A Polar Tetragonal Tungsten Bronze with Colossal Second‐Harmonic Generation

Abstract A polar tetragonal tungsten bronze, Pb1.91K3.22□0.85Li2.96Nb10O30 (□: vacancies), has been successfully synthesized by a high temperature solid‐state reaction. Single crystal and powder X‐ray diffraction indicate that the structure of Pb1.91K3.22□0.85Li2.96Nb10O30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO6, LiO9, PbO12, and (Pb/K)O15 polyhedra. While NCS Pb1.91K3.22□0.85Li2.96Nb10O30 undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb1.45K3.56Li3.54Nb10O30 (P4/mbm) once heated to 1200 °C. Powder second‐harmonic generation (SHG) measurements with 1064 nm radiation indicate that Pb1.91K3.22□0.85Li2.96Nb10O30 exhibits a giant phase‐matchable SHG intensity of ≈71.5 times that of KH2PO4, which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well‐aligned NbO6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.


DOI: 10.1002/advs.202301374
from the fundamental light, which is a pivotal property in a variety of advanced industries. [2] Although a number of superb NLO materials such as LiNbO 3 , KH 2 PO 4 (KDP), KTiOPO 4 , etc. have been developed to date, [3] discovering novel functional materials with strong SHG response and wide transparency window is still an ongoing challenge. Thus far, introducing NCS chromophores with large polarization, such as two families of cations susceptible to second-order Jahn-teller (SOJT) distortions, for example, d 0 transition metal cations in the octahedral coordination environments (Ti 4+ , V 5+ , Nb 5+ , Mo 6+ ) and stereochemically active lone pair cations (Pb 2+ , Sb 3+ , Bi 3+ , etc.), and -delocalized anionic groups (NO 3 − , CO 3 2− , BO 3 3− ) have been widely utilized during the synthesis to ensure high-performance NLO materials. [4] Unfortunately, however, the majority of the reaction products are still found to be thermodynamically stable centrosymmetric (CS) structures owing to the facile alignment of the local asymmetric units in an antiparallel manner. Therefore, exploring suitable frameworks with well-aligned building units along a specific direction is extremely important.
Rigid frameworks of tungsten bronzes (TBs) consist of cornersharing MO 6 (M = Ti 4+ , V 5+ , Nb 5+ , etc.) octahedra, from which various cations and vacancies are observed in the constituting channels. In particular, since the structural distortion of TB can be enhanced from the distorted MO 6 octahedra as well as the polarizable cations and vacancies in the channels, the material has a perfect structural feature as a terrific NLO material. In fact, a few polar TB materials such as Pb 2. 15 (Li 0 [5] Previously reported TB with distorted NbO 6 octahedra and polarizable cations, Pb 0.91 K 1.72 Li 1.46 Nb 5 O 15 , might be also expected to exhibit a strong SHG response. [6] In this work, we have successfully synthesized a novel polar tetragonal tungsten bronze (TTB), Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 , via a high temperature solid-state reaction. The reported TTB material reveals a remarkably strong SHG intensity of ≈71.5 times that of KDP, which is the strongest SHG response in the visible range among the reported NLO materials thus far. We believe that exploring TBs with proper chemical compositions may accelerate  6 and Nb(2)O 6 octahedra with C 4 octahedral distortions and b) extended structure in the ab-plane. Three types of polyhedra, that is, LiO 9 , PbO 12 , and (Pb/K)O 15 reside at 3-, 4-, and 5-MR channels, respectively (dark green, Pb/K; light green, Pb; yellow, Li; orange, Nb; red, O). the development of novel NLO materials with extremely strong SHG efficiency.

Results and Discussions
Single crystal X-ray diffraction (SC-XRD) indicates that Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 crystallizing in the NCS polar tetragonal space group, P4bm (No. 100) reveals a 3D TTB structure containing two Nb, one disordered Pb/K, one partially occupied Pb, one partially occupied Li, and five O atoms in an asymmetric unit ( Figure S1, Supporting Information). Two unique Nb 5+ cations, Nb(1) and Nb(2) are connected by six oxygen atoms, forming NbO 6 distortive octahedra with the Nb−O distances of 1.84(5)-2.20(5) Å (Table S2, Supporting Information). Both Nb(1)O 6 and Nb(2)O 6 octahedra exhibit one short, one long, and four intermediate Nb-O bonds in C 4 distortive octahedral environments, in which the highly unsymmetrical NbO 6 octahedral units are attributed to the SOJT effect (Figure 1a). Each distorted Nb(1)O 6 and Nb(2)O 6 polyhedron shares its corners through oxygen atoms and constitutes a 3D framework structure containing 3-(3-MR), 4-(4-MR), and 5-membered ring (5-MR) channels (Figure 1a Figure S6, Supporting Information). However, differential scanning calorimetry (DSC) and temperature-dependent in situ PXRD suggest that the material exhibits a reversible phase transition at ≈460°C (Figure 2a The change of unit cell parameters for phases measured at different temperatures has been more closely analyzed by the GSAS-II program using the in situ PXRD data ( Figure 2c). While the unit cell parameters for a and b gradually increase with temperature, those for c gradually decrease up to 450°C. However, the unit cell parameters sharply change as soon as it passes the phase transition temperature, 460°C. As seen in the PXRD patterns and final Rietveld refinement plots measured at 480°C, the hightemperature CS phase (P4/mbm) is clearly distinct from the NCS phase (P4bm) (Figures S8 and S9  and Figure S10, Supporting Information). SC-XRD on a colorless rod-shaped crystal obtained upon fast cooling from the high temperature indicates that the decomposed material is Pb 1 Figure  S11, Supporting Information). Besides, the small moments arising from the slightly distorted NbO 6 octahedra effectively cancel in the CS structure when taken as a whole (Table S4, Supporting Information).
Powder SHG measurements using 1064 nm radiation indicate that NCS Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 exhibits an extremely large SHG intensity of about 71.5 times that of KDP and the type-I phase-matching behavior (Figure 3a,b). It should be noticed that to the best of our knowledge, the measured SHG intensity of the title material is the strongest in the visible range among all the reported NLO materials (Table S5, Supporting Information).
To understand the origin of the observed giant SHG efficiency of the title material, several calculation methods have been utilized. First, calculations on the magnitude of out-of-center distortion (Δ d ) for Nb(1)O 6 and Nb(2)O 6 octahedra result in values of 0.31 and 0.36, respectively (Figure 4a). [9] To quantify the intrinsic moments arising from the distorted NbO 6 octahedra, we further calculated their dipole moments by using a simple bond valence method. By doing so, local dipole moments of 3.42 D and 5.80 D have been obtained for Nb(1)O 6 and Nb(2)O 6 , respectively. In addition, since all the moments arising from Nb(1)O 6 and Nb(2)O 6 octahedra point toward the −c direction, a net polarization of 38.08 D occurs along the [00−1] direction from the framework (Figure 4a and Table 1; Figures S6 and S7, Supporting Information). Also, it has been known that TB structures containing vacancies in the channels might enhance local structural distortions. [5b,10] Thus, the tolerance factor (t) calculations have been performed to account for the role of vacancies in the structure. [5b, 10,11]  Pb/K, respectively. Thus, tolerance factors for the respective sites and the overall material, that is, t Pb , t Pb/K , and t total are calculated to be 0.779, 0.991, and 0.919, respectively (Table S8, Supporting  Information). Here, the structural distortion of the title material with the t total value of 0.919 should be attributable to the vacancies in the unfilled sites in the channels, which significantly enhance structural distortions in the title compound. Density functional theory (DFT) calculations were also performed to investigate the relationship between the structure and optical properties. Band structure calculations and partial density of state (PDOS) suggest that Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 has a direct band gap of ≈2.64 eV (Figure 4b and Figure S13, Supporting Information), which is smaller than the measured optical band gap because of the discontinuity of exchangecorrelation energy. [12] While the valence band maximum (VBM) is mainly due to 2p orbitals from O atoms, the conduction band minimum (CBM) includes large contributions from Nb 4d orbitals, O 2p orbitals, and Pb 6p orbitals. The PDOS also indicates that the nonlinear and linear optical properties of Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 mostly come from the NbO 6 , PbO 12 , and Pb/KO 15 polyhedra because the optical properties are closely related to the behavior of the electrons near the VBM and CBM. It is well known that strong interactions between Pb 6s orbitals and O 2p orbitals result in a high contribution of the Pb 6s orbital character in the VBM, forming an asymmetric electron density. [13] However, there is little contribution of 6s orbitals from Pb in VBM, suggesting that 6s electrons on Pb are stereochemically inert (Figure 4b). The coordination environment of Pb can be visualized by the electron localization function (ELF) diagrams ( Figure 4c and Figure S14, Supporting Information). As can be seen in the ELF diagrams, the limited stereoactivity of lone pairs on Pb 2+ cations is consistent with the result of PDOS; however, it can be clearly seen that the electron density of highly polarizable Pb 2+ cations is severely distorted. In addition, strong interatomic interactions between the NbO 6 octahedra and highly polarizable heavy metal cations such as Pb 2+ or K + are clearly visualized from the ELF diagrams. Overall, the extremely strong SHG efficiency for the reported material is attributed to the constructive addition of well-ordered moments arising from the distorted NbO 6 octahedra, structural distortions in the constituting channels with vacancies, and strong interactions of the framework with highly polarizable cations (Figure 4d).

Conclusion
A polar TTB, Pb 1 exhibits a giant type-I phase-matchable SHG of 71.5 × KDP that is the largest intensity in the visible range among the reported NLO materials thus far. A closer structural investigation along with several calculation methods suggests that the remarkably strong SHG efficiency for the title material is attributed to the synergistic effect from the highly aligned moments originating from the distorted NbO 6 octahedra, vacancy-driven structural distortions in the constituting channels, and strong interactions of the framework with highly polarizable cations. We do believe that the discovery of the novel polar TTB material with the extremely large SHG could further accelerate to open a new way toward various industries utilizing innovative coherent lights.
Although the crystal structure of the related tetragonal TB, Pb 0.91 K 1.72 Li 1.46 Nb 5 O 15 was previously reported, [6] high quality single crystals were grown and the structure of Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 was determined to better understand the structure-NLO properties relationship. Single crystals of the title compound were successfully grown by a high temperature solution method with LiBO 2 as a flux. The polycrystalline product and LiBO 2 were initially mixed with 7:3 molar ratio using a mortar and pestle. The homogeneous mixture was transferred to a platinum crucible and heated to 950°C for 10 h. After heating, the reaction product was slowly cooled to 650°C at a rate of 3°C h −1 , then the furnace was turned off to cool the temperature rapidly to room temperature. After washing extra LiBO 2 using 2 m HCl solution, colorless rod-shaped single crystals of Pb (P4/mbm) were selected for the SC-XRD measurements. The SC-XRD data were collected by using a Bruker D8 QUEST diffractometer with graphite monochromated Mo K radiation ( = 0.71703 Å) at the Advanced Bio-Interface Core Research Facility, Sogang University. The data reduction and absorption correction for the obtained data were performed through the SAINT [14] and SADABS [15] software, respectively. The crystal structures were solved using SHELXS-2013 [16] and refined by SHELXL-2013 [17] implemented in WinGX-2014. [18] PXRD data were obtained by a Rigaku Mini-Flex 600 with 40 kV and 15 mA using Cu K ( = 1.5406Å) radiation at room temperature. Temperature-dependent in situ PXRD patterns were collected using the benchtop heating stage (BTS-500, Anton-Paar) at the temperature range of 25-500°C. Synchrotron powder diffraction pattern was collected on the 2D supramolecular crystallography beamline in Pohang Acceleration Laboratory at room temperature in the 2 range of 3°-65°using synchrotron radiation ( = 0.68880 Å). The Rietveld refinements were performed by using GSAS software [19] with an initial model obtained from the SC-XRD data. Final Rietveld refinement plots and the detail crystallographic results for the title compounds are in Figures S18, S19 and Tables S1-S3, Supporting Information, respectively. IR spectrum was obtained by a Thermo Fisher Scientific Nicolet iS50 spectrometer in the range of 400-4000 cm −1 at room temperature. The ground sample was placed on the diamond attenuated total reflectance crystal.
UV-vis spectrum was recorded on a JASCO V-650 in the range of 200-700 nm at room temperature. The band gap for the title compound was calculated using the Kubelka-Munk equation. [8] TGA and DSC measurements were performed using a SCINCO TGA N-1000 and TA Q2000 DSC thermal analyzer, respectively. The polycrystalline sample was placed on an alumina crucible and heated to 900°C (500°C for DSC) at a rate of 10°C min −1 under flowing Ar.
Field emission scanning electron microscopy and EDX Spectroscopy (FE-SEM/EDX) results were collected by using a JEOL Benelux JSM-7100F attached to an Oxford Instruments NanoAnalysis AZtecEnergy. The observed heavier atoms' ratio from the EDX results for Pb 1 The quantitative analysis for Li + and K + was conducted through an ICP-OES using an Agilent ICP-OES 5900 spectrometer. The title compound was dissolved in a hydrofluoric acid and nitric acid for ICP-OES measurements. Calculated (experimental): Li + , 1.05% (1.14%); K + , 6.45% (6.74%) ( Table  S9, Supporting Information). 7 Li solid-state MAS NMR analysis was performed on a Bruker AVANCE III HD 400 MHz in a 9.4 T magnetic field at the Western Seoul Center, Korea Basic Science Institute. The polycrystalline sample was placed in a 4 mm HXY-MAS probe with a sample spinning frequency of 10 kHz. The 7 Li MAS NMR spectrum was collected at a Larmor frequency of 155.506 MHz with a delay of 10 s. Chemical shift was referenced to aqueous LiCl solution at 0 ppm.
DFT calculations were performed to investigate the relationship between the structure and optical properties of the title compound. Because the crystal structure contains disordered Pb/K sites, a supercell software was employed to generate an appropriate supercell structure. [20] The band structure and DOS were calculated using the CASTEP package, [21] which employs norm-conserving pseudopotentials and the Perdew-Burke-Ernzerhof functionals for all elements (Pb, K, Li, Nb, and O). [22] An energy cutoff of ≈925.20 eV was used, and the total energy convergence threshold was set to 10 −6 Ry. The Brillouin zone was sampled with a k-point separation of 0.03 Å −1 . ELF plots were calculated using the Quantum Espresso package [23] and visualized with the VESTA program. [24] Powder SHG measurements were performed through a modified Kurtz-Perry nonlinear optical system. [25] After sieving into distinct particle sizes, each graded sample was packed into capillary tubes (o.d. = 2.0 mm and i.d. = 1.8 mm) and irradiated by using DAWA Q-switched Nd:YAG laser (1064 nm). The SHG light (532 nm) was detected by a Hamamatsu photomultiplier tube and monitored by a Tektronix TDS 1012 oscilloscope.
The stability of the TB structure has been estimated by the tolerance factor (t) as in the case of perovskite. [11,26]  (2) where t Pb , t Pb/K , and t total are the geometric tolerance factors, r Pb , r Pb/K , r Nb , and r O are the Shannon's ionic radii [27] for Pb, Pb/K, Nb, and O, respectively. A t total value of 1 assumes no structural distortions in the framework. Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen (Germany), on quoting the depository numbers CSD-2239720-2239721.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.