From AgTeO₂F and Ag₂(TeO₂F₂) to Ag₃F₃(TeF₆)(TeO₂)₁₂: the first silver tellurite oxyfluorides with linear and nonlinear optical properties

Abstract

The first examples of silver fluorotellurites, namely, AgTeO₂F and Ag₂(TeO₂F₂), and the first silver fluoride tellurite, Ag₃F₃(TeF₆)(TeO₂)₁₂, have been obtained successfully under mild hydrothermal conditions. AgTeO₂F displays a new 2D structure composed of a 2D silver oxide layer strengthened by 1D [TeO₂F]_∞ chains. Ag₂(TeO₂F₂) exhibits a new 3D construction consisting of a 3D silver oxyfluoride framework with 1D polyhedral ring channels occupied by the isolated TeO₂F₂ groups. Ag₃F₃(TeF₆)(TeO₂)₁₂ features the first 3D neutral [TeO₂]_∞ open framework with 8- and 4-MPR channels along the a-, b- and c-axes. AgTeO₂F and Ag₂(TeO₂F₂) crystallize in the centrosymmetric space group while Ag₃F₃(TeF₆)(TeO₂)₁₂ is in a non-centrosymmetric space group. Ag₃F₃(TeF₆)(TeO₂)₁₂ shows a unique nonlinear optical property with an SHG intensity of about 70% that of commercial KH₂PO₄, while AgTeO₂F and Ag₂(TeO₂F₂) present an apparent linear optical property and their birefringence is calculated as 0.078 and 0.032@1064 nm, respectively. This work further confirms that fluorination in tellurium(IV) oxides can greatly enrich the structural chemistry and optical properties of metal tellurites.

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Introduction

Second harmonic generation (SHG) materials have continued to be in great demand due to their capacity of laser frequency conversion, material micromachining, photolithography and so on.^1–9 The first requirement for an SHG material is a non-centrosymmetric (NCS) structure since it is a physical quantity described by an odd-order tensor.^10–14 The lone pair cation of Te(IV), with a 5s² electronic configuration, can exhibit three different kinds of asymmetric coordination mode, namely, TeO₃ trigonal pyramid, TeO₄ seesaw-like group, and TeO₅ tetragonal pyramid, when it coordinates with oxygen ligands due to the hybridization between the s- and p-orbitals of the cation and anions.¹⁵ These polar tellurite groups can stimulate the formation of NCS structures, making metal tellurites good candidates for SHG materials.^16–20 Metal tellurites can also display plentiful structure types because the polar oxyanions can join together by condensation to generate 0D clusters,^21–23 1D chains,^24–27 2D layers^28,29 and even 3D frameworks.^30,31

Fluorine, as the most electronegative element, could replace oxygen ligands due to their similar ionic radii.^32–34 When fluorine atoms are introduced into oxide compounds, fluoride compounds often can display excellent comprehensive performance and abundant structure types,^35–38 such as RbTeMo₂O₈F (27 × KDP, 3.63 eV),³⁹ Ba(MoO₂F)₂(TeO₃)₂ (7.8 × KDP, 2.96 eV),⁴⁰ and BaF₂TeF₂(OH)₂ (3 × KDP, 5.9 eV)⁴¹ in metal tellurites. Fluorine-containing tellurite compounds (or tellurite oxyfluorides) can be classified into fluoride tellurites and fluorotellurites according to the connection mode of the fluorine element. In fluorotellurites, the fluorine atoms are connected with the lone pair cation of Te(IV); in other words, there are Te(IV)–F bonds in the structures, such as Bi₃F(TeO₃)(TeO₂F₂)₃⁴² and HgTeO₂F(OH).⁴³ BaF₂TeF₂(OH)₂ is the first reported SHG material in metal fluorotellurites, which has proved to be a promising UV nonlinear optical material.⁴¹ The Te–F bonds in BaF₂TeF₂(OH)₂ play an important role in the wide band gap. In fluoride tellurites, there are no Te(IV)–F bonds, and the fluorine atoms are linked with the other cations, such as d⁰ transition metals (TM), RbTeMo₂O₈F³⁹ and Ba(MoO₂F)₂(TeO₃)₂,⁴⁰ and alkali metals, Li₇(TeO₃)₃F.⁴⁴ RbTeMo₂O₈F containing a MoO₅F octahedron features the strongest SHG material of metal tellurites at both the visible and near-infrared ranges, and the MoO₅F octahedron contributed the most to the SHG efficiency of RbTeMo₂O₈F. Therefore, fluorination in tellurites not only broadens the band gap of metal tellurites, but can also increase the SHG intensity of crystals. However, compared with tellurite oxides, tellurite oxyfluorides, including fluoride tellurites and fluorotellurites, are still rare due to their synthetic difficulty. It is still very necessary to enrich the structural chemistry and the SHG crystal types in metal tellurite oxyfluorides.

To increase the success rate of NCS structures, the d¹⁰ TM cation Ag⁺ was chosen to balance the charge due to its large polar displacement.^28,45–48 After an extensive survey, we found that no silver tellurites with the fluorine element have been reported yet. Our efforts in the silver–tellurite–fluorine system successfully result in the first examples of silver fluorotellurites and the first silver fluoride tellurite, namely, AgTeO₂F and Ag₂(TeO₂F₂), and Ag₃F₃(TeF₆) (TeO₂)₁₂. Among these compounds, Ag₃F₃(TeF₆) (TeO₂)₁₂ crystallized in an NCS space group and presented a moderate SHG response of 0.7 × KH₂PO₄ (KDP) at 1064 nm. Herein, we report their syntheses, crystal structures, and thermal and optical properties from theoretical and experimental aspects.

Experimental section

Reagents

Silver fluoride (AgF, 98+%, AR), tellurium dioxide (TeO₂, 99.9%, AR) and hydrofluoric acid (HF, 40+%, AR) were obtained commercially and used as received. Caution! Hydrofluoric acid is toxic and corrosive! It must be handled with extreme caution and with the appropriate protective equipment and training.

Syntheses

The three compounds were successfully synthesized via a mild hydrothermal method based on the following chemical proportions: AgF (0.127 g, 1.0 mmol), TeO₂ (0.160 g, 1.0 mmol), hydrofluoric acid (0.25 mL) and 1 mL of deionized water for AgTeO₂F; AgF (0.127 g, 1.0 mmol), TeO₂ (0.080 g, 0.5 mmol), hydrofluoric acid (0.25 mL) and 1.5 mL of deionized water for Ag₂(TeO₂F₂); AgF (0.102 g, 0.8 mmol), TeO₂ (0.256 g, 1.6 mmol), hydrofluoric acid (0.25 mL) and 0.5 mL of deionized water for Ag₃F₃(TeF₆)(TeO₂)₁₂. It is worth noting that the amount of deionized water is critical in these reactions besides the proportions of the raw materials. The mixtures were placed in a 23 mL Teflon liner equipped with a stainless-steel autoclave. These samples were heated to 220 °C, maintained for 48 h, and then cooled to 30 °C at 2.3 °C h⁻¹. After washing with deionized water, the products were dried in air at room temperature. As shown in Fig. S1,† the three compounds exhibit three different morphologies: lamellar AgTeO₂F, columnar Ag₂(TeO₂F₂) and octahedral Ag₃F₃(TeF₆)(TeO₂)₁₂, and the three kinds of crystals were obtained in yields of about 58%, 43% and 75% (based on Te), respectively. The existence and distribution of Ag, Te and F were demonstrated via the elemental distribution maps (Fig. 1). As shown in Fig. S2,† the experimental powder X-ray diffraction (PXRD) patterns are in good agreement with the calculated ones, so their purities are certified.

Fig. 1 SEM images of AgTeO₂F (a), Ag₂(TeO₂F₂) (b) and Ag₃F₃(TeF₆)(TeO₂)₁₂ (c) and their elemental distribution maps.

Single-crystal structure determination

Single-crystal X-ray diffraction data of AgTeO₂F, Ag₂(TeO₂F₂) and Ag₃F₃(TeF₆) (TeO₂)₁₂ were collected on an Agilent Technologies SuperNova dual-wavelength CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Cell refinement and data reduction were conducted with CrysAlisPro and absorption correction based on the multi-scan method was applied.⁴⁹ The structures were determined by direct methods and refined by full-matrix least-squares fitting on F² using the SHELXL-2017 software package.⁵⁰ The atoms were refined with anisotropic thermal parameters. The detailed crystallographic data of the three structures are listed in Table 1, and some selected atomic coordinates, bond lengths and angles are listed in Tables S1–S3.† Detailed information on the crystal structure of the three compounds can be obtained from the Cambridge Crystallographic Data Center.

Table 1 Summary of crystal data and structural refinements for AgTeO₂F, Ag₂(TeO₂F₂) and Ag₃F₃(TeF₆)(TeO₂)₁₂

Molecular formula	AgTeO2F	Ag2(TeO2F2)	Ag3F3(TeF6)(TeO2)12
a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2}^1/2.
Formula weight	286.47	413.34	2575.07
Crystal system	Monoclinic	Orthorhombic	Cubic
Space group	P21/c	Pbca	P4̄3n
Temperature (K)	292.38(10)	293.39(10)	294.15
F(000)	728	2880	2245
a/Å	8.7523(14)	15.6642(13)	11.3998(5)
b/Å	6.3759(9)	6.8862(5)	11.3998(5)
c/Å	5.4505(8)	16.5328(12)	11.3998(5)
α (°)	90	90	90
β (°)	91.396(14)	90	90
γ (°)	90	90	90
V/Å^3	304.07(8)	1783.3(2)	1481.47(19)
Z	4	16	2
D c (g cm^−3)	6.258	6.158	5.773
GOF on F^2	1.008	1.073	1.138
Flack factor	—	—	0.48(9)
R 1, wR2 [I > 2σ(I)]^a	0.0223, 0.0439	0.0306, 0.0704	0.0200, 0.0434
R 1, wR2 (all data)^a	0.0267, 0.0462	0.0377, 0.0750	0.0251, 0.0460

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Powder X-ray diffraction

Powder X-ray diffraction (PXRD) data of the three crystals were recorded on a Rigaku Miniflex600 diffractometer equipped with graphite-monochromated Cu Kα radiation at room temperature. The 2θ range is set to 5–70°, and the scan step size is 0.02°.

Energy-dispersive X-ray spectroscopy

Microprobe elemental analysis was conducted with the aid of a field-emission scanning electron microscope (JSM6700F) outfitted with an energy-dispersive X-ray spectroscope (Oxford INCA).

Spectroscopic measurements

The IR spectrum was recorded using a Magna 750 FT-IR spectrometer under an air background, and the selected range is 4000–400 cm⁻¹. The UV-vis-NIR diffuse reflection spectrum was measured using a PerkinElmer Lambda 950 UV-vis-NIR spectrophotometer with a BaSO₄ powder board as a reference for 100% reflectance, and the recording range was 200–2000 nm. Absorption data were calculated from the diffuse reflection data by the Kubelka–Munk function: α/S = (1 − R)²/2R, in which α and S represent the absorption coefficient and the scattering coefficient, respectively. The band gap value can be given by extrapolating the absorption edge to the baseline in the α/S versus energy graph.

Thermal analysis

A NETZCH STA 449F3 thermal analyzer was used to analyze the thermal stability of the three crystals. The three samples were heated in alumina crucibles from 20 °C to 1000 °C under nitrogen gas at a rate of 15 °C min⁻¹.

SHG measurements

Powder SHG measurements were carried out using a modified method of Kurtz and Perry. An irradiation laser beam (λ = 1.064 μm) is generated with a Nd:YAG solid-state laser equipped with a Kurtz and Perry setup.⁵¹ The SHG signal oscilloscope traces of the Ag₃F₃(TeF₆)(TeO₂)₁₂ and KDP samples in a particle size range (150–210 μm) were both recorded.

Results and discussion

Structure of AgTeO₂F

Equivalent silver and tellurium sources resulted in the production of AgTeO₂F. AgTeO₂F crystallizes in the monoclinic space group P21/c. Its asymmetric unit contains 1 Ag, 1 Te, 2 O and 1 F atoms (Table S1†). The Ag(1) atom is 4-coordinated into a distorted tetrahedron AgO₄ with the lengths of Ag–O bonds ranging from 2.319(4) to 2.478(5) Å. The Te(1) atom connects with 3 O and 1 F atoms, forming a polar TeO₃F group in a seesaw-like configuration. The Te–O bond distances are in the range of 1.832(4)–2.162(4) Å and the Te–F bond length is 2.029(4) Å, which are consistent with the reported metal tellurites.^43,52–57 The calculated total bond valences for Ag and Te are 0.825 and 4.031, indicating their oxidation states of +1 and +4 respectively (Table S2†).

AgTeO₂F presents a new 2D layered structure. Within the structure, the AgO₄ tetrahedra were corner- and edge-shared into a silver oxide layer parallel to the bc plane (Fig. 2a). The TeO₃F groups were corner-shared into a zigzag chain along the c-axis (Fig. 2b). The fluorotellurite 1D chains were linked on the both sides of the silver oxygen layers via Te–O–Ag bonds, forming the 2D layered structure of AgTeO₂F (Fig. 2c). The lone pairs of tellurites were pointed to the space between layers. The interlayer distance was calculated as 8.75 Å.

Fig. 2 Silver oxide layer in the bc plane (a), the 1D chain of Te–O–F (b), and the layer structure of AgTeO₂F (c).

Structure of Ag₂(TeO₂F₂)

When we increased the ratio of silver, a new compound of Ag₂(TeO₂F₂) was achieved. Ag₂(TeO₂F₂) presents a new 3D construction composed of a 3D silver oxygen framework strengthened by TeO₂F₂ units. The structure crystallized in the orthorhombic space group Pbca. Its asymmetric unit includes 4 Ag, 2 Te, 4 O and 2 F atoms. Both of the Te(IV) cations were four-coordinated in seesaw-like TeO₂F₂ groups with the Te–O and Te–F bonds in the ranges of 1.848–1.869 Å and 2.021–2.068 Å, respectively (Fig. S3a and S3b†). The four Ag⁺ cations were connected with both O and F atoms, forming Ag(1)O₃F, Ag(2)O₃F₂, Ag(3)O₄F, and Ag(4)O₃F polyhedra, respectively. The Ag–O and Ag–F bond distances were in the ranges of 2.19(6)–2.663(8) and 2.458(6)–2.68(6) Å, respectively. The oxidation states of the Ag and Te atoms were proved to be +1 and +4, respectively. The calculated total bond valences for Ag(1), Ag(2), Ag(3), Ag(4), Te(1) and Te(2) are 0.769, 0.961, 0.711, 0.809, 4.007, and 4.066, respectively.

The Ag(1)O₃F polyhedra were corner-shared in a Ag(1) chain along the b-axis, so were the Ag(2) polyhedra (Fig. 3a and b). The Ag(3)O₄F and Ag(4)O₃F polyhedra were corner- and edge-shared into a 1D Ag(3)Ag(4) chain along the b-axis with four-member polyhedral rings (MPRs) (Fig. 3c). The Ag(3)Ag(4) chains were connected by the Ag(1) chain to form a 2D layer parallel with the ab plane (Fig. S3c†), which were further linked by the Ag(2) chains to a 3D silver oxyfluoride open framework with 1D 18-MPR channels along the b-axis (Fig. S3d†). The isolated Te(1)O₂F₂ and Te(2)O₂F₂ units were filled in the 18-MPR channels to strengthen the 3D structure of Ag₂(TeO₂F₂) (Fig. 3d). Due to the blocking effect of TeO₂F₂ groups, the 18-MPR channels have been shrunk to 10-MPR channels with the lone pairs of tellurites pointed to the center of them.

Fig. 3 Ag(1)–O–F zigzag chain (a), Ag(2)–O–F wave chain (b), 1D Ag(3)–Ag(4)–O–F chains (c), and 3D structure of Ag₂(TeO₂F₂) (d).

Structure of Ag₃F₃(TeF₆)(TeO₂)₁₂

A mixed valence tellurium compound of Ag₃F₃(Te^VIF₆)(Te^IVO₂)₁₂ was obtained when we adjusted the reaction conditions further. Ag₃F₃(TeF₆)(TeO₂)₁₂ is crystallized in the cubic space group P4̄3n. There are 1 Ag, 2 Te, 2 O and 2 F atoms in the asymmetric unit. Tetravalent Te(1) is coordinated with four O²⁻ anions into a TeO₄ quadrangular pyramid with the Te–O bond lengths ranging from 1.827(4) to 2.107(5) Å while the hexavalent Te(2) atom connects with 6 F atoms to form a TeF₆ octahedron with a Te–F bond length of 1.906(17) Å (Fig. S4a†). The Ag⁺ cation is 8-coordinated into AgO₈ polyhedra with the length of Ag–O bonds ranging from 2.589(7) to 2.559(7) Å (Fig. S4b†). The oxidation state of Ag, Te(1) and Te(2) atoms was proved to be +1, +4 and +6, respectively, based on the calculated total bond valences of 1.000, 4.078 and 6.180 for Ag, Te(1) and Te(2), respectively.

Ag₃F₃(TeF₆)(TeO₂)₁₂ features an interesting 3D structure formed by the 3D Ag₃F₃(TeO₂)₁₂ framework embedded with the isolated (TeF₆) octahedra. The tetravalent Te(1)O₄ units were interconnected into a 3D neutral [TeO₂]_∞ open framework by corner-sharing with 8- and 4-MPR channels along the a-, b- and c-axes (Fig. 4a). The Ag(1) cations were located in the two different 1D channels to form the 3D cationic framework of [Ag₃(TeO₂)₁₂]³⁺ (Fig. S4c†). Interestingly, the silver cations were situated at the sites with symmetry of 222, namely, the twelve axis-center and the six face-center of the unit cell. The body center of the cubic structure was empty, and was filled by the isolated F(2) atoms and the (TeF₆) octahedron to balance the charge and support the framework, respectively (Fig. 4b). So, the structure of Ag₃F₃(TeF₆)(TeO₂)₁₂ can be described as a 3D construction composed of a 3D Ag₃F₃(TeO₂)₁₂ framework embedded with the neutral (TeF₆) octahedra.

Fig. 4 The 3D skeleton with two different 1D channels (a), and the structure chart of AgO₈ and TeF₆ residing in the centre of 8-MPRs and 4-MPRs (b).

From the elucidation of the structures, we can find that the quantity and site of the fluorine element are totally different in the three tellurite oxyfluorides. In AgTeO₂F, only one oxygen was replaced in the TeO₃F groups while two oxygen atoms were substituted in TeO₂F₂ of Ag₂(TeO₂F₂). Only the Te(VI) groups were fluoride in Ag₃F₃(TeF₆)(TeO₂)₁₂. We think that the difference should be caused by the different synthesis conditions, especially from the molar ratio of AgF to TeO₂. When the ratio of AgF/TeO₂ was 1/1, AgTeO₂F was obtained, of which the tellurite was a mono-substituted TeO₃F group. If the ratio was increased to 2/1, Ag₂(TeO₂F₂) was isolated and disubstituted TeO₂F₂ groups were found. When the ratio was decreased to 1/2 and the content of solvent water was halved, partial Te(IV) oxidized into Te(VI) and mixed valence Ag₃F₃(TeF₆)(TeO₂)₁₂ was achieved. In the structure of Ag₃F₃(TeF₆)(TeO₂)₁₂, only Te(VI) groups were fluoride, which can be explained by the Hard–Soft–Acid–Base (HSAB) theory. Compared with a Te(IV) ion, Te(VI) has higher charge and a smaller radius, so it has stronger acidity. The fluorine element with strong electronegativity can be regarded as a hard base, which is liable to coordinate with cations with strong acidity. This work revealed that the fluorination in tellurium(IV) oxides can enrich the structural chemistry of metal tellurites greatly.

Thermal analyses

Thermogravimetric analyses (TGA) were performed to explore the thermal behavior of the three compounds. From Fig. 5a we can find that there are slight weight losses of about 0.94% and 0.97% in the ranges of 294–494 °C and 271–509 °C for AgTeO₂F and Ag₂(TeO₂F₂), respectively, corresponding to the escape of partial F₂ molecules. We checked the thermal stability of AgTeO₂F and Ag₂(TeO₂F₂) at 20, 300, 400 and 500 °C, respectively, by PXRD measurements. From the results shown in Fig. S5,† we can find that the PXRD patterns of the two samples at 300 °C could not match the patterns at room temperature, which indicates that the two crystals have been decomposed at 300 °C. The release of the TeO₂ molecule occurred at about 662 °C and 689 °C for AgTeO₂F and Ag₂(TeO₂F₂), respectively. The thermogravimetric-mass spectrometry (TG-MS) measurement was performed for Ag₃F₃(TeF₆)(TeO₂)₁₂ (Fig. 5b). The TG curve of Ag₃F₃(TeF₆)(TeO₂)₁₂ involves two steps of weight-losses. The first one (exp. 6.6%) in the range of 328 to 524 °C is consistent with the release of 4.5 F₂ molecules (cal. 6.7%). The second weight loss that occurred above 684 °C can be attributed to the evaporation of the TeO₂ molecules, which did not complete even at 1000 °C, just like the situations in AgTeO₂F and Ag₂(TeO₂F₂). The ion current curves of Ag₃F₃(TeF₆)(TeO₂)₁₂ indicate the existence of the fluorine element and the absence of the OH⁻ group or the H₂O molecule, which further proved the correctness of the formula.

Fig. 5 TGA curves of AgTeO₂F and Ag₂(TeO₂F₂) (a), and the TG-MS curves of Ag₃F₃(TeF₆)(TeO₂)₁₂ (b).

IR and UV-vis-NIR spectra

The IR spectra revealed that no obvious absorption was found in the region 4000–800 cm⁻¹ for the three compounds, indicating no hydroxy bonds in the structures (Fig. S5†). Their vibration bands were focused over the range of 400–780 cm⁻¹, which can be attributed to the Te–O and Te–F vibrations. These assignments correspond to the reported tellurites.^52,58–63

The UV-vis-NIR diffuse-reflectance spectra of the three compounds show that they have no obvious absorption in the range of 500–2000 nm (Fig. 6). The ultraviolet absorption cutoff edges of AgTeO₂F, Ag₂(TeO₂F₂) and Ag₃F₃(TeF₆)(TeO₂)₁₂ were 283, 328 and 276 nm, respectively, and their optical band gaps were measured as 3.41, 3.22 and 3.69 eV, respectively.

Fig. 6 UV-vis-NIR diffuse reflectance spectra of AgTeO₂F (a), Ag₂(TeO₂F₂) (b) and Ag₃F₃(TeF₆)(TeO₂)₁₂ (c).

SHG measurements

For Ag₃F₃(TeF₆)(TeO₂)₁₂, measurement of the powder frequency-doubling effect was carried out by the method of Kurtz and Perry, employing a Q-switched Nd:YAG laser under 1064 nm radiation. A sieved sample (70–100 mesh) was used to assess its second-order susceptibility coefficient. Powder SHG examination revealed that Ag₃F₃(TeF₆)(TeO₂)₁₂ displayed a moderate frequency-doubling efficiency of about 0.7 × KDP (Fig. 7).

Fig. 7 The oscilloscope traces of the SHG signals for the samples (150–210 μm) of Ag₃F₃(TeF₆)(TeO₂)₁₂ and KDP under laser irradiation at 1064 nm.

Theoretical calculation

To access the birefringence of AgTeO₂F and Ag₂(TeO₂F₂), their linear optical properties have been studied using CASTEP based on DFT methods. Such work has not been performed for Ag₃F₃(TeF₆)(TeO₂)₁₂ due to its isotropic nature.

Fig. 8 presents the band structures along the high symmetry points of the first Brillouin zone for AgTeO₂F and Ag₂(TeO₂F₂). The state energies of the CBs and highest VBs of the two structures are shown in Table S4.† For AgTeO₂F, the maximum of VBs is situated at the Z point and the minimum of CBs is placed at the G point with a band gap of 2.29 eV, indicating that it is an indirect band gap compound (Fig. 8a). For Ag₂(TeO₂F₂), the top of the VBs is located at the X point while the bottom of CBs is placed at the G point with a band gap of 1.64 eV, which shows that it also is an indirect band gap material (Fig. 8b). The calculated band gaps are much smaller than the experimental results, which was caused by the discontinuity of the exchange–correlation functional.⁶⁴ Therefore, scissor operators are 1.12 eV and 1.58 eV for AgTeO₂F and Ag₂(TeO₂F₂), respectively, which are used in the following optical property calculations.

Fig. 8 Calculated band structures of AgTeO₂F (a) and Ag₂(TeO₂F₂) (b).

The total and partial density of states are displayed in Fig. 9. The TDOS and PDOS of the two structures are very similar. In AgTeO₂F and Ag₂(TeO₂F₂), the VBs in the lower energy from −20 to −17 eV mostly originate from O 2s and Te 5s5p. The CBs in the higher energy between 3.0 and 10.0 eV are composed of Te 5s5p and O 2p states primarily. To assign the electronic bands explicitly, we have concentrated our attention on the top of VBs and the bottom of CBs close to the Fermi level, which are able to clarify the majority of the bonding character. In AgTeO₂F and Ag₂(TeO₂F₂), we can find that the O-2p states and F-2p states match well with the states of Te-5p, which indicates the strong Te–O and Te–F bonding interactions. For AgTeO₂F and Ag₂(TeO₂F₂), the maximum of valence bands are mainly from the O-2p and Ag-4d non-bonded states, while the minimum of valence bands come from the empty Te-5p and Ag-5s orbitals primarily (Fig. 9). Therefore, the band gaps of AgTeO₂F and Ag₂(TeO₂F₂) are determined by O, Ag and Te atoms.

Fig. 9 The total and partial density of states for AgTeO₂F (a) and Ag₂(TeO₂F₂) (b).

The linear optical response properties were calculated by the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω). The dispersion curves of refractive indices based on the formula n²(ω) = ε(ω) displayed strong anisotropy. The frequency-dependent refractive indices for AgTeO₂F and Ag₂(TeO₂F₂) are shown in Fig. 10. For AgTeO₂F, the refractive indices follow the order of n₀₁₀ > n₀₀₁ > n₁₀₀. The birefringence of AgTeO₂F is 0.078 at 1064 nm. For Ag₂(TeO₂F₂), the refractive indices are in a sequence of n₀₀₁ > n₀₁₀ > n₁₀₀ at 1064 nm. The birefringence of Ag₂(TeO₂F₂) was calculated to be 0.032 at 1064 nm, which is much smaller than that of AgTeO₂F. The large birefringence of AgTeO₂F can be attributed to the regularly arranged TeO₃F groups.

Fig. 10 Calculated refractive index dispersion curves of AgTeO₂F (a) and Ag₂(TeO₂F₂) (b).

Conclusions

The first cases of silver tellurite oxyfluorides, namely, AgTeO₂F, Ag₂(TeO₂F₂) and Ag₃F₃(TeF₆)(TeO₂)₁₂, have been successfully synthesized through a facile hydrothermal method. AgTeO₂F and Ag₂(TeO₂F₂) exhibit the first examples of silver fluorotellurites and the NCS Ag₃F₃(TeF₆)(TeO₂)₁₂ is the first silver fluoride tellurite. The tellurite groups in these structures display three different dimensional structures, namely, 1D [TeO₂F]_∞ chains in AgTeO₂F, 0D (TeO₂F₂) groups in Ag₂(TeO₂F₂) and a 3D [TeO₂]_∞ architecture in Ag₃F₃(TeF₆)(TeO₂)₁₂. The neutral [TeO₂]_∞ open framework with 8- and 4-MPR tunnels along three crystallographic axes is first reported here. In addition, the NCS Ag₃F₃(TeF₆)(TeO₂)₁₂ can exhibit a moderate powder SHG effect of about 0.7 times that of commercial KDP. The birefringence of AgTeO₂F was calculated to be 0.078@1064 nm, which is much larger than that of Ag₂(TeO₂F₂) (0.032@1064). This work has enriched the syntheses, structures and optical properties of tellurite oxyfluorides further. Other related works about metal fluorotellurites and fluoride tellurites are underway.

Author contributions

Bo Zhang: investigation, validation and writing – original draft; Jia-Hang Wu: validation; Chun-Li Hu: formal analysis; Ya-Feng Li: validation; Fang Kong: conceptualization, supervision and writing – review and editing; Jiang-Gao Mao: supervision and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.: 91963105, 22031009 and 21921001).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, important bond distances and angles, simulated and measured PXRD patterns, infrared spectra, and computational method. CCDC 2214235–2214237. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02272a

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