Polymorphism of BaTeMo2O9: A New Polar Polymorph and the Phase

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Polymorphism of BaTeMo2O9: A New Polar Polymorph and the Phase Transformation Junjie Zhang, Zhonghan Zhang, Weiguo Zhang, Qingxin Zheng, Youxuan Sun, Chengqian Zhang, and Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China

bS Supporting Information ABSTRACT: A new polar polymorph of BaTeMo2O9, R-BaTeMo2O9, has been synthesized by spontaneous crystallization with molten flux based on the TeO2MoO3 solvent. The two polymorphous phases of BaTeMo2O9 are referred to as β- and R-BaTeMo2O9 according to their crystallization temperature from high to low. The structure of R-BaTeMo2O9 contains two-dimensional (Te2Mo4O18)4 anionic layers interleaved with Ba2+ cations. Each anionic layer is composed of Mo4O20 tetramers connected to TeOx (x = 3, 4) polyhedra. The Mo6+ and Te4+ are in asymmetric coordination environments attributable to second-order JahnTeller (SOJT) effects. Second-harmonic generation (SHG) measurements using 1064 nm radiation show that R-BaTeMo2O9 is phasematchable. Polarization measurements indicate that R-BaTeMo2O9 is not ferroelectric, i.e., the polarization is not “switchable”. In addition, the powder X-ray patterns of the BaTeMo2O9 samples obtained from solid-state reactions revealed that BaTeMo2O9 undergoes an irreversible phase transition from β-BaTeMo2O9 to R-BaTeMo2O9 at ∼570 °C. BaMoO4 is found to lower the energy barrier and stimulate the transformation. Furthermore, structural distortions of both polymorphs were calculated to gain a better understanding of the structureproperty relationships. Crystal data: R-BaTeMo2O9, orthorhombic, space group Pca21 (No. 29), a = 14.8683(2) Å, b = 5.6636(1) Å, c = 17.6849(3) Å, V = 1489.21(4) Å3, and Z = 8. KEYWORDS: polymorphism, phase transformation, noncentrosymmetric (NCS), polar materials, second-order JahnTeller (SOJT) effects

’ INTRODUCTION During the past few decades, considerable interest has focused on the design and synthesis of noncentrosymmetric (NCS) and polar materials, especially inorganic oxides because of their technologically important functional properties, such as piezoelectricity, pyroelectricity, ferroelectricity, and second-order nonlinear optical (NLO) activity.1 In searching for new NCS and polar oxides, effective asymmetric building units are often utilized.2 These units contain distorted MO6 octahedra (M = d0 transition metal ions, e.g., Mo6+, W6+, V5+, and Nb5+),3 asymmetric AOxE polyhedra (A = cations with stereochemically active lone pairs, e.g., I5+, Te4+, Se4+, and Pb2+, and E = lone pair),4 d10 cations (e.g., Zn2+ and Sn4+) with large polar displacement,5 and NCS borate π-orbital systems [e.g., (BO3)3, (B3O6)3, and (B3O7)5].6 With respect to the MO6 octahedra and AOxE polyhedra, second-order JahnTeller (SOJT) effects have been invoked to explain the local polar coordination environments.7 Polar oxides obtained using the above-mentioned strategies include K3V5O14,3b Ag2Mo3Te3O16,3c Na2Te3Mo3O16,3g BaTeMo2O9 (hereafter β-BaTeMo2O9),3h Pb3SeO5,4b ZnSnO3,5b Cd4BiO(BO3)3,6b Pb2B5O9I,6c and Cs2TeMo3O12.8 In each of these compounds, the local polarizations “constructively” add, resulting in a macroscopically polar material. To measure the direction-dependent physical properties such as piezoelectricity and nonlinear optical properties and fully r 2011 American Chemical Society

understand the structureproperty relationships, bulk single crystals of β-BaTeMo2O9,9 Na2TeW2O9,10 and Cs2TeMo3O1211 have been grown successfully using the top-seeded solution growth (TSSG) method. Our group’s previous contribution to the growth and characterization of β-BaTeMo2O9 has shown that it is not only a potential NLO crystal in mid-IR, but also a very promising piezoelectric crystal at room temperature.9,12 To make devices such as optical parametric oscillators (OPO) and crystal oscillators, large-scale and high-quality single crystals are needed. In the course of growing such β-BaTeMo2O9 single crystals, we discovered another novel polar polymorph of BaTeMo2O9: R-BaTeMo2O9. Polymorphism13 is of fundamental significance in understanding crystal packing, crystal structure prediction, and the mechanisms of crystallization.14 Although polymorphism has been extensively explored in many inorganic materials, examples of polymorphism in inorganic oxides containing cations susceptible to SOJT effects are limited. Until now, only a few have been reported, including R-Cs2I4O114c and β-Cs2I4O11,15 R-KMoO3(IO3)16 and β-KMoO3(IO3),17 R-Cu(VO)(SeO3)218 and β-Cu(VO)(SeO3)2,18 and R-KVO2(IO3)2(H2O)19 and β-KVO2(IO3)2(H2O);19 however, Received: May 27, 2011 Revised: July 12, 2011 Published: July 25, 2011 3752

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Chemistry of Materials

Figure 1. Photograph of R-BaTeMo2O9 crystals.

there have been no reports about their phase transformations. In addition, few reports are found about the polymorphism and phase transformations of polar oxides containing cations susceptible to SOJT effects. We deem that it is very important and interesting to study the polymorphism in BaTeMo2O9 as well as its phase transformations, which may provide new insight into controlling of polymorphism. In this contribution, we report the synthesis, structure, characterization, and functional properties of R-BaTeMo2O9. The phase transformations between R- and β-BaTeMo2O9 are investigated by combination of powder X-ray diffraction, differential thermal analysis (DTA), thermogravimetric analysis (TGA), and isothermal annealing. The structureproperty relationships of both polymorphs are also discussed.

’ EXPERIMENTAL SECTION Synthesis. Polycrystalline samples of R-BaTeMo2O9 were synthesized by traditional solid-state reaction techniques. Suitable stoichiometric ratios of BaCO3 (Tianjin XinChun Chemical Reagent Research Institute, 99.99%), TeO2 (Sinopharm Chemical Reagent Co., Ltd., 99.99%), and MoO3 (Sinopharm Chemical Reagent Co., Ltd., 99.5%) were thoroughly ground and then packed into columns. The columns were heated in air from room temperature to 580 °C at a rate of 20 °C/min and allowed to dwell at the temperature for 24 h, and then quenched in air. The solid was ground, packed, and sintered thrice using the procedures mentioned above. The phase purity of the resultant yellow solid was confirmed by powder X-ray diffraction. To study the effect of reaction conditions on polymorph formation, a series of experiments at different reaction temperatures (550590 °C) and reaction times (0.572 h) were conducted. Crystal Growth. Millimeter-sized single crystals of R-BaTeMo2O9 were prepared by flux method (shown in Figure 1). A Pt crucible containing BaCO3, TeO2, and MoO3 in the molar ratio of 1:3.4:4 was placed in the center of a vertical, programmable temperature furnace. The mixture was heated to 650 °C and held for 3 days in order to melt the powders into a homogeneous liquid solution. A Pt wire was dipped into the solution, and the temperature was decreased from 650 to 540 °C at a rate of 1 °C/h. This resulted in high-quality crystals grown by spontaneous nucleation along the Pt wire during the slow cooling process. Powder X-ray diffraction confirmed that the crystals were pure R-BaTeMo2O9. Single-Crystal X-ray Diffraction. A colorless prism-shaped crystal of R-BaTeMo2O9 (0.14 mm 0.13 mm 0.11 mm) was used for single-crystal X-ray data collection. Data were collected using a

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Bruker SMART APEX-II diffractometer equipped with a CCD area detector using graphite-monochromated Mo KR radiation (λ = 0.71073 Å). Preliminary lattice parameters and orientation matrices were obtained from three sets of frames. Data integration and cell refinement were done by the INTEGRATE program of the APEX2 software20 and multiscan absorption corrections were applied using the SCALE program for area detector.20 The program XPREP21 provided two possible space groups: Pca21 (No. 29) and Pbcm (No. 57). The centrosymmetric space group Pbcm (No. 57) was ruled out because of the second-order nonlinear activity [when R-BaTeMo2O9 samples were irradiated by a Q-switched Nd:YAG solid-state laser (1064 nm, 10 kHz, 10 ns), obvious green light was observed]. The structure was solved in the space group Pca21 (No. 29) by direct methods and refined by full matrix least-squares methods on F2 using SHELX.21 All of the atoms in the structure were refined with anisotropic thermal parameters, and the refinements converged for I > 2σ (I). All calculations were performed using the SHELXTL crystallographic software package.21 A symmetry analysis on the model using PLATON22 revealed that no obvious space group change was needed. Details of crystal parameters, data collection, and structure refinement are listed in Table 1. The final refined atomic coordinates and isotropic thermal parameters are summarized in Table S1 of the Supporting Information. Selected bond distances (Å) and angles (deg) are given in Table S2 of the Supporting Information. Additional information in the form of CIF has also been supplied as Supporting Information, and the CIF has been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany [Fax: (49) 7247808666. E-mail: [email protected]], depository number CSD 422545. Powder X-ray Diffraction. The powder X-ray diffraction (PXRD) data were collected using a Bruker-AXS D8 ADVANCE X-ray diffractometer at room temperature (Cu KR radiation). Data were collected in the 2θ range of 1575° with a step size of 0.02° and a step time of 3 s. Elemental Analysis. The elemental mass percentage of the barium, tellurium, and molybdenum in R-BaTeMo2O9 was detected by using energy dispersive X-ray (EDX) analysis (Analytical UHR FESEM SU-70, Hitachi High-Technologies Corp.). Found: Ba, 24.53%; Te, 21.37%; Mo, 36.10%. The elemental analysis was also performed using a ZSX Primus II X-ray fluorescence spectrometer (Rigaku Americas Instrument). The mass percents for barium, tellurium, and molybdenum were 23.60%, 21.07%, and 31.52%, respectively. The calculated values for R-BaTeMo2O9: Ba, 22.86%; Te, 21.24%; Mo, 31.94%. Infrared and Raman Spectroscopy. The infrared spectra of Rand β-BaTeMo2O9 were recorded on a Matteson FTIR 5000 spectrometer in the range 4004000 cm1 range, with the sample mixed with dry KBr and pressed into a pellet. The Raman spectra were recorded at room temperature on a Thermo-Nicolet NEXUS 670 spectrometer equipped with an InGaAs detector with the powder sample placed in a capillary tube. Excitation was provided by an Nd:YAG laser at a wavelength of 1064 nm, and the output laser power was 1.000 W. The spectral resolution was 4.000 cm1, and 64 scans were collected. UVVis Diffuse Reflectance Spectroscopy. UVvisible diffuse reflectance data for R- and β-BaTeMo2O9 were collected by a Shimadzu UV 2550 recording spectrophotometer equipped with an integrating sphere over the spectral range 200800 nm. BaSO4 was used as a reference material. Reflectance spectra were converted to absorbance with the KubelkaMunk function.23 Thermal Stability. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) for R- and β-BaTeMo2O9 were carried out on a Diamond TG/DTA analyzer (Perkin-Elmer Instruments). The samples were placed in a platinum crucible and heated at a rate of 10 °C/min from room temperature to 650 °C under flowing nitrogen. After the DTA/TGA measurement, powder X-ray diffraction was taken of the residue in the platinum pan. Specific heat of the 3753

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Table 1. Crystallographic Data and Structure Refinement for r-BaTeMo2O9 and β-BaTeMo2O9a empirical formula

R-BaTeMo2O9

β-BaTeMo2O9

fw (g/mol)

600.82

temperature (K)

293(2) Mo KR, λ = 0.71073 Å

radiation, wavelength cryst syst. space group

orthorhombic Pca21 (No. 29)

monoclinic P21 (No. 4)

unit cell dimens (Å)

a = 14.8683(2)

a = 5.5346(1)

b = 5.6636(1)

b = 7.4562(1)

c = 17.6849(3)

c = 8.8342(1) β = 90.897(1)

a

unit cell volume (Å3)

1489.21(4)

Z, Dcalcd (g/cm3)

8, 5.360

2, 5.474

abs coeff (μ) (mm1) F(000)

12.411 2112

12.676 528

cryst dimens (mm3)

0.14 0.13 0.11

0.11 0.10 0.09

2θmax

74.30°

74.24°

limiting indices

24 eh e 23, 7 ek e 9, 21 el e 29

9 eh e 9, 11 ek e 11, 13 el e 14

364.517(9)

no. reflns collected/unique

14719/5906

6765/3045

R(int)

0.0211

0.0170

abs correction

semiempirical from equivalents

refinement method data/restraints/param

5906/1/236

full-matrix least-squares on F2 3045/1/119

GOF on F2

1.107

1.160

R indices [I > 2σ (I)]b

R1 = 0.0205, wR2 = 0.0383

R1 = 0.0162, wR2 = 0.0360

R indices (all data)

R1 = 0.0220, wR2 = 0.0387

R1 = 0.0163, wR2 = 0.0361

flack param

0.064(16)

0.031(10)

extinction coeff

0.00233(4)

0.0305(6)

largest diff. peak and hole (e Å3)

1.298 and 0.948

1.406 and 1.153

From ref 9b. b R1 = ∑ ||Fo| |Fc||/∑ |Fo|, wR2 = {∑ [w(Fo2 Fc2)2]/∑ [w(Fo2)2]}1/2

β-BaTeMo2O9 single crystal was measured by the method of differential scanning calorimetry using a simultaneous thermal analyzer (TGA/DSC1/1600HT analyzer) made by Mettler Toledo Instruments in the temperature range between 545 and 575 °C at a heating rate of 5 °C/min. Second Harmonic Generation Measurement. Powder secondharmonic generation (SHG) measurements were performed on a modified KurtzNLO24 system. The SHG efficiency of the powder sample has been shown to depend strongly upon particle size, and thus, polycrystalline samples of R-BaTeMo2O9, β-BaTeMo2O9, and KDP were ground and sieved into distinct particle size ranges (2545, 4562, 6275, 75109, 109150, 150212, and 212250 μm). The sieved KDP powders were used as a reference. The samples were pressed between glass microscope cover slides and secured with tape in 1 mm thick aluminum holders containing an 8 mm diameter hole. They were then placed in a lighttight box and irradiated with a pulsed infrared beam (10 ns, 3 mJ, 10 Hz) from a Q-switched Nd:YAG laser of wavelength 1064 nm. A cutoff filter was used to limit background flash-lamp light on the sample, and an interference filter (530 ( 10 nm) was used to select the second harmonic for detection with a photomultiplier tube attached to a RIGOL DS1052E 50-MHz oscilloscope. No index-matching fluid was used in any of the experiments. Polarization Measurement. The polarization was performed on a Radiant Technologies Precision Premier II Ferroelectric Test System with a TREK high voltage amplifier. The samples were pressed into pellets (∼13 mm diameter, ∼1 mm thick) and sintered at 300 °C, well below the decomposition temperatures, for 10 h for both polymorphs. Silver paste was applied to both sides as electrodes and cured at 200 °C for 3 h. To measure the potential ferroelectric behavior, polarization

measurements were done at room temperature under a static electric field of 15 kV/cm at various frequencies (1, 10, 100, and 1000 Hz).

’ RESULTS AND DISCUSSION Polymorphism and Phase Transformation. Polycrystalline BaTeMo2O9 samples were synthesized using traditional solid-state reaction techniques. The room temperature powder X-ray diffraction patterns of the resultant solids obtained at various temperatures (550590 °C) are shown in Figure 2. In addition to the reactions at 550 and 590 °C used to prepare pure samples of β-BaTeMo2O9 and R-BaTeMo2O9, respectively, reactions at 560, 570, and 580 °C were carried out to establish the role of the reaction temperature in the formation of the two phases. In the 72 h solid-state reactions, temperatures below 570 °C resulted in product of β-BaTeMo2O9, while higher temperatures led to product of R-BaTeMo2O9. At the temperature of 570 °C, the majority of the product was β-BaTeMo2O9; however, a small amount of R-BaTeMo2O9 emerged. The appearance of R-BaTeMo2O9 at 570 °C suggests an existence of a phase transformation from β-BaTeMo2O9 to R-BaTeMo2O9. To confirm the existence of the phase transformation, a series of reactions were carried out at 585 °C with different reaction times from 0.5 to 72 h. The PXRD patterns were measured on each obtained sample, and the results are shown in Figure 3. In the early stage (0.5 h), large proportions of β-BaTeMo2O9 and small amounts of R-BaTeMo2O9, BaMoO4, and Te-containing amorphous products formed. With increasing 3754



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Figure 2. PXRD traces showing the influence of reaction temperature on polymorph formation. Note that in the 72 h solid-state reactions, temperatures below 570 °C resulted in product of β-BaTeMo2O9, while higher temperatures led to product of R-BaTeMo2O9. At the temperature of 570 °C, the majority of the product was β-BaTeMo2O9; however, a small amount of R-BaTeMo2O9 emerged.

Figure 3. Thermal evolution of the PXRD patterns showing the role of reaction time (0.572 h) in the progression of the phase transformation from β-BaTeMo2O9 to R-BaTeMo2O9. (/ = β-BaTeMo2O9; pink diamond = R-BaTeMo2O9; blue circle = BaMoO4).

Figure 4. DTA and TGA data for R-BaTeMo2O9. Extrapolated onset temperature is indicated. Note that there is one endothermic peak on the heating curve of DTA and no weight loss on the curve of TGA.

reaction time, the proportions of β-BaTeMo2O9 and BaMoO4 decreased, and correspondingly, those of the R-BaTeMo2O9 increased. In the end, all the β-BaTeMo2O9 and BaMoO4 disappeared and pure R-BaTeMo2O9 was obtained. This demonstrated that, during the reactions, the monoclinic β-BaTeMo2O9 forms first and transforms to the orthorhombic R-BaTeMo2O9. To better understand the results of the X-ray measurements, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed on the two polymorphs. For

R-BaTeMo2O9, as shown in Figure 4, only one clear endothermic peak at T = 592.68 °C is observed from the DTA heating curve. In addition, there is no weight loss in the TGA curve. These results indicate that R-BaTeMo2O9 potentially melts congruently. However, powder X-ray diffraction data of the residue in the platinum pan after DTA and TGA measurements demonstrated that R-BaTeMo2O9 decomposed to BaMoO4 as well as some unidentified amorphous products (Figure S1 of the Supporting Information). Thus, R-BaTeMo2O9 melts incongruently. Similar to the result of R-BaTeMo2O9, no phase transition endothermic peak was detected on β-BaTeMo2O9, which matches well with the reference.9a To further confirm the conclusion, the temperature variation 10 °C/min was decreased to 1 °C/min, and still no phase transition endothermic peak was observed from the DTA curves of both R- and β-BaTeMo2O9. As discussed above, evidence of the solidsolid transformation between the two polymorphs was not observed through DTA. There are two possible reasons: (i) such a transition would be much too slow, just like the case of BBO,25 and it cannot be detected in such a short time through DTA, and (ii) the phase transition belongs to second order phase transition. Further investigation indicates that there is no discontinuity on the heat capacity versus temperature curve of the β-BaTeMo2O9 crystal around 570 °C, from which the possibility of the second-order phase transition can be precluded (Figure S2 of the Supporting 3755



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Figure 5. Ball-and-stick diagram of R-BaTeMo2O9 in the ac-plane.

Information). Isothermal annealing experiments were carried out on the β-BaTeMo2O9 powders in the R-BaTeMo2O9 stable temperature range to see whether the phase transformation from β-BaTeMo2O9 to R-BaTeMo2O9 can occur. Unexpectedly, the solid-state phase transformation was not observed (Table S3 of the Supporting Information), which seems to contradict the result of solid-state reaction where monoclinic β-BaTeMo2O9 transforms to orthorhombic R-BaTeMo2O9 in the R-BaTeMo2O9 stable temperature range. However, it is worth noting that there are small amounts of R-BaTeMo2O9, BaMoO4, and Te-containing amorphous products besides β-BaTeMo2O9 in the 585 °C reaction within a short time ( 0.8; following the criteria proposed by the author, these figures correspond to strong distortion. Table 2 summarizes the direction and magnitude of the Mo6+ distortions. It is of interest to note that the magnitudes of out-of-center distortion of MoO6 octahedra in R-BaTeMo2O9 are all larger than those of β-BaTeMo2O9, which might lead to a stronger SHG response of R-BaTeMo2O9 than that of β-BaTeMo2O9. 3757



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Table 2. Direction and Magnitude of the SOJT Distortion of the MoO6 Octahedra and TeOx (x = 3, 4) Polyhedra in R- and β-BaTeMo2O9 octahedron distortion, Δd

dipole moment magnitude

compd.

species

R

MoO6

β

direction

magnitude

x (a)

y (b)

z (c)

debye

104 esu 3 cm/Å3

C3[111]

1.385

0.6953

6.7558

2.5942

7.2701

390.55

C3[111]

1.432

2.0139

6.6993

3.3143

7.7409

415.84

C3[111] C3[111]

1.379 1.464

1.0903 1.9837

6.2860 7.1244

2.7052 3.4081

6.9297 8.1429

372.26 437.44

TeO4

2.9017

11.0592

4.6579

12.3460

663.22

TeO3

2.2219

9.1377

4.8141

10.5646

567.53

unit Cell

0

0

0.556

3.73

MoO6

C3[111]

1.258

2.0442

0.7697

5.2901

5.6937

312.39 361.71

C3[111]

1.276

5.5516

2.5841

2.3568

6.5925

TeO4

9.2109

5.6026

1.3911

10.8889

597.44

unit Cell

0

14.834

0

406.95

Figure 9. UVvis diffuse reflectance spectroscopy plots for R- and βBaTeMo2O9. Band gaps are ∼3.12 eV (R-BaTeMo2O9) and ∼2.95 eV (β-BaTeMo2O9), respectively.

Figure 8. Ball-and-stick representation of the MoO6 and TeOx (x = 3, 4) polyhedral. Dark blue and green arrows indicate the approximate direction of the dipole moments for MoO6 and TeOx (x = 3, 4) polyhedra, respectively. Black arrows indicate the approximate direction of the net dipole moments for Mo4O20 tetramer. The red arrow represents the direction of the net dipole moment for R-BaTeMo2O9.

The direction and magnitude of the distortion in MoO6 octahedra and TeOx (x = 3, 4) polyhedra may also be quantified by determining the local dipole moments.27 This method uses a bondvalence approach to calculate the direction and magnitude of the dipole moments. As to the TeOx (x = 3, 4) polyhedra, the lone pair is given a charge of 2 and localized 1.25 Å from the Te4+ cation.28 Using this methodology, we were able to calculate the dipole moment of MoO6 octahedra and TeOx (x = 3, 4) polyhedra in R- and β-BaTeMo2O9. As shown in Table 2, the magnitudes of dipole moment of MoO6 octahedra in R-BaTeMo2O9 are considerably larger than those in β-BaTeMo2O9. This is in excellent agreement with the result of the out-of-center distortion calculation. The direction of the dipole moment of the

TeOx (x = 3, 4) polyhedra is estimated to be in the opposite direction of the stereoactive lone pair.27a,b Noticeably, the magnitude of the dipole moment of TeO4 polyhedra in R-BaTeMo2O9 is also much larger than that in β-BaTeMo2O9. The magnitude of dipole moment of TeOx (x = 3, 4) polyhedra in R-BaTeMo2O9 is remarkably larger than the corresponding average value27b (8.67 debye for TeO3 and 8.57 debye for TeO4). However, β-BaTeMo2O9 has a larger net dipole moment by 2 orders of magnitude than that of R-BaTeMo2O9. As shown in Figure 8, various dipole moments point in a nearly antiparallel manner in R-BaTeMo2O9, while in β-BaTeMo2O9 the individual bond dipoles align in an adductive manner. These distinct arrangements of dipole moment might lead to a weak SHG response of R-BaTeMo2O9 but a strong one of β-BaTeMo2O9, which is well consistent with our SHG measurements (see the Functional Properties section). Spectroscopic Studies. The infrared and Raman spectra of R-BaTeMo2O9 are similar to those of β-BaTeMo2O9 (Figures S5 and S6 of the Supporting Information). They reveal TeO, MoO, and TeOMo vibrations between 400 and 1000 cm 1 in both the IR and Raman spectra. The MoO vibrations are observed between 800 and 930 cm1. The TeO 3758



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Figure 10. (a) Phase-matching, that is, SHG intensity vs particle size curve for R-BaTeMo2O9. The curve drawn is to guide the eye and is not a fit to the data. (b) Oscilloscope traces of the SHG signals for the powder (109150 μm) of KDP and R- and β-BaTeMo2O9.

vibrations are observed between 600 and 800 cm1. The bands occurring from 400 to 600 cm1 may be assigned to MoO, TeO, and TeOMo stretches. These assignments are in agreement with the previous report.3c,g,h The UVvis diffuse reflectance spectra for R- and β-BaTeMo2O9 are shown in Figure 9. R-BaTeMo2O9 exhibits an energy band gap of 3.12 eV, while β-BaTeMo2O9 has a band gap of 2.95 eV. This result is in agreement with the fact that R-BaTeMo2O9 single crystal is colorless and that β-BaTeMo2O9 single crystal is brown. In addition, it might be expected that the single crystal of R-BaTeMo2O9 has a UV absorption edge less than 400 nm. Functional Properties: Second-Harmonic Generation (SHG) and Polarization Measurements of r-BaTeMo2O9. The noncentrosymmetric and polar crystal structure of RBaTeMo2O9 prompts us to measure its second-order NLO properties. Panel (a) of Figure 10 shows the curve of the SHG signal intensity versus particle size for ground R-BaTeMo2O9 crystals. As is shown, the second-harmonic intensity increases with particle size and plateaus at a maximum value, which indicates that the material is Type I phase-matchable. Comparison of the second-harmonic signal produced by R-BaTeMo2O9 sample with KDP sample in the same particle range from 109 to 150 μm reveals that R-BaTeMo2O9 exhibits a very limited SHG response of about 0.2 KDP (under the same conditions, βBaTeMo2O9 exhibits very large SHG response of about 2.5 KDP), as shown in panel (b) of Figure 10. According to Chen’s anionic group theory, the SHG response in a material is attributable to the anionic group.29 For R-BaTeMo2O9, the distorted MoO6 octahedra and TeOx (x = 3, 4) polyhedra are responsible for its large local SHG effects; however, the arrangement of the fundamental building blocks (MoO6 octahedra and TeOx (x = 3, 4) polyhedra) is in an unfavorable manner so that the SHG coefficients are almost canceled (Table 2 and Figure 8). Consequently, the overall SHG efficiency of R-BaTeMo2O9 is merely about 0.2 KDP and remarkably smaller than that of β-BaTeMo2O9. Because both polymorphs are not only noncentrosymmetric but also polar, they possess a permanent dipole moment indicating the possibility of ferroelectric behavior. Although “hysteresis” loops are measured (shown in Figure S7 of the Supporting Information),30 the materials are not ferroelectric. In other words, for both R- and β-BaTeMo2O9, the macroscopic polarization cannot be reversed under an external electric field. To understand the irreversibility, the local polarizations in the

two polymorphs need to be examined. The macroscopic polarity in both polymorphs is attributable to the local polarity in the MoO6 octahedra and TeOx (x = 3, 4) polyhedra. For the d0 transition metal Mo6+ in the octahedral coordination environment, it is feasible to envision the cation to be switched from one face to the opposite. With respect to the polarization reversal of TeOx (x = 3, 4) polyhedron, the entire polyhedron must be inverted, which is not possible without substantial bond rearrangement or bond breaking. Thus, polarization inversion of the TeOx (x = 3, 4) polyhedron is energetically very unfavorable and would not occur. For both polymorphs of BaTeMo2O9, although the local polarity in the MoO6 octahedron can be switched, the overall dipole moment of MoO6 octahedra along the polar axis (4.6 105 esu 3 cm/Å3 for R-BaTeMo2O9 and 1.0 102 esu 3 cm/Å3 for β-BaTeMo2O9) is much smaller than that of TeOx (x = 3, 4) polyhedra (42.0 105 esu 3 cm/Å3 for RBaTeMo2O9 and 3.1 102 esu 3 cm/Å3 for β-BaTeMo2O9), respectively. In other words, the reversibility of the polarization of MoO6 octahedra cannot change the direction of the overall polarization of MoO6 and TeOx (x = 3, 4) polyhedra. Therefore, polarization associated with R- and β-BaTeMo2O9 cannot be reversed, and the materials are not ferroelectric.

’ CONCLUSION We identified a new polar polymorph of BaTeMo2O9, RBaTeMo2O9. The structure of R-BaTeMo2O9 features a twodimensional layer consisting of Mo-centered [MoO6] distorted octahedra and TeOx (x = 3, 4) polyhedra. It exhibits very limited SHG response, about 0.2 times that of KDP, and is phasematchable. Although the dipole moments of MoO6 octahedra and TeOx (x = 3, 4) polyhedra are very large, the arrangement of various dipole moments is nearly in an antiparallel manner, resulting in remarkably smaller SHG efficiency than that of βBaTeMo2O9. Polarization measurements indicate that both Rand β-BaTeMo2O9 are nonferroelectric. R-BaTeMo2O9 has a band gap of ∼3.12 eV. In addition, β-BaTeMo2O9 undergoes an irreversible phase transition at ∼570 °C to R-BaTeMo2O9. The solid-state reactions and further analysis suggest that BaMoO4 plays a key role in the phase transition, which lowers the energy barrier and stimulates the transformation. We are in the process of growing large crystals of R-BaTeMo2O9, intending to understand the mechanisms of crystallization of the two polymorphs and explore new properties of the R-BaTeMo2O9 single crystals. 3759



’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic file for R-BaTeMo2O9 in CIF format, atomic coordinates and thermal parameters, selected bond distances and angles, ball-and-stick diagram of 2D anionic layer, PXRD of the residue after DTA and TGA measurements, IR and Raman spectra, polarization versus electric field plots for R-BaTeMo2O9, specific heat versus temperature curve of the single crystal β-BaTeMo2O9 in the temperature range 545575 °C, production of β-BaTeMo2O9 powder after annealing, IR and Raman spectra, polarization versus electric field plots for β-BaTeMo2O9 (PDF), and dipole moment calculations (Excel). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +86-531-88574135. Tel: +86531-88364963.

’ ACKNOWLEDGMENT The authors thank Dr. Shangqian Sun (Shandong University) for his useful advice in the IR/Raman assignment, Prof. Ning Ye (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) and Prof. Zhanggui Hu (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for their help with the SHG measurements, and Dr. MingLei Zhao (Shandong University) for his help with the polarization measurements. The authors also gratefully acknowledge financial support by the National Natural Science Foundation of China (Grants 51021062 and 50990061), the 973 Program of the People’s Republic of China (Grant 2010CB630702), and the Graduate Innovation Foundation of Shandong University (GIFSDU). ’ REFERENCES (1) (a) Authier, A. International Tables for Crystallography. Physical Properties of Crystals; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003; Vol. D. (b) Nye, J. F. Physical Properties of Crystals: Their Representation by Tensors and Matrices; Oxford University Press: New York, 1985. (c) Marder, S. R.; Stucky, G. D.; Sohn, J. E. Materials for Nonlinear Optics: Chemical Perspectives; ACS Symposium Series 455; American Chemical Society: Washington, DC, 1991. (d) Waser, R.; B€ottger, U.; Tiedke, S. Polar Oxides: Properties, Characterization, and Imaging; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2005. (2) Brammer, L. Chem. Soc. Rev. 2004, 33, 476–489. (3) (a) Chang, H. Y.; Kim, S. W.; Halasyamani, P. S. Chem. Mater. 2010, 22, 3241–3250. (b) Yeon, J.; Kim, S.-H.; Halasyamani, P. S. Inorg. Chem. 2010, 49, 6986–6993. (c) Zhou, Y.; Hu, C. L.; Hu, T.; Kong, F.; Mao, J. G. Dalton Trans. 2009, 5747–5754. (d) Chang, H. Y.; Kim, S. H.; Ok, K. M.; Halasyamani, P. S. Chem. Mater. 2009, 21, 1654–1662. (e) Chang, H. Y.; Sivakumar, T.; Ok, K. M.; Halasyamani, P. S. Inorg. Chem. 2008, 47, 8511–7. (f) Chen, X. A.; Zhang, L.; Chang, X. N.; Xue, H. P.; Zang, H. G.; Xiao, W. Q.; Song, X. M.; Yan, H. J. Alloys Compd. 2007, 428, 54–58. (g) Chi, E. O.; Ok, K. M.; Porter, Y.; Halasyamani, P. S. Chem. Mater. 2006, 18, 2070–2074. (h) Ra, H. S.; Ok, K. M.; Halasyamani, P. S. J. Am. Chem. Soc. 2003, 125, 7764–7765. (i) Goodey, J.; Ok, K. M.; Broussard, J.; Hofmann, C.; Escobedo, F. V.; Halasyamani, P. S. J. Solid State Chem. 2003, 175, 3–12. (j) Goodey, J.; Broussard, J.; Halasyamani, P. S. Chem. Mater. 2002, 14, 3174–3180. (4) (a) Hu, T.; Qin, L.; Kong, F.; Zhou, Y.; Mao, J.-G. Inorg. Chem. 2009, 48, 2193–2199. (b) Kim, S. H.; Yeon, J.; Halasyamani, P. S. Chem.

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Polymorphism of BaTeMo2O9: A New Polar Polymorph and the Phase

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