A new I₃O₉³⁻ group constructed from IO₃⁻ and IO₅⁵⁻ anion units in Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]

Abstract

An alkali metal gallium iodate, Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] (CGIO), was synthesized by the traditional hydrothermal method. Interestingly, CGIO revealed a novel I₃O₉³⁻ fundamental building block (FBB) constituted by two IO₃ tetrahedrons and one IO₅ polyhedron through sharing O atoms. The new I₃O₉³⁻ trimer forms a 0D [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ anion cluster together with the [Ga₂O₁₁]¹⁶⁻ dimer and isolated IO₃⁻ units. The UV-vis-NIR diffuse reflectance spectrum of CGIO shows a wide bandgap of 4.05 eV, and the thermal stability could reach above 400 °C. First-principles calculations explicated that the optical properties of CGIO mainly originated from the iodate anion units with a calculated birefringence of 0.04 at 1064 nm.

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Introduction

Inorganic functional materials have become a research hotspot due to their wide applications in the fields of pyroelectricity, birefringence, piezoelectricity, nonlinear optics, and so on.^1–4 Recently, metal iodates have drawn lots of attention because of their structural diversity based on I⁵⁺ with a stereochemically active lone pair of electrons and constituted by multiple fundamental building blocks (FBBs), such as IO₃⁻, IO₄³⁻, IO₅⁵⁻ units, etc.^5–7 These FBBs could further form polyiodates providing various structural combinations, for instance, the I₂O₅ dimer in HBa_2.5(IO₃)₆(I₂O₅),⁸ the I₃O₈⁻ trimer in NaI₃O₈,⁹ the I₃O₉³⁻ trimer in A₂BiI₅O₁₅ (A = K⁺ or Rb⁺),¹⁰ the I₄O₁₁²⁻ tetramer in HBa(IO₃)(I₄O₁₁),⁸ and the I₅O₁₄³⁻ pentamer in REI₅O₁₄ (RE = Y and Gd).¹¹

To explore novel inorganic iodates with a large band gap for a wide working spectra region, one of the feasible approaches is to introduce a main group cation with a distorted octahedral structure into the crystal structure. For example, by replacing the V⁵⁺ cation with a Ga³⁺ cation, Pb₂GaF₂(SeO₃)₂Cl (ref. 12) achieved an increased band gap from 2.41 eV (Pb₂VO₂(SeO₃)₂Cl)¹³ to 4.32 eV. Via introducing the distorted Ga-centered octahedron, the band gaps of α- and β-Ba₂[GaF₄(IO₃)₂](IO₃) (4.61 eV and 4.35 eV)¹⁴ were widened by 0.29 eV and 0.03 eV, respectively, compared with Ba(IO₃)F (4.32 eV).¹⁵ Meanwhile, in consideration of the inexistence of d–d or f–f electron transitions for alkaline metals, it is commonly accepted that alkaline metals were also in favour of widening the band gap, e.g., the band gaps within compounds ABi₂(IO₃)₃F₅ (A = NH₄, K, Rb and Cs) (3.78–3.88 eV)^16,17 are larger than that of Bi₃OF₃(IO₃)₄ (3.7 eV).¹⁸

Therefore, following the above ideas, we found a new gallium iodate compound, Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] (CGIO), by using a mild temperature hydrothermal method. CGIO possesses a zero-dimensional (0D) structure consisting of Cs⁺ cations and novel [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ polyanions, which are constituted by a [Ga₂O₁₁]¹⁶⁻ dimer, I₃O₉³⁻ trimer, and IO₃⁻ triangular pyramids. Different from the previously reported I₃O₉³⁻ group, the newly discovered I₃O₉³⁻ unit in CGIO is constructed from two IO₃⁻ units and one IO₅⁵⁻ unit. Besides, CGIO also displayed a wide band gap (4.05 eV) and good thermal stability (∼400 °C). First-principles calculations were also performed to investigate the structure–property relationship in detail.

Experimental section

Reagents

Cs₂CO₃ (Aladdin, 99.9%), Ga₂O₃ (Aladdin, 99.99%), and I₂O₅ (Aladdin, 98%) were purchased from commercial sources without further purification.

Synthesis

Cs₂CO₃ (0.756 g, 2.32 mmol), Ga₂O₃ (0.0815 g, 0.43 mmol), I₂O₅ (1.162 g, 3.44 mmol), and 4 mL deionized water were placed into a Teflon liner (30 mL), and sealed in an autoclave and kept at 220 °C for 5 days. After that, the autoclave was cooled down to room temperature at a rate of 3 °C h⁻¹. The crystals of CGIO were washed with deionized water and ethyl alcohol and later dried in air. The photograph of the crystals is illustrated in Fig. 1.

Fig. 1 X-ray powder diffraction pattern of CGIO and the grown crystals are displayed in the insert picture.

Powder X-ray diffraction

The powder X-ray diffraction (PXRD) data of the target sample was successfully collected on a SmartLab 9 KW X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å) in the 2θ range of 10–70° with a scan step width of 0.01° and a fixed counting time of 1.5 s per step at room temperature. The powder XRD pattern is shown in Fig. 1, indicating that the diffraction peaks of the experiment are in agreement with the calculated data, which clearly demonstrate the phase purity.

Energy dispersive X-ray spectroscopy

A field emission scanning electron microscope (FESEM, Quanta FEG 250) made by FEI was applied to analyse the microprobe elementals of CGIO and revealed that the crystal contained Cs, Ga, I, and O elements and the average molar ratio was 6.4 : 3.37 : 15.12 : 75.12, which is in agreement with that determined from single crystal XRD studies (Fig. S1†).

Single crystal X-ray determination

A Rigaku XtaLAB PRO single crystal diffractometer with Mo Kα radiation at 299 K was used to collect the single crystal X-ray diffraction data of crystal CGIO, and the data reduction and integration of CGIO were carried out using the SAINT program.¹⁹ The crystal structure was figured out by direct methods and refined using least-squares minimization with the ShelXLT²⁰ refinement package through the use of Olex2.²¹ The potential missing symmetry of the structure was examined by the program PLATON.²² The crystallographic data and structure refinements are shown in Table 1. The atomic coordinates, equivalent isotropic displacement parameters, and selected bond lengths and angles are given in Tables S1–S3 in the ESI.†

Table 1 Crystal data and structure refinements of Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] (CGIO)

a R 1 = Σ||Fo| − |Fc||/Σ|Fo|, ωR2 = [Σω(Fo^2 − Fc^2)2/Σω(Fo^2)^2]^1/2.
Formula	Cs3[Ga2O(I3O9)(IO3)4(HIO3)]
Crystal system	Monoclinic
Space group	P21/m
a (Å)	7.9191 (4)
b (Å)	21.1203 (7)
c (Å)	8.6696 (4)
α (°)	90
β (°)	111.691 (5)
γ (°)	90
Z	2
ρ calc (g cm^−3)	4.817
μ (mm^−1)	15.259
F(000)	1704
Reflections collected	24 656
R 1, ωR2 (all data)^a	0.0369/0.0775

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Thermal analysis

Thermogravimetric (TG) and differential thermal analysis (DTA) were measured in the temperature range of 50–650 °C with a heating rate of 5 °C min⁻¹ under the flow of N₂ by using a NETZSCH STA thermal analyzer instrument.

UV-vis-NIR diffuse reflectance spectroscopy

The UV-vis-NIR diffuse reflectance spectrum was measured in the range from 200 nm to 1200 nm with a UH4150 UV-vis-NIR spectrophotometer with an integrating sphere at room temperature. The reflectance spectrum was converted to the absorption spectrum by using the Kubelka–Munk function F(R) = (1 − R)²/2R, where R is the reflectance.^23,24 The band gap was deduced based on the straightforward extrapolation method.²⁵

Infrared spectroscopy

The infrared spectrum was measured in the spectral region from 400 cm⁻¹ to 4000 cm⁻¹ with a Nicolet iS50 FT-IR spectrometer with attenuated total reflection (ATR) at room temperature.

Raman spectroscopy

The Raman spectrum was investigated in the 0–1000 cm⁻¹ range with an Alpha300 RA at an excitation wavelength of 532 nm at room temperature.

Computational method

Utilizing a plane-wave pseudopotential total energy package based on density functional theory (DFT), the electronic band structure and optical properties of CGIO were performed by CASTEP.^26,27 The exchange–correlation energy was described by using the functional developed by Perdew–Burke–Ernzerhof (PBE) within the generalized gradient approximation (GGA) form.^28,29 The effective interactions between atom cores and valence electrons were modelled using the optimized norm-conserving pseudopotentials³⁰ in the Kleinman–Bylander³¹ form for all the elements. The valence electron configurations of the atoms were Cs 5s²5p⁶6s¹, Ga 3d¹⁰4s²4p¹, I 5s²5p⁵, O 2s²2p⁴ and H 1s¹. Without compromising the computational accuracy, the adoption of a relatively small basis set was allowed. The high kinetic energy cutoff of 880 eV and the k-point meshes of Monkhorst–Pack³² in the Brillouin zones of 3 × 1 × 3 were chosen. The calculation results show that the calculation parameters are accurate enough for this study.

Results and discussion

Crystal structure

CGIO crystallizes in the centrosymmetric (CS) space group P2₁/m (no. 11) with the cell parameters: a = 7.9191 (4) Å, b = 21.1203 (7) Å, c = 8.6696 (4) Å, β = 111.691 (5)° and Z = 2. The crystal structure of CGIO features a novel 0D cluster [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ polyanion, composed of a [Ga₂O₁₁]¹⁶⁻ dimer, five IO₃⁻ triangular pyramids, and one new type of polyiodate anion I₃O₉³⁻, with alkali metal cations filling the structural void to maintain the charge balance (as shown in Fig. 2).

Fig. 2 (a) Iodate primitives in the crystal CGIO; (b) [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ polyanion; (c) crystal structure of CGIO along the b axis.

Interestingly, there can be seen as three kinds of I–O polyhedra in CGIO, namely, the IO₃ tetrahedron, IO₅ polyhedron, and a novel I₃O₉³⁻ anion unit which is composed of two IO₃ and one IO₅ polyhedron structures by sharing O atoms (Fig. 2a). As shown in Fig. 2b, there exists a neutral molecule HIO₃ in CGIO, similar to β-RbIO₃(HIO₃)₂ (ref. 33) and K₄[(VO)(IO₃)₅]₂(HIO₃)(H₂O)₂·H₂O.³⁴ The bond distances of I–O range from 1.764 (6) Å to 2.469 (4) Å, which are consistent with metal polyiodate compounds reported previously,^35,36 such as Cs₂I₄O₁₁ (1.783 Å to 2.441 Å).³⁷

The Ga atom is octahedrally coordinated by six O atoms from four IO₃⁻ units, one IO₅⁻ unit, and one terminal O atom, forming the GaO₆⁹⁻ group. Two GaO₆⁹⁻ groups are bridged by the terminal O atom to form a slightly distorted [Ga₂O₁₁]¹⁶⁻ dimer group (Fig. 2b), with Ga–O bond lengths ranging from 1.951 (4) Å to 2.008 (3) Å. The magnitude of the out-of-center distortion (Δd) was calculated to be approximately 0.1085.³⁸ One Cs⁺(1) and two Cs⁺(2) cations are filled between the two adjacent [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ anion groups to maintain the charge balance, and finally form a 0D cluster Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] frame construction (Fig. 2c). It also can be seen that all the 0D clusters in the Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] structure are arranged in an anti-parallel manner, leading to the centrosymmetric space group.

The Cs⁺(1) cation is coordinated with ten O atoms, whereas the Cs⁺(2) cation is coordinated with eleven O atoms. The bond lengths of Cs–O are in the range of 3.174 (4) Å to 3.554 (4) Å for Cs(1)–O and 3.109 (5) to 3.464 (6) for Cs(2)–O, respectively. Cs(1)O₁₀ and Cs(2)O₁₁ are bridged by O atoms from IO₃⁻ and IO₅⁵⁻ units to form a [Cs₂O₁₉]³⁶⁻ dimer (as shown in Fig. S2†). Bond valence sum (BVS) calculations^39,40 verified that the oxidation states of Cs, Ga, I, O and H are 1.062, 3.028, 5.127, 1.950 and 0.953, respectively. It is obvious that the corresponding coordination numbers of atoms are reasonable.

Notably, the I₃O₉³⁻ in CGIO is totally different from the previously reported ones. In A₂BiI₅O₁₅ (A = K⁺ or Rb⁺), the I₃O₉³⁻ consists of one IO₃⁻ unit and two IO₄³⁻ units via sharing two O atoms in the way shown in Fig. 3a.¹⁰ Meanwhile in CGIO, the newly discovered I₃O₉³⁻ unit consists of two IO₃⁻ units and one IO₅⁵⁻ unit, in which the two IO₃⁻ units linked with IO₅⁵⁻ by sharing adjacent O atoms in the IO₅⁵⁻ group. Their polyhedral structures are exhibited in Fig. 3b.

Fig. 3 The I₃O₉³⁻ type in A₂BiI₅O₁₅ (A = K⁺ or Rb⁺) (a) and crystal CGIO (b).

Thermal analysis

The TG-DTA curves are shown in Fig. S3.†CGIO shows good thermal stability up to ∼400 °C, and it exhibits two main steps of weight losses. The first weight loss is about 41.97% in the temperature range of 100–500 °C caused by the volatilization of one molecule of HIO₃, two molecules of I₂ and four of molecules O₂ (cal. 41.55%), which is also consistent with the endothermic peaks at 419.67 °C in the DTA curve. The second weight loss obtained by the experiment (9.76%) occurred in the temperature region from 500–550 °C which is attributed to the volatilization of 0.5 molecules of I₂ and 2 molecules of O₂ (cal. 9.77%) which is consistent with the endothermic peaks at 538.91 °C in the DTA curve.

UV-vis-NIR diffuse reflectance spectroscopy

The UV-vis-NIR diffuse reflectance spectrum is shown in Fig. 4, and it exhibits a wide experimental band gap of 4.05 eV, comparable with Rb₂Ge(IO₃)₆ (E_g = 4.06 eV)⁴¹ and K₂Ga(IO₃)₄(H_0.5IO₃)₂ (E_g = 4.02 eV),⁴² but much larger than other iodates Ba₃Ga₂(IO₃)₁₂ (E_g = 3.06 eV),⁴³ NH₄[MoO₃(IO₃)] (E_g = 3.26 eV),⁴⁴ Rb₂Ga(IO₃)₄(H_0.5IO₃)₂ (Eg = 3.67 eV),⁴² Cs₂Ti(IO₃)₆ (E_g = 3.2 eV),⁴⁵ Cs₂Ge(IO₃)₆ (E_g = 3.32 eV),⁴⁶ and CsVO₂F(IO₃) (E_g = 2.39 eV).⁴⁷ Compared with K₂BiI₅O₁₅ (E_g = 3.50 eV) containing different I₃O₉³⁻ groups, the band gap of CGIO (E_g = 4.05 eV) is enhanced to 4.05 eV with the synergistic effect of alkali and main group metals of Cs⁺ and Ga³⁺ cations.

Fig. 4 UV-vis-NIR diffuse reflectance spectrum of CGIO.

Infrared spectroscopy and Raman spectroscopy

As shown in the IR spectrum (Fig. S4†), the characteristic absorption peaks of crystal CGIO in the range from 400–670 cm⁻¹ are denoted as the I–O vibrations;^48,49 the peaks at 787 cm⁻¹, 814 cm⁻¹, and 837 cm⁻¹ are attributed to the Ga–O vibrations,¹⁴ and the peak at 1057 cm⁻¹ is attributed to the vibrations of the IO₃⁻ group;⁴⁹ the peaks in the region of 1643–3391 cm⁻¹ can be assigned to the characteristic peaks of O–H vibration.⁴² In the Raman spectrum (Fig. S5†), the peaks in the range of 780–845 cm⁻¹ are attributed to the Ga–O vibrations; the characteristic peaks between 450 cm⁻¹ and 780 cm⁻¹ are considered as the vibrations of I–O; the weak peaks below 300 cm⁻¹ can be denoted as the characteristic peaks of the Ga–O group.^43,50

Theoretical calculations

First-principles calculations performed by the CASTEP package are carried out to understand the relationship between the structure and properties of CGIO. By utilizing the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) form, the electronic band structure, partial density of states and birefringence are accurately evaluated (Fig. 5 and 6). The calculated electronic band structure is shown in Fig. 5a, revealing that CGIO possesses an indirect band gap of 4.05 eV, which is consistent with the experimental value.

Fig. 5 The calculated band structure (a) and the partial density of states (b) of CGIO.

Fig. 6 Calculated refractive indices of CGIO.

In order to clearly understand the origin and contribution units of the electronic structure, the partial density of states (PDOS) of the compound was further calculated. The PDOS of CGIO is shown in Fig. 5b and it is obvious that the top states of the valence band (VB) (−5 eV to 0 eV) are mainly attributed to I 5p and O 2p orbitals, while the bottom states of the conduction band (CB) (0 eV to 5 eV) are primarily made up of the unoccupied I 5p and O 2p orbitals. As the optical properties are mainly determined by the electron transitions between the band gaps, it can be deduced that the optical properties of CGIO are mainly determined by IO₃⁻ and I₃O₉³⁻ anionic units. The calculated bond orders of the I–O bond by population analysis are 0.27–0.32 e for IO₃⁻ and 0.04–0.4 e for IO₅⁵⁻, respectively, indicating that the I–O bonds in IO₃⁻ have stronger covalent characteristics than those in IO₅⁵⁻.

As shown in Fig. 6, the refractive indices n and birefringence Δn of CGIO are calculated to evaluate the optical anisotropy. According to the refractive index curves, CGIO is a biaxial crystal with n_x > n_z > n_y and the calculated birefringence (Δn = n_x − n_y) is 0.04 at 1064 nm.

Conclusions

In conclusion, a new alkali metal gallium iodate crystal, namely, Cs₃[Ga₂O(I₃O₉)(IO₃)₄(HIO₃)] (CGIO), with a novel 0D cluster [Ga₂O(I₃O₉)(IO₃)₄(HIO₃)]³⁻ polyanion was obtained by the traditional hydrothermal method. CGIO exhibits a wide band gap of 4.05 eV, and thermogravimetric measurement indicated it possess a high thermal stability above 400 °C. Meanwhile, the theoretical calculations revealed that the birefringence of crystal CGIO can reach 0.04 at 1064 nm and the optical properties originate from the IO₃⁻ and I₃O₉³⁻ anionic units. This work would enrich the structural diversity in iodates with wide band gaps by effectively combining different fundamental building blocks.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank X. Li for discussion. This work was supported by the National Natural Science Foundation of China (Grant No. 51702232, 51890864, 52002288).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2107844. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ce01234g

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