Crystallographic Investigations into the Polar Polymorphism of

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Crystallographic investigations into the polar polymorphism of BaTeW2O9: Phase transformation, controlled crystallization, and linear and nonlinear optical properties Conggang Li, Zeliang Gao, Peng Zhao, Xiangxin Tian, Haoyuan Wang, Qian Wu, Weiqun Lu, Youxuan Sun, Deliang Cui, and Xutang Tao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01750 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Crystal Growth & Design

Crystallographic investigations into the polar polymorphism of BaTeW2O9: Phase transformation, controlled crystallization, and linear and nonlinear optical properties Conggang Li, † Zeliang Gao,*, † Peng Zhao,†,‡ Xiangxin Tian,† Haoyuan Wang,† Qian Wu,† Weiqun Lu, † Youxuan Sun,† Deliang Cui† and Xutang Tao*,† † State

Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China

‡ Beijing

Sinoma Synthetic Crystals Co. Ltd., Beijing 100018，P. R. China

ABSTRACT: Controlling polymorphism crystallization can be used to tune the intrinsic properties of a material without introducing any other atoms and is, therefore, of important technological significance. Although diversity in polymorphisms has been extensively studied, an intensive understanding of the relationships between their structure and properties is limited. Herein, a systematic investigation on the phase transformation, synthesis and structure, controlled growth, as well as the functional properties was carried out for the polar metastable polymorph -BaTeW2O9, which was grown via the top-seeded solution growth (TSSG) method using TeO2-WO3 as a flux. -BaTeW2O9 crystallizes in an acentric space group P21 (No. 4, a = 5.499 (6) Å, b = 7.469 (9) Å, c = 8.936 (10) Å, and Z = 2), with a two-dimensional layered structure consisting of WO6 octahedra linked to TeO4 polyhedra. A notable phase transformation occurred from β-BaTeW2O9 to α-BaTeW2O9 at approximately 608 °C and was demonstrated by the DSC analysis combined with the insitu powder X-ray pattern measurements. This phase transformation was irreversible. This compound exhibited strong second harmonic generation (SHG) response of 1.5 × KTP with a type-I phase-matching characteristic. Moreover, it also showed an excellent thermal stability, and possessed a greatly broad transparent range from 0.325 to 5.70 μm with a large bandgap of 3.50 eV. In addition, electronic structure calculations revealed that WO6 and TeO4 are critical for producing a substantial NLO response. These outstanding attributes indicate that β-BaTeW2O9 is a promising nonlinear optical (NLO) material.

INTRODUCTION Controlling polymorphism crystallization is of great interest for tuning performance and providing intriguing platforms for comprehending the versatile crystalchemistry origin.1-3 It is generally acknowledged that polymorphs have identical chemical compositions but typically possess different variations, which exhibit versatile physical properties embodied in the crystal morphology, chemical reactivity, and functional performance.4 Polymorphs are common and exist in organic and inorganic materials, which have practical applications in pharmaceuticals, dyestuffs, foods, and molecular chemistry.5-8 In particular, the polymorphs that exhibit polar behavior in inorganic materials, are believed to be the ideal platform for investigating the impacts of atomic packing on the functional properties, which is worthy of studying deeply. As a new focus of polymorphism, polar polymorphic compounds have become attractive because they not only possess diversified structures, but also display a variety of useful physical properties, such as frequency conversion, piezoelectricity, ferroelectricity and pyroelectricity, that are of great significance in many practical optoelectronic fields.9-22 The key to designing new polar compounds

depends on a deeper understanding of the structure−property relationships. Many research efforts have been devoted to the chemical design and synthesis of polar materials. For example, Halasyamani et al. 23,24 created a host of polar materials such as BaTeM2O9 (M = Mo6+ or W6+), and Ba3ZnB5O10PO4. Novel kinds of polar oxides, including Li7(TeO3)3F, Bi(IO3)F2, KPb2(PO3)5 and CsB4O6F were also recently discovered by the Mao and Pan’ groups.25-28 Unfortunately, most of these materials are mainly focused on analogues with similar structures and different chemical compositions. A clear physical insight into the characterizations of polar polymorphism from an experimental point of view remains limited. This research is further hindered by the difficulty of obtaining large-sized single crystals and investigating optical and other physical properties. The polar polymorphic materials of BaTeMo2O9 (BTM), including monoclinic β-BTM and orthorhombic α-BTM, have been reported recently by our research group, and have attracted considerable interest for their outstanding optoelectronic properties, such as their nonlinear optical, piezoelectric and electro-optical properties.29-34 However, there are always undesirable colors and inclusions in the polar β-BTM crystals due to the self-flux effect.

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Meanwhile, the molybdate β-BTM has low values of thermal conductivity, which greatly obstructs further possible applications. Previous investigations have shown that tungstate compounds not only have lower viscosity, but also possess higher thermal conductivity and stability.35 Inspired by this, as an analogue, BaTeW2O9 (BTW) prompted our great interest. The tungstate BaTeW2O9 (BTW) is another polymorphism, and has two phases: orthorhombic α-BTW (high-temperature phase) and polar monoclinic β-BTW (low-temperature phase).36 Aside from its intriguing features that are similar to BTM, the greater thermal stability and the absence of color impurities compared with those of BTM, makes BTW a more promising material for optoelectronic applications. The polar polymorph -BTW was synthesized by Halasyamani et al. 23 which shows a strong SHG response (500 × SiO2) and thus provides a potentially novel avenue for the research of not only the phase transformation, but also optoelectronic applications. However, the former research was mainly concentrated in the polycrystalline forms of the polar monoclinic -BTW. On the other hand, it is worth noting that our group has recently reported the growth of large-size and high-quality orthorhombic BTW single crystals with optical properties, but an undesirable centrosymmetric structure was obtained.36 Therefore, to further explore the mechanism of phase transition and evaluate optoelectronic applications, a systematic experimental understanding of the polar polymorphism BTW (β-BTW) is highly desirable. In the present study, the metastable polymorph -BTW crystals were grown via the modified top-seeded solution growth technique using TeO2-WO3 as a flux. We systematically investigated the single-crystal structure, thermal behaviors, and linear and nonlinear optical (NLO) properties, and electronic properties of the -form, as well as the phase transformation between α- and β-BTW. Moreover, structural distortions and the asymmetrical polyhedra were analyzed and discussed to illustrate the relationship between the properties and the crystal structure. These behaviors may also provide better understandings for further studying polymorphisms as well as offer new avenues for the controllable crystal growth of these materials. EXPERIMENTAL SECTION Synthesis. Polycrystalline samples of -BTW were prepared through a high temperature solid-state reaction method. BaCO3, (Alfa Aesar, 99.9%), TeO2 (Sinopharm Chemical Reagent Co., Ltd., 99.99%), and WO3 (Alfa Aesar, 99.8%) were obtained from commercial sources and were used as precursors. The required amounts of precursors were mixed homogeneously, thoroughly ground and then packed into columns. Then, the columns were preheated to 400 °C at a rate of 5 K/min and then heated at a rate of 1 K/min from 400 to 530 °C in air for 96 h. Afterwards, the columns were cooled to 350 °C at a rate of 5 K/h and rapidly cooled to room temperature by switching off the furnace. Repeated grindings were carried out using the whole process until the pure powder was obtained. The phase purity of the final product

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was identified by X-ray powder diffraction. To inspect the effect of reaction conditions on the polymorph formation, several rounds of experiments at various reaction temperatures (530-650 ºC) were also implemented. Powder X-ray diffraction. X-ray powder diffraction (XRD) measurements of the target samples were collected on a Bruker D8 Advance Diffractometer, using the Cu-Ka radiation (λ = 1.54056 Å) in the angular range of 2 = 10–80º, with a step size of 0.02° and a step time of 0.2 s at room temperature. Growth of β-BTW single crystals. Although the bulk growth of -BTW single crystals has been investigated by our research group,36 until now, none have investigated the growth and properties of bulk -BTW single crystals due to the extreme difficulty in obtaining bulk single crystals. To solve this problem and further explore its intriguing properties, we probed the appropriate phase region and employed the modified top-seeded solution growth (TSSG) technique to grow -BTW single crystals. The single crystals of β-BTW were grown using a TeO2–WO3 mixture as a selfflux. After several attempts, a mixture of BaCO3, TeO2 and WO3 with a molar ratio of 1:8:8 was placed in a platinum crucible that was placed inside the middle of a vertical, programmable temperature furnace. Then the mixture was heated to 900 °C and kept at that temperature for 72 h to ensure that the powder melted into a homogeneous solution and then cooled to 700 °C at a rate of 10 °C h−1. A platinum rod was dipped into the 700 °C solution, and the temperature was decreased to 685 °C at a rate of 0.3 °C h−1, which resulted in some small β-BTW crystals grown by spontaneous nucleation along the platinum rod during the slow cooling process. Some hand-picked crystals were employed as seeds to grow large-sized single crystals through a TSSG technique. The saturation point of the solution was determined by observing the growth or dissolution of seed crystals on the surface of the solution. At the temperature 10 °C higher than the melting point, a crystal seed attached to a platinum rod was slowly introduced into the solution surface. Afterwards, it was quickly cooled down to the temperature point and followed by slow cooling at a rate of 5-7 °C per day until the end of the growth. Finally, the single crystal was lifted out of the solution, and then cooled to room temperature at a rate no more than 10 °C/h. Single-crystal structure determination. A single crystal of transparent β-BTW (0.1 mm× 0.09 mm× 0.07 mm) was selected for single-crystal X-ray data collection on a Bruker AXS SMART three-circle diffractometer equipped with an APEX II CCD detector and Mo Kα radiation (λ = 0.71073 Å). Data integration and cell refinement were performed using the SAINT program of the APEX2 software, and multiscan absorption corrections were applied by the SCALE program for area detector.37 The structures were checked by direct methods and refined by full matrix least-squares methods on F2. All atoms were refined with anisotropic thermal parameters, and calculations were implemented by the SHELXTL crystallographic software package.38 Table 1 listed the final details of crystal parameters, data collection, and structure refinement.


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High-resolution X-ray diffraction (HRXRD) measurements. The crystalline quality was evaluated through HRXRD, based on the measurements of a BrukerAXS D5005HR diffractometer with Cu-Kα radiation (λ = 1.54056 Å) and a four-bounce Ge (220) monochromator. A (003)-oriented wafer of the β-BTW single crystal, mechanically polished on both sides, was used for the HRXRD measurements.

250–325, and 325–425 µm). The microcrystalline KTiOPO4 powders with the corresponding particle size ranges served as reference samples. To further measure the SHG intensity of β-BTW and KTP, the comparative SHG signals at the same powder size (109−150 μm) of KTP and β-BTW were recorded. The tests were implemented at room temperature using a pulsed beam generated by a Q-switched Nd:YAG solid-state laser (10 ns, 1 Hz).

Thermal analysis. The melting behaviors of β-BTM were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using a simultaneous thermal analyser (TGA/DSC1/1600HT, Mettler-Toledo Instruments). The samples were enclosed in a platinum crucible, heated from room temperature to 950 ºC at a rate of 10 ºC/min with flowing nitrogen.

Theoretical calculations. To fully comprehend the relationship between the electronic structure and the optical properties of β-BTM, the first principle electronic structural calculations were performed on the basis of the density functional theory (DFT).43 The Perdew–Burke–Ernzerhof (PBE) function was employed to calculate the exchange– correlation potential, with an energy cutoff of 350.0 eV under the generalized gradient approximation (GGA),44 as implemented in the Vienna Ab initio Simulation Package (VASP).45 The convergence threshold was set to an energy of 1 × 10−2 eV and a force of 0.2 eV/Å. The experimental singlecrystal structural data of β-BTW were used for the theoretical calculations. The Brillouin zone integration employed a 1 × 1 × 1 Monkhorst−Pack mesh for the geometry optimizations.

Raman spectroscopy. The Raman spectra measurement was performed at room temperature using a Thermo-Nicolet NEXUS 670 spectrometer equipped with an InGaAs detector. The β-BTM powder placed in a capillary tube was used as the measured sample. Excitation was applied by a Nd:YAG laser at a wavelength of 1064 nm, and the output laser power was 1.000 W. The spectral resolution was 4.000 cm-1, and 64 scans were collected. Density measurement. Density is also an important reflection in differentiating different polymorphs in single crystals.32,39 The density of β-BTM at room temperature was measured by the Archimedes method.40 Forty milliliters of distilled water was poured into an empty beaker. Without touching the bottom of the beaker, a β-BTM crystal attached to a silver filament was immersed into the water. Finally, the density was determined based on the following formula: 𝜌𝑒𝑥𝑝 = 𝑚0𝜌𝑤𝑎𝑡𝑒𝑟/(𝑚0 ― 𝑚1), where 𝑚0 denotes the test crystal weight in air, 𝑚1 denotes the sample weight dipped into distilled water, and 𝜌𝑤𝑎𝑡𝑒𝑟 denotes the density of deionized water (𝜌𝑤𝑎𝑡𝑒𝑟 = 0.9973𝑔/𝑐𝑚3).40 The entire process was repeated several times to acquire an average value. Linear optical measurements. The UV-vis diffuse reflectance measurements of β-BTM were implemented at 298 K using a Shimadzu UV 2550 recording spectrophotometer equipped with an integrating sphere over the spectral range 200–800 nm, which can provide the UV or visible cut-off edge. BaSO4 was used as a standard material. Then reflectance spectra were converted to absorbance through the Kubelka–Munk function.41 To further determine the transparency range of β-BTM, the room temperature transmission spectra for β-BTM were also recorded using a Hitachi U-4100 UV/vis/IR spectrometer from 200 to 2000 nm and a Nicolet NEXUS 670 FTIR spectrometer in the range of 2000−7800 nm. A thick slab of the sample (4 × 4 × 1 mm3) was cut to carry out the measurements with both sides polished. Second-harmonic generation measurement. The powder SHG responses for β-BTW were checked using the method described by Kurtz−Perry.42 As the SHG efficiency of the powder sample depends directly upon particle size, the β-BTW sample was powdered to an approximate spherical shape and graded by standard sieves to obtain distinct particle size ranges (25–48, 48–75, 75–109, 109–150, 150–250,

RESULTS AND DISCUSSION Synthesis and phase transformation. The preparation of a polycrystalline sample of monoclinic β-BTW was performed using traditional solid-state reaction techniques. XRD analysis was applied to check the phase purity of the product. After many attempts, we found that pure polycrystalline β-BTW can be obtained with the reaction temperature of no more than 550 ºC (seen in Figure S1), which agrees well with the previous report.23 However, a higher temperature, approximately 600 °C, promoted the generation of α-BTW, which indicated that the temperature for the β to α-BTW phase transformation is approximately 600 °C, as shown in Figure 1a. According to our experiments, α-BTW easily appeared at higher temperatures and formed a stable structure. The isothermal annealing experiments at different temperatures and times were also implemented on the α-BTW powders to verify the reversibility of the phase transformation from α-BTW to β-BTW. However, the solidstate phase transformation was not observed, suggesting an irreversible transformation between the β- and α-BTW polymorphs (shown in Table S1). To further investigate the phase transformation and evaluate their thermal stability, DSC - TGA measurements for both of α-BTW and β-BTW were carried out as presented in Figure 2. It is worth noting that the DSC curve of α-BTW shows only one clear endothermic peak at 784 ºC, which is consistent with the reference,36 and a tiny endothermic peak at approximately 608 ºC and one sharp peak with a maximum at 784 ºC was observed for β-BTW upon heating to 950 ºC. Moreover, there was no weight loss in the TGA curve at approximately 608 ºC. The present results clearly indicate that β-BTW potentially melts incongruently with a high melting point of 784 ºC and undergoes a structural phase transformation at 608 ºC. To further confirm its incongruent melting characteristic, polycrystalline samples of β-BTW were heated to 900 °C. After the powder melted, it was cooled to room temperature to analyze the powder XRD patterns. As expected, the XRD



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Figure 1. (a) The calculated and experimental powder XRD patterns at different temperature. (b) The in-situ XRD pattern of βBTW varying from 500 – 700 ºC. (c) The enlarged patterns of β-BTW in the 2 range of 10–30 and 30-50 degrees.

Figure 2. (a) and (b) DSC and TGA data for -BTW and -BTW respectively. The DSC curves of -BTW indicate the phase transformation point at around 608 ºC.


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Figure 3. (a, b) Photographs of the as-grown β-BTW single crystals through spontaneous crystallization and by using non-oriented seed, respectively. (c) Z-plate (4×4×1mm2) cuts from the β-BTW single crystal. (d) Predicted growth morphology of the β-BTW crystal based on the BFDH method.

diffractograms of the melted samples are considerably different from those of the original powders (presented in Figure S2), which unquestionably confirms the incongruent melting behavior for β-BTW. Therefore, large β-BTW single crystals must be grown by the flux method, which has been verified by our crystal growth experiments. To grasp further insights into the phase transitions and the associated structural transformations, the in-situ XRD pattern measurements for β-BTW were also carried out ranging from 500 - 700 °C, and the intuitive results are gathered in Figure 1b. Combined with enlarged patterns in the 2 range of 10–50 degrees (Figure 1c), the evolution from β-BTW to pure αBTW polymorph can be seen clearly with their transformation temperature at approximately 600 ºC, which corresponds to the result of its DSC measurement. It is noteworthy is that the same situation also occurs in the Cd3B2O6 and Pb2Ba4Zn4B14O31 compounds reported recently,46,47 but it is different from previous investigations on BTM, in which the phase transformation from β-BTM to -BTM cannot be observed in the -BTM stable temperature without any additives. 31 Raman spectroscopy. Raman spectroscopy was used to probe the coordination geometry of the title compounds.

The Raman spectra of β-BTW are presented in Figure S3. As a result, the Raman spectrum of the investigated β-BTW exhibited similar absorption bands with -BTW ranging from 400 to 1000 cm−1.36 The Raman absorption bands at 800-950 cm-1 could be assigned to the stretching and bending vibrations from W-O bonds, which are consistent with other tungsten molybdenum. The bands observed at 600 and 800 cm-1 could be attributed to Te-O vibrations. Furthermore, the bands in the range of 400–600 cm-1 can be regarded as the effect of the bending modes of the W–O, Te–O bonds and W–O–Te bridges (Table S2). The assignments are consistent with previous reports.31,36 The Raman spectra further confirm the existence of the TeO4 and WO6 units, which matches the results obtained from the single-crystal X-ray diffraction. Controlled crystal growth. The flux growth method is particularly preferable as it can allow crystal growth at a temperature well below the melting point of the target solution. Meanwhile, crystals grown from the flux method have a euhedral habit and a reasonably lower degree of dislocation density.48 Therefore, the selection of the flux for β-BTW crystal growth is pivotal. To determine the crystallization region of β-BTW, the phase diagram in the



BaO−TeO2−WO3 system was primarily investigated by using the same equipment.

systematically

After many attempts, the crystallization region of different products was identified. Figure S4 clearly shows that the crystallization region of β-BTW is very narrow, whereas a large crystalline region is observed for -BTW. This naturally leads to the fact that the undesirable -BTW is easy to crystallize instead of β-BTW. We also have found the BTW phase always appeared even in the crystallization region of β-BTW (Figure S5), indicating that the former is more stable and, thus, β-BTW can be regarded as a quasimetastable phase in a high temperature solution, which poses great challenges for obtaining β-BTW single crystals. To improve upon the negative situation, the crystallographic effect of the cooling rate on crystallization was systematically studied, because the change in the cooling rate has a direct impact on the crystal quality and morphology and, more importantly, it can also control the generation of the polymorphism in the flux method, according to previous studies.3,49 To determine the polymorph of BTW, we carried out a large series of experiments and explorations with different cooling rates of the solution. After having a great deal of attempts, we found that employing a slow cooling rate, the undesired stable polymorph -BTW was prone to appear, whereas the desired β-BTW was obtained only when adopting a faster cooling rate. The specific process of the investigated samples is illustrated in Figure S6, which apparently shows that BTW crystals with orthorhombic or monoclinic structures were obtained depending on the cooling rate during the crystal growth. More specifically, with a slow cooling rate of  0.5 °C/d and 1-3°C/d, -BTW crystals were obtained (Figure S6a-6b); while using a faster cooling rate of 5-7 °C/d and 810°C/d led to crystallization of the quasi-metastable phase βBTW (Figure S6d-6e). When the cooling rate was approximately 4°C/d, large proportions of -BTW and small amounts of β-BTW products formed (Figure S6c). Consequently, the dependence of the polymorphs of BTW on the cooling rate is now established. These interesting findings also exist in the crystal growth of ZnSnP2 compound by flux method. 50,51 It is evident from the picture that increasing the cooling rate of the solution caused a disappearance of the -BTW and an appearance of the quasi-metastable phase β-BTW. Therefore, to obtain β-BTW single crystals, suitable cooling rate (5-7 ºC/d) should be utilized. Based on our optimized experimental growth conditions, some small β-BTW single crystals were obtained by spontaneous nucleation, as shown in Figure 3a. Bulk crystals with suitable size and quality for seed crystal preparation were obtained after several circles of seed cultivation and selection. Then, high-quality transparent β-BTW crystals with a size of 11 × 8 × 7 mm3 were grown successfully during the cooling process using the TSSG technique, as illustrated in Figure 3b. Meanwhile, the theoretical morphology of the β-BTW crystal was also determined by using the Materials Studio Modeling program with its structural parameters based on the BFDH method (Figure 3d).52-54 The difference between the experimental observed and predicted morphology could be assigned to growth conditions such as flux system, cooling rate, and seed

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orientation. To determine the crystalline perfection of the asgrown β-BTW single crystal, rocking curve measurements were performed. It is clear from Figure 4 that the shape of the diffraction peak is symmetrical and the full width at half maximum (FWHM) is as low as 33.5 arcsec, indicating that the as-grown crystal is of high quality. This result also confirms that the β-BTW crystal is sufficient to implement the experimental measurements of intrinsic physical properties.

Figure 4. High-resolution X-ray diffraction rocking curve, and the corresponding FWHMs for the β-BTW wafer. Structure characterization. The β-BTW crystal structure was first reported by Halasyamani et al in 2003.23 To provide better insights into the crystal structure, single-crystal X-ray diffraction data were rerecorded on a small piece of the asgrown crystal with high quality. The crystallographic data and structural refinement parameters for β-BTW are listed in Table 1. β-BTW crystallizes into a polar space group, P21 (No. 4), which is analogous to that of β-BTM. The cell parameters were refined as a =5.499 (6), b = 7.469 (9), and c = 8.936 (10) Å, which is consistent with the reported results.23 Selected bond lengths and angles for the β-BTW crystal structure have been deposited in the supplementary information (Table S3). The analysis of the crystal structure shows that the target compound shows two-dimensional layered structures consisting of WO6 octahedra linked to TeO4 polyhedra with Ba2+ cations between the layers for maintaining charge balance. The asymmetric unit contains one crystallographically independent Ba atom, one independent Te atom, two independent W atoms, and nine independent O atoms (presented in Figure 5a). As shown in Figure 5b, W6+ cations are six coordinated to form distorted octahedron, while Te4+ cations are coordinated to four oxygen atoms in a distorted “seesaw” configuration, both of which are in asymmetric coordination environments attributable to SOJT distortions. These distortions could result in local dipole moments for WO6 and TeO4 polyhedra. Within the asymmetric unit, Te atoms are 4-fold-coordinated by four O atoms to form TeO4 polyhedra with Te−O bond lengths varying from 1.869(22) to 2.190(19) Å. Local asymmetric coordination environments presented in the Te4+ cations are assigned to the stereo-active lone pairs. With respect to WO6 octahedron, the bond distances for W(1)-O and W(2)-O range from 1.736(26) to 2.168(22) and 1.719(28) to 2.207(21) Å respectively, and all of them are in the normal ranges.


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Crystal Growth & Design Table 1. Crystal Data and Structure Refinement for β-BaTeW2O9 and -BaTeW2O9.

crystal

-BTW

-BTW36

Empirical formula

BaTeW2O9

BaTeW2O9

Mr

776.64

776.64

Crystal system

Monoclinic

Orthorhombic

space group

P21 (No. 4)

Pnma (No. 62)

Temperature (K)

296

296

a, b, c (Å)

5.499 (6), 7.469 (9), 8.936 (10)

13.9824(8), 5.5774(3), 9.8671(5)

β (°)

90.362 (9)

90.00

V (Å3)

367.0 (7)

769.49 (7)

Z

2

4

Dcalcd, g/cm3

7.028

6.704

µ (mm−1)

40.49

38.624

F(000)

656

1312

Crystal size (mm)

0.10× 0.09× 0.07

0.08× 0.06× 0.05

Rint

0.059

0.0287

GOF on F2

1.086

1.268

R indices [I＞2(I)]a

R1=0.0438, wR2=0.1029

R1=0.0175, wR2=0.0367

R indices (all data)

R1=0.0447, wR2=0.1036

R1=0.0186, wR2=0.0370

Extinction coeff

0.067 (4)

0.00418 (13)

CCDC

1868012

953707

To gain further insight into the structure of β-BTW, the structures between both the - and β-BTW phases were compared in detail. - and β-BTW exhibit different crystallographic systems and space groups; however, their space groups are related by the crystallographic groupsubgroup relations, which provide the possibility of phase transformation between the two polymorphs. The compounds can be viewed as members of a homologous series (BaTeW2O9)n, where n = 1 and 2 for the asymmetric units of β-BTW and -BTW, respectively (See Table 1). It is noteworthy that the length for the crystallographic a axis of a-BTW is twice as long as that of β-BTW, that is, a () =2.54 × a (), while the remaining other two lengths are almost equal, b () =0.75 × b (), c () =1.1 × c (). This also matches well the relationship of their volumes; namely, two cells of βBTW are equivalent to one cell of α-BTW (Figure 5b-5c). This situation, to some extent, leads to similar densities and powder X-ray patterns. As illustrated in Figure 5e, -BTW has a three-dimensional (3D) framework structure composed of the distorted W-O6 octahedral connected with Te-O4 polyhedra on the corner O atoms, which is significantly different from that of β-BTW (Figure 5d). Furthermore, in BTW, the Te4+ cations are in a distorted tetrahedral

environment owing to the stereo-active lone-pair, and the W6+ cations exhibit two kinds of distorted octahedral environments, which are ascribed to SOJT distortions. However, there exists an inversion center for the distributions of these asymmetric polyhedra in a-BTW, resulting in a centrosymmetric Pnma, which markedly differs from those of β-BTW. Density measurement. Density can also be used as an indicator to identify the polymorph in a single crystal. 55 The density of β-BTW at room temperature was measured to be 6.98 g/cm3, slightly smaller than the calculated value (7.028 g/cm3) using its crystal structure. The density of β-BTW is apparently approximately the same value as that of -BTW (6.723 g/cm3), which is consistent with the relationship of their different structures.36 Linear optical properties. The optical band gap of compounds is one of the important factors influencing the laser-damage threshold.56-59 The UV-vis diffuse reflectance spectrum for β-BTW was performed, as displayed in Figure 6a. The result of the band structure calculation implies an indirect band gap of β-BTW. Absorbance was calculated by



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Cs2TeW3O12 (0.41-5.31 µm),35 Cs2TeMo3O12 (0.43-5.38 µm),64 Na2Te3Mo3O16 (0.42-5.4 µm),65 MgTeMoO6 (0.36-5.2 µm),66 and CdTeMoO6 (0.345-5.40 µm),67 (addressed in Table 2), which makes the β-BTW crystal relatively more attractive in further industrial and optical applications. Second-harmonic generation. As β-BTW crystallizes into the NCS structure, powder SHG measurement technique was implemented to investigate its nonlinear optical properties. PSHG measurements on β-BTW with 1064 nm radiation revealed a very strong SHG efficiency (approximately 1.5 times that of KTP), as showed in Figure 7a. Notably, the SHG intensity increased with increasing particle size before reaching the maximum value independent of the particle size, which matched well with the type-I phasematching behavior based on the rules proposed by Kurtz and Perry (see Figure 7b).42 The overall performance of β-BTW were more desirable compared to many other molybdate/tungstate oxide crystals, including β-BaTeMo2O9 (2.5×KDP), Cs2TeMo3O12 (1.5×KTP), MgTeMoO6 (1.5×KTP), Cs2TeW3O12 (1.5×KTP) and CdTeMoO6 (2×KTP), which also listed in Table 2.

Figure 5. (a) W(1)O6 polyhedra, W(2)O6 polyhedra and TeO4 polyhedra in -BTW. The arrows indicate the approximate direction of the dipole moments. (b) and (c) Unit cell diagrams for -BTW and -BTW with the ball-and-stick representation, respectively. (d) and (e) 3D framework structure for the -BTW and -BTW crystal, respectively. using the reflectance spectra based on the following equation 𝛼ℎ𝑣 = A(ℎ𝑣 ― 𝐸𝑔)𝑛/2 where , h, , Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. The band gap energy of β-BTW is found to be approximately 3.50 eV, which is slightly larger than the band gap of α-BTW (3.35 eV) and thus indicates a higher laser damage threshold.60,61 To further determine the accurate value of the absorption cutoff edge, the UV-vis-NIR and mid-IR transmission spectra of the α-BTW and β-BTW crystals were obtained at room temperature with light along c-axes. As Figure 6b illustrates, the transmission spectra of β-BTW crystal is broad enough to cover that of α-BTW crystal. The UV absorption edge of βBTW was located near 325 nm, which is more reliable than that obtained from the diffuse reflectance test of the powder samples. The β-BTW crystal possessed high mid-IR transparency up to 5.4 µm, and the IR absorption edge was extended to 5.7 µm covering a critical atmospheric transparent window (3–5 µm). It is gratifying to see that the broad transmission range (325 nm - 5.70 µm) for β-BTW is better than other previously reported molybdenum tellurite compounds, such as -BaTeMo2O9 (0.38-5.53 µm),32 βBaTeMo2O9 (0.5-5.0 µm),29,62 Na2TeW2O9 (0.36-5.0 µm),63

Structure–property relationship. It is well known that physical properties are closely related to the structures for a material.68-70 To further investigate the relationship between the structure and corresponding second-order nonlinear optical properties of β-BTW, we analyzed the magnitude of distortion for WO6. Both W(1)O6 and W(2)O6 distort in the local C3 [111] direction, and the magnitudes of out-of-center distortions were calculated using the approach proposed by Halasyamani et al.23,71 The magnitude of the distortion was calculated as follows. d 

(W  O1)  (W  O 4) (W  O 2)  (W  O5) (W  O3)  (W  O6)  + cos 1 cos  2 cos 3

where Δd is the magnitude of the distortion, and the corresponding bond lengths in the equation are presented in Figure 5a. The obtained Δd values for W(1)O6 and W(2)O6 octahedra in β-BTW are 1.074 and 1.066, respectively, both of which are obviously larger than 0.8 reported by previous studies,72 and correspond to a strong distortion. With respect to the dipole moments, the vector summation over the TeO4 geometry showed a net dipole moment of 10.74 Debye, which was apparently larger than the corresponding average value (8.57 Debye).73 Thus, it is reasonable that the strong SHG efficiency is attributable to the cooperative effect of WO6 and TeO4 polyhedra, which is also in good agreement with the previous findings.23 On the other hand, to shed light on the microscopic mechanism of the optical nonlinearity, band structure calculations were also implemented to further insights into the electronic structure of the β-BTW by using the total-energy code CASTEP, based on density functional theory (DFT). The β-BTW was an indirect transition semiconductor with a calculated band gap 3.19 eV, slightly smaller than the measured values (3.50 eV) by 0.31 eV. The discrepancy is attributed to the self-consistent band structure calculation within DFT. In general, PBE functional calculations underestimate the value of the band gap.74,75 As displayed in the energy band structure representation (Figure 8a), the valence band maxima (VBM) occur along the B direction, and conduction band minima (CBM) is located


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Figure 6. (a) UV−visible diffuse reflectance spectroscopy data. The insert is (αhν)1/2−(hν) curve for the β-BTW single crystal. (b) UV-vis and IR Transmittance spectra for the β-BTW and α-BTW single crystal, respectively.

Figure 7. (a)The oscilloscope traces of the SHG signals for β-BTW in the particle size of 109−150µm, and with KTP as a reference. (b) Phase-matching curves for β-BTW. The solid curves are drawn as a guide for the eyes.

Figure 8. Electronic structure for β-BTW. (a) Energy band structure. (b) TDOS and PDOS curves. The Fermi level is set at 0.0 eV (dashed vertical line).



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Table 2. Powder SHG, Band gap (eV), Thermal Stability (ºC) and Transmission (µm) of β-BTW, Comparing with Other Molybdenum Tellurite NLO Materials. Compounds

Powder SHG

Band gap

Thermal stability

Transmission

β-BTW (This work)

1.5 × KTP

3.50 eV

784 ºC

0.325 - 5.7 µm

α-BaTeMo2O932

0.2 × KDP

3.12 eV

592.68 ºC

0.38 - 5.53 µm

β-BaTeMo2O929,62

2.5 × KDP

2.78 eV

602 ºC

0.50 - 5.0 µm

Na2TeW2O963

500 × α-SiO2

3.45 eV

670 ºC

0.36 - 5.0 µm

Cs2TeW3O1235

1.5 × KTP

2.89 eV

820 ºC

0.41 - 5.31 µm

Cs2TeMo3O1264

2 × KTP

2.88 eV

494.95 ºC

0.43 - 5.38 µm

Na2Te3Mo3O1665

500 × α-SiO2

2.95 eV

468 ºC

0.42 - 5.4 µm

MgTeMoO666

1.5 × KTP

3.12 eV

682.35 ºC

0.36 - 5.2 µm

CdTeMoO667

2.0 × KTP

3.59 eV

755 ºC

0.345 - 5.4 µm

at the E point, resulting in the β-BTW indirect band gap. Based on the total and partial densities of states (TDOS, PDOS, respectively) projected onto the constituent atoms, the detailed electronic structure can be determined, as illustrated in Figure 8b, from which several characteristics can be indicated: (i) The TDOS can be divided into five regions. Below -15 eV, the energy level is mainly contributed by the Ba 5s and O 2s orbitals, which are quite difficult to excite and almost contribute nothing to the optical property related with the electron transition across the band gap. (ii) The region ranging from approximately -12 to -8 eV is mainly dominated by the Ba 5p and Te 5s states with small amounts of the O 2p state. (iii) The levels in both the valance bands (VB) and conduction bands (CB) near the Fermi level (−6.5 eV to 0 eV and 2.5 to 6 eV) are mainly composed of the W 5d, O 2p and Te 5p states, which play a leading role in the linear and nonlinear optical properties of the crystal. Therefore, the WO6 octahedra and TeO4 polyhedra contribute substantially to the SHG effect, while BaO6 polyhedral contribute less, corresponding to the experimental results. It is worth noting that the PDOS of W 5d states are split into several peaks and become a wide band, corresponding to the split in the orbitals of the d0 electrons due to the SOJT. CONCLUSIONS In summary, we thoroughly studied the polymorphism in BTW for the first time, which undergoes an irreversible phase transformation from polar β-BTW to centrosymmetric -BTW at approximately 608 ºC. Notably, the polymorphism crystallization of BTW can be controlled by tuning the cooling rate, and on this basis bulk single crystals of β-BTW are cultivated successfully via the TSSG technique. The tunability of polymorphisms offers a promising route to engineer the crystal growth for the benefit of application.

Meanwhile, rocking curve measurements revealed a FWHM of 33.5 arcsec for the (003) reflection indicating a highquality crystal. β-BTW displays a two dimensional structure with an indirect band gap as large as 3.5 eV, implying that it will exhibit high laser damage threshold. Moreover, β-BTW possesses excellent thermal stability and a remarkable SHG efficiency of ～ 1.5 × KTP with type-Ι phase-matchablity. Markedly, it also shows a great wide transmittance region in the range of 0.325–5.7 µm, which is more suitable for NLO applications. All of these current investigations render that β-BTW is of fundamental scientific interest as well as an outstanding candidate for NLO applications. These behaviors also provide a better understanding for the further studies of these materials.

ASSOCIATED CONTENT Supporting Information. [CCDC1868012 for β-BTeW2O9 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]; Calculated and experimental powder X-ray diffraction patterns for -BTW; The product of powder -BTW after annealing at various temperatures and times; The experimental patterns of powder XRD of the residue in the platinum pan after melt at 900 ºC; Room temperature spontaneous Raman spectra of -BTW; Observed Raman wavenumbers (cm-1) and vibrational assignments for -BTW; The crystallization region of crystallized product; Photographs of the as-grown -BTW crystals; The crystals obtained with different cooling rate; Bond Distances (Å) and Angles (deg.) for β-BTW crystal.

AUTHOR INFORMATION


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Corresponding Authors * E-mail: [email protected] (X.T.) * E-mail: [email protected] (Z.G.)

Present Addresses State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China.

Author Contributions All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This research is financially supported by the National Natural Science Foundation of China (Grant Nos. 51321091, 51572155 and 11504389), Shandong Provincial Natural Science Foundation, China (ZR2014EMM015), Independent Innovation Foundation of Shandong University, IIFSDU. National key Research and Development Program of China (2016YFB1102201), the Foundamental Research Founds of Shandong University (2017JC044) and the 111 Project 2.0 (Grant No: BP2018013). The authors also thank Xiufeng Cheng for her help in the DSC-TGA measurements and Shaojun Zhang (Shandong University) for his help with the SHG measurements.

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For Table of Contents use only

Crystallographic investigations into the polar polymorphism of Ba2TeW2O9: Phase transformation, controlled crystallization, linear and nonlinear optical propertie Conggang Li, † Zeliang Gao,*, † Peng Zhao,†,‡ Xiangxin Tian,† Haoyuan Wang,† Qian Wu,† Weiqun Lu, † Youxuan Sun,† Deliang Cui† and Xutang Tao*,† † State

Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China

‡ Beijing

Sinoma Synthetic Crystals Co. Ltd., Beijing 100018，P. R. China

CORRESPONDING AUTHOR EMAIL ADDRESS: [email protected]

The phase transformation from β-BaTeW2O9 to -BaTeW2O9 was thoroughly studied. The polymorphism crystallization of BTW can be controlled by tuning the cooling rate, which offers a promising route to engineer the crystal growth for the benefit of application. Besides, the linear and nonlinear optical properties were also investigated, indicating β-BaTeW2O9 is a promising NLO material.


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Crystallographic Investigations into the Polar Polymorphism of

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