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Polymorphic Smooth Interfaces Formation based on the Bi-Phasic BaTeMo2O9 Using Top Multi-Seeded Growth Qian Wu, Zeliang Gao, Haoyuan Wang, Conggang Li, Weiqun Lu, Xiangxin Tian, Youxuan Sun, Na Lin, Sheng-Qing Xia, and Xutang Tao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00523 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Crystal Growth & Design

Polymorphic Smooth Interfaces Formation Based on the BiPhasic BaTeMo2O9 Using Top Multi-Seeded Growth Qian Wu, Zeliang Gao*, Haoyuan Wang, Conggang Li, Weiqun Lu, Xiangxin Tian, Youxuan Sun, Na Lin, Shengqing Xia, and Xutang Tao* State Key Laboratory of Crystal Materials & Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan 250100, P. R. China ABSTRACT: An interesting phenomenon of two polymorphic heterojunctions is reported in this paper. The selective growth of the individual phases of high-quality α- and β-BaTeMo2O9 (BTM) crystals were successfully achieved using an optimized flux system and growth parameters. A bi-phasic BTM crystal based on a multi-seeding technique used to control the nucleation and subsequent growth was realized in a one-flux system. The formation of a grain boundary, similar to a heterojunction, between the α- and βBTM crystals emerged as an unavoidable consequence of this process, and the factors that led to the formation of the grain boundaries in the multiseeds were examined. Lattice mismatch and interfacial binding energy calculation verified that the BTM crystals were grown together rather than being physically adhered, and the chains at the grain boundary were linked by O5Mo-O-MoO5 dioctahedrons corner sharing O atoms at the eutectic boundary. The screw dislocation growth mechanism was confirmed for β-BTM with the growth unit of a single cell, as well as the similarities in structures for both phases. Additionally, a “pseudo-liquid-phase epitaxial” growth model was used to describe the formation mechanism of bi-phasic BTM based on a “virtual supercell” of β-BTM. The successful implementation of polymorphic BTM growth may provide information regarding polymorph control, and the heterojunction between the two polymorphic phases may demonstrate interesting future applications.

INTRODUCTION Polymorphs are different crystalline forms of the same pure chemical compound, and polymorphism is common in many inorganic, organic, and pharmaceutical substances.1-7 Although identical in chemical composition, polymorphs differ in their solubility, dissolution rate, melting point, physicochemical stability, and many other properties.8 The successful design of crystalline organic solids requires control over molecular packing, whereas the widespread occurrence of polymorphism in organic crystals makes the packing of organic molecules especially difficult to rationalize and predict.9 Especially in the pharmaceutical industry, the bioavailability, drug efficacy, and safety can be directly affected by the presence of polymorphism in drugs.8, 10-13 Additionally, many new methods for controlling polymorph formations have been developed to select pure forms. These methods include conventional and unconventional techniques, such as sublimation, solvent evaporation, polymer-induced heteronucleation, and capillary growth.13-28 Furthermore, inorganic polymorphic materials are of academic and commercial interest due to their technologically relevant functional properties, such as ferroelectricity, pyroelectricity, nonlinear optical behavior, and optoelectricity.29-37 The nucleation stage is known to be the decisive step for determining the form of a certain polymorph. The achievement of supersaturation is considered to be the driving force for both crystal nucleation and growth, which depends on, for example, the temperature, solution composition, cooling rate, etc.38-39 Noncentrosymmetric and polar materials of orthorhombic αBTM 40 and monoclinic β-BTM41-42, which belong to polymorphic materials,43 have attracted considerable interest for their excellent optoelectronic properties. Our group’s previous contribution to the characterization of α-BTM has shown that it exhibits stable physicochemical characteristics and a wide transmission band, namely, a large refractive index, and is an excellent candidate for polarized optical and acoustic-optical materials.40, 44-46 Additionally, β-BTM shows excellent piezoelectric and Raman properties in

the mid-IR region and could be an excellent candidate for photoelectric applications and Raman laser output.47-55 However, the growth of individual-phase BTM crystals is rather complex due to their polymorphism. The control of high-quality single-phase polymorphs during the growth process is necessary for a variety of optoelectronic applications. The crystal growth and the structural similarity of β-BTM and α-BTM have been previously reported,40, 42 but the growth of biphasic BTM crystal using multiseeds has not been reported. Multi-seeding techniques have been previously used to fabricate large and complex materials and structures with decreased processing time, and various types of artificial grain boundaries have been obtained by varying the angle between the two seeds.56-59 Inspired by the above, electrical and microstructural homogeneity may be exhibited at the grain boundary of a bi-phasic BTM crystal grown by the multiseeding technique, which may have promising applications in polymorphic heterojunctions. Additionally, the grain boundaries of bi-phasic BTMs may provide new insight into interfacial structures. In this contribution, the growth of polymorphic β- and α-BTM crystals in which the indivisible phase can be controlled has been successfully achieved based on an optimized flux system by adjusting the supersaturation conditions. Furthermore, a bi-phasic BTM crystal was successfully grown using two seeds tided together in an identical furnace through optimization of the flux system to obtain a suitable solubility and viscosity for the α- and β-BTM solution. Moreover, the grain boundaries were investigated by lattice mismatch calculation, atomic force microscopy (AFM), Laue diffraction, and scanning electron microscopy (SEM), along with the analysis of the interfacial binding energy at the eutectic grain boundary. A model for the “pseudo-liquid-phase epitaxial” growth was described to illustrate the formation of the bi-phasic BTM crystal.

EXPERIMENTAL SECTION

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Crystal Growth. High-quality bulk crystals of α-BTM and βBTM were grown using a TeO2-MoO3 mixture as the flux, as previously reported.40, 42 Different from the former experimental parameters, the flux for the growth of α-BTM and β-BTM was optimized. To optimize the flux, Pt crucibles containing BaCO3, TeO2, and MoO3 at molar ratios of 1:4:4 and 3:11:12 were placed in the center of a vertical, temperature-programmable furnace for the synthesis of α-BTM and β-BTM, respectively. Then, seeds oriented along the [010] direction were dipped into the solution after determination of the saturation point for α-BTM and β-BTM. The preparation of the bi-phasic BTM crystal was carried out using a TeO2-MoO3 mixture with a molar ratio of 1.18:1 as the flux. A platinum crucible containing BaCO3, TeO2, and MoO3 in a molar ratio of 1:3.3:3.9 was heated to 650 °C and maintained for 3 days to form a homogeneous solution. The saturation temperature was determined by observing the growth or dissolution of these seeds on the surface when the temperature was decreased to 540 °C. The two seeds oriented along the [010] and [100] direction or the (204) and (012) planes were tied together and dipped into the solution at 3 °C above the saturation temperature. The solution was then cooled to the saturation temperature in 5 hours. Subsequently, the crystals were grown by cooling the solution at a rate of 0.1-0.4 °C/d with a rotation speed of 25 rpm (at the beginning of the growth). Then, the rate slowly changed to 0.20.45 °C/d with a rotation speed of 16 rpm as the bulk crystal continued to grow. The as-grown crystals were removed and suspended over the surface of the solution and cooled to room temperature at a rate of 2-5 °C/h. The Solubility of the β- and α-BTM Solutions. The solubility of a solution plays a crucial role in crystal growth. Briefly, polycrystalline BTM was placed into a TeO2-MoO3 flux at 650 °C and held for 3 days to form a homogeneous solution. The saturation temperature was determined by observing the growth and dissolution of the crystal seeds on the surface of the solution. Then, the amount of polycrystalline BTM was increased, and the saturation temperature was determined. This procedure was repeated several times. The Melt Viscosity of β- and α-BTM Solutions. Crystal growth is a phase transition process, accompanied by the movement of atoms, changes in the cluster structure, and restructuring of the coordinating atoms. All processes mentioned above are related to the transport properties of the melt.60 The viscosity is a critical property reflecting the mass transfer of the elements inside the reaction melt. To better study the growth mechanism, the viscosities of both phases of the β- and α-BTM crystals were determined using an Orton RSV-1600-type rotary high-temperature viscometer (Orton, USA). This viscometer has a measurement range from 50 cP to 4×107 cP, a temperature variation of less than ± 0.1 K, and viscosity error of ± 1 %. Raw material solutions of the β- and α-BTM crystals in their own growth concentrations was utilized during sample preparation. The samples were heated to 620 °C at a heating rate of 8 °C/min to ensure a homogeneous solution. Then, the viscosity versus temperature curves were measured at a cooling rate of 1 °C/min. The Calculation of the Lattice Mismatch. As described, the growth mode of bi-phasic BTM crystal can be treated as epitaxial growth on a substrate. To obtain the growth rate of the faces, the lattice mismatch was calculated. The lattice mismatch represents the differences in the attachment rates of the solute molecules into the kink sites. To investigate the relationship between α- and βBTM along axis-direction, the lattice mismatch was calculated in this study. The definition of the lattice mismatch can be expressed as61:

Page 2 of 11 δ=

as − ae ae

(1)

where as is the lattice parameter of the substrate without stress, ae represents the lattice parameter of the epitaxy, and δ is the lattice mismatch. The Computational Details of the Interfacial Binding Energy. The geometry optimization calculation was carried out based on density functional theory (DFT) using the Perdew–Burke– Ernzerhof (PBE) functional and the generalized gradient approximation (GGA) for the exchange correlation function, as implemented in the Vienna Ab-initio Simulation Package (VASP). A 1 × 2 × 1 supercell was used for the structural optimizations. The energy cutoff was set to 350 eV, and the convergence threshold was set to an energy of 10-2 eV and a force of 0.2 eV/Å. The atoms at the bottom of the bulk were fixed to simulate the actual reaction process of the crystal structure. The Brillouin zone integration used a 1 × 1 × 1 Monkhorst-Pack mesh for the geometry optimizations. The atoms in the structure of the grain boundary were rearranged through the DFT calculation. A new bipyramidal structure has been obtained. Then the final calculated structure in the form of CIF was shown in Supporting Information. Atomic Force Microscopy Measurements. A morphological analysis based on Atomic force microscopy (AFM) was used to further study the micromorphology and the growth mechanism of α- and β-BTM at the interface using a Nanoscope IIIa Multi-Mode AFM (Digital Instruments Co.). A high concentration, a low cooling rate and a short growth cycle were required to prepare the βBTM crystal, as the micromorphology and growth striation of the (100) plane of the crystal was more easily exposed with a short growth cycle. Scanning Electron Microscopy (SEM) and Laue BackDiffraction Measurements. SEM measurements at the grain boundary of the bi-phasic BTM crystal were performed on a Hitachi S-4800 ultrahigh resolution (UHR) field-emission (FE) SEM, and the images were obtained at an acceleration voltage of 5.0 kV. A Laue diffractometer manufactured by Multi-wire Laboratories with a W target was used to obtain Laue patterns at the grain boundaries. Agglomeration and a number of new diffraction points were observed at the interface in the SEM and the Laue back-diffraction patterns.

RESULTS AND DISCUSSION Controlled Growth of Polymorphic BTM based on the Solubility and Viscosity Curves of the Solution. The flux growth technique readily allows crystal growth at a temperature well below the melting point of the solute, produces euhedral characteristics and leads to a reasonably lower degree of dislocation density.52 Well-developed block-like single crystals of α- and βBTM with dimensions up to 78 × 65 × 43 mm3 and 54 × 53 × 47 mm3 were obtained by the top-seeded solution growth method, as shown in Figure 1(a) and (b), respectively. Both crystals exhibited good quality when the optimized TeO2-MoO3 flux was used. The sizes of the crystals were dependent on the solute amount and the growth period. In addition, the solubility curves of α- and β-BTM under identical conditions were obtained to investigate the growth characteristics of these two BTM crystals when changing the amount of polycrystalline BTM, as shown in Figure 2(a). Several intersections were observed in the solubility curves. α-BTM is a hightemperature phase, and β-BTM is a metastable phase at our selected growth concentration corresponding to the saturation temperature above 540 °C. A slower cooling rate and an invariable temperature environment, with dynamic adjustment of rotation speed, may be favorable to obtain the α-BTM crystal, whereas β-


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Figure 1. Photograph of the as-grown (a) α-BTM and (b) β-BTM crystals with b-axis-oriented seeds.

Figure 2. (a) The solubility curves of the bi-phasic BTM (α- and β-BTM) crystal in the flux system (TeO2:MoO3=1.2:1). (b) The viscosity measurements of β- and α-BTM in their growth concentrations.

Figure 3. The as-grown bi-phasic BTM crystal with two linked seed crystals oriented in certain directions: (a) α-BTM oriented along the [010]-aixs and β-BTM oriented along the [100]-axis and (b) α-BTM in the (204) plane and β-BTM in the (012) plane. BTM was easy to crystallize with a faster cooling rate and a certain temperature gradient environment according to our growth experience. High-viscosity solution do not facilitate crystal growth because of the time-consuming growth process and the possible inclusion of ionic impurities in the crystal. We found that lower concentrations and excess amounts of TeO2 can reduce the viscosity of the solution. Therefore, the mixture ratios of the TeO2-MoO3 flux were changed from 1.2:1 and 1:1 to 1.5:1 and 4:3 for α- and βBTM crystals, respectively. In Figure 2(b), the viscosity of the βand α-BTM crystals are similar from 460 °C to 600 °C, whereas the viscosity of both BTM crystals dramatically increase at temperatures below 490 °C. Therefore, the growth temperature should

be controlled at above 500 °C, because a higher viscosity adversely affects the growth process, lengthening the growth time. In addition, the formation of a large number of impurities can be caused by the higher precipitation rate of the solute during this growth process, which may lead to the distribution of impurities in the crystal or on the surface of the crystal. Meanwhile, a slower cooling rate is necessary for the growth of high-quality single crystals. The viscosity of α-BTM is marginally lower than that of β-BTM in the range of 500~600 °C, as shown in Figure 2(b). We speculated that the concentration selected for β-BTM is slightly lower than that for α-BTM, which means that the viscosity increased as the content of BaCO3 decreased. The phasecontrollable growth of high-quality α- and β-BTM can be



achieved based on the solubility and viscosity curves of the solution above 540 °C. The high-temperature phase α-BTM crystal crystallizes prior to that of the metastable phase in a constant temperature environment. In summary, an appropriate temperature ladder and appreciable fast cooling rate is advantageous for the formation of β-BTM. As a result, the controllable growth of αand β-BTM can be achieved. The theoretical calculations reveal that both phases exhibit similar indirect band gaps of 2.93 eV (β-BTM) and 3.12 eV (α-BTM), which agrees with the measured values in a previous paper.62 The α-BTM structure should be compared with that of β-BTM, as they belong to different space groups; however, their space groups are related by their crystallographic group-subgroup relations.40, 42 If it is assumed that ββ=90.897°≈90°, the “2” structure of β-BTM can be identified as belonging to the “pseudo mm2” point group, and thus, β-BTM can be treated as α-BTM. Interestingly, α- and β-BTM have the same 2θ (2θ=23.391°) and d-spacing (d=3.8) corresponding to the (204) and (012) lattice planes, which are perpendicular to the [010] and [100]-aixs of α- and β-BTM, respectively. A concentration was selected from the solubility curves to grow a bi-phasic BTM crystal in which the same saturation point would be obtained for both α- and β-BTM. In addition, gradually increasing cooling rate and a dynamic adjustment of the rotating speed during growth were adopted to stabilize the supersaturation for the crystallization of α- and β-BTM. Then, the previous rotation speed was dynamically adjusted during growth to ensure the desired amount of crystallization. Considering both the theoretical basis and analysis mentioned above, the controlled growth of the bi-phasic BTM crystal was achieved based on the appropriate cooling rate, thermal field, and solution composition, as shown in Figure 3. The Computation of the Lattice Mismatch and Interfacial Binding Energy of β- and α-BTM. A 2.28 % experimental lattice mismatch produces a coherent interface that corresponds to an in-direction epitaxial relationship between the [100]β-BTM and [010]α-BTM planes, which is in good agreement with the vertical relationship that they were perpendicular to the in-plane of (012)βBTM and (204)α-BTM, respectively. Moreover, the lattice mismatch in the other directions are much larger than 2.28 %, which would be inconducive to the formation of a coherent grain boundary. Perhaps this is the most possible motivation for forming the eutectic grain boundary along [100]β-BTM and [010]α-BTM. Furthermore, some crystal structures of the polymorphs are more energetically stable than their isolated molecules,63 which is beneficial for the phase-controlled growth of the polymorphs. In the computation of the interfacial binding energy, the calculated lattice parameters for α-BTM were selected to be a=34.8129 Å, b=11.518 Å, and c=36.9059 Å, whereas the calculated lattice parameters of β-BTM were a=35.0232 Å, b=11.224 Å, and c=36.6681 Å. The calculated lattice constant conforms to lattice matching requirements, which agrees with the experimental value. Figure 4 shows the calculated eutectic structure for the (012)β-BTM and (204)α-BTM planes. There are two different growth units at the eutectic grain layers based on the computation, which was performed using the following equation:

Ead (204−102) = Ecom − Eα − BaTeMo2O9 (204) − Eβ − BaTeMo2O9 (012)

(2)

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the interfacial binding energy of the bi-phasic BTM crystal was found to be approximately -7.99 eV from our DFT computations. This finding suggests that a new layer can easily form with the rearrangement of the exposed atoms in the α- and β-BTM crystals.64 Figure 4(b) shows the theoretical rearrangement of the eutectic layer in the bi-phasic BTM crystal obtained from the DFT computation (see the Supporting Information). In Figure 4(b), the surfactant MoO6 octahedra in the α- and β-BTM crystals were interconnected by corner sharing an O atom to form Mo2O11 chains running along the b direction of α-BTM and the a direction of β-BTM. These chains are linked into 2D layers by the O5MoO-MoO5 bipyramidal structure. The new rearrangement of the O5Mo-O-MoO5 octahedra constructed the new interfacial layer. In contrast, the connection of TeOx (x=3, 4) polyhedra was relatively strong without recombination. The Ba2+ cations found between the layers maintained charge balance without bonding with O2-, Te4+, or Mo6+ ions. As a result of the chemical bonds formed at the interface, the newly grown structures have a solid mechanical and electrical connection between the α- and β-BTM crystals. The Growth Mechanism of the Bi-Phasic BTM Crystal. The growth steps of β-BTM are shown in Figure 5. From the AFM images in Figure 5(a), the screw dislocation growth mechanism was confirmed in the (100) plane of β-BTM, and the perfect spiral growth hillock presents a rectangle with its b axis parallel to the longitudinal axial direction of the rectangle and its c axis parallel to the short axial direction. A certain radial direction along the b axis exhibits weak anisotropy, whereas the straight steps along the c axis display strong anisotropy, which implies that the [100] direction during crystal growth demonstrates anisotropy. The heights of the steps on either side of the dislocation source were 0.587 nm and 0.606 nm, as shown in Figures 5(c) and 5(d), respectively, which is in good agreement with the cell length, a=0.5535 nm. The mentioned above results indicate that growth unit oriented-[100] is a single cell. Frank et al. analyzed the effect of stress on the stability of a crystal based on the thermodynamics and proposed that a hollow core was present in the center of the spiral growth hillock with a sufficiently large Burgers vector during the formation of the screw dislocation.65 Then a hollow core centered in the growth hillock was observed in the (100) plane of the β-BTM crystal, which is shown in Figures 5(b). The 0.686 nm depth of the hollow core also agrees with the length of the cell parameter, a=0.5535 nm. The growth mechanism of β-BTM, which was in the form of a unit cell, provides reasonable motivation for the accumulation of β-BTM unit cells at the boundary of the as-grown α-BTM crystal. The Laue Diffraction and SEM of the BTM Crystal. In contrast to the Laue diffraction patterns of the α- and β-BTM crystals, the numerous new diffraction points observed in the Laue diffraction patterns of the bi-phasic BTM crystal indicate that a new type of structure was formed at the grain boundary (Figure 6(a-c)). The lower intensity in the diffraction patterns of the α- and β-BTM crystals may be caused by the arrangement of atoms at the grain boundary, which suggests poor-quality crystals. SEM measurements were used to further characterize the grain boundary of the bi-phasic BTM crystal. In Figures 6(d) and 6(e), the grain boundary was clean and visible, with some adhesions at the grain boundary. In summary, we speculated that the α- and β-BTM crystals grew together rather than being simply attached to each other by the flux.

where Ecom, Eα-BTM (204) and Eβ-BTM (012) are the total binding energy of the bi-phasic BTM crystal, the (204) planes of α-BTM and the (012) planes of β-BTM, respectively. The interfacial binding energy was calculated using the energy of the ion-electron system based on DFT. The total binding energy in bi-phasic BTM was approximately -4656.02 eV, and -2401.36 eV and -2246.67 eV were obtained for α-BTM and β-BTM, respectively. Additionally,


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Figure 4. The structure and morphology of the bi-phasic BTM crystals. (a) Schematic illustration of the growth of the bi-phasic BTM crystal. (b) The structure of α- and β-BTM crystals. (c) The structure of the eutectic grain boundary obtained from the computation.

Figure 5. The AFM morphology analysis in the (100)-plane of the β-BTM crystal with a high concentration, low cooling rate and short growth cycle. (a) Screw dislocation growth hillock, (b) A hollow core of the β-BTM crystal, (c) The heights steps of 0.587 nm on one side, (d) The steps heights of 0.587 nm on either side of the dislocation source.



Page 6 of 11

Figure 6. Laue patterns of β-BTM: (a) Laue pattern of β-BTM, (b) Laue pattern of the grain boundary, and (c) Laue pattern of α-BTM. (d) The photograph of the grain boundary in dual-phasic BTM (e) SEM images of the grain boundary in the bi-phasic BTM crystals grown by the flux method.

Figure 7. The atomic structure of the α-BTM (Pca21) and β-BTM (P21) crystals at the interface.

Figure 8. A model of the molecular arrangement at the interface of the bi-phasic BTM crystal. “Pseudo-liquid-phase epitaxy” growth model of the biphasic BTM crystal. In liquid-phase epitaxy (LPE),66-71 growth occurs through the addition of new molecules at the step edges, which usually generates microstructures of different morphologies,

such as steps, islands, spirals or pyramids, which influence the physical properties of the layers.72-76 However, flux growth involves a high-temperature melt of an inorganic compound used as the solvent for crystallization, which induces nucleation by the seed crystal.61 Usually, combinations of inorganic compounds in the flux are used to form an even lower melting eutectic, reducing the melting point of the high-temperature solution.61 Compared with the growth mechanism of LPE, the flux growth mechanism can be viewed as “pseudo-liquid-phase epitaxial growth” simultaneously combined with the crystallization mechanism of β-BTM. In the formation of the bi-phasic BTM crystal, α-BTM starts to grow prior to the start of β-BTM growth when lower supersaturation is achieved by cooling the molten homogeneous solution. Immediately, the β-BTM particles are precipitated out from the highly supersaturated liquid solution on the as-grown surface of α-BTM, which acts like a substrate with increasing supersaturation. During this time, the α-BTM surface can be treated as non-homogeneous or even as non-continuous epitaxial layers containing crystallographic defects, resulting in the driving force


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for β-BTM growth. These crystals belong to different space groups; however, their space groups are related by their crystallographic group-subgroup relations. Since the combined growth mechanism was confirmed, the structural model at the interface is suggested to be as follows. A “virtual supercell” consisting of four lattices of β-BTM was introduced to account for the contact surface of the two components based on the relationship of the cell parameters of the two polymorphs. Specifically, four cells of βBTM are equivalent to one cell of α-BTM, as shown in Figure 7. The four cells of β-BTM can be viewed as a “super-lattice” deposited at the uneven interface of the as-grown α-BTM, which formed first because it is the high-temperature phase. The transport of the “growth unit” in the interfacial layer was achieved by a convection-diffusion process. A virtual cell is located along the interface, allowing for contact of the crystalline lattices, forming a quasi-coherent boundary. Accordingly, an uneven step interface, as shown in Figure 8, was formed.

for the help of the measurement of the Atomic Force Microscope of the β-BTM.

REFERENCES (1) Threlfall, T. L. Analysis of Organic Polymorphs: A Review. Analyst. 1995, 120, 2435-2460. (2) Desiraju, G. R. Cryptic crystallography. Nat. Mater. 2002, 1, 7779. (3) Desiraju, R. G. Crystal engineering: A brief overview. J. Chem. Sci. 2010, 122, 667-675. (4) David, F.; Labes, M. M.; Weissberger, A. Physics and Chemistry of the Organic Solid State; Interscience Publisher: New York, USA, 1963-1967.

CONCLUSIONS

(5) Dunitz, J. D. Phase transitions in molecular crystals from a chemi-

In summary, based on the control of the solubility and solution viscosity of α- and β-BTM crystals, the controlled growth of two polymorphic high-quality BTM crystals was successfully achieved. A bi-phasic BTM crystal with linked seed crystals was successfully obtained by using the concentration at the intersection of the solubility curves at 540 °C. The Laue diffraction and SEM patterns of the grain boundary, a lattice mismatch of 2.28 %, and the interfacial binding energy of -7.99 eV demonstrated that the two polymorphic phases were partly grown together rather than attached to each other by the flux. Rearrangement of the atoms showed the MoO6 octahedra were interconnected by corner sharing O atoms to form O5Mo-O-MoO5 di-octahedrons along the eutectic grain boundary. A new growth model for the “pseudoliquid-phase epitaxial” growth was determined to illustrate the growth mechanism of the bi-phasic BTM crystal. These investigations of the eutectic grain boundary provide a better understanding of the boundaries of such eutectic materials, which may be some interesting future applications in the heterojunction.

cal viewpoint. Pure Appl. Chem. 1991, 63, 177-185. (6) Rao, C. N. R.; Rao, K. J. Phase Transitions in Solid, McGrawHill: New York, USA, 2nd edn, 1978. (7) Yu, L. Polymorphism in Molecular Solids: An Extraordinary System of Red, Orange, and Yellow Crystals. Acc. Chem. Res. 2010, 43, 1257-1266. (8) Llinàs, A.; Goodman, J. M. Polymorph control: past, present and future. Drug Discov. Today 2008, 13, 198-210. (9) Zerkowski, J. A.; MacDonald, J. C.; Whitesides, G. M. Polymorphic Packing Arrangements in a Class of Engineered Organic Crystals. Chem. Mater. 1997, 9, 1933-1941. (10) Cruz-Cabeza, A. J.; Bernstein, J. Conformational Polymorphism. Chem. Rev. 2014, 114, 2170−2191.

ASSOCIATED CONTENT Supporting Information

(11) Brittain, H. G.; Fiese, E. F. Effect of pharmaceutical processing

Crystallographic information (CIF file) for the contact of the grain boundary in the bi-phasic BTM. The Supporting Information is available free of charge on the ACS Publications website.

in Pharmaceutical Solids, Drugs and the Pharmaceutical Science,

on drug polymorphs and solvates, in: H. Brittain (Ed), Polymorphism

Marcel Dekker, New York, USA, 1999. (12) Chemburkar, S. R.; Bauer, J.; Deming, K.; Spiwek, H.; Patel, K.; Morris, J.; Henry, R.; Spanton, S.; Dziki, W.; Porter, W.; Quick, J.;

AUTHOR INFORMATION

Bauer, P.; Donaubauer, J.; Narayanan, B. A.; Soldani, M.; Riley, D.;

Corresponding Author

McFarland, K. Dealing with the impact of ritonavir polymorphs on

*E-mail: [email protected]; [email protected]

the late stages of bulk drug process development. Org. Proc. Res.

Author Contributions

Dev. 2000, 4, 413–417.

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

(13) Bugay, D. E. Characterization of the solid-state: spectroscopic

Funding Sources Notes

(14) Kulkarni, S. A.; McGarrity, E. S.; Meekes, H.; Horst, J. H. Ison-

The authors declare no competing financial interest.

techniques. Adv. Drug Deliv. Rev. 2001, 48, 43–65.

icotinamide self-association: the link between solvent and polymorph nucleation. Chem. Commun. 2012, 48, 4983–4985.

ACKNOWLEDGMENT

(15) Huang, J.; Chen, S.; Guzei, I. A.; Yu, L. Discovery of a Solid

The authors gratefully acknowledge financial support by the National Natural Science Foundation of China (Grant No. 51321091, 51772170, 21573129), National Key Research and development Program of China (Grant No. 2016YFB1102201), The Program of Introducing Talents of Disciplines to Universities in China (111 Program No. b06015). We also thank Dr. Junjie Zhang for the help with the solubility of the BTM solution; Dr. Weiguo Zhang

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For Table of Contents Use Only Polymorphic Smooth Interfaces Formation Based on the Bi-Phasic BaTeMo2O9 Using Top Multi-Seeded Growth Qian Wu, Zeliang Gao*, Haoyuan Wang, Conggang Li, Weiqun Lu, Xiangxin Tian, Youxuan Sun, Na Lin, Shengqing Xia, and Xutang Tao* State Key Laboratory of Crystal Materials & Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan 250100, P. R. China CORRESPONDING AUTHOR EMAIL ADDRESS: [email protected]

High quality and large-sized single phase BaTeMo2O9 crystals and bi-phase BaTeMo2O9 crystal using the top multiple-seeded growth method were successfully grown and the grain boundary were investigated. The interfacial binding energy of -7.99 eV demonstrated that the eutectic grain boundary were interconnected by rearrangement of O5Mo-O-MoO5 di-octahedrons. Then a “pseudo-liquid-phase epitaxial” growth was determined to illustrate the growth mechanism of the bi-phasic BaTeMo2O9 crystal based on the Atomic Force Measurement.

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