WANG Hi-Jun JIAO Zhi-Wei SUN Tong-Qing LIANG Rui CHEN Shung LIU Feng-Bin YANG Yue

a (College of Mechanic and Materials Engineering,North China University of Technology, Beijing 100144, China)

b (School of Physics, Nankai University, Tianjin 300071, China)

c (Gansu Provincial Key Laboratory of Fine Chemicals, Lanzhou 730020, China)

1 INTRODUCTION

Phosphates play an important role in nonlinear optical (NLO) materials due to their prominent optical properties[1-3].KH2PO4(KDP) and KTiOPO4(KTP) are two typical phosphate NLO materials for their outstanding phase-matching behaviors, second harmonic generation (SHG) efficiencies and stabilities.But except the above two traditional NLO materials, rare investigations and discoveries about phosphates were reported owing to the absence of deep-UV transparency and high NLO activity.

Recently, Luo et al.have synthesized a new phosphate compound RbBa2(PO3)5with a short UV absorption edge (163 nm) and relatively significant

SHG response (1.4 KDP)[2], which attracted immense interest in finding new phosphate NLO materials.Soon after, a series of new phosphate NLO materials with short UV cut-off edges and relatively larger SHG responses have been obtained including LiCs2PO4(174 nm), Cs2Ba3(P2O7)2(176 nm), Ba3P3O10Cl (180 nm), Ba5P6O10(167nm),etc.[3-7].Such investigations suggest that phosphate could be as important as the borates in exploring new deep UV NLO materials.

LiZnPO4has three different phase structure α, β and δ-LiZnPO4, and all of them crystallize in the noncentrosymmetric space group[8-10].β-LiZnPO4was originally observed in Li3PO4-Zn3(PO4)2phase diagram by Trevino and West[11].However, its characterization and optical properties have not been studied systematically yet.In the present work,β-LiZnPO4single crystal was successfully synthesized.The structure and optical properties including UV cut-off edge and SHG response were reported.For a deeper understanding of the band structures and electronic states, theoretical calculations based on DFT using CASTEP were performed.

2 EXPERIMENTAL

2.1 Sample preparation

A mixture of LiOH·H2O (35 mmol) and Zn3(NO3)2·6H2O (10 mmol) was dissolved and stirred in 30 mL water, and the pH of the mixture was adjusted to 3.0 with H3PO4.The solution was sealed in a 25 mL Telfon-lined stainless-steel container and heated at 453 K for 72 h, and then cooled to room temperature at a rate of 2 K/h.The samples were washed by ionized water and dried at ambient environment.Transparent crystals up to millimeter size were obtained to select for structure analysis and further optical tests.The comparison of the experimental and the calculated powder XRD patterns is shown in Fig.1.The XRD pattern of the experimental product is in great agreement with the calculated one using the single crystal data,confirming that the obtained final product is pure β-LiZnPO4.

Fig.1. Comparison of the experimental and calculated XRD patterns of β-LiZnPO4

2.2 Characterization

Phase tests were carried out on a Bruker D8 Advance diffractometer using CuKα radiation (λ =0.15405 nm) in the 2θ range of 10～70°, with a step of 0.02°.A crystal with dimensions of 0.35mm ×0.14mm × 0.13mm was chosen for structure determination.Single-crystal X-ray test was performed on an Xcalibur and Gemini diffractometer with MoKα (λ = 0.71073 nm), using an ω-2θ scan mode at 108.6 K.A total of 1281 independent reflections were collected, of which 1258 were observed with I ＞ 2σ(I).The structure was solved with SIR2004 program by direct methods and refined with the SHELXTL refined package using Olex2[12].All of the atoms were refined using full-matrix leastsquares techniques with anisotropic thermal parameters and finally converged at I ＞ 2σ(I).The validity about the structure was carefully checked with PLATON[13].The selected bond distances and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

UV-Vis diffuse reflectance spectrum was recorded on a U-3900 spectrophotometer with BaSO4as the standard sample.The reflectance data (R) were treated by the following Kubelka-Munk function:F(R) = (1 – R)2/(2R)[14].Powder SHG response of β-LiZnPO4was tested using a 10 ns Q-switched Nd:YAG laser (λ = 1064 nm) based on Kurtz-Perry method[15].The signals were recorded by fiber spectrophotometer for further analysis.Samples and crystalline KDP were ground and sieved into the following particle size ranges: 10～15, 15～30,30～50, 50～98, 98～125, 125～180 μm.KDP here was used to serve as a reference.

Theoretical calculations based on density function theory (DFT) were finished by CASTEP package.The exchange correlation interaction was treated by the generalized gradient approximation (GGA)[16]and Perdew-Burke-Ernzerhof (PBE)[17]function.The optimized normal-conserving pseudopotential(NCP) modeled the ion-electron interactions for every element[18].The cut-off energy was set to 340 eV and a Monkhorst-Pack k-point sampling of 5×5×5 was used to perform the numerical integration of the Brillouin zone.Other convergent parameters were fitted to the default values.Energy and electron states calculations for β-LiZnPO4were carried out after geometry optimization.The following valence-electron configurations were adopted in the calculation: Li-1s22s1, Zn-3d104s2, P-3s23p3and O-2s22p4.

3 RESULTS AND DISCUSSION

3.1 Crystal structure characterization

β-LiZnPO4crystallizes in the noncentrosymmetric R3 space group of trigonal system with a = b =13.6490(4) ?, c = 9.1123(3) ?, γ = 120.00°, Z = 18,V = 1470.13(8) ?3, Mr= 167.28, Dc= 3.401 g/cm3,F(000) = 1440, the final R = 0.0187 and wR =0.0497 for 1258 independent reflections (I ＞ 2σ(I)).The structure contains two unique cation positions,two P atoms and eight oxygen atoms.The atom coordination environments of β-LiZnPO4were depicted in Fig.2.All of the Li, Zn and P atoms are four-coordinated and connected to each other through bridging oxygen atoms.In the LiO4tetrahedron, the Li(1)–O distances range from 1.925(18)to 1.989(9) ?, and the Li(2)–O distances change from 1.895(18) to 1.975(16) ?.The Zn–O bond lengths in the ZnO4tetrahedron vary from 1.933(3)to 1.974(3) ?.The P–O bond lengths in the PO4tetrahedron fall in the 1.530(3)～1.550(3) ? range.All of the bond lengths of Zn–O, P–O and Li–O are consistent with the compounds that have been previously reported and do not show remarkably larger distortion[4,19].The O–M–O (M = Li, Zn, P)angles vary from 102.5(5)° to 117.9(5)°, indicating that the tetrahedra in the structure are slightly distorted.

Fig.2. Coordination environments of the cations in β-LiZnPO4

In the structure, three four-coordinated atoms connect with each other through the corner-sharing oxygen atoms, forming an alternation of hexagon and quadrilateral shape-like structure viewed along the c axis (Fig.3a).As shown in Fig.3b and c, both of the two shapes along the c axis are constituted by LiO4, ZnO4and PO4tetrahedra in alternation of the ABCABC··· mode.LiO4, ZnO4and PO4tetrahedral units are linked through common corners to form a three-dimensional (3D) framework.PO4tetrahedra are separated by the ZnO4and LiO4units.It is believed that the almost perfect regular tetrahedron and relatively loose atomic stacking are negative factors to large SHG responses (＞1×KDP) according to the traditional anionic group concept.But the subsequent optical tests indicate that β-LiZnPO4exhibits relatively larger SHG response (≈1.2×KDP).This is probably due to the unique structure of β-LiZnPO4, which is constructed by the isolated PO4,LiO4and ZnO4units.According to Luo et al.[2],SHG responses of the phosphate follow the condensation degree trend of [P2O7]4?＜ [P3O10]5?＜[PO3]∞.Furthermore, all the PO4, LiO4and ZnO4tetrahedral units are aligned along the c-axis, which may also play a prominent contribution to the effective SHG response of β-LiZnPO4.

Fig.3.Coordination polyhedral graphs along the c axis.(a) Structure stacked charts of β-LiZnPO4.(b) and (c) 3D frameworks of hexagon and quadrilateral shape-like structures

3.2 UV diffuse reflectance spectrum and SHG tests

The UV-Vis diffuse reflectance spectrum of the as-prepared sample is shown in Fig.4.The inset graph is the corresponding energy gap.The UV-Vis diffuse-reflectance spectrum shows the optical band gap of β-LiZnPO4is 5.6 eV, corresponding to the UV absorption edge of 220 nm.

Fig.4. UV diffuse reflectance spectrum recorded from 190 to 800 nm.The inset is the corresponding band gap(The broad peak between 260 and 340 nm was the result of the changing of the lamp)

Fig.5 presents the correlation between the SHG response intensity and the particle size.With the increasing particle sizes, the SHG intensity of β-LiZnPO4increases initially and decreases afterwards with a maximum at a range of 50～98 μm.The largest SHG response of β-LiZnPO4is about 1.2 times that of the KDP sample.β-LiZnPO4has larger SHG response than the new phosphate NLO materials Rb2Ba3(P2O7)2[2], KLa(PO3)4[18], LiCaPO4[20],and CsLa(PO3)4[21].The phase-matching curve demonstrates the β-LiZnPO4crystal is non-phasematchable at 1064 nm.

Fig.5. Powder SHG intensity measurements of β-LiZnPO4.The sieved KDP powders were used as a reference

3.3 Theoretical calculations

For a better understanding about the intrinsic mechanism of the optical properties, we performed the first-principals theoretical calculations.The calculated energy gap result is shown in Fig.6.The bottom of the conduction bands (CBs) and the top of the valance bands (VBs) both locate at the G point.It reveals that β-LiZnPO4has a direct bandgap of 4.3 eV, which is significantly smaller than the experiment value (5.6 eV) due to the insufficient estimation and description about the eigenvalues of the electronic states in generalized gradient approximation (GGA) mode[4].So a scissors value of 1.3 eV was adopted in the following calculations[2,4,22].The total and partial density of states (TDOS and PDOS)were calculated and the results are depicted in Fig.7.

Fig.6. Calculated band structures of β-LiZnPO4

Fig.7. Total and partial density of states of β-LiZnPO4

According to the PDOS, the bottom of CBs from 4.5 to 8 eV is the mixture of Zn-4s and P-3p states.Meanwhile, the top region of VBs from ?5 eV to the Fermi level is the major contribution of O-2p states while there is little distribution of Zn-3d and P-3p states, indicating the existence of Zn–O and P–O covalent bonds.The Zn-3d, O-2p states account for the dominant part of VBs from ?8 to ?5 eV and there is a small quantity of O-2s and P-3s3p states.The bands from ?22 to ?18 eV are mainly com-prised by O-2s2p and P-2s3p states, which further confirms the strong P–O covalent interaction in β-LiZnPO4.The Li-2s states occupy the bands from?44 to ?42 eV.The SHG response has significant relationships with the electronic transitions between the top VBs and bottom CBs regions, and the main contribution for SHG response region is located at the narrow range (?3 to 0 eV) of VBs near the Fermi level.As to the NLO crystal β-LiZnPO4, the total density of states in the narrow range is mainly occupied by the 2p orbitals of oxygen atoms and a small proportion of Zn-3d and P-3p states, so the P?O and Zn?O groups determine the SHG effect.In the structure of β-LiZnPO4, all the PO4and ZnO4tetrahedral units are preferentially aligned along the same direction, increasing the microscopic secondorder susceptibility and yielding the relatively larger SHG response.So, we can draw the conclusion that the SHG response of β-LiZnPO4mainly originates from the isolated and neatly arranged PO4and ZnO4units in the crystal structure.

4 CONCLUSION

In summary, β-LiZnPO4was successively synthesized via one pot hydrothermal method.The structure shows a noncentrosymmetric structure constructed by corner-sharing O atoms of LiO4,ZnO4and PO4tetrahedra.The isolated PO4and ZnO4features lead to a relatively larger SHG effect.The crystal exhibits a UV absorption edge at 220 nm.The SHG response is about 1.2 times that of KDP,but it is not phase-matchable at the wavelength of 1064 nm.Theoretical calculations based on DFT demonstrate the SHG response of β-LiZnPO4originates from the PO4and ZnO4groups.The work has provided meaningful guides for further investigations on the UV even deep UV phosphate NLO materials.

ACKNOWLEDGEMENTS

We deeply appreciated Prof.Chengzhi Xie of Medical University of Tianjin for the crystal structure revise suggestions and Prof.Dongxiang Zhang of Institute of Physics, Chinese Academy of Sciences for the SHG tests.

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