B–Ø bands of BØ4 tetrahedron) because the two bonds will break when the BIBO crystal melts. Thus, the faces only containing B–Ø or B4–Ø bonds possess weaker strength of binding. According to the BIBO crystal structure, the faces only containing Bi–Ø bonds are parallel to the {001} faces, the faces only containing Bi–Ø and B4–Ø bonds are parallel to the {110}, {111} and {102} faces. Therefore, these faces should be the large developed faces in the final crystal morphology. The predicted growth habit is in good agreement with the experimental results reported by Becker et al.9,10 The cases of the (001) and (011) faces are shown in Fig. 6.
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Fig. 6 Packing arrangement of the BIBO crystal structure as viewed along the b axis (top) and the c axis (bottom). The (001) face is parallel to the faces rich in Bi–Ø bonds; the (110) face is parallel to the faces rich in Bi–Ø bonds and B4–Ø bonds. Dark, medium and light grey balls denote oxygen, bismuth and boron atoms, respectively. |
Investigation on the crystal growth mechanism is crucial for us to identify the factors affecting the quality of single crystals, and thus to develop new growth techniques for producing high-quality crystals. Our work has proved that high-temperature Raman spectroscopy is the effective tool for investigating the crystal growth mechanism. We believe that the method has the potential to widen our knowledge of crystal growth micro-processes.
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