New High-Pressure Polymorph of the Nonlinear Optical Crystal

Communication pubs.acs.org/crystal

New High-Pressure Polymorph of the Nonlinear Optical Crystal BaTeMo2O9 Dehong Yu,*,† Dehui Sun,†,‡ Maxim Avdeev,† Qian Wu,‡ Xiangxin Tian,‡ Qinfen Gu,§ and Xutang Tao*,‡ †

Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China § Australian Synchrotron, Clayton, VIC 3168, Australia ‡

S Supporting Information *

ABSTRACT: High-pressure synchrotron X-ray diffraction on the nonlinear optical crystal α-BaTeMo2O9 (α -BTM) has revealed a new polymorph of γ-BTM having a monoclinic space group of P21. This first order phase transition from the orthorhombic Pca21 to the new monoclinic P21 structure occurs at 3.8 GPa pressure. The mechanism of the phase transition under high pressure is attributed to the anisotropic pressure response along different crystallographic directions.

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which stack parallel to the c direction. The Ba2+ cations are found between the layers to maintain charge balance.10 Though the two polymorphs are similar in terms of layered structure, they have different asymmetric units of (BaTeMo2O9)m, with m = 1 for β-BTM and m = 2 for α-BTM. That is, β-BTM contains two symmetry-independent MoO 6 octahedra and one TeO4 polyhedron unit, while α-BTM is built up of four MoO6 octahedra and two TeOx (x = 3, 4) polyhedral units.10 A recent high-pressure Raman scattering study provided evidence of a phase transition near 3.5 GPa pressure for orthorhombic α-BTM;11 however, the detailed structure of the new phase is not yet known. In this communication, we report on the structure of a new polymorph of BTM formed from α-BTM under pressure. The α-BTM powder sample was ground from single crystals grown by a TSSG method with a TeO2−MoO3 mixture (TeO2/MoO3 = 6:5) as the flux, as detailed in ref 12. Synchrotron powder X-ray diffraction (PXRD) on α-BTM powder under high pressure at room temperature was carried out at the Australian Synchrotron Powder Diffraction Beamline. The X-ray wavelength used was 0.6205 Å with a beam size of 140 μm × 140 μm. Pressure was generated by a standard diamond-anvil cell and determined using the shift of the fluorescence line of the ruby.13 The diffraction patterns were collected using a MarCCD 165 area detector. The intensity versus 2θ (from 6° to 21°) diffraction pattern was obtained by integrating the 2D powder image using the program FIT2D.14 Rietveld refinements were carried out on the obtained

oncentrosymmetric (NCS) compounds have attracted significant research interest due to their multifunctional properties such as ferroelectricity, piezoelectricity, dielectric behavior and nonlinear optical (NLO) phenomena.1,2 Recently, a new class of NCS materials was synthesized with d0 transition metal cations (Mo6+ or W6+) and lone-pair cations (Se4+ or Te4+). Both kinds of ions are susceptible to the second-order Jahn−Teller effect, which is attributed to the unique properties of the materials.3,4 BaTeMo2O9 (β-BTM), first synthesized by Ra et al. through the solid-state reaction method, is one of these NCS materials and its powder second-harmonic-generation efficiency is about 600 times that of α-SiO2.5 Large and high-quality single crystals were successfully grown by a top-seeded solution growth (TSSG) method and the properties characterized by Tao’s group.6−9 This has paved the road leading to industry applications of the BTM materials. The general structure of β-BTM, belonging to the monoclinic P21 space group, is a layered structure consisting of MoO6 octahedra linked to TeO4 tetrahedra with Ba2+ cations sitting in between the layers to maintain the charge balance.6 A different polymorph of orthorhombic BTM (α-BTM) with space group Pca21 has been discovered in the course of growing the β-BTM, and it was found that a mixture of β-BTM, α-BTM, and BaMoO4 can be transferred to a pure α-BTM phase through a solid-state reaction.10 The crystal structure of α-BTM also exhibits a layered structure of (Mo4Te2O18)4− where the layers are composed of MoO6 octahedra and TeOx (x = 3, 4) polyhedra. The MoO6 octahedra share corners to form Mo4O20 tetramers, which are further interconnected to form long zigzag chains running along the b direction. These chains are linked by TeOx (x = 3, 4) polyhedra to form two-dimensional (2D) layers, © 2015 American Chemical Society

Received: March 26, 2015 Revised: May 13, 2015 Published: June 17, 2015 3110

DOI: 10.1021/acs.cgd.5b00421 Cryst. Growth Des. 2015, 15, 3110−3113

Crystal Growth & Design

Communication

phases are listed in Table S1 together with some Rietveld refinement parameters. As compared with the orthorhombic αBTM phase, the monoclinic γ-BTM phase loses two symmetry operations, and the number of the independent atomic positions is doubled. Therefore, the asymmetric unit of the new γ-BTM has the formula “Ba4Te4Mo8O36” with m = 4. The ball-and-stick diagrams of the crystal structures of α-BTM and γ-BTM are shown in Figure S1. The lattice constants, unit-cell volume, γ angle, and relative change of the lattice constants as a function of pressure are shown in Figure 3. To show the reversibility, the corresponding

diffraction pattern with the General Structure Analysis System (GSAS) program and EXPGUI front-end.15,16 From the pressure-dependent XRD patterns, as shown in Figure 1, there is no obvious change apart from peaks shifting

Figure 1. Selection of pressure-dependent synchrotron radiation PXRD patterns from powder α-BTM with 2θ from 6.5° to 13°. Black lines denote the low-pressure phase, and red lines correspond to the high-pressure phase. Asterisks mark the positions for the extra peaks.

to higher angles up to 3.3 GPa. At 4.1 GPa, several extra peaks not allowed by the Pca21 space group appear around 2θ = 7°, 10°, and 12° (marked by asterisks) These peaks develop further up to 8.2 GPa. Upon releasing the pressure, the original low pressure diffraction pattern is recovered as the pressure decreases to 3.5 GPa, indicating a reversible phase transition process. The refined diffraction data are shown in Figure 2a,b for 3.3 and 4.1 GPa, respectively. Refinement on the low-pressure Figure 3. Pressure dependence of lattice parameters of α-BTM and γBTM phases. Load, data taken with increasing pressure; unload, data taken with decreasing pressure. The dashed lines are the linear fits of the pressure-dependent lattice parameters a, b, and c for both phases. The magenta vertical dashed lines indicate the transition pressure. The error bars are smaller than the symbols.

data for the pressure released cycle is also included. The parameters at ambient pressure, marked as blue, are from ref 6, for consistency. As indicated by the vertical dash line, all pressure-dependent parameters change slope at pressure around 3.8 GPa, with rapid decrease of the γ angle by about 1.2° from 90°. This clearly indicates that a first-order phase transition occurs at around 3.8 GPa. The compression coefficients, determined by the slope of lattice constants versus pressure divided by the initial values of the corresponding lattice constants (shown in Supporting Information) are listed in Table S2 for the two phases along the three directions. As indicated in Figure 3f and Table S2, for the α-BTM phase it is much easier to be compressed along b direction. This anisotropic response to pressure provides the basis for the phase transition. After transforming to the γ-BTM, the response to pressure becomes more isotropic. In order to understand the phase-transition mechanism, we investigate the change of Mo−O−Mo bond angles as a function of pressure. As defined in Figure S2a, four bond angles of Mo4−O1−Mo1, Mo1−O8−Mo4, Mo3−O17−Mo2, and Mo2−O13−Mo3, for α-BTM, represent the angles between neighboring octahedra along the Mo4O20 tetramer chains. After phase transition, for γ-BTM, the two neighboring Mo4O20 tetramer chains deform independently. The four bond angles become eight as shown in Figure S2b.

Figure 2. Synchrotron PXRD patterns at two pressures (a) 3.3 GPa, αBTM, and (b) 4.1 GPa, γ-BTM, with 2θ from 6.5° to 13°. Plus (+) symbols are experimental data. Red lines represent the calculations from Rietveld refinement. Blue lines are the difference between experiment and refinement, and vertical ticks are the calculated positions of Bragg reflections.

diffraction data confirmed the standard orthorhombic α-BTM structure with Pca21 space group. For the high-pressure phase, the appearance of extra diffraction peaks in the high-pressure phase implies that the symmetry of the crystal structure decreases at high pressures. However, the known structure of βBTM cannot match the high-pressure diffraction pattern at all. Further analysis based on the relationship of interplanar space for a monoclinic system (Supporting Information) shows that a possible scenario leading to the split of (110) peak is a non-90° γ angle. Based on a monoclinic P21 space group with the c-axis as the unique one, having non-90° γ angle, we can refine the highpressure data very well as shown in Figure 2b for 4.1 GPa pressure. We name this new high-pressure phase γ-BTM. The CIF file of γ-BTM at 4.1 GPa pressure can be found in the Supporting Information. The lattice parameters for the two 3111

DOI: 10.1021/acs.cgd.5b00421 Cryst. Growth Des. 2015, 15, 3110−3113

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transition is a result of the opposite pressure dependence of the bond angles of Mo−O−Mo in two perpendicular directions as marked by the red line (increase) and arrows (decrease) due to the opposite effects from pressure along a- and b-directions, as schematically indicated in Figure 5. This anisotropic pressure dependence is associated with the different JT distortions from different MoO6 octahedra.

Below the phase transition, all these angles decrease as pressure increases, as indicated in Figure S3. At the phasetransition pressure, these angles undergo a sudden change with different pressure dependence along different directions. For example, the angles of Mo1−O8−Mo4, Mo2−O13−Mo3, Mo5−O19−Mo8 and Mo6−O35−Mo7, along roughly the same direction (Figure S3b), decrease with pressure after the sudden reduction, while the other four angles of Mo4−O1− Mo1, Mo8−O26−Mo5, Mo3−O17−Mo2, and Mo7−O31− Mo6, nearly in the perpendicular direction, increase with pressure after the sudden initial increment. The opposite pressure dependence of these corresponding angles causes a slight twist of the Mo4O20 tetramer chains in ab plane and thus contributes largely to the phase transition with pressure. The Jahn−Teller (JT) distortion parameter of MoO6 octahedra is defined as17 6

δJT =

∑ (di(Mo− O) − dMo− O )2 i=1

(1)

Here, di(Mo−O) (i = 1 to 6) are the six Mo−O distances of the MoO6 octahedra, and dMo − O is the average value of these bond lengths. The derived JT distortion parameters for each MoO6 octahedra based on the refined Mo−O bond lengths (Figures S4 and S5) are presented in Figure 4. Below the phase

Figure 5. Illustration of the mechanism of pressure-induced phase transition.

In summary, high-pressure synchrotron PXRD on the nonlinear optical crystal α-BTM has revealed a new polymorph of γ-BTM having a monoclinic lattice with the space group of P21. The detailed structure information for the new polymorph has been obtained through Rietveld refinement. The phase transition is first order and occurs at around 3.8 GPa pressure. The phase-transition mechanism is understood in terms of anisotropic response to pressure of the initial α-BTM structure in association with the JT distortions of MoO6 octahedra. The new structure discovered is consistent with the observations from a Raman spectroscopic study of α-BTM under pressure.11

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Figure 4. Pressure dependence of JT distortion parameters.

Experimental method; refinement details; CIF files of γ-BTM; ball-and-stick diagram and Mo4O20 tetramer chains of crystal structure of α-BTM and γ-BTM; the comparison of refined results between α-BTM and γ-BTM; the compression coefficient of α-BTM and γ-BTM; pressure-dependent Mo− O−Mo angles and Mo−O bonds length. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00421.

transition, all JT distortion parameters decrease linearly with pressure, and much larger distortions occur for the Mo2O6 and Mo4O6 octahedra than the other two of Mo1O6 and Mo3O6 octahedra, as a result of different orientation of each octahedra with respect to the crystallographic directions. This anisotropic pressure dependence continues above the phase transition. However, significant change happens for the JT distortion of the Mo2O6 and Mo4O6 octahedra at the phase-transition pressure. This indicates that the MoO6 octahedra having larger initial JT distortions are more susceptible to pressure deformation, which provides a trigger for the phase transition under pressure. The above discussions have indicated clearly that the effect of pressure on the phase transition is dominant in the ab plane in relation to the deformation of the tetramer chain. The pressure along a direction tends to flatten the tetramer chain, while the pressure along b direction tends to squeeze the tetramer chain. These competing effects become more significant and cause a higher level of frustration for the atoms to respond to the pressure effect with continuing pressure increase. Upon reaching the phase-transition pressure, around 3.8 GPa, the frustration is released by reaching a new equilibrium state (new phase) with angle of γ becoming less than 90°. This phase

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 51321091, 51202128) and the Program of Introducing Talents of Disciplines to Universities in China (111 Program No. b06015). The experiment was performed at the Powder Diffraction Beamline, Australian Synchrotron. D.S. would like to acknowledge the financial support from ANSTO during his research visit. D.Y. 3112

DOI: 10.1021/acs.cgd.5b00421 Cryst. Growth Des. 2015, 15, 3110−3113

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would like to thank Garry Mcintyre for his valuable comments on the manuscript.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00421 Cryst. Growth Des. 2015, 15, 3110−3113