Top-Seeded Solution Crystal Growth, Morphology, Optical and

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Top-Seeded Solution Crystal Growth, Morphology, Optical and Thermal Properties of Ba3(ZnB5O10)PO4 (BZBP) Hongwei Yu, Jacqueline Cantwell, Hongping Wu, Weiguo Zhang, Kenneth R. Poeppelmeier, and P. Shiv Halasyamani Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00529 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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Crystal Growth & Design

Top-Seeded Solution Crystal Growth, Morphology, Optical and Thermal Properties of Ba3(ZnB5O10)PO4 Hongwei Yu,† Jacqueline Cantwell,‡ Hongping Wu,† Weiguo Zhang,† Kenneth R. Poeppelmeier,‡ and P. Shiv Halasyamani*,† †

Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States

‡

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3133, United States E-mail: [email protected] (P. S. Halasyamani)

Abstract: Ba3(ZnB5O10)PO4 (BZBP) single crystals were grown successfully by a

top-seeded solution growth (tssg) method. High-resolution X-ray diffraction rocking curve measurements reveal a FWHM of 34.56'' of a BZBP single crystal grown from a [101]-oriented seed. The refractive indices from the UV to the NIR region were measured and revealed a birefringence of 0.04179 - 0.03059 in the wavelength range of 253.6 2325.4 nm. In addition the type-I and type-II phase-matching range for second and third harmonic generation were calculated based on the fitted Sellmeier equations. In order to further evaluate the potential application of Ba3(ZnB5O10)PO4, the thermal properties including specific heat, thermal diffusivity and thermal conductivity were also measured along different crystallographic axes.

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1 Introduction As a key constituent of the solid-state laser, a nonlinear optical (NLO) material can efficiently expand laser wavelengths by cascaded frequency conversion. These materials play an indispensable role in many different fields, such as semiconductor photolithography, laser micromachining, and modern scientific instrumentation.1-10 NLO materials can be classified into ultraviolet-visible (UV-vis),11-17 infrared (IR),18-23 and deep-ultraviolet (deep-UV) groups.24-32 Over the past 50 years, through intense research, many UV-vis and IR NLO materials, such as KBe2BO3F2 (KBBF),4 β-BaB2O4 (β-BBO),11 LiB3O5 (LBO),12 CsLiB6O10 (CLBO),13 CsB3O5 (CBO),14 LiNbO3,15 KTiOPO4,16 AgGaQ2 (Q = S, Se),18 and ZnGeP2 have been discovered.19 These materials have largely satisfied the frequency-conversion requirements from the visible to the IR. However, in UV and deep-UV region, all these NLO crystals have some drawbacks. For example, β-BBO possesses a large birefringence (∆n = 0.113 at 1064 nm) resulting in a large walk-off effect;11 CLBO exhibits softness and is hygroscopic nature making the cutting and polishing of crystal difficult;33 whereas KBBF has a strong layered crystal habit and requires highly toxic BeO in the synthesis, both of which limit its commercialized use.34,35 Therefore, designing, synthesizing and especially growing new high performance NLO materials that can be used in UV and deep-UV region is still of current academic and technological interest.34-49

Recently, we determined that combining borate and phosphate groups in the same material is a good strategy for discovering new UV and deep-UV NLO materials.50-54 For example, the absorption edge of BPO4 is 130 nm.52 However, its very small birefringence, 0.005,55 renders the material useless for NLO applications. In BPO4, the BO4 and PO4 tetrahedra are connected to form a three-dimensional framework with weak anisotropic polarization that is thought to reduce the birefringence of the material.56, 57 Recently, we reported on the design and discovery of Ba3(ZnB5O10)PO4 (BZBP).47 This material retains the positive attributes of the borate and phosphate polyhedra - a strong NLO response and wide transparency. However in BZBP, unlike BPO4, the borate and phosphate polyhedra are not connected. We demonstrated that BZBP exhibits strong second-harmonic generation responses at 1064 nm and 532 nm, and a deep-UV


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absorption edge (180 nm).47 However, high quality, large single crystals are necessary to accurately evaluate any NLO applications and to measure fundamental properties. Herein, we report on the crystal growth of BZBP by the top-seeded solution growth method. In addition, the crystal morphology, rocking curve, linear and nonlinear optical properties, as well as thermal expansion, specific heat, thermal conductivity, and thermal diffusivity are reported.

2 Experimental Section 2.1 Synthesis and Crystal Growth. Our previous work demonstrated BZBP melts congruently, but the relatively high melting temperature (~940 oC) and viscosity are detrimental to obtaining high quality crystals from a stoichiometric melt. Therefore, we focused on the top-seeded solution growth (tssg) method to grow large BZBP crystals. In order to avoid introducing any impurities, the self-flux system B2O3-ZnO was used. As described in our previous paper,47 polycrystalline BZBP was synthesized by standard solid state methods. A mixture with a molar ratio of BZBP: B2O3: ZnO = 1: 3.5: 0.7 was placed in a platinum crucible and heated to 980 °C for 15 h. This resulted in a clear homogeneous solution that was used for the crystal growth. Initially, the spontaneous crystallization method was used to obtain BZBP seeds for the crystal growth. With these seeds, a saturation temperature of 850 °C was determined by observing the growth or dissolution of the seed crystals when soaking in the melt. A BZBP seed was dipped into the surface of the solution at 5 °C higher than the saturation temperature (855 °C), followed by decreasing the temperature to the saturation point over 30 min. From the saturation temperature, the solution was cooled at a rate of 0.2 °C per day until the desired crystal size was obtained. The single crystal was pulled out of the solution surface and cooled to room temperature at a cooling rate of 10 oC/h. After a well-shaped crystal was obtained, this crystal was indexed and cut into seeds with different crystallographic directions. To study the effect of seed orientation on the quality of the single crystals, seed crystals with different crystallographic directions, [100], [010], and [001], were used. 2.3 High-Resolution X-ray Diffraction. A polished BZBP sample oriented along the (101) with dimensions 5 × 6 × 3 mm3 was used for the high-resolution X-ray diffraction


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measurement. The measurement was made on a Rigaku ATXG with a 4-bounce Ge-220 crystal system, a Cu rotating anode generator, and a multilayer optic. The Κα2 radiation is eliminated by a slit. The rocking curve measurements were made by changing the angle between the X-ray beam and the sample surface (the ω scan) around the Bragg diffraction peak position θ. 2.4 Refractive Index Measurements. The refractive indices and dispersion of BZBP crystal were determined by the minimum deviation method with 16 different monochromatic sources from 253.7 to 2325.4 nm (Trioptics Spectromaster HR, Germany). Since BZBP crystallizes in the orthorhombic crystal system, two right-angle prisms with a vertex angle of 30° were cut, polished and used for measuring the three principle refractive indices (nx, ny and nz) (see Figure 1).

Figure 1. Photographs and morphologies of the BZBP crystal prisms.


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2.5 Thermophysical Property Measurements. The cuboid-shaped BZBP crystal with three edges along crystallographic a, b and c axis was cut for the measurement of thermal expansion coefficients of BZBP. This measurement was done on a NETZSCH DIL 402 PC dilatometer in the temperature range of 40−600 °C in air. The sample lengths along the a-, b-, and c-directions were 4.16, 5.04 and 3.96 mm, respectively, and the heating rate was 5 °C/min. The specific heat was measured by using the method of differential scanning calorimetry with a simultaneous thermal analyzer (TGA/DSC1/1600HT, Mettler-Toledo Inc.). The measurement was carried out from 25 to 400 °C with a heating rate of 10 °C/min and sapphire was used as a reference. Thermal diffusivity measurements were carried out on a NETZSCH LFA 457 NanoFlash. Three wafers perpendicular to a- b- and c- directions with similar dimensions of 4 × 4 × 1 mm3 were cut and coated with graphite on both sides. The temperature range was 25–400 °C and heating rate was 10 °C min-1.

Results and discussion Crystal growth and morphology. With the TSSG method, the seed orientations have a great impact on the growth rate, morphology and quality of the crystals.58-63 In order to obtain the optimal seed direction, a theoretical morphology analysis was initially carried out. The theoretical morphology of BZBP crystal was calculated by the Mercury program (Figure S1).64 The facets {010}, {101}, {110}, {001}, and {100} are the prominent faces in the theoretical morphology of BZBP crystal and the facets {010} and {101} have largest area in the external morphology. According to the Bravais-Friedel-Donnay-Harker (BFDH) theory,65 the prominent faces of a crystal and its area will be determined by the growth rate of the crystal faces. The {010} and {101} faces of BZBP crystal will have a slow growth rate, whereas the {001} face will have a fast growth rate.


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Figure 2. The photographs and morphologies of the as-grown BZBP crystals with different oriented seeds: (a) for [001]-oriented seed, (b) for [010]-oriented seed, and (d) for [101]-oriented seed

In order to obtain high crystal quality, the [001]-oriented seed was initially used. With the [001]-oriented seed, a BZBP crystal with dimensions of 31 × 23 × 7 mm3 was grown (Figure 1a). Although the crystal with [001]-oriented seed has well-shaped {001}, {010} and {101} facets, the crystal contains obvious growth striations on its upper surface (Figure 1a). The growth striations may be attributable to the facets along the [001]-direction that from the theoretical morphology is not the {001} plane, but rather the large {101} planes and two relatively small {011} planes (Figure S2a). During the crystal growth although a [001]-oriented seed was used, these (-10-1), (10-1), (0-1-1) and (01-1) facets would be expressed on the surface, that leads to growth striations on the upper surface. To improve upon the crystal quality, the [010] and [101]-oriented seeds were used next. With [010]-oriented seed, a BZBP crystal with size dimensions of 35 × 20 × 5 mm3 was obtained with {010}, {110}, {011} and {001} facets (Figure 1b). With the [101]-oriented seed, a BZBP crystal with dimensions of 34 × 15 × 8 mm3 was obtained and the {101}, {110}, {011} and {010} facets are prominent (Figure 1c). The two crystals with [010] and [101]-oriented seeds have the similar morphologies with their calculated theoretical morphologies (Figures S2b and S2c), and these two crystals have high optical quality (Figures 1b and 1c). The crystal grown with the [101]-oriented seed has a thicker dimension and exhibits a more regular shape compared with the crystal


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grown with the [010]-oriented seed. The quality of the crystal with [101]-oriented seed was also measured by high-resolution X-ray diffraction. The FWHM of the rocking curve of this crystal is 34.56'' (Figure 2). The rocking curve data indicate that the [101]-oriented seed is optimal for high quality BZBP crystal growth.

Figure 3. Rocking curve of the BZBP single crystal.

Refractive Indices. The refractive indices from UV to NIR region are summarized in Table 1. BZBP belongs to a biaxial crystal class, orthorhombic, with the principle optical axes x, y, z parallel to the crystallography axes, c, b, a, respectively. The order of the refractive indices along three principle optical axes are nx

Top-Seeded Solution Crystal Growth, Morphology, Optical and

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