Investigation on the Structure of a LiB3O5–Li2Mo3O10 High

Article pubs.acs.org/IC

Investigation on the Structure of a LiB3O5−Li2Mo3O10 HighTemperature Solution for Understanding the Li2Mo3O10 Flux Behavior Songming Wan,*,† Guimei Zheng,†,‡ Yanan Yao,†,‡ Bo Zhang,†,‡ Xiaodong Qian,†,‡ Ying Zhao,§ Zhanggui Hu,§ and Jinglin You∥ †

Anhui Key Laboratory for Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China ‡ University of Science and Technology of China, Hefei 230026, China § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China ∥ School of Material Science and Engineering, Shanghai University, Shanghai 200072, China S Supporting Information *

ABSTRACT: LiB3O5 is the most widely used nonlinear optical crystal. Li2Mo3O10 (a nominal composition) is a typical flux used to produce large-sized and high-quality LiB3O5 crystals. The structure of the LiB3O5−Li2Mo3O10 high-temperature solution is essential to understanding the flux behavior of Li2Mo3O10 but still remains unclear. In this work, high-temperature Raman spectroscopy combined with density functional theory (DFT) was applied to study the LiB3O5− Li 2 Mo 3 O 10 solution structure. Raman spectra of a LiB 3 O5 − Li4Mo5O17−Li2Mo4O13 polycrystalline mixture were recorded at different temperatures until the mixture melted completely. The solution structure was deduced from the spectral changes and verified by DFT calculations. When the mixture began to melt, its molybdate component first changed into the Li2Mo3O10 melt; meanwhile, the complicated molybdate groups existing in the crystalline state transformed into Mo3O102− groups, which are formed by three corner-sharing MoO3Ø−/MoO2Ø2 (Ø = bridging oxygen atom) tetrahedra. When LiB3O5 dissolved in the Li2Mo3O10 melt, the crystal structure collapsed into polymeric chains of [B3O4Ø2−]n. Its basic structural unit, the B3O4Ø2− ring, coordinated with the Mo3O102− group to form a MoO3·B3O4Ø2− complex and a Mo2O72− group. On the basis of the LiB3O5−Li2Mo3O10 solution structure, we discuss the LiB3O5 crystal growth mechanism and the compositional dependence of the solution viscosity.

1. INTRODUCTION

intense and ultrafast laser systems demand wide-aperture and high-quality LiB3O5 crystals, stimulating us to find better techniques to grow LiB3O5 crystals. LiB3O5 melts incongruently and has to be grown by the flux method. B2O3 was the most commonly used flux in the 1990s.9 However, the high viscosity of the LiB3O5−B2O3 hightemperature solution often leads to a low growth rate, entrapment of the solution, and the inclusion of foreign phases, and therefore, the obtained LiB3O5 crystals are limited in size and often exhibit poor and variable optical quality. A significant advance in LiB3O5 crystal growth was achieved by using lithium molybdate fluxes.10−12 With these fluxes, the viscosity of the high-temperature solution decreases remarkably, and the obtained LiB3O5 crystals have much larger size and higher quality.

The lithium triborate (LiB3O5) crystal is the best selection for generating the second and third harmonics of the Nd:YAG laser because of its appealing properties such as relatively large nonlinear optical coefficients, a wide transparency range, a high damage threshold, a large acceptance angle, good chemical stability, and good mechanical properties.1,2 Recently, particular attention has been paid to the crystal application in superintense and ultrafast laser systems.3−5 For example, by using a 100 mm × 100 mm × 17 mm LiB3O5 crystal, Liang and co-workers constructed a high-energy and high-conversionefficiency laser system that achieved a peak power of 1.02 PW (1 PW = 1015 W) with a pulse duration of 32 fs (1 fs =10−15 s).6 Such works are expected to open new prospects for studying the physics of intense fields and nonlinear quantum electrodynamics and for developing new technological applications, including laser particle acceleration, laser-assisted thermonuclear fusion, and new laser medical therapies.7,8 The super© 2017 American Chemical Society

Received: January 8, 2017 Published: March 2, 2017 3623

DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630

Article

Inorganic Chemistry It is well-known that the selection of the flux is of primary importance for crystal growth from high-temperature solution. The successful application of lithium molybdate fluxes for LiB3O5 crystal growth aroused our keen interest to study the lithium molybdate flux behavior. According to the Li2O− B2O3−MoO3 ternary phase diagram, the compositions of all of the lithium molybdate fluxes for LiB3O5 crystal growth are close to Li2Mo3O10.12 Hence, a LiB3O5−Li2Mo3O10 high-temperature solution was selected as a typical example for studying the lithium molybdate flux behavior in this paper. In order to deeply understand the flux behavior and exquisitely control the crystallization process, clarifying the structure of the high-temperature solution serves as the initial task. However, to the best of our knowledge, no research has been carried out to investigate the structure of LiB3O5− Li2Mo3O10 or any other borate−molybdate high-temperature solution. Molecular vibrational spectroscopy is an effective tool to obtain information about the molecular structure.13−16 Recent progress in in situ attenuated total reflectance infrared (ATRIR) spectroscopy identified the structural evolution of both KH2PO4 (KDP) and NH4H2PO4 (ADP) at the nucleation stage in aqueous solution.14−16 In addition, chemical bonding theory of single-crystal growth, which emphasizes the vital role of the chemical bonding process at the growing interface during crystal growth, has been widely applied to guide the crystal growth of inorganic crystalline materials with multiscale sizes.17,18 Raman spectroscopy is another common experimental technique for in situ structural investigation of melts/hightemperature solutions at the molecular scale, and it has been employed to investigate the structures of some alkali-metal molybdate melts.19,20 With the use of pulsed laser sources and the confocal technique, Raman spectra with good signal-tonoise were collected.21,22 However, the Raman bands of a melt/ high-temperature solution are hard to identify because of their low intensity and significant broadening. In order to deal with this problem, a new density functional theory (DFT) method has been developed and successfully applied to resolve the Raman spectra of some borate/niobate melts.23,24 In this paper, high-temperature Raman spectroscopy together with the DFT method and chemical bonding theory is used to investigate the structure of the LiB3O5−Li2Mo3O10 solution. On the basis of the results, the flux behavior of Li2Mo3O10 is discussed.

Li2Mo3O10 and LiB3O5 polycrystalline samples were weighed, thoroughly mixed in an agate mortar, and then transferred into a platinum boat. Each mixture was heated to the temperature at which the mixture just melted completely and held at that temperature for 30 min to ensure good homogeneity. Subsequently, the transparent solution was air-quenched under ambient conditions to produce a Li2Mo3O10−LiB3O5 glass sample. All of the glass samples were characterized by Raman spectroscopy. LiB3O5 crystal slices were cut from a LiB3O5 boule provided by the Beijing Centre for Crystal Research and Development, Chinese Academy of Sciences. 2.2. X-ray Diffraction and Raman Spectrum Measurements. All of the polycrystalline samples were identified by XRD analyses, which were performed on a computer-controlled MXPAHF diffractometer with graphite-monochromatized Cu Kα radiation (λ = 1.54056 Å). All of the Raman spectra were recorded on a Jobin Yvon LABRaman HR800 spectrometer equipped with a confocal microscope. The 532 nm line of a Q-switched pulsed SHG-Nd:YAG laser was employed as the excitation source with an output power of about 1.0 W. The laser spot diameter was less than 2 μm. The scattering light was collected in a backscattering configuration with an integration time of 200 s for the polycrystalline and glass samples and 100 s for the high-temperature solutions. An intensive charge couple device (ICCD) was used to monitor the melting and crystallization processes. 2.3. Computational Methods. All of the calculations were carried out by the DFT method using a plane-wave basis set and normconserving pseudopotentials, as implemented in the Cambridge Sequential Total Energy Package (CASTEP) code.27 The Wu− Cohen functional of the generalized gradient approximation (GGA− WC) was adopted to describe the exchange−correlation potential.28 The valence electron configurations for the Li, Mo, and O pseudopotentials were 2s1, 4s24p64d55s1, and 2s22p4, respectively. In these calculations, the convergence criteria for the energy change, maximum force, maximum stress, and maximum displacement tolerances were set to be 10−5 eV/atom, 0.05 eV/Å, 0.1 GPa, and 0.002 Å, respectively. The kinetic energy cutoff and the self-consistent field (SCF) convergence criteria were set to be 800 eV and 10−6 eV/ atom, respectively. According to the results of total energy convergence tests, the Brillouin zone integrations were done over a 3 × 3 × 1 Monkhorst−Pack grid for the Li2Mo3O10 melt and over a 3 × 3 × 2 Monkhorst−Pack grid for the Li2Mo2O7 melt (see Table S1, Figure S2, Table S5, and Figure S4 in the Supporting Information for more details). Raman frequencies of the Li2Mo3O10 and the Li2Mo2O7 melts were obtained by diagonalization of dynamical matrices computed by density functional perturbation theory (DFPT).29 Raman activity tensors were calculated using a hybrid DFPT−finite displacement method.30 All of the calculated Raman intensities were corrected by Bose−Einstein factors corresponding to the experimental temperatures (850 K for the Li2Mo3O10 melt and 1000 K for the Li2Mo2O7 melt) and the wavelength of the excitation source (532 nm). The Raman bands were broadened by a Gaussian line shape function with a full width at half-maximum (fwhm) of 1 cm−1.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A Li2Mo3O10−LiB3O5 polycrystalline sample (with a Li2Mo3O10/LiB3O5 molar ratio of 0.65:0.57) was synthesized by a conventional solid-state reaction method. All of the starting materials (Li2CO3, H3BO3, and MoO3, analytical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification. Li2CO3 (3.954 g), H3BO3 (7.234 g), and MoO3 (14.682 g) were weighed, ground thoroughly in an agate mortar, and then transferred into a closed corundum crucible. The mixture was preheated at 450 °C for 12 h and then heated at 680 °C for 24 h with an intermediate grind. As a result, a white polycrystalline powder was produced. Li2Mo4O13, Li4Mo5O17, and LiB3O5 polycrystalline samples were synthesized following the procedures reported in the literature.25,26 All of the polycrystalline products were identified by Xray diffraction (XRD) and Raman spectroscopy. The Li2Mo3O10−LiB3O5 polycrystalline sample was heated in a 2 mm × 5 mm × 10 mm platinum boat by a homemade microfurnace to obtain a Li2Mo3O10−LiB3O5 solution. Li2Mo3O10−LiB3O5 glass samples with various Li2Mo3O10/LiB3O5 molar ratios were prepared by a conventional melt-quenching method. Appropriate amounts of

3. RESULTS AND DISCUSSION A Li 2 Mo 3 O 1 0 −LiB 3 O 5 polycrystalline mixture at a Li2Mo3O10:LiB3O5 molar ratio of 0.65:0.57 was selected to investigate the Li 2 Mo 3 O 10 flux behavior because this composition is located at the center of the LiB3O5 crystalline region in the Li2O−B2O3−MoO3 ternary phase diagram.12 According to the results reported by Solodovnikov et al., only four lithium molybdate compounds, namely, Li4MoO5, Li2MoO4, Li4Mo5O17, and Li2Mo4O13, exist at room temperature.25,31,32 Therefore, the LiB3O5−Li2Mo3O10 polycrystalline powder should be a mixture of LiB3O5, Li4Mo5O17, and Li2Mo4O13. This judgment was verified by the XRD analysis (see Figure S1 for more details). The Raman spectrum of the polycrystalline powder is shown in Figure 1, together with those of LiB3O5, Li4Mo5O17, and Li2Mo4O13. The characteristic 3624

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melt;19 we thus deem that the Li2Mo3O10 melt is also made up of the Mo3O102− group. The DFT method was used to verify the above deduction. One Mo3O102− group, together with two Li+ cations, was placed into a triclinic unit cell, and then the geometry of the structure was optimized. During the optimization, the unit cell parameters and atomic positions were relaxed in order to obtain the most stable conformation. After the geometry optimization, the Raman spectrum was calculated. The results are shown in Figure 3. The optimized Mo3O102− group

Figure 1. Raman spectra of (a) the Li2Mo3O10−LiB3O5 polycrystalline mixture, (b) Li2Mo4O13, (c) Li4Mo5O17, and (d) LiB3O5.

Raman bands of both Li4Mo5O17 and Li2Mo4O13 are present in the polycrystalline powder spectrum, further demonstrating that Li2Mo3O10 is a mixture of Li4Mo5O17 and Li2Mo4O13. In addition, the figure reveals that the Raman activity of the LiB3O5 vibrational modes is much lower than that of the Li4Mo5O17 and Li2Mo4O13 modes. The Li2Mo4O13−Li4Mo5O17 polycrystalline powder, along with a little LiB3O5 crystal slice (serving as the seed crystal), was put into a platinum boat and heated slowly in a homemade microfurnace. The Raman spectra as a function of temperature are shown in Figure 2. With increasing temperature, all of the

Figure 3. (a) Experimental and (b) calculated Li2Mo3O10 melt Raman spectra. Inset: the Li2Mo3O10 melt structural model. The calculated Raman bands were broadened by a Gaussian line shape function with a fwhm of 1 cm−1.

maintains a chain-type structure; the Mo−O bond lengths and O−Mo−O angles coincide well with the reported values (see Figure S3 and Tables S2 and S3 for more details). Two different types of molybdenum−oxygen bonds are included in the group. One pertinent to the terminal oxygen atoms ranges from 171.3 to 177.7 pm in length, which is interpreted as the MoO double bond; another pertinent to the bridging oxygen atoms is about 190 pm, which is interpreted as the Mo−Ø single bond. The calculated Raman spectrum shown in Figure 3 is consistent with the experimental one, further confirming that the anion motif of the Li2Mo3O10 melt is the Mo3O102− group. On the basis of the calculated results, the Li2Mo3O10 melt Raman spectrum is interpreted (see Table S4 for more details). The Raman bands below 400 cm−1 are mixtures of MoO/ Mo−Ø wagging vibrations and Li+ cation motions. The bands in the range of 400−500 cm−1 mainly arise from Li+ cation motions. The Raman bands above 700 cm−1 are attributed to the mixed stretching vibrations of MoO and Mo−Ø bonds (the bands at 757, 855, 864, 876, and 955 cm−1) or the stretching vibrations of MoO bonds (the bands at 891, 893, 938, 952, and 976 cm −1 ). The atomic displacements corresponding to six intense bands are depicted in Figure 4. With increasing temperature, the Li2Mo3O10−LiB3O5 polycrystalline powder gradually transformed into a solution. When the LiB3O5 crystal slice began to melt, the temperature was slowly decreased to allow the crystal to grow. Finally, a crystal− solution equilibrium system was established (Figure 5). The solution Raman spectra were collected during the dissolution process. The measurement positions and their corresponding Raman spectra are presented in Figures 5 and 6, respectively. In comparison with the Li2Mo3O10 melt spectrum, (1) the strongest peak in the solution spectrum sharpens and shifts to lower frequency; (2) the shoulder peak located around 880 cm−1 disappears; and (3) the Raman peak located around 700

Figure 2. (a−c) Raman spectra of Li2Mo3O10 polycrystalline powder recorded at (a) room temperature, (b) 340 °C, and (c) 540 °C. (d) Raman spectrum of a Li2Mo3O10 melt recorded at 570 °C.

Raman bands red-shift and decrease in intensity, which is due to a general temperature effect caused by anharmonicity.33 When the temperature was close to the eutectic temperature of the Li2Mo4O13−Li4Mo5O17 mixture (541 °C),31 the polycrystalline mixture began to melt. Meanwhile, the characteristic Raman bands of Li2Mo4O13 and Li4Mo5O17 disappeared (see Figure 2d), indicating that a new molybdate group formed in the process. Voronko and co-workers deduced the K2Mo3O10 melt structure on the basis of the spectral similarity between the K2Mo3O10 crystal and the K2Mo3O10 melt.19,20 The anion motif in the K2Mo3O10 melt is the Mo3O102− group, which is formed by three corner-sharing MoO3Ø−/MoO2Ø2 (Ø = bridging oxygen atom) tetrahedra. In contrast to K2Mo3O10, Li2Mo3O10 is a mixture comprising two different crystal structures at room temperature; therefore, we cannot deduce the Li2Mo3O10 melt structure in a similar manner as used in the study of the K2Mo3O10 melt structure. Fortunately, the Raman spectrum of the Li2Mo3O10 melt is very close to that of the K2Mo3O10 3625

DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630

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Figure 5. In situ observation of the dissolution of LiB3O5 in the Li2Mo3O10 melt (a) at the beginning of (the solution temperature was about 670 °C), (b) during (the solution temperature was about 735 °C), and (c) after the dissolution process (the solution temperature was about 730 °C). Positions A, B, and C were selected to collect Raman spectra.

Figure 4. Atomic displacements corresponding to six strong bands in the calculated Li2Mo3O10 melt Raman spectrum.

cm−1 decreases in intensity. These results indicate that the Mo3O102− group, the anion motif of the Li2Mo3O10 melt, transforms into another molybdate group. As already mentioned above, the Mo3O102− group contains two types of molybdenum−oxygen bonds, the MoO double bond and the Mo−Ø single bond. Hence, the breakage of the weak Mo−Ø bond is a potential pathway for understanding the structural transformation of the Mo3O102− group. The possible products of the breakage are the MoO42− and Mo2O6 groups (formed by a MoO3Ø− tetrahedron and a MoO2Ø+ triangle) or the MoO3 and Mo2O72− groups (formed by two corner-sharing MoO3Ø− tetrahedra). The MoO42− group and the Mo2O72− group have been determined to be the anion motifs of the K2MoO4 melt and the K2Mo2O7 melt, respectively. Thus, we compared the LiB3O5−Li2Mo3O10 solution Raman spectrum (Figure 6c) with the K2MoO4 and K2Mo2O7 melt Raman spectra,19,20 and found that the solution spectrum is obviously different from the K2MoO4 melt spectrum but quite similar to the K2Mo2O7 melt spectrum. The comparison indicates that the LiB3O5−Li2Mo3O10 solution contains the Mo2O72− group, which means that the Mo3O102− group breaks into the Mo2O72− group and the MoO3 group in the dissolution process (Figure 7). Like Li2Mo3O10, Li2Mo2O7 does not exist in crystalline form; thus, the Li2Mo2O7 melt structure cannot be deduced from the “Li2Mo2O7” crystal structure. Therefore, the Li2Mo2O7 melt structure and Raman spectrum were studied by the DFT method in a similar manner as used for the Li2Mo3O10 melt. One Mo2O72− group, together with two Li+ cations, was placed

Figure 6. Solution Raman spectra recorded as LiB3O5 dissolved in the Li2Mo3O10 melt. Spectra (a), (b), and (c) correspond to the positions A, B, and C shown in Figure 5, respectively.

Figure 7. Structural transformations occurring in the LiB3O5− Li2Mo3O10 solution.

into a triclinic unit cell. After geometry optimization, the Raman spectrum was calculated. The results are shown in 3626

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2:1, 6:1, and 12:1) were prepared by a conventional meltquenching method to explore the structure of LiB3O5 in the LiB3O5−Li2Mo3O10 solution. Their Raman spectra are shown in Figure 9. The wide Raman bands confirm the glass nature of

Figure 8 (see Figure S5 and Tables S6−S8 for more details). The calculated spectrum is consistent with the experimental

Figure 8. Comparison of (a) the LiB3O5−Li2Mo3O10 solution spectrum and (b) the calculated Li2Mo2O7 melt spectrum. The calculated Raman bands were broadened by a Gaussian line shape function with a fwhm of 1 cm−1. Inset: the Li2Mo2O7 melt structural model.

Figure 9. Raman spectra of four LiB3O5−Li2Mo3O10 glass samples with various LiB3O5/Li2Mo3O10 molar ratios: (a) 1:1, (b) 2:1, (c) 6:1, and (d) 12:1.

these samples. With an increase in the LiB3O5 content, four Raman bands appear near 480, 780, 1270, and 1450 cm−1. All four Raman bands can be attributed to the characteristic vibrations of the LiB3O5 melt rather than those of the LiB3O5 crystal (Figure 10).36 According to our recent work, the LiB3O5

one, confirming the existence of the Mo2O72− group in the LiB3O5−Li2Mo3O10 solution. On the basis of the DFT results, the spectral changes following the dissolution of LiB3O5 in the Li2Mo3O10 melt are explained. After the Mo3O102− group breakage, (1) the highest-frequency 976 cm−1 band, related to the −MoO2− symmetrical stretching vibration (see Figure 4), disappears, which makes the strongest Raman peak sharpen and red-shift; (2) the strongest band of the Li2Mo2O7 melt redshifts and merges with its low-frequency shoulder peak to form one peak; and (3) the Raman bands around 760 cm−1 (Li2Mo3O10) are attributed to the bending vibrations of the bridging oxygen atoms (see Figure 4). The calculated intensity of the band in the Li2Mo2O7 melt spectrum (772 cm−1) is lower than that in the Li2Mo3O10 melt spectrum (757 cm−1), and thus, the band is hard to observe in the LiB3O5−Li2Mo3O10 solution spectrum. According to the structural transformation shown in Figure 7, the MoO3 group is another product of the Mo3O102− group breakage. The strongest Raman band related to the MoO3 group arises from the stretching vibration of the terminal Mo O bonds with a frequency near 980 cm−1.34 The frequency is significantly higher than that of the strongest Raman band for Mo2O72−. Therefore, if the MoO3 group coexists with the Mo2O72− group in the LiB3O5−Li2Mo3O10 solution, two sharp peaks or one broad peak should be present in the range of 950−1000 cm−1. However, only one sharp peak is present in this spectral range in the solution spectrum, which means that the MoO3 group is fairly probable to transform into another group. In order to explain the whereabouts of the MoO3 molecule, we should clarify the structure of LiB3O5 in the LiB3O5− Li2Mo3O10 solution. However, the Raman bands corresponding to LiB3O5 are difficult to identify in the solution spectrum because of their low intensity and significant broadening. In principle, the structure of a melt/high-temperature solution should be similar to that of the glass obtained by quickly cooling the melt/high-temperature solution, and the glass Raman bands are much easier to identify than the melt bands.35 Therefore, the glass Raman spectrum is often used to understand the melt structure. In the present work, four glass samples with different LiB3O5/Li2Mo3O10 molar ratios (1:1,

Figure 10. Raman spectra of (a) the LiB3O5−Li2Mo3O10 glass sample with a LiB3O5/Li2Mo3O10 molar ratio of 12:1, (b) the LiB3O5 melt, and (c) the LiB3O5 crystal.

melt is made up of boron−oxygen polymeric chains whose structural unit is the B3O4Ø2− six-membered ring. The 480 cm−1 band mainly arises from the wagging vibration of the B3O4Ø2− rings; the 780 cm−1 band is related to the breathing vibration of the B3O4Ø2− rings; the bands in the range of 1200−1600 cm−1 originate from the stretching vibrations of the intraring B−O bonds and the extraring B−O bonds.36,37 Therefore, the [B3O4Ø2−]n chain is the primary boron−oxygen species included in the LiB3O5−Li2Mo3O10 glass samples and therefore the primary boron−oxygen species included in the LiB3O5−Li2Mo3O10 solution. Each B3O4Ø2− ring contains a terminal oxygen atom that has three lone pairs of electrons and can thus behave as an electronpair donor. Meanwhile, the Mo atoms in the Mo3O102− group contain 4d vacant orbitals and can thus serve as electrophilic centers. Therefore, the B3O4Ø2− ring tends to coordinate to a Mo atom to form a complex. Considering that the Mo2O72− group is present and the MoO3 group is absent in the LiB3O5− 3627

DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630

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sharp peak with the same Raman frequency was found in the LiB3O5 crystal melting process (Figure 12a).41 The peak has been assigned to the B−Ø bending vibration of the BØ4− tetrahedron in a B5O6Ø4− group. In view of the fact that the terminal oxygen atom in the MoO3·B3O4Ø2− complex can combine with the neighboring three-coordinate boron atom to form a BØ4− unit, a possible structural transformation of the MoO3·B3O4Ø2− complex is proposed, as shown in Figure 13.

Li2Mo3O10 solution, we propose a mechanism to describe the structural transition following the dissolution of LiB3O5 in the Li2Mo3O10 melt. When the LiB3O5 crystal melts, the threedimensional crystal structure collapses into the [B3O4Ø2−]n chains, and the terminal oxygen atom in the B3O4Ø2− ring attacks a molybdenum atom in the Mo3O102− group and forms a MoO3·B3O4Ø2− complex. At the same time, a Mo2O72− group is produced (see Figure 11). Electronegativity (EN) is defined

Figure 13. Structural transformation of the MoO3·B3O4Ø2−complex in the LiB3O5−Li2Mo3O10 solution.

As a result, a BØ4− unit similar to that in the B5O6Ø4− group is formed. The well-defined and highly symmetrical BØ4− unit in the new structure (MoO2(μ-O)2B3O3Ø2−) gives rise to the special shape of the 480 cm−1 band. A simple structural model (see Figure S6 and Table S9 for more details) has been constructed to simulate the MoO2(μ-O)2B3O3Ø2− Raman spectrum. A Raman band located at 477 cm−1 is present in the calculated spectrum (see Figure S7); the frequency is very close to the experimental value (480 cm−1). The calculation also reveals that the 477 cm−1 band arises from the bending vibration of the BØ4− unit in the MoO2(μ-O)2B3O3Ø2− complex, which is consistent with our analysis. The above calculated results further support the presence of the MoO3· B3O4Ø2− complex in the LiB3O5−Li2Mo3O10 solution. The LiB3O5−Li2Mo3O10 solution structure can be used to understand the LiB3O5 crystal growth mechanism. At high temperatures, LiB3O5 reacts with Li2Mo3O10 to form the MoO3·B3O4Ø2−complex. As the temperature decreases, a phase separation process takes place. As a result, the complex decomposes into B3O4Ø2−. The B3O4Ø2− groups, acting as the crystal growth units, connect with each other to form the LiB3O5 crystal structure following the steps described previously.42,43 The LiB3O5−Li2Mo3O10 solution structure can also be used to explain the correlation between the solution composition and viscosity. For a borate melt, the interaction between terminal oxygen atoms and three-coordinate boron atoms is a major cause of high viscosity.44 With the addition of a MoO3-based flux, such as Li2Mo3O10, to the LiB3O5 melt, the terminal oxygen atom in the B3O4Ø2− group combines with the molybdate group to form the MoO3·B3O4Ø2− complex, which weakens the interaction between [B3O4Ø2−]n chains and thus decreases the viscosity. On the basis of this viewpoint, we infer that the LiB3O5−Li2Mo3O10 solution viscosity will decrease to a minimum when the amount of LiB3O5 is equal to that of Li2Mo3O10. The viscosity of a LiB3O5 lithium molybdate solution as a function of the LiB3O5/lithium molybdate molar ratio was measured by Nikolov et al.,5 and their experimental result is consistent with our inference.

Figure 11. Interaction between a B3O4Ø2− ring and a Mo3O102− group.

as an attracting force between the atom and electron. In order to take into consideration the real environments surrounding the constituent elements, a series of novel EN scales have been proposed.38−40 In the LiB3O5−Li2Mo3O10 solution, EN acts as the essential driving force for the coordination between the [B3O4Ø2−]n polymeric chain and the Mo3O102− group, which promotes the formation of MoO3·B3O4Ø2− and Mo2O72−. The two products contain only one kind of molybdenum−oxygen structural unit, MoO 3 Ø −; thus, the LiB 3 O 5 −Li 2 Mo 3 O 10 solution exhibits only simple spectral features, similar to those of the Mo2O72− group. It is noteworthy that an unusual sharp peak located around 480 cm−1 is present in the LiB3O5−Li2Mo3O10 solution Raman spectrum (Figure 6c or 12b). In our previous work, a similar

Figure 12. Raman spectra recorded (a) during the LiB3O5 crystal melting process and (b) in the LiB3O5−Li2Mo3O10 solution. 3628

DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630

Article

Inorganic Chemistry

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4. CONCLUSIONS High-temperature Raman spectroscopy together with DFT calculations has been used to investigate the structure of a LiB3O5−Li2Mo3O10 high-temperature solution in order to understand the flux behavior of Li2Mo3O10. The anion motif of the Li2Mo3O10 melt is the Mo3O102− group, which is formed by three corner-sharing MoO3Ø−/MoO2Ø2 tetrahedra. As LiB3O5 dissolves in the Li2Mo3O10 melt, the LiB3O5 crystal structure collapses into polymeric chains of [B3O4Ø2−]n. Its structural unit, the B3O4Ø2− ring, then coordinates to the Mo3O102− group to produce a MoO3·B3O4Ø2− complex and a Mo2O72− group. These results can be used to understand the LiB3O5 crystal growth mechanism and to explain the compositional dependence of the viscosity of the LiB3O5− lithium molybdate solution. We believe that these results can help us to deeply understand the flux behavior of molybdates in other borate crystal growth systems and then to optimize the crystal growth conditions to improve the crystal quality.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00041. Experimental XRD pattern of the LiB3O5−Li2Mo3O10 polycrystalline powder, convergence tests for the Li2Mo3O10 and Li2Mo2O7 melt total energies with respect to the Γ-centered k-point grids and the kinetic energy cutoffs, the optimized structural models of the Li2Mo3O10 and Li2Mo2O7 melts and their geometrical parameters, calculated vibrational modes of the Li2Mo3O10 and the Li2Mo2O7 melts and their frequencies, and the optimized structural model of MoO2(μO) 2B3O 3Ø2 − and its geometrical parameters and calculated Raman spectrum (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Songming Wan: 0000-0002-8923-7914 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (Grants 51372246 and 51132005). The DFT calculations were partially performed at the Center for Computational Science, CASHIPS.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630

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DOI: 10.1021/acs.inorgchem.7b00041 Inorg. Chem. 2017, 56, 3623−3630