Synthesis, crystal structure and luminescence of [(CH3)3S]2ZrCl6

Abstract The current work reports on the synthesis, crystal structure and optoelectronic properties of (Me3S)2ZrCl6, prepared by reacting the solid precursors (Me3S)Cl and ZrCl4 in pyrex tubes at 150 °C under vacuum. According to X-ray powder diffraction and Rietveld analysis, (Me3S)2ZrCl6 crystallizes in the cubic space group Pa-3 (No. 205) with a = 12.4664(1) Å. The crystal structure consists of isolated trigonal pyramidal trimethylsulfonium cations and octahedral [ZrCl6]2- anions with weak hydrogen bonds among them and no signs of structural disorder. This 0D-material is stable in air and dissolves in water and dimethylformamide. Raman spectroscopy shows characteristic vibrational modes for the organic and inorganic moieties over the frequency range of 5–3200 cm−1. UV-Vis spectroscopy reveals a large band gap of 5.1 eV and a broadband luminescence with emission maximum at 465 nm in the solid state. The luminescent properties of (Me3S)2ZrCl6 are discussed and compared with those of similar inorganic or metal-organic compounds.


Introduction
Organic-inorganic compounds have attracted considerable interest for their optoelectronic applications [1].In particular, metal halide Perovskites of the stoichiometry ABX 3 (usually A ¼ methylammonium, formamidinium or Cs, B ¼ divalent Sn or Pb, X ¼ Cl, Br or I) are studied as absorbers and hole transporters in third generation solar cells [2] and as luminescent materials [3].The main challenges to address are the toxicity of metal (e.g.Pb) and the stability of the organic cation in humid conditions.We have previously reported the use of the trimethylsulfonium ((CH 3 ) 3 S) cation in hybrid Perovskite compounds as a substitute for the hygroscopic cations [4].This led to the new series of (Me 3 S)PbX 3 (X ¼ Cl, Br, I) compounds, which are very stable in ambient conditions but have large band gaps of over 3 eV [5,6].Therefore, they perform poorly as absorbers in solar cells with a maximum power conversion efficiency of 2.22% for a (Me 3 S)PbI 3 -based device [7].Except for the common ABX 3 stoichiometry, the A 2 BX 6 stoichiometry (B is a tetravalent metal) has also been extensively studied with promising electronic properties based on the theoretical calculations for Cs 2 MX 6 (M ¼ Sn, Te, Zr; X ¼ Br, I) [8].In this context, [(CH 3 ) 4 N] 2 ZrCl 6 has been recently synthesized with strong and excitation-dependent luminescence [9], whereas our research group also reported a work on the (Me 3 S) 2 SnX 6 compounds (X ¼ Cl, Br, I) with fine tuning of the band gap based on the halogen [10].In the current work, we focus on the analogous Zr(IV) compound, namely (Me 3 S) 2 ZrCl 6 .

Materials
ZrCl 4 was purchased from Sigma-Aldrich with purity >99.9% and was used without purification.(Me 3 S)Cl was synthesized by reacting methyl chloroformate with dimethyl sulfide in a 1:2 molar ratio in a pressure bottle at 80 � C overnight [11].The white crystals of (Me 3 S)Cl were washed with diethyl ether and then stored in a desiccator under vacuum.

Synthesis of (Me 3 S) 2 ZrCl 6
2.5 mmol (582.5 mg) of ZrCl 4 and 5 mmol (563 mg) of (Me 3 S)Cl were ground together and loaded in a pyrex tube, which was flame-sealed under vacuum, and subsequently heated in a furnace at 150 � C for 72 h.The resulting white product was washed with dichloromethane and dried in air.Yield: 1.1 g (ca.96%).

X-ray powder diffractometer
X-ray powder diffraction (XRPD) analysis was performed for the title compound using a Rigaku Smartlab diffractometer that operates in Bragg-Brentano geometry with Cu K a1 (k ¼ 1.5406 Å) and Cu K a2 (k ¼ 1.5444 Å) radiation.Data were collected over the angular range 5 � � 2h � 80 � counting for 3 s at each step of 0.03 � in detector position.A Rietveld refinement was performed using the FULLPROF software [12].Only the positions of Cl, S and C atoms were left to refine freely.Hydrogen atoms were constrained for methyl groups.All isotropic thermal displacement values were fixed to 0.03166 Å 2 .Illustration of the crystal structure was performed in the VESTA software [13].

Raman spectroscopy
Raman spectra of the polycrystalline powder were recorded at the backscattering geometry on a Renishaw InVia Raman microscope equipped with 1200 and 2400 lines/ mm diffraction grating, a high sensitivity Peltier-cooled charge coupled device (CCD), a motorized xyz microscope stage and an x20 magnification lens.All measurements were made at room temperature with a 2 cm −1 resolution, using either a solid-state laser emitting at 785 nm, allowing off-resonance Raman acquisitions with a power density of ca.0.05 mW lm −2 or the 514.5 nm line of an Ar ion laser for excitation employing ca.0.10 mW lm −2 on the sample with no signs of laser-induced modifications observed.A Rayleigh rejection notch filter at 514.5 nm was used when necessary, allowing measurements down to 5 cm −1 .Each spectrum represents the average of 15 scans to improve the signal-to-noise ratio over the spectral range 5-3200 cm −1 for the 514.5 nm line and 100-3200 cm −1 for the 785 nm line.

UV-Vis spectroscopy
UV-Vis absorption spectra on a thin film of the solid material were collected on a Perkin-Elmer L19 spectrometer.Steady-state photoluminescence spectra were collected on a Jobin Yvon Fluorolog spectrometer with an excitation wavelength of 240 nm and the sample powder that was placed into a capillary tube.Excitation spectra were collected as well on this instrument.Absorption spectra were also recorded in dimethylformamide solution.

Structural analysis
The white polycrystalline powder of (Me 3 S) 2 ZrCl 6 is stable in ambient air.This behavior is common for A 2 ZrCl 6 compounds (Cs 2 ZrCl 6 [14], [(CH 3 ) 4 N] 2 ZrCl 6 [9] and [(C 6 H 5 ) 2 P-CH 2 -P(H)(C 6 H 5 ) 2 ] 2 ZrCl 6 [15]) in contrast to the chemical instability of the precursors ZrCl 4 and (Me 3 S)Cl in ambient conditions due to hydrolysis.The compound is soluble in polar solvents such as water and dimethylformamide and poorly soluble in less polar solvents, such as ethanol, acetone and dichloromethane.The crystal structure consists of isolated units of regular (CH 3 ) 3 S þ trigonal pyramids and slightly distorted [ZrCl 6 ] 2-octahedra in the cubic space group Pa-3 (No. 205) with a ¼ 12.4664(1) Å and d ¼ 1.5871 g cm −3 (Figure 1).In the [ZrCl 6 ] 2-octahedra, the Cl-Zr-Cl angles are at 89.8(1) � and 90.1(1) � for adjacent Cl sites, and the Cl-Zr-Cl angles are at 180 � for opposite Cl sites.All Zr-Cl bond lengths equal to 2.496(4) Å and the hydrogen bond lengths (H-Cl) between anions and cations range from 2.809(4) to 2.997(5) Å.The Zr-S interatomic distances range from 5.271(4) to 5.823(4) Å.The Rietveld plot of the powder diffraction pattern is shown in Figure 2 and the atomic parameters are given in   Table 1.Only one very weak impurity peak is found at 2h ¼ 14.9 � that remains unindexed.As a side note, reaction temperatures over 150 � C cause gradual, thermal decomposition of the (Me 3 S) cation and reduce the reaction yield.Moreover, synthesis in water or dimethylformamide solution is also possible, although high quality crystals were not obtained for single-crystal diffraction.The title 0D material is isostructural to many other A 2 BX 6 compounds such as [(CH 3 ) 4 N] 2 ZrCl 6 [9], (Me 3 S) 2 SnCl 6 [10], (NH 4 ) 2 SeBr 6 [16] and (Me 3 Se) 2 TeCl 6 [17].This occurs because the A 2 BX 6 structure contains large cuboctahedral voids that accommodate the (Me 3 S) cation in the expected cubic symmetry without any distortion of the inorganic framework.In contrast, the ABX 3 stoichiometry causes steric hindrance for large cations and prohibits the cubic 3D perovskite structure, found for instance in CsPbCl 3 [18].

Vibrational properties
The Raman spectra of (Me 3 S) 2 ZrCl 6 are shown in Figure 3. Distinct areas of the organic and the inorganic moieties are found.The vibrational modes appearing above ca.280 cm −1 correspond to (CH 3 ) 3 S þ , whereas the modes of the inorganic part of (Me 3 S) 2 ZrCl 6 are at lower frequencies.In particular, the peak at 67 cm −1 is attributed to lattice vibration whereas the peak at 166 cm −1 is attributed to the asymmetric bending of the Cl-Zr-Cl bonds.The peaks at 285 and 320 cm −1 are attributed to the Zr-Cl asymmetric and symmetric stretches, respectively.Similar vibrational modes were reported for the isostructural (Me 3 S) 2 SnCl 6 compound [6] with a minor peak shift, due to the large mass of Sn compared to Zr atoms.
From the viewpoint of transport properties, low frequency vibrations at 67 cm −1 may minimize the lattice contribution to the thermal conductivity (j lattice ) of the compound via phonon scattering.This effect is commonly found in open-framework materials (bulk or nanostructured) [19] and has also been proposed for the A 2 BX 6 family [20,21].This improves the thermoelectric efficiency of semiconductors, given by the equation ZT ¼ T S 2 r j −1 (where ZT is the dimensionless figure of merit, T is temperature in Kelvin, S is Seebeck coefficient in V K −1 , r is electrical conductivity in X −1 m −1 and j is the thermal conductivity in W m −1 K −1 ).The latter term contains the contribution of electrons and lattice, j ¼ j electron þ j lattice .

Optoelectronic properties
With regard to electronic properties, the title compound shows semiconducting behavior with a direct, large band gap of 5.1 eV with an absorption maximum at 243 nm and two more weak peaks at 315 nm and 380 nm (Figure 4a).These values are similar to the other 0D-hexachlorozirconates, such as [(CH 3 ) 4 N] 2 ZrCl 6 with absorption maxima at 262 nm, 333 nm and 382 nm [9]; the absorption spectra are almost unaffected by the organic cation.On the other hand, the halogen atom plays a decisive role in the electronic properties for this class of materials.For comparison, heavier halogens in the cubic Cs 2 ZrX 6 (X ¼ Br, I) compounds are expected to exhibit significantly smaller band gaps, according to theoretical calculations [8,22].The absorption spectrum is also recorded in dimethylformamide solution of (Me 3 S) 2 ZrCl 6 (Figure 4b), with two strong bands at 315 nm and 364 nm (the spectrum is cut off below 270 nm due to the strong absorption of the solvent).These absorption bands are attributed to metal-to-ligand and ligand-to-metal transitions of the six-fold coordinated Zr atoms to Cl atoms.

Conclusion
The new, low-cost, stable and environmentally-friendly (Me 3 S) 2 ZrCl 6 is of considerable interest for luminescent applications.The presence of the (Me 3 S) cation renders it more soluble in organic solvents (e.g.ethanol, dimethylformamide) than its inorganic analogues, such as Cs 2 ZrCl 6 , which facilitates the fabrication process of thin-film devices.As in the case of Zr(IV) oxides, e.g.Eu-doped La 2 Zr 2 O 7 [25], these materials may be doped with different elements, in particular with halogen atoms in (Me 3 S) 2 ZrCl 6 and alter the emission wavelength.Moreover, thermoelectric efficiency could be another field to explore in a broader perspective of physical applications, thanks to semiconducting behavior of the material and the low-frequency vibrational modes of the inorganic framework.

Figure 3 .
Figure3.(a) Raman spectrum of (Me 3 S) 2 ZrCl 6 using the 785 nm excitation laser.The inset shows enlarged the region between 100 and 1200 cm −1 and (b) Raman spectrum of (Me 3 S) 2 ZrCl 6 using the 514 nm excitation laser which allows to collect data below 100 cm −1 .

Figure 4 .
Figure 4. (a) UV-Vis absorption spectrum for (Me 3 S) 2 ZrCl 6 in the solid state, (b) UV-Vis absorption spectrum for (Me 3 S) 2 ZrCl 6 in DMF solution and (c) Photoluminescence spectrum using an excitation source of 240 nm for (Me 3 S) 2 ZrCl 6 in the solid state.

Table 1 .
Wyckoff sites, fractional atomic coordinates and isotropic thermal displacement values for (Me 3 S) 2 ZrCl 6 based on the Rietveld refinement.