Na₁₁Ta₈P₇O₄₃: (Ta₈O₃₃) bi-capped triangular prisms connected by PO₄ groups resulting in a phase-matched second harmonic response

Abstract

Much effort has been devoted to synthesizing nonlinear optical crystals with efficient second harmonic responses. Herein, a tantalum phosphate crystal Na₁₁Ta₈P₇O₄₃ with a moderate second harmonic response was obtained using self-fluxes of NaH₂PO₄ and Na₂HPO₄. Na₁₁Ta₈P₇O₄₃ belongs to the Cc space group of monoclinic systems with cell parameters of a = 8.5710(2) Å, b = 15.1144(4) Å, c = 28.7712(7) Å, β = 92.304(2)°, Z = 4. Its structural features were unique (Ta₈O₃₃) bi-capped triangular prisms uniformly oriented with a PO₄ connection in the ab plane, which were further joined through additional PO₄ groups in the c direction, resulting in moderate nonlinear effects. In addition, this crystal has the characteristics of phase-matchable second harmonic generation, a shorter cutoff edge than the known materials LiTaO₃ and KTiOPO₄, and high transmittance from the visible to the near infrared band, and may serve as a potential nonlinear optical crystal.

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1. Introduction

Nonlinear optical (NLO) crystals can broaden the wavelength range of laser radiation by means of laser frequency doubling, sum frequency generation, difference frequency generation, optical parametric oscillation and so on.¹ During the past decades, NLO crystals with efficient second harmonic generation (SHG) have attracted interest, and great efforts have been made to synthesize NLO crystals aimed at specific applications. Excellent and practical second-order NLO crystals not only must satisfy the non-centrosymmetric symmetry, but also have to satisfy the required properties of large nonlinear optical coefficients, phase-matching, a high laser damage threshold, a wide optical transparency window, high transmittance and stable physicochemical properties; they must also be easy to grow into large-sized transparent crystals and easy to process.² Currently, most of the commercially used NLO crystals are inorganic, such as KH₂PO₄ (KDP), KTiOPO₄ (KTP), LiNbO₃ (LN), β-BaB₂O₄ (BBO), LiB₃O₅ (LBO) and KBe₂BO₃F₂ (KBBF).^3–8 However, they all have some shortcomings, e.g. KBBF is toxic and difficult to grow, KTP crystals cannot completely cover the entire 3–5 μm mid-infrared region, and LiNbO₃ has a low laser damage threshold, and so on. According to prior experiences with titanates (BaTiO₃ ⁹vs. KTiOPO₄), one of us proposed that the more isolated the titanium group in the crystal, the shorter the UV transmittance cutoff edge and the higher the laser-induced damage threshold (LIDT). And a new titanate Rb₄Li₂TiOGe₄O₁₂ (RLTG)¹⁰ has been successfully synthesized and it does show a shorter UV cutoff edge and higher laser-induced damage threshold than KTP (RLTG: 0.28 μm, 6.8 GW cm⁻²vs. KTP: 0.35 μm, 3.4 GW cm⁻²).

At the same time, phosphate is also a suitable system for exploring NLO crystals because of its rich structure (including the isolated group (P_nO_3n+1)⁽ⁿ⁺²⁾⁻, ring (P_nO_3n)ⁿ⁻, and chain (PO₃)_∞),¹¹ low growth temperature and a short UV cutoff edge. However, the anisotropy of the phosphate group is small, as are its birefringence and nonlinear susceptibility. Therefore, the introduction of octahedral-coordinated d⁰ transition metal cations with larger distortion and larger anisotropy will help to obtain a non-centrosymmetric crystal with large birefringence and large nonlinear susceptibility (second-order Jahn–Teller effect).¹²

A related scientific research study to explore the octahedral-coordinated d⁰ transition metal phosphate system has been in progress. In 2022, two kinds of non-centrosymmetric tantalum phosphate crystals, Na₃TaP₂O₉ ¹³ and Na₅Ta₈P₅O₃₅,¹⁴ were discovered by our group. Both crystals had the characteristics of phase-matchability, a large band gap, a wide transmission range and high transmittance. Here, we report a new tantalum phosphate crystal Na₁₁Ta₈P₇O₄₃ (denoted as NTPO), in which eight distorted TaO₆ octahedra are connected into a unique (Ta₈O₃₃) bi-capped triangular prism. (Ta₈O₃₃) units are connected by PO₄ groups, indicating that the Ta⁵⁺ ions in NTPO are more separated than those in LiTaO₃.¹⁵ In this paper, the synthesis, structure, characterization methods and theoretical calculation for NTPO are presented in detail.

2. Experimental section

2.1. Synthesis

The polycrystalline powder of the Na₁₁Ta₈P₇O₄₃ crystal was prepared by mixing the chemicals of NaH₂PO₄ (Shanghai Aladdin Biochemical Technology Co., Ltd, 99.0%), Na₂HPO₄ (Shanghai Macklin Biochemical Co., Ltd, 99.99%) and Ta₂O₅ (Shanghai Aladdin Biochemical Technology Co., Ltd, 99.99%) according to a stoichiometric ratio of 3 : 4 : 4. Then, the mixture was ground evenly and loaded into a platinum crucible of diameter 20 mm × 20 mm, which was slowly heated to 200 °C overnight to decompose NaH₂PO₄ and Na₂HPO₄. Finally, the sample was slowly heated to 850 °C and kept at this temperature for about 500 minutes to obtain the Na₁₁Ta₈P₇O₄₃ polycrystalline powder, which was ground several times during the process.

Single crystals of Na₁₁Ta₈P₇O₄₃ were obtained in an open system by the flux method using self-fluxes NaH₂PO₄ and Na₂HPO₄. The compounds NaH₂PO₄, Na₂HPO₄ and Ta₂O₅ were thoroughly mixed at masses of 2.0 g, 1.8 g and 0.5 g, respectively, and placed into a platinum crucible. Then, the crucible was placed in a programmable electric furnace, heated to 850 °C and kept for 24 hours to melt the content completely. Finally, the crucible was cooled to 600 °C with a cooling rate of 2 °C per hour, and then the furnace was switched off. Some millimeter-sized colorless single crystals were obtained.

2.2. Single crystal structure determination

X-ray diffraction data of a transparent, crack-free single crystal Na₁₁Ta₈P₇O₄₃ was gathered at 293(2) K using a Rigaku Synergy diffractometer. Data collection, reduction and cell refinement were all completed using CrysAlisPro software. At the Olex2¹⁶ interface, the crystal structure of Na₁₁Ta₈P₇O₄₃ was analyzed using SHELXT-2018¹⁷ (intrinsic phasing method) and refined using SHELXL-2014¹⁸ (full matrix least squares on F²). No additional symmetry was found in the ADDSYM routine of the PLATON¹⁹ program. Crystal data, structure refinement and specific crystallographic information for Na₁₁Ta₈P₇O₄₃ can be found in Table 1 and Tables S1–S3,† respectively.

Table 1 Crystal data and structure refinement of Na₁₁Ta₈P₇O₄₃

Formula	Na11Ta8P7O43
Formula weight	2605.28
Temperature/K	293(2)
Crystal system	Monoclinic
Space group	Cc
a/Å	8.5710(2)
b/Å	15.1144(4)
c/Å	28.7712(7)
β/°	92.304(2)
Volume/Å^3	3724.17(16)
Z	4
ρ calc/g cm^−3	4.647
μ/mm^−1	23.967
F (000)	4616.0
Radiation	MoKα (λ = 0.71073)
Independent reflections	7344 [Rint = 0.0356, Rsigma = 0.0450]
Goodness-of-fit on F^2	1.025
Final R indexes [I ≧ 2σ (I)]	R 1 = 0.0244, wR2 = 0.0505
Flack parameter	0.023(8)
Final R indexes [all data]	R 1 = 0.0259, wR2 = 0.0510

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2.3. Powder X-ray diffraction (PXRD)

The powder X-ray diffraction of the polycrystalline powder Na₁₁Ta₈P₇O₄₃ was tested using a Bruker D8 Focus diffractometer. Diffraction data were collected in the 2θ range of 5–75°. The results show that the experimental diffraction pattern of Na₁₁Ta₈P₇O₄₃ is in line with the simulated pattern of the solved structure (cif file) (Fig. 1). The Le Bail fitting of the experimental XRD pattern using the FULLPROF program is given in the ESI (Fig. S1†).^20–22

Fig. 1 The simulated and experimental PXRD patterns of Na₁₁Ta₈P₇O₄₃.

2.4. Thermal analysis

Thermogravimetry and differential scanning calorimetry (TG-DSC) were performed with 10 mg of NTPO samples in an Al₂O₃ crucible. Then the sample was heated from 50 °C to 1150 °C at a rate of 10 K min⁻¹ and finally cooled at 10 K min⁻¹ (NETZSCH STA 449F3).

2.5. Diffuse reflectance and infrared spectra

Using BaSO₄ as a reference, the diffuse-reflectance spectra were recorded in the range of 200–2500 nm (Cary 7000 UV-vis-NIR spectrophotometer) and the infrared spectra were recorded in the range of 500–4000 cm⁻¹ (Varian Excalibur 3100 spectrometer).

2.6. SHG tests

The SHGs of Na₁₁Ta₈P₇O₄₃ were tested at the wavelength of 1064 nm using a Q-switched Nd:YAG laser adopting the Kurtz–Perry method.²³ The crystalline Na₁₁Ta₈P₇O₄₃ powder was ground and sieved into different particle size ranges to compare with the corresponding sizes of KDP (50–100, 100–150, 150–224, 224–300 and 300–355 μm).

2.7. Computational details

Using the CRYSTAL17 code²⁴ and the B3LYP functional,²⁵ the electronic band structure of Na₁₁Ta₈P₇O₄₃ was computed with a mesh of 6 × 6 × 2 k points in the irreducible reciprocal cell. The pure electronic static optical properties (ω = 0) were calculated with the coupled perturbation Kohn–Sham method.^26–28

3. Results and discussion

3.1. Structure description

NTPO belongs to the Cc space group of monoclinic systems, and its cell parameters are a = 8.5710(2) Å, b = 15.1144(4) Å, c = 28.7712(7) Å, β = 92.304(2)°, V = 3724.17(16) ų (Table 1). There are 4 chemical formulas within the unit cell of NTPO (Z = 4); all atoms are in the general position. As can be seen from the cell volume, NTPO is a very large structure with the basic structural units of the TaO₆ octahedron and PO₄ tetrahedron. In this structure, all the TaO₆ octahedra are distorted. The eight distorted TaO₆ octahedra form a unique (Ta₈O₃₃) bi-capped triangular prism by sharing O atoms; these interesting structural units were first seen in tantalum phosphate crystals (Fig. 2a). Each (Ta₈O₃₃) unit was connected to six adjacent (Ta₈O₃₃) ones by twelve PO₄ tetrahedra to form an infinite two-dimensional (Ta₅₆P₁₂O₂₄₃)_∞ layer (Fig. 2b). The layers were again joined by isolated PO₄ tetrahedra, forming a three-dimensional network, in which Na⁺ ions fill the voids (Fig. 2c). The bond lengths of Na–O, Ta–O and P–O bonds in NTPO were in the range of 2.203(11)–2.959(11) Å, 1.909(10)–2.078(8) Å and 1.485(9)–1.561(9) Å, respectively. The calculated bond valence sums (BVS) of Na, Ta, P and O were 0.8749–1.1868, 4.9348–5.1124, 4.9782–5.0768 and 1.8229–2.2208, respectively, which were in line with their ideal values.

Fig. 2 (a) The structural units, (b) the (Ta₅₆P₁₂O₂₄₃)_∞ plane layer, and (c) the crystal structure of NTPO. Orange – Na, blue – Ta, bright green – P and red – O.

3.2. Thermal analysis

NTPO has no obvious thermal effect in the process of heating and cooling, and its weight decreases significantly from about 900 °C (Fig. S2†). The powder XRD spectra show that NTPO is stable below 850 °C, begins to partially decompose at 900 °C, and completely decomposes into Na₂Ta₄O₁₁ at 1350 °C (Fig. S3†).

3.3. Optical properties

The infrared spectrum of the NTPO crystal is shown in Fig. 3(a) and mainly shows the vibrations of the PO₄ group. The absorption observed at the frequencies 997–1190 cm⁻¹ and 972 cm⁻¹ was attributed to the stretching vibrations of ν₃ and ν₁ of the P–O bonds, respectively. The absorption near the frequency 555–638 cm⁻¹ was attributed to the bending vibration of ν₄.²⁹ And the absorption observed at the frequencies 794 cm⁻¹ and 705 cm⁻¹ was attributed to the stretching vibrations of ν_as and ν_s of the Ta–O bonds, respectively.³⁰ The UV-vis-NIR spectrum of the NTPO crystal is shown in Fig. 3(b), which has a high transmittance of about 92% in the visible to near-infrared bands. And NTPO still has a reflectance of 31.89% at 261 nm, illustrating that its cutoff edge is less than 261 nm.

Fig. 3 The infrared spectra (a) and UV-vis-NIR diffuse reflectance spectra (b) of NTPO.

3.4. NLO properties

The Kurtz–Perry method was adopted to measure the SHG response of NTPO, which was increased with the increase in particle size. NTPO exhibited a SHG response of about 1× KDP, and was phase-matchable (Fig. 4). On the basis of the anionic group theory,³¹ the distorted TaO₆ octahedra and PO₄ tetrahedra were the principal sources of the SHG response of the NTPO crystal. Among them, the distortions of the TaO₆ octahedra were small compared to those in LiTaO₃ (0.162 Å),³² as reflected by bond length differences (Δr) of Ta1–O (0.135 Å), Ta2–O (0.128 Å), Ta3–O (0.053 Å), Ta4–O (0.092 Å), Ta5–O (0.117 Å), Ta6–O (0.128 Å), Ta7–O (0.161 Å), and Ta8–O (0.133 Å), respectively, averaging 0.118 Å. When these distorted TaO₆ octahedra joined to form a (Ta₈O₃₃) unit, the distortions of some of them pointed in opposite directions, therefore cancelling their contributions to the SHG effect. Nevertheless, the (Ta₈O₃₃) unit has an approximate symmetry of D_3h, similar to that of the (B₃O₆)³⁻ unit in BBO, and the arrangement of the entire ab plane of the crystal is also similar to that of BBO (Fig. S4†). Notably, the SHG response of NTPO is greater than that of the tantalum phosphates discovered by our group previously, such as Na₃TaP₂O₉ (0.6× KDP) and Na₅Ta₈P₅O₃₅ (0.5× KDP). Moreover, if Ta⁵⁺ cations are replaced by Nb⁵⁺ cations, niobium phosphates with the corresponding structures are expected to exhibit a greater NLO–SHG response.³³

Fig. 4 The SHG curve of NTPO with KDP of different particle sizes as a reference.

3.5. Theoretical calculations

According to theoretical calculations, NTPO makes an electronic transition from the Γ point (0,0,0) to the C point (1/2,1/2,0) with an indirect band gap of 5.55 eV. The density of states shows that the top of the valence band and the bottom of the conduction band are mainly contributed by the O and Ta orbitals, respectively (Fig. S5†). From the diffuse reflectance spectra, the cutoff edge is less than 261 nm, which is shorter than the cutoff edge of LiTaO₃ (280 nm) and KTP (350 nm).³⁴ Considering Kleinman symmetry,³⁵ NTPO belongs to the Cc space group of the m point group with six independent SHG coefficients (d₁₁ = −0.63 pm V⁻¹, d₁₂ = 0.78 pm V⁻¹, d₁₃ = −0.08 pm V⁻¹, d₃₁ = −0.07pm V⁻¹, d₃₂ = 0.10 pm V⁻¹ and d₃₃ = 0.03 pm V⁻¹). Among them, the largest SHG coefficient d₁₂ was about twice the KDP coefficient d₃₆ and one-third of the BBO coefficient d₁₁, which also indicates that the degree of distortion of the (Ta₈O₃₃) unit is not large enough. In addition, NTPO belongs to the biaxial crystals, with refractive indices of n_x = 1.6839, n_y = 1.6872 and n_z = 1.7444. The relatively large birefringence of Δn = 0.0605 can satisfy the phase matching conditions in the visible range, which is also consistent with our test results. The original results of NTPO are shown in the ESI.†

4. Conclusion

In summary, a tantalum phosphate Na₁₁Ta₈P₇O₄₃ with a phase-matched second harmonic response was synthesized by the flux method. Notably, the second-harmonic response was mainly due to the uniform orientation of the (Ta₈O₃₃) bi-capped triangular prisms through PO₄ tetrahedron connections. Simultaneously, this kind of crystal has a shorter cutoff edge than LiTaO₃ and KTP and high transmittance from the visible to near infrared bands, and thus may serve as a potential nonlinear optical crystal. And in future work Ta⁵⁺ ions can be replaced by Nb⁵⁺ ions to obtain niobium phosphate crystals with a greater NLO–SHG response.

Author contributions

Ziyan Lv: sample preparation, data curation and analysis, and writing – original manuscript. Rukang Li: supervision, funding acquisition, theoretical calculations, data analysis, and review and editing of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52172010).

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Footnote

† Electronic supplementary information (ESI) available: TG-DSC; X-ray diffraction patterns; fractional atomic coordinates and equivalent isotropic displacement parameters (Ų); anisotropic displacement parameters (Ų); selected bond lengths (Å); and the original calculation results. CCDC \ 2249067. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt02207b

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