Growth, Properties, and Theoretical Analysis of KBa2(PO3)5 Single

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Growth, Properties, and Theoretical Analysis of KBa(PO) Single Crystal Pai Shan, Tongqing Sun, Hongde Liu, Shiguo Liu, Shaolin Chen, Xuanwen Liu, Yongfa Kong, and Jingjun Xu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00758 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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Crystal Growth & Design

Growth, Properties, and Theoretical Analysis of KBa2(PO3)5 Single Crystal

Pai Shan1, Tongqing Sun1*, Hongde Liu1, Shiguo Liu1, Shaolin Chen1,2, Xuanwen Liu3, Yongfa Kong1,2,4, and Jingjun Xu1,2,4 1

The MOE Key Laboratory of Weak-Light Nonlinear Photonics and School of

Physics, Nankai University, Tianjin 300457, China 2

3

Teda Institute of Applied Physics, Nankai University, Tianjin 300457, China School of Resources and Materials, Northeastern University at Qinhuangdao,

Qinhuangdao 066004, China 4

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan,

030006, China

* Corresponding author. School of Physics, Nankai University, 94 Weijin Road, Nankai District, Tianjin 300071, China E-mail: [email protected]. (T. Sun)

ACS Paragon Plus Environment


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Abstract

In this study, we used a high-temperature solution to grow KBa2(PO3)5 (KBP), which is a potential deep-ultraviolet (UV) nonlinear optical crystal. It is a congruent melting compound that solidifies in a non-centrosymmetric crystal system that is monoclinic with the Pc space group. KBP exhibits a powder second-harmonic response at 1064 nm incident radiation, with an efficiency of 0.9 × KDP. KBP exhibits a wide transparency range, as the cutoff absorption edge in the UV region is as short as 167 nm. In addition, we performed Raman and IR spectroscopies on the reported material, which confirmed the existence of 1D [PO3]∞ chains in KBP. We also performed first-principles calculations to elucidate that KBP’s nonlinear optical activity stemmed from the 1D [PO3]∞ chains, and that the optical properties stemmed from the P–O groups.


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Growth, Properties, and Theoretical Analysis of KBa2(PO3)5 Single Crystal

In recent years, many researchers have searched for new ultraviolet (UV) and deep-UV nonlinear optical (NLO) materials because of their potential uses in laser communication, semiconductor lithography, and surgery.1-3 New materials with good UV-NLO performance should satisfy the following optical property requirements:4, 5 a large, optically transparent window into the UV region, a relatively larger effective second-harmonic generation (SHG) coefficient that is comparable to KH2PO4 (KDP), an average birefringence (0.06−0.1) to complete the phase-matching condition, and a high laser damage threshold. Note that perfect optical properties are not sufficient for NLO crystal’s practical applications, and that additional requirements should be imposed, such as chemical and thermal stability, simplicity of growth and mechanical treatment. Hence, KBe2BO3F2 (KBBF) is unique, because it can be practically used to directly produce a deep-UV coherent light by SHG.4,

6

Unfortunately, the KBBF

crystal is difficult to produce, not only because it contains beryllium, which may lead to the development of symptoms similar to those of pneumonia and cancer, but also for its strongly layered growth habit.7-9 Therefore, it is necessary to find beryllium-free

UV

and

deep-UV

NLO

crystals,

such

as

Li4Sr(BO3)210,

Rb3Li3B3O10F11, and K3Ba3Li2Al4B6O20F12, which successfully balance the above conflicting factors. Recently, some promising beryllium-free deep-UV NLO phosphates were reported, such as Ba3P3O10X (X=Cl, Br)13, RbBa2(PO3)514, Ba5P6O2015, CsLa(PO3)416, and borate-phosphate Ba3(ZnB5O10)PO417. Our extensive search in phosphates



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resulted in the finding of KBa2(PO3)5 (KBP) as a new NLO crystal. Martin C. et al. first discovered KBP in 1975,18 and, to the best of our knowledge, no additional studies on the KBP compound have been reported recently. Big alkaline and alkaline-earth metal cations are easy to polarize and possess short absorption edges.14 Moreover, KBP is structurally analogous to RbBa2(PO3)5, meaning that its optical properties may also be analogous to RbBa2(PO3)5. In this work, we used a high-temperature solution to obtain KBP single crystals with a K2O–P2O5 flux. KBP consists of flexible 1D [PO3]∞ chains, and exhibits a moderate SHG efficiency of 0.9 × KDP. Besides measuring its optical properties, we performed first-principles calculations on the reported material.

We adopted the high-temperature solid-state reaction technique to prepare the polycrystalline samples of KBP. For KBP, we thoroughly ground and calcined stoichiometric amounts of BaCO3, K2CO3, and NH4H2PO4 at 750°C for 48 h in air. The phase purity of KBP was confirmed using powder X-ray diffraction (PXRD) with copper Kα radiation (Figure 1). We used a high-temperature solution to grow single crystals of KBP, using K2O–P2O5 as the flux system. We thoroughly mixed K2CO3, BaCO3, and NH4H2PO4 with a molar ratio of 1:1:4 and ground them in an agate mortar. We packed the concoction into a Φ 60 mm × 60 mm Pt crucible, heated it to 950°C at 20°C/h in an electric furnace, and then held it for 24 h to ensure a homogeneous melt. Subsequently, we quickly cooled it to the initial crystallization temperature (716°C), and further cooled it at 1°C/10 h. When big KBP crystals developed at the top of the melt, we cooled the furnace down to room temperature at 30°C/h. We obtained colorless and transparent KBP crystals (Figure 2a).


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As reported in Ref 18, KBP crystallizes in the acentric Pc space group of monoclinic system. And KBP is isostructural with RbBa2(PO3)5. In an unsymmetrical unit, there are one K atom, two Ba atoms, five P atoms, and fifteen O atoms. Figure 3e shows the crystal structure of KBP projected onto the (010) planes. The crystal configuration of KBP is composed of infinite one-dimensional (1D) [PO3]∞ chains, which are repeatedly formed by five independent [PO4]3− tetrahedra that are linked by corner-sharing (Figure 3d), with K and Ba atoms filling the gaps (Figure 3e). Each K atom bonds with ten oxygen atoms (Figure 3a), with the K−O distances changing from 2.7256 Å to 3.4335 Å. The Ba1 atom is encircled by eight oxygen atoms (Figure 3b), with the Ba−O bond lengths changing from 2.6359 Å to 3.0296 Å. The Ba2 atom is also 8-coordinated, and the Ba−O bond lengths of 2.6512 Å to 2.9312 Å (Figure 3c). Our bond valence calculation results were 0.938, 2.071, and 2.046 for K, Ba1 and Ba2, respectively. The big K and Ba atoms formed the unsymmetrical polyhedra, and combining them with 1D [PO3]∞ chains resulted in the crystallographic asymmetry of KBP. Furthermore, the differential scanning calorimetry (DSC) and thermogravimetric (TG) analyses revealed that the KBP crystal had good thermal stability. Figure 4 shows the DSC/TG curves of the KBP crystal. The DSC data revealed a sharp endothermal peak at 818 °C for the heating curve and one exothermal peak in the process of cooling at a lower temperature, which was likely because of a retarded nucleation of the solid from the melt. Also, no obvious weight loss was observed in the TG analysis from room temperature to 900°C. These results indicated that the



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KBP crystal congruently melts at 818°C. Thus, the stoichiometric melt can be used to grow big KBP crystals. Furthermore, the KBP crystal is easy machining. Figure 2b shows a polished KBP single crystal. Lastly, we placed a KBP crystal in water for three weeks and did not observe any decomposition or degradation. This implied that KBP crystal was immune to moisture. We documented the infrared spectrum (Figure 5) using a Thermo Nicolet Magna IR-560ESP FT-IR spectrometer in the range of 400−4000 cm−1. We observed IR vibration characteristic bands of [PO3]∞, which was consistent with previous reports.19,

20

We attributed the absorption bands of 1236–1271 cm–1 to the

antisymmetric stretching vibration vas(O–P–O), whereas we attributed the band around 1130 cm–1 to the symmetrical vibration vs(O–P–O). Also, we ascribed the band at 1028 cm–1 to the vas(P–O–P), and found that the band at 904 cm–1 was due to its symmetrical vibration vs(P–O–P). We carried out Raman spectrum of KBP on a Renishaw inVia confocal Raman microscope, using the excitation laser wavelength of 785 nm. The Raman spectrum (Figure 6) had two characteristic strong peaks at around 1150 and 657 cm−1. We ascribed the strong peak at around 1150 cm−1 to the vs(O–P–O), which corresponded to the motion of the non-bridging oxygen, whereas the strong peak at around 657 cm−1 belonged to the vs(P–O–P), which matched reported values21. Both IR and Raman spectra confirmed the existence of 1D [PO3]∞ chains in the structure of KBP. To evaluate the frequency doubling capabilities of KBP, we carried out powder SHG tests on KBP using a Q-switched Nd:YAG laser (λ = 1064 nm) according to the Kurtz-Perry method22. SHG efficiencies rely on particle size, so we crushed and sieved polycrystalline samples into several ranges: 15–38.5, 38.5–60, 60–75, 75–98,


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98–125, 125–154, and 154–180 µm. We used the commercial KDP sample as our reference, which was powdered and sieved into the same size ranges. The SHG measurements on the samples showed that KBP had a moderate SHG response of 0.9 times that of the KDP sample. The SHG efficiency variation with the particle size revealed that the KBP was type I phase-matchable (Figure 7). We measured both the UV-vis-NIR and deep-UV optical transmittance spectra on the same polished crystal wafer with a thickness of 1.1 mm (Figure 2b) at room temperature. The optical transmittance spectra indicated that KBP exhibited a wide transparency range, as the UV cut-off absorption edge was as short as 167 nm (Figure 8). This implied that the band gap was 7.43 eV. Even at 200 nm, the transmittance was nearly 73%. This indicated that KBP could serve as a potential deep-UV NLO material. To learn more about the band structure and optical properties of KBP, we performed theoretical calculations with the density functional theory (DFT) by CASTEP.23,

24

The generalized gradient approximation (GGA)25 with the

Perdew-Burke-Ernzerhof (PBE)26 function was used to process the exchange correlation interaction. The optimized normal-conserving pseudopotential (NCP) modeled the ion-electron interactions for every element, and the following valence-electron configurations were adopted in the calculation: K-3s23p64s1, Ba-5s25p66s2, P-3s23p3, O-2s22p4. The cutoff energy was set to 830 eV, and a Monkhorst-Pack k-point sampling of 4×3×2 was used to perform the numerical integration of the Brillouin zone. As often recommended, we used over 300 empty bands while calculating the optical properties. Figure 9 shows that the top of the valence bands (VBs) was found at the A point, while the bottom of the conduction bands (CBs) was at the G point. Thus, KBP is an indirect gap insulator. The calculated band gap of KBP (5.33 eV) was smaller than



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that observed in experiments (7.43 eV) due to the insufficient description of DFT-GGA on the eigenvalues of the electronic states,27 so a scissor value of 2.10 eV was added in the later optical properties calculation. Figure 10 shows the dispersions of total and partial density of states (TDOS, PDOS). The bottom of the CBs from 5.3 to 8.7 eV was composed of mixed orbitals on all constituent atoms. However, the top region of the VBs was predominantly made up of O-2p states, and small amounts of O-2s and P-3s3p states, implying that there were covalent bonds between O and P. The bands from −11.8 to −6.9 eV mainly stemmed from K-3p, and Ba-5p states, and some O-2p states mixed with small amounts of O-2p and P-3s3p states. The bands from −22.5 to −16.1 eV were predominantly derived from O-2s and P-3s3p states, and showed quite a wide hybridization between the O-2s and P-3s3p states, which indicated that there were strong P−O covalent bonds in the KBP compound. The bands from −25.1 to −23.2 eV were made up of the solitary inner-shell state with Ba-5s. The optical properties of a crystal arise from the electronic transition between the states near the energy band gap,5,

28

so it was the P–O bonds that mainly

influenced the optical properties of KBP (e.g., the SHG effect and the NLO activity of KBP originated from the 1D [PO3]∞ chains). This was analogous to RbBa2(PO3)5, which corresponded to similar crystal structures. The second-order nonlinear coefficients could be deduced from a classical anharmonic oscillator (AHO) model29, as this method was used to study other NLO materials.30, 31 The KBP crystallizes in class m, and its nonlinear coefficients include ten non-zero tensors. Further, only six independent tensors (d11, d12, d13, d15, d24, and d33) were left because of the constraints of Kleinman’s symmetry. The values of these six tensors at 1064 nm (1.165 eV) for KBP varied within the range of 7.50×10−10 to 7.79×10−10 esu. We found that these numbers were similar to that of our experimental


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results, which were 0.9 times of that of KDP (d36 = 1.1×10−9 esu). In conclusion, we characterized a deep-UV NLO crystal KBP. The KBP crystal achieved desirable optical properties and displayed a markedly short deep-UV absorption edge of 167 nm. KBP exhibited a moderate powder SHG response at 1064 nm incident radiation, with an efficiency of 0.9 × KDP, and was type I phase-matchable. KBP congruently melted, which would be beneficial to grow large, high optical quality crystals. Its chemical stability and mechanical durability would also allow it to be easily processed through cutting and polishing. All of these excellent characteristics of KBP crystals demonstrated that it had potential as an SHG material for deep-UV wavelength applications. Acknowledgements The National Natural Science Foundation of China (grant no. 21271109), the National Basic Research Program of China (grant no. 2013CB328706), the Program for Changjiang Scholars and Innovative Research Team at the University of China (IRT_13R29), and the 111 Project of China (grant no. B07013) supported this study. We would like to thank Prof. Ye Tao and Dr. Yan Huang of Institute of High Energy Physics, Chinese Academy of Sciences for measuring the deep-UV transmittance spectrum, as well as Dr. Jiwei Qi of TEAD Applied Physics Institute, Nankai University for valuable discussions. References (1) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Bulk characterization methods for non-centrosymmetric

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For Table of Contents Use Only

Growth, Properties, and Theoretical Analysis of KBa2(PO3)5 Single Crystal Pai Shan, Tongqing Sun*, Hongde Liu, Shiguo Liu, Shaolin Chen, Xuanwen Liu, Yongfa Kong, and Jingjun Xu

First, we introduced the significance and value of conducting research on KBa2(PO3)5 (KBP). Second, we described KBP crystal growth and polycrystalline samples synthesis methods. Third, we confirmed the existence of 1D [PO3]∞ chains. We carried out powder SHG tests on KBP, with an efficiency of 0.9×KDP, while the UV absorption edge was found to be located at 167 nm.


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Figure 1. Simulated and experimental powder X-ray diffraction patterns for KBP. 76x58mm (300 x 300 DPI)



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Figure 2. (a) Photo of the as-grown KBP crystals. (b) Photo of the polished KBP crystal. 76x32mm (300 x 300 DPI)


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Figure 3. Coordination of oxygen atoms around (a) K (b) Ba1 and (c) Ba2 cations (d) 1D [PO3]∞ chain. (e) View of the structure of KBP down the b axis (green, Ba; white, K; purple, P; red, O) 76x57mm (300 x 300 DPI)



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Figure 4. The DSC-TG curves of the KBP crystal 76x54mm (300 x 300 DPI)


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Figure 5. The IR spectrum of KBP 76x56mm (300 x 300 DPI)



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Figure 6. The Raman spectrum of KBP 76x56mm (300 x 300 DPI)


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Figure 7. SHG intensity vs particle size at 1064 nm. The drawn solid curves are not fitted to the data and just serve to guide the eyes. 76x58mm (300 x 300 DPI)



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Figure 8. The deep-UV and UV-vis-NIR (inset panel) optical transmittance spectra of KBP 76x54mm (300 x 300 DPI)


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Figure 9. The band structure of KBP 76x55mm (300 x 300 DPI)



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Figure 10. The total and partial density of states of KBP 76x90mm (300 x 300 DPI)


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Growth, Properties, and Theoretical Analysis of KBa2(PO3)5 Single

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