Synthesis and Characterization of New Sr 3 (BO 3 ) 2 Crystal for Stimulated Raman Scattering Applications

: A new kind of borate crystalline material Sr 3 (BO 3 ) 2 with a similar calcite type structure was designed and synthesized by solid state reaction method, moreover, the single crystal growth was attempted with traditional Czochralski pulling method. Crystal phase of Sr 3 (BO 3 ) 2 was investigated by using X-Ray powder diffraction (XRPD) at room temperature and found similar to Ca 3 (BO 3 ) 2 crystal with space group of R-3c. The phase stability was studied by means of thermogravimetric differential thermal analysis (TG/DTA) and high temperature XRPD up to 1350 ◦ C, where an obvious endothermic peak was observed in DTA curve around 1250 ◦ C, and weak splits of diffraction peaks were found at temperatures above 1250 ◦ C, indicating the existence of structure transformation for Sr 3 (BO 3 ) 2 crystal. Raman properties were studied experimentally and theoretically by using density functional perturbation theory, though the strongest frequency shift of Sr 3 (BO 3 ) 2 crystal (900 cm − 1 ) was comparable to that of Ca 3 (BO 3 ) 2 (927 cm − 1 ), the line width of the strongest Raman peak obtained for Sr 3 (BO 3 ) 2 (5.72 cm − 1 ) was much lower than Ca 3 (BO 3 ) 2 (7.01 cm − 1 ), indicating a larger Raman gain for Sr 3 (BO 3 ) 2 crystal, which would be favorable for stimulated Raman scattering application.


Introduction
As one of the earliest nonlinear optical processes, in recent years, stimulated Raman scattering (SRS) has become an important and efficient method for frequency conversion to fulfill various applications including optical molecular imaging, structure analysis, biology monitoring, and medical treatment [1][2][3][4][5][6][7].Different from the optical frequency doubling, SRS is a third-order nonlinear optical process with many advantages such as larger angular acceptance, and no walk-off angle and phase-matching (PM) direction [8].In the early years, SRS experiments were mostly based on liquids and gases including CH 4 , N 2 , and benzene etc. [9].However, some flaws such as poisonousness and instability restrict their application in SRS.Besides the liquids and gases SRS mediums, solid materials, especially crystals, were applied in SRS.Since the 1980s, several new artificial crystals-Such as Ba(NO 3 ) 2 , BaWO 4 and YVO 4 crystals-Have been studied for SRS applications [10][11][12][13][14][15].These crystals were revealed to possess many merits for SRS process, including larger Raman gain coefficient, stable thermal and mechanical properties, good chemical stability, non-deliquescence, and small volume etc.However, the relatively low anti-optical damage threshold and the long cut-off wavelength restricted their applications for SRS in the ultraviolet waveband [16,17].
SRS is an inelastic scattering process with inevitable energy loss.Therefore, both the Raman gain coefficient and the anti-optical damage threshold are critical parameters for selecting Raman

Structure Analysis and New Crystal Design
The Sr 3 (BO 3 ) 2 was first reported by Richter and Müller in 1980, and revealed to belong to trigonal system and R-3c space group (ICSD No. 93395) [25], and the lattice parameters were reported to be a = b = 9.046 Å and c = 12.566 Å [26]. Figure 1 presents the crystal structure of Sr 3 (BO 3 ) 2 , where the [BO 3 ] 3− group were observed to form a regular triangle in the same plane normal to the crystallographic c-axis, and the three terminal oxygen atoms were linked with Sr atoms, which were similar to Ca 3 (BO 3 ) 2 crystal (Figure 1a,b).It can be observed from Figure 1c,d that the [BO 3 ] 3− groups were parallel to each other.The twisty Sr-O octahedron and the triangular [BO 3 ] 3− group were shown in Figure 1e,f, respectively.It was noticed that the Mg 3 (BO 3 ) 2 crystal belongs to the Pnmn space group (point group mmm), and the lattice parameters were reported to be a = 5.4014 Å, b = 8.4233 Å, c = 4.5071 Å, α = β = γ = 90 • , and V = 205.1 Å 3 [27].It can be presumed that the variation of ionic radius resulted in the change of lattice parameters, leading to the structure transformation from trigonal to orthorhombic when the ion varied from Sr 2+ to Mg 2+ .
The potential Raman properties of Mg 3 (BO 3 ) 2 , Ca 3 (BO 3 ) 2 , and Sr 3 (BO 3 ) 2 crystals were theoretically studied within the framework of density functional perturbation theory by using the Cambridge Sequential Total Energy Package (CASTEP) code [28].The exchange and correlation potential were described in a generalized gradient approximation of Perdew-Burke-Erenzerhof functionality (GGA-PBE) [29], and the interaction between the valence electrons and the core electrons was described by norm-conserving pseudopotentials [30].The cutoff energy and Monkhorst-Pack k-point were set to 380 eV and 4 × 4 × 4. The primitive cells containing 22 atoms were used to reduce the computational cost.All the configurations were fully optimized before the Raman spectrum calculation with the error of a and c lattices within 2%.As it can be observed in Figure 2, the strongest Raman shifts for Mg 3 (BO 3 ) 2 , Ca 3 (BO 3 ) 2 , and Sr 3 (BO 3 ) 2 crystals appeared at 895, 909, and 886 cm −1 , respectively, associated with the symmetrical stretching vibration of [BO3] 3− group.It was noticed that the Ca 3 (BO 3 ) 2 and Sr 3 (BO 3 ) 2 crystals showed more plentiful peaks than Mg 3 (BO 3 ) 2 , which might be associated with their different crystal structures, as both the Ca 3 (BO 3 ) 2 and Sr 3 (BO 3 ) 2 crystals belong to the same space group R-3c, while Mg 3 (BO 3 ) 2 belongs to space group Pnmn.The plentiful peaks of Ca 3 (BO 3 ) 2 and Sr 3 (BO 3 ) 2 crystals would be beneficial for achieving applicable wavelengths in the field of SRS.

Experimental
The polycrystalline Sr3(BO3)2 was prepared by using solid state reaction method, where the starting materials were SrCO3 (99.95%) and H3BO3 (99.99%) powders, which were weighed in stoichiometric ratio, according to the chemical reaction equation below, 3SrCO3 + 2H3BO3 = Sr3(BO3)2 + 3CO2↑ + 3H2O In order to compensate the evaporation of B2O3 during the solid-state reaction and crystal growth processes, an excess of H3BO3 (1.0 wt %) was added to the starting materials.All the starting materials were fully mixed for at least 24 h to ensure homogeneity.Then mixed raw materials were sintered at 1000 °C for 10 h to decompose H3BO3 and SrCO3 completely.After that the sintered raw materials were ground, mixed again and pressed into pieces, which were charged into an alumina crucible and sintered at 1250 °C for at least 40 h to synthesize the polycrystalline Sr3(BO3)2.Ca3(BO3)2 and Sr3(BO3)2 crystals showed more plentiful peaks than Mg3(BO3)2, which might be associated with their different crystal structures, as both the Ca3(BO3)2 and Sr3(BO3)2 crystals belong to the same space group R-3c, while Mg3(BO3)2 belongs to space group Pnmn.The plentiful peaks of Ca3(BO3)2 and Sr3(BO3)2 crystals would be beneficial for achieving applicable wavelengths in the field of SRS.

Experimental
The polycrystalline Sr3(BO3)2 was prepared by using solid state reaction method, where the starting materials were SrCO3 (99.95%) and H3BO3 (99.99%) powders, which were weighed in stoichiometric ratio, according to the chemical reaction equation below, In order to compensate the evaporation of B2O3 during the solid-state reaction and crystal growth processes, an excess of H3BO3 (1.0 wt %) was added to the starting materials.All the starting materials were fully mixed for at least 24 h to ensure homogeneity.Then mixed raw materials were sintered at 1000 °C for 10 h to decompose H3BO3 and SrCO3 completely.After that the sintered raw materials were ground, mixed again and pressed into pieces, which were charged into an alumina crucible and sintered at 1250 °C for at least 40 h to synthesize the polycrystalline Sr3(BO3)2.

Experimental
The polycrystalline Sr 3 (BO 3 ) 2 was prepared by using solid state reaction method, where the starting materials were SrCO 3 (99.95%)and H 3 BO 3 (99.99%)powders, which were weighed in stoichiometric ratio, according to the chemical reaction equation below, In order to compensate the evaporation of B 2 O 3 during the solid-state reaction and crystal growth processes, an excess of H 3 BO 3 (1.0 wt %) was added to the starting materials.All the starting materials were fully mixed for at least 24 h to ensure homogeneity.Then mixed raw materials were sintered at 1000 • C for 10 h to decompose H 3 BO 3 and SrCO 3 completely.After that the sintered raw materials were ground, mixed again and pressed into pieces, which were charged into an alumina crucible and sintered at 1250 • C for at least 40 h to synthesize the polycrystalline Sr 3 (BO 3 ) 2 .
A J40 single crystal growth furnace was used to grow Sr 3 (BO 3 ) 2 single crystal.The prepared polycrystalline Sr 3 (BO 3 ) 2 blocks were put into an Ir crucible (ø60mm) in the furnace, where the atmosphere was controlled to be a mixture of 2 vol % air and 98 vol % N 2 .It is important to find out the melting point of Sr 3 (BO 3 ) 2 in the first run, so the heating rate was kept slow enough when the floating compounds on the melt got shrinking.The melt was maintained at 50 • C higher than the observed melting point for several hours to make the melt homogeneous.Then the temperature was slowly decreased for seeding.Ca 3 (BO 3 ) 2 crystal seed along the c-axis was used to grow Sr 3 (BO 3 ) 2 single crystal.It was found that the melting point of Sr 3 (BO 3 ) 2 crystal was higher than Ca 3 (BO 3 ) 2 (1470 • C [31]), so the seeding process was carried out within a short period in order to get rid of seed fusing.During the crystal growth, the pulling rate was controlled at 0.4 mm/h and the rotation rate was varied from 15 to 20 rmp.For the purpose of decreasing the thermal stress in as-grown crystal, a long period (~100 h) was implemented to cool down the Sr 3 (BO 3 ) 2 crystal to room temperature.
The thermogravimetric and differential thermal analysis (TG-DTA) for Sr 3 (BO 3 ) 2 was carried out with a Diamond TMA thermal mechanical analyzer, provided by Perkin Elmer Corporation.The heating rate was 10 • C/min and the flow rate of N 2 was 50 mL/min.
In order to study the crystal structure and possible phase transition of Sr 3 (BO 3 ) 2 , the synthesized polycrystalline Sr 3 (BO 3 ) 2 and the grown Sr 3 (BO 3 ) 2 crystal blocks were selected for X-ray Powder Diffraction (XRPD) characterization (Bruker D8 Advance X-Ray Diffractometer equipped with Cu-Kα radiation and a semiconductor array detector (Bruker LynxEye)).High temperature structure analysis for Sr 3 (BO 3 ) 2 crystal was carried out by using an in situ XRPD equipped with a high temperature in situ attachment (Anton Paar HTK-2000N).The tested temperature was operated from 1150 • C to 1350 • C, and the diffraction spectra were recorded with step of 50 • C. The heating rate was controlled to be 10 • C/min and the retention time at each desired temperature was 5 min.
The powder Raman spectra of the grown Sr 3 (BO 3 ) 2 crystal obtained by Cz method was measured from 20 to 7500 cm −1 by an FT-IR & Raman Spectrograph (NEXUS 670) with a high resolution of 0.09 cm −1 .The used exciting source was a 1064 nm laser pumped by a Nd:YVO 4 crystal.

Single Crystal Growth
The Sr 3 (BO 3 ) 2 crystal was grown by using Cz-pulling method.It was observed that the Sr 3 (BO 3 ) 2 crystal was transparent in the furnace after pulling out of the melt, however, crystal cracks slowly emerged during the following cooling process.As crystal cracks spread, the Sr 3 (BO 3 ) 2 crystal was finally broken into pieces, as shown in Figure 3.This phenomenon always occurred no matter how the component or temperature gradient was adjusted.
Crystals 2017, 7, 125 4 of 9 A J40 single crystal growth furnace was used to grow Sr3(BO3)2 single crystal.The prepared polycrystalline Sr3(BO3)2 blocks were put into an Ir crucible (ø60mm) in the furnace, where the atmosphere was controlled to be a mixture of 2 vol % air and 98 vol % N2.It is important to find out the melting point of Sr3(BO3)2 in the first run, so the heating rate was kept slow enough when the floating compounds on the melt got shrinking.The melt was maintained at 50 °C higher than the observed melting point for several hours to make the melt homogeneous.Then the temperature was slowly decreased for seeding.Ca3(BO3)2 crystal seed along the c-axis was used to grow Sr3(BO3)2 single crystal.It was found that the melting point of Sr3(BO3)2 crystal was higher than Ca3(BO3)2 (1470 °C [31]), so the seeding process was carried out within a short period in order to get rid of seed fusing.During the crystal growth, the pulling rate was controlled at 0.4 mm/h and the rotation rate was varied from 15 to 20 rmp.For the purpose of decreasing the thermal stress in as-grown crystal, a long period (~100 h) was implemented to cool down the Sr3(BO3)2 crystal to room temperature.
The thermogravimetric and differential thermal analysis (TG-DTA) for Sr3(BO3)2 was carried out with a Diamond TMA thermal mechanical analyzer, provided by Perkin Elmer Corporation.The heating rate was 10 °C/min and the flow rate of N2 was 50 mL/min.
In order to study the crystal structure and possible phase transition of Sr3(BO3)2, the synthesized polycrystalline Sr3(BO3)2 and the grown Sr3(BO3)2 crystal blocks were selected for X-ray Powder Diffraction (XRPD) characterization (Bruker D8 Advance X-Ray Diffractometer equipped with Cu-Kα radiation and a semiconductor array detector (Bruker LynxEye)).High temperature structure analysis for Sr3(BO3)2 crystal was carried out by using an in situ XRPD equipped with a high temperature in situ attachment (Anton Paar HTK-2000N).The tested temperature was operated from 1150 °C to 1350 °C, and the diffraction spectra were recorded with step of 50 °C.The heating rate was controlled to be 10 °C/min and the retention time at each desired temperature was 5 min.
The powder Raman spectra of the grown Sr3(BO3)2 crystal obtained by Cz method was measured from 20 to 7500 cm −1 by an FT-IR & Raman Spectrograph (NEXUS 670) with a high resolution of 0.09 cm −1 .The used exciting source was a 1064 nm laser pumped by a Nd:YVO4 crystal.

Single Crystal Growth
The Sr3(BO3)2 crystal was grown by using Cz-pulling method.It was observed that the Sr3(BO3)2 crystal was transparent in the furnace after pulling out of the melt, however, crystal cracks slowly emerged during the following cooling process.As crystal cracks spread, the Sr3(BO3)2 crystal was finally broken into pieces, as shown in Figure 3.This phenomenon always occurred no matter how the component or temperature gradient was adjusted.

Thermogravimetric and Differential Thermal Analysis
The thermogravimetric and differential thermal analysis were measured from 20 • C to 1350 • C. Figure 4 gives the measured TG-DTA curves, where the mass loss was less than 0.6% over the tested temperature range (the red line in Figure 4) and an endothermic peak in DTA curve (the blue line in Figure 4) was observed at around 1250 • C. According to previous reports, the melting point for Ca 3 (BO 3 ) 2 was 1479 • C [31], higher than 1285 • C for Sr 3 (BO 3 ) 2 [32].However, in this work, the melting point for Sr 3 (BO 3 ) 2 was judged to be higher than Ca 3 (BO 3 ) 2 , since the Ca 3 (BO 3 ) 2 crystal seed melted rapidly when dipped into the Sr 3 (BO 3 ) 2 melt during the seeding process.Therefore, the endothermic peak observed at round 1250 • C was presumed to be associated with a phase transition.

Thermogravimetric and Differential Thermal Analysis
The thermogravimetric and differential thermal analysis were measured from 20 °C to 1350 °C. Figure 4 gives the measured TG-DTA curves, where the mass loss was less than 0.6% over the tested temperature range (the red line in Figure 4) and an endothermic peak in DTA curve (the blue line in Figure 4) was observed at around 1250 °C.According to previous reports, the melting point for Ca3(BO3)2 was 1479 °C [31], higher than 1285 °C for Sr3(BO3)2 [32].However, in this work, the melting point for Sr3(BO3)2 was judged to be higher than Ca3(BO3)2, since the Ca3(BO3)2 crystal seed melted rapidly when dipped into the Sr3(BO3)2 melt during the seeding process.Therefore, the endothermic peak observed at round 1250 °C was presumed to be associated with a phase transition.

Structure and Phase Transition Analysis
Figure 5 shows the XRPD pattern of as-grown Sr3(BO3)2 crystals as well as that of polycrystalline sample prepared by solid state reaction for comparison, where all the refraction peaks of the as-grown bulk crystal sample and polycrystalline sample were in consistent with the standard diffraction data (JCPDS No. 31-1343), indicating the Sr3(BO3)2 samples before and after Cz growth possess the same crystal phase.The cell parameters for Sr3(BO3)2 crystals were analyzed utilizing Jade Version 6 with necessary steps including phase retrieval, subtracting the background and stripping the Kα2-lines with cubic spline, smoothing and fitting all profiles [33].In the end, the cell parameters were obtained to be a = b = 9.02895(0.002125)Å, c = 12.54646(0.002462)Å, α = β = 90°, γ = 120°, and V = 885.78(4.72)Å 3 , slightly lower than the reported values [26].

Structure and Phase Transition Analysis
Figure 5 shows the XRPD pattern of as-grown Sr 3 (BO 3 ) 2 crystals as well as that of polycrystalline sample prepared by solid state reaction for comparison, where all the refraction peaks of the as-grown bulk crystal sample and polycrystalline sample were in consistent with the standard diffraction data (JCPDS No. 31-1343), indicating the Sr 3 (BO 3 ) 2 samples before and after Cz growth possess the same crystal phase.The cell parameters for Sr 3 (BO 3 ) 2 crystals were analyzed utilizing Jade Version 6 with necessary steps including phase retrieval, subtracting the background and stripping the Kα2-lines with cubic spline, smoothing and fitting all profiles [33].In the end, the cell parameters were obtained to be a = b = 9.02895(0.002125)Å, c = 12.54646(0.002462)Å, α = β = 90 • , γ = 120 • , and V = 885.78(4.72)Å 3 , slightly lower than the reported values [26].

Thermogravimetric and Differential Thermal Analysis
The thermogravimetric and differential thermal analysis were measured from 20 °C to 1350 °C. Figure 4 gives the measured TG-DTA curves, where the mass loss was less than 0.6% over the tested temperature range (the red line in Figure 4) and an endothermic peak in DTA curve (the blue line in Figure 4) was observed at around 1250 °C.According to previous reports, the melting point for Ca3(BO3)2 was 1479 °C [31], higher than 1285 °C for Sr3(BO3)2 [32].However, in this work, the melting point for Sr3(BO3)2 was judged to be higher than Ca3(BO3)2, since the Ca3(BO3)2 crystal seed melted rapidly when dipped into the Sr3(BO3)2 melt during the seeding process.Therefore, the endothermic peak observed at round 1250 °C was presumed to be associated with a phase transition.

Structure and Phase Transition Analysis
Figure 5 shows the XRPD pattern of as-grown Sr3(BO3)2 crystals as well as that of polycrystalline sample prepared by solid state reaction for comparison, where all the refraction peaks of the as-grown bulk crystal sample and polycrystalline sample were in consistent with the standard diffraction data (JCPDS No. 31-1343), indicating the Sr3(BO3)2 samples before and after Cz growth possess the same crystal phase.The cell parameters for Sr3(BO3)2 crystals were analyzed utilizing Jade Version 6 with necessary steps including phase retrieval, subtracting the background and stripping the Kα2-lines with cubic spline, smoothing and fitting all profiles [33].In the end, the cell parameters were obtained to be a = b = 9.02895(0.002125)Å, c = 12.54646(0.002462)Å, α = β = 90°, γ = 120°, and V = 885.78(4.72)Å 3 , slightly lower than the reported values [26].In order to further determine the phase transition for Sr 3 (BO 3 ) 2 crystal, high temperature XRPD measurement was carried out.Figure 6 presents the XRPD patterns of Sr 3 (BO 3 ) 2 crystal samples measured from 1150 • C to 1350 • C. It was found from Figure 6a that the main diffraction peaks of Sr 3 (BO 3 ) 2 sample were still consistent with the standard data, while some splitting peaks appeared at the diffraction angles (θ) of 28.70 • , 34.34 • , and 40.02 • when the temperature increased above 1250 • C, corresponding to (113), (300), and (220) planes (JCPDS No. 31-1343), respectively.It was found that the diffraction angles (θ) for ( 113), (300), and (220) planes were all shifted to high angle region then to low angle region, as shown in Figure 6b.
It is known that the relationship between cell parameters and interplanar spacing (d) for crystals with R-3c space group was as follows: where a, V, and α are cell parameters; h, k, and l are indices of crystal face; and d hkl is the interplanar spacing of (hkl) plane.According to Bragg equation (2dsinθ = nλ), the values of interplanar spacing d for different crystal planes were also related to the diffraction angle θ.Therefore, the variation tendency of 2-Theta could be attributed to the change of d values.In order to further determine the phase transition for Sr3(BO3)2 crystal, high temperature XRPD measurement was carried out.Figure 6 presents the XRPD patterns of Sr3(BO3)2 crystal samples measured from 1150 °C to 1350 °C.It was found from Figure 6a that the main diffraction peaks of Sr3(BO3)2 sample were still consistent with the standard data, while some splitting peaks appeared at the diffraction angles (θ) of 28.70°, 34.34°, and 40.02° when the temperature increased above 1250 °C, corresponding to (113), (300), and (220) planes (JCPDS No. 31-1343), respectively.It was found that the diffraction angles (θ) for ( 113), (300), and (220) planes were all shifted to high angle region then to low angle region, as shown in Figure 6b.
It is known that the relationship between cell parameters and interplanar spacing (d) for crystals with R-3c space group was as follows:   In order to illustrate the change of crystal structure for Sr3(BO3)2, the variations of cell parameters as a function of temperature up to 1350 °C were draw in Figure 7, where a distinct drop was found at 1250 °C.The split of diffraction peaks observed at temperatures above 1250 °C and the significant change of cell parameters at 1250 °C indicating a new crystal phase was formed, which was in accordance with the TG-DTA experimental results.Therefore, the conclusion can be drawn that the In order to illustrate the change of crystal structure for Sr 3 (BO 3 ) 2 , the variations of cell parameters as a function of temperature up to 1350 • C were draw in Figure 7, where a distinct drop was found at 1250 • C. The split of diffraction peaks observed at temperatures above 1250 • C and the significant change of cell parameters at 1250 • C indicating a new crystal phase was formed, which was in accordance with the TG-DTA experimental results.Therefore, the conclusion can be drawn that the endothermic peak in DTA curve observed around 1250 • C should be a reflection of phase transition other than its melting point.

Raman Spectra Analysis
Figure 8 gives the powder Raman spectra of Sr3(BO3)2 crystal together with the results of Ca3(BO3)2 crystal for comparison.It was noted that the strongest Raman peak for Sr3(BO3)2 crystal (900 cm −1 ) was located at a similar position with that of Ca3(BO3)2 crystal (927 cm −1 ), corresponding to the symmetrical stretching vibration of [BO3] 3− .The line widths of the strongest peak were obtained and found to be on the order of 5.72 cm −1 and 7.01 cm −1 for Sr3(BO3)2 and Ca3(BO3)2, respectively.On account of the Raman gain coefficient being inversely proportional to the line width [34,35], the Sr3(BO3)2 crystal might possess even larger Raman gain than Ca3(BO3)2 crystal.

Raman Spectra Analysis
Figure 8 gives the powder Raman spectra of Sr 3 (BO 3 ) 2 crystal together with the results of Ca 3 (BO 3 ) 2 crystal for comparison.It was noted that the strongest Raman peak for Sr 3 (BO 3 ) 2 crystal (900 cm −1 ) was located at a similar position with that of Ca 3 (BO 3 ) 2 crystal (927 cm −1 ), corresponding to the symmetrical stretching vibration of [BO 3 ] 3− .The line widths of the strongest peak were obtained and found to be on the order of 5.72 cm −1 and 7.01 cm −1 for Sr 3 (BO 3 ) 2 and Ca 3 (BO 3 ) 2 , respectively.On account of the Raman gain coefficient being inversely proportional to the line width [34,35], the Sr 3 (BO 3 ) 2 crystal might possess even larger Raman gain than Ca 3 (BO 3 ) 2 crystal.endothermic peak in DTA curve observed around 1250 °C should be a reflection of phase transition other than its melting point.

Raman Spectra Analysis
Figure 8 gives the powder Raman spectra of Sr3(BO3)2 crystal together with the results of Ca3(BO3)2 crystal for comparison.It was noted that the strongest Raman peak for Sr3(BO3)2 crystal (900 cm −1 ) was located at a similar position with that of Ca3(BO3)2 crystal (927 cm −1 ), corresponding to the symmetrical stretching vibration of [BO3] 3− .The line widths of the strongest peak were obtained and found to be on the order of 5.72 cm −1 and 7.01 cm −1 for Sr3(BO3)2 and Ca3(BO3)2, respectively.On account of the Raman gain coefficient being inversely proportional to the line width [34,35], the Sr3(BO3)2 crystal might possess even larger Raman gain than Ca3(BO3)2 crystal.

Conclusions
A new kind of Raman crystal Sr 3 (BO 3 ) 2 was synthesized by solid state reaction method and Cz pulling method.The crystal structure of Sr 3 (BO 3 ) 2 crystal was analyzed by XRPD method and found to possess trigonal symmetry (space group R-3c) similar to Ca 3 (BO 3 ) 2 .The structure stability was studied by means of TG/DTA and high temperature XRPD up to 1350 • C, where the Sr 3 (BO 3 ) 2 crystal was found to show a phase transition around 1250 • C, which induced cracking during crystal cooling.Raman properties were studied theoretically and experimentally, where the Sr 3 (BO 3 ) 2 crystal was found to show strong frequency shift at 900 cm −1 , comparable to that of Ca 3 (BO 3 ) 2 (927 cm −1 ).In addition, the line width of the strongest Raman peak of Sr 3 (BO 3 ) 2 (5.72 cm −1 ) was lower than Ca 3 (BO 3 ) 2 (7.01 cm −1 ), indicating that Sr 3 (BO 3 ) 2 might possess a larger Raman gain coefficient, which is beneficial for the SRS process.
a, V, and α are cell parameters; h, k, and l are indices of crystal face; and dhkl is the interplanar spacing of (hkl) plane.According to Bragg equation (2dsinθ = nλ), the values of interplanar spacing d for different crystal planes were also related to the diffraction angle θ.Therefore, the variation tendency of 2-Theta could be attributed to the change of d values.

Figure 7 .
Figure 7. Variation of cell parameters of Sr3(BO3)2 as a function of temperature.

Figure 7 .
Figure 7. Variation of cell parameters of Sr 3 (BO 3 ) 2 as a function of temperature.

Figure 7 .
Figure 7. Variation of cell parameters of Sr3(BO3)2 as a function of temperature.
Table 1 lists the d hkl values for different crystal planes calculated from XRPD data measured at different temperatures.
Table 1 lists the dhkl values for different crystal planes calculated from XRPD data measured at different temperatures.

Table 1 .
Part of cell parameters and interplanar spacing d values of Sr 3 (BO 3 ) 2 crystal under different temperatures.

Table 1 .
Part of cell parameters and interplanar spacing d values of Sr3(BO3)2 crystal under different temperatures.

Table 1 .
Part of cell parameters and interplanar spacing d values of Sr3(BO3)2 crystal under different temperatures.