Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D framework at the nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

Supporting information for this article is given via a link at the end of the document. Abstract: Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn 4 Si 2 O 7 Cl 2 . We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H 2 evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids


Introduction
Mixed-anion metal compounds contain metal cations bounded to two different anions. [1,2]The resulting heteroleptic cation-centered polyhedra provide an additional dimension to access ionic conductivity, [3] superconductivity, [4] magnetic, [5] optical [6] and (photo)catalytic properties [7] that provide new horizons for batteries, [3] nonlinear optics, [8] water splitting [7,9] and optoelectronics. [10]Among these materials, metal oxyhalides are a largely uncovered territory compared to the vast library of oxides and halides because their composition space is hard to explore by conventional solid-state synthesis methods at high temperatures, where they often decompose by halogen loss. [11]opochemical, [5] solvothermal [12] and flux syntheses [13,14] enable to decrease the reaction temperatures and to isolate original oxyhalides by accessing new reaction pathways.Yet, the structural diversity of oxyhalides remains limited.][17] Stability is further undermined by the prominent contribution of Cl 3p states at the top of the valence band, which makes them prone to photocorrosion, apart some Bi-based materials. [14,18]Besides stability considerations, aiming at oxychlorides with 3D structures instead of layered ones can yield new phenomena, like spin-induced high-T c multiferroicity. [19]erein, we present a reaction pathway towards a chemically and thermally stable metal oxychloride based on a new polar 3D tetrahedral framework (Figure 1).This material shows strongly anisotropic hole mobility along the polar axis and photoelectrocatalytic activity for H 2 production from water.
To explore how 3D connectivity can yield new oxychlorides, we design a reaction path by following 3 principles.First, we use molten salts media at lower temperature (420 °C) than usual flux syntheses of oxyhalides, [13,14] to avoid thermal decomposition of the product during synthesis, while enabling the recovery of metastable solids. [20]As liquid media, molten salts also increase the nucleation and crystallization rates compared to solid-state reactions, [21] thus giving access to nano-objects, [21] which are scarce among oxyhalides and exacerbate the impact of the surface on the properties, especially for catalysis.Second, we use [ SiO 4 ] tetrahedral connectors already observed in the few metalsilicon oxyhalides reported (27 entries in the Inorganic Crystal Structure Database excluding Rare Earths), [17] especially in Cd 4 Si 2 O 7 F 2 , [22] which has been the only oxyhalide of silicon and a post-transition metal before the present work. [17]As metal cation, we focus on Zn 2+ because its parent ZnO and ZnCl 2 structures are also built on tetrahedral units, and compounds of d 10 metal cations are of interest for photocatalysis. [23]Zinc has never been reported in a non-hydrated, thermally stable oxychloride compound. [24,25]e anticipate that [SiO 4 ] connectors could bridge Zn units through Si-O-Zn bonds by reacting with [ZnCl 4 ] tetrahedra.Molten salts based on ZnCl 2 have actually been described as polymeric liquids made of interconnected [ZnCl 4 ] tetrahedra, [26] so that the structure of the solid could be directly related to the liquid structure by using such salts.Third, in order to control independently the Si/Zn ratio and O stoichiometry, we use a specific oxygen-free silicon precursor: sodium silicide Na 4 Si 4 (Figure S1).[30] It contains [Si 4 ] 4-clusters that are readily oxidized by oxygenated species.Its reactivity has not been explored to yield oxo-species with controlled oxygen content.This combination of Zn and Si into an oxychloride framework could deliver a new Earth-abundant photocatalyst

Results and Discussion
We choose the eutectic molten salt medium ZnCl 2 :NaCl (68:32 mol., melting point = 260 °C) to enable a wide temperature range in the liquid state (see detailed procedure in Supplementary information SI).Oxygen is provided in a small proportion either by using ZnCl 2 ꞏ1.33H 2 O impurities in commercial ZnCl 2 (overall Si/Zn, O/Si and O/Zn molar ratios are 0.05, 2.4 and 0.13, respectively, according to thermogravimetric analysis (TGA, Figure S2 in SI)), or by using anhydrous ZnCl 2 (99.999%) with addition of a small amount of water (H 2 O/ZnCl 2 mol.ratio 0.415).After washing of the salts, the reaction performed at 420 °C yields a grey powder made of high aspect ratio nanowires with 20 to 50 nm diameter and hundreds of nanometers to micrometers length (Figures 1A-B, S3 and S4), with a Brunauer-Emmett-Teller (BET) surface area of 17 m 2 g -1 .Energy dispersive X-ray spectroscopy (EDS) indicates a Zn:Si:Cl composition of 4:2:2 (Figure S5).Scanning transmission electron microscopy (STEM)-High-angle annular dark field (HAADF) imaging and STEM-EDS elemental mapping (Figure S6) show the homogeneous distribution of Zn, Si, O and Cl.The nanowires were already detected within the salt medium, before washing (Figure S7).
The powder X-ray diffraction (XRD) pattern could not be indexed along any structure reported.Increasing the reaction time up to 100 h or thermally annealing the powders only resulted in an increase of the nanowires length and did not deliver bulk single crystals (Figure S8).The small size of the nanowires was detrimental to the deconvolution of XRD peaks for precise assessment of their relative intensities, thus precluding identification of any structural model.To overcome this limitation, structure analysis was carried out by precession-assisted electron diffraction tomography (PEDT), a 3D single crystal electron diffraction technique [31] enabling to solve the structure of materials with sub-micrometer size (see detailed description of PEDT and XRD results in supplementary information S-5 and S-10).Despite diffuse scattering (Figure S9) indicating structural disorder, the reconstructed reciprocal space enabled to retain a A-centered orthorhombic lattice with unit cell parameters a  5.2 Å, b  17.5 Å and c  20.7 Å. Ab initio structure solution by the charge flipping method [32] was performed from the data acquired on two nanowires for high completeness of the data set (Table S1).
According to symmetry analysis, [33] the polar A2mm group was retained with a structure encompassing 21 independent atoms revealed from the electrostatic potential map (Figure S10).This structural model was refined (Tables S2 and S3) with the stoichiometry Zn 4 Si2O 7 Cl 2 by considering dynamical diffraction effects. [34]This new structure is fully supported by calculations of interatomic distances and bond valence strengths (Table S4).One can identify ZnO 3 Cl tetrahedra bridged by Cl atoms (Figure 1D-H).(ZnO 3 ) 2 Cl dimers are bridged by dimers of corner-sharing [SiO 4 ] tetrahedra.The polar axis is the a axis along which all tetrahedra apexes are oriented (Figure 1F).This axis also corresponds to the channels delimited alternatively by 7 tetrahedra (4Zn+3Si) and 6 tetrahedra (4Zn+2Si).In the latter case, one chlorine atom points towards the center of the channels.A Rietveld analysis conducted on a synchrotron powder XRD (SXRD) pattern of the whole powder (Figure 2A, Table S5) confirms the structural model issued from PEDT on only a few nanowires.Analysis of the anisotropic size broadening of the SXRD peaks yields reconstructed needle-like crystallites grown along the [100] direction (Figure 2A), in full agreement with electron microscopy (Figures 1 and S11).
Zn 4 Si 2 O 7 Cl 2 is one of the rare solids and the first metal oxyhalide crystallizing in the polar A2mm space group (Table S6).It is also the only one built only from coordination tetrahedra.This 3D interconnected framework obviously deviates from the trend of oxychlorides to grow 2D structures.
The crystal structure of this new compound is confirmed by  coordination SiO 4 2 . [35]The O-K edge at 533 eV is typical of O 2- species.The Zn L 3 , L 2 and L 1 edges are also typical of Zn 2+ oxo species at 1021, 1044 and 1194 eV (Figure S13), respectively. [35]-ray photoelectron spectroscopy (XPS, Figure S14 and Table S7) and Fourier Transform Infrared spectroscopy (Figure S15) are consistent with Zn 2+ and Si 4+ O 4 units as well as with Zn-Cl bonds.
The 29 Si NMR spectrum (Figure 2B) exhibits a wide and low intensity signal below the narrow signals assigned to crystalline Zn 4 Si 2 O 7 Cl 2 .This feature could correspond to an amorphous component.[38] The PDF refinement of the experimental G(r) curve is fully consistent with the PEDT crystal structure (Figure 2D, reliability factor R w = 28.1%).An additional correlation distance is found at 2.30 Å and attributed to amorphous silicon impurities, as shown by the improved reliability factor when amorphous Si is considered (R w = 23.8%, Figure S16).This component then accounts for the wide and weak signal observed on the 29 Si NMR spectrum.We ascribe this amorphous Si component to the side-decomposition of Na 4 Si 4 .The Si 0 /Si 4+ ratio is 0.05 according to XPS, which corresponds to ~0.6 wt.% of amorphous silicon (Figure S17).
The Zn 4 Si 2 O 7 Cl 2 structure is built from SiO 4 and ZnO 3 Cl tetrahedral building blocks that can be traced back to the reaction mixture.Indeed, while [SiO 4 ] units are readily formed by the oxidation of Na 4 Si 4 by small amounts of water, Zn-based units originate from molten ZnCl 2 , which is a polymeric liquid made of ZnCl 4 tetrahedra linked together by corners and edges. [26]Mixing ZnCl 2 with NaCl brings Na + ions that partially break the tetrahedra network [39] and yield small clusters of ZnCl 4 tetrahedra where zinc cations are linked by chloride ions, especially (Cl 3 Zn)Cl(ZnCl 3 ) dimers where the 6 Cl terminating groups can react with [SiO 4 ] building blocks for growing a zinc oxychloride framework.Note that using water-free zinc chloride (99.999 % purity) without incorporation of water results in metallic zinc as the major crystalline phase, which originates from the reduction of ZnCl 2 by Na 4 Si 4 in the absence of oxygen and water.
Thermogravimetric analysis (Figure S18) shows the high thermal stability in air or nitrogen of Zn 4 Si 2 O 7 Cl 2 , which decomposed at 600 °C into ZnO and silica by chlorine loss.In the title compound, Cl is locked by bridging two ZnO 3 Cl tetrahedra within structural channels, which ensures enhanced thermal stability than layered metal oxychlorides, where Cl is located between the metal-oxo layers weakly bonded to each other, with resulting lower temperature of chlorine loss and decomposition (~450 °C). [40]No thermal degradation is observed on annealing at 500 °C under argon for one week or air for one day (Figure S19).Further, the nanowires show no degradation after dispersion (Figure S20) for one year in water or methanol and high stability in alkaline and mildly acidic (pH 4) conditions, although the compound decomposes in more acidic medium.
Upon UV excitation at room temperature, we observed a strong blue emission (Figure 3A), with an emission band centered at about 2.8 eV (450 nm).A shoulder around 3.4 eV (370 nm) clearly detected at low temperature is ascribed to bandto-band relaxation in Zn 4 Si 2 O 7 Cl 2 and is indicative of the band gap.The 450 nm broad emission band is assigned to recombination through Si-related shallow defect centers. [41,42]Such defects induce long lifetimes, [42] as observed for the 2.  To investigate the role of the structure polarity on charge transport, we have calculated the effective mass of the charge carriers (Table S9).The effective mass of electrons (m* e ) is isotropic.Its low value of ~0.4 m 0 highlights the high mobility of photo-generated electrons.It is one order of magnitude lower than for holes (m* h ) (Figure 3C), which then promotes the separation of photoinduced electron-hole pairs.The DFT-calculated dipole moment of the unit cell is oriented along the polar [100] direction and exhibits a high value of 16.5 e Å, thus yielding a strong builtin electric field along the polar axis. [43,44]The polarity of the structure is also linked to the pronounced acentric character of the heteroleptic tetrahedral units ZnO 3 Cl.m* h shows a strong anisotropy (Table S9) with the smallest values along the [100] direction, parallel to the dipole moment.The built-in electric field is then along the direction of the easiest hole migration, which further enhances the separation of charge carriers and the photoelectrocatalytic activity. [23,45]The total density of states (DOS) and projected DOS (PDOS) are shown in Figure 3D with additional orbital resolved PDOS in Figure S23 focusing on the heteroleptic unit ZnO 3 Cl.The dispersive conduction band encompasses contributions of all elements, in particular Zn s and p states at the conduction band minimum (CBM).The valence band (VB) consists mainly in O 2p and Cl 3p orbitals hybridized with Zn states (Figure 3F).Si states are found lower in the VB.Most of the Cl contribution is at the top of the VB from just below the Fermi level to about -6 eV with a major contribution centered around -1 eV.O states are found in a larger range from -8 to 0 eV, in the same region and even at slightly higher energy than Cl, contrary to what would be expected given the lower electronegativity of Cl compared to O and the knowledge on previous oxyhalides. [14,18]This situation is beneficial for stability in water and photocatalytic water splitting since O states at the VB maximum (VBM) restrain oxidation of the halide anion and then stabilize the material versus self-decomposition. [18]According to Mulliken electronegativities, [46,47] the band edges should encompass the redox potentials of water couples.
We then assessed the possibility of doping the framework.The synthesis was performed in a glassy carbon crucible and yielded a powder with identical XRD pattern (Figure S24) and dark color.XPS (Figure S25) indicates an additional contribution to the C1s signal at 283 eV, which is attributed to a carbide environment. [48]This XPS component accounts for a carbon/zinc elemental ratio of 0.1.The band structure of this doped material was further studied by electrochemical impedance spectroscopy.A Mott-Schottky plot (Figure 4A) recorded at pH 7.4 exhibits a positive slope, indicating an n-type semiconductor behavior for Cdoped Zn 4 Si 2 O 7 Cl 2 .From this plot, the flat band potential E FB is evaluated at -0.12 V vs.The reversible hydrogen electrode (RHE, pH = 7.4) (Figure 4A).With a difference of 0.2 V between E FB and the CBM in a n-type semiconductor, [49] the CBM lies at ca. -0.3 V vs. RHE (Figure 4B).By taking into account the band gap of 3.3 eV, the VBM is located at 3.0 V vs. RHE at pH 7.4.
Undoped Zn 4 Si 2 O 7 Cl 2 yielded scattered Mott-Schottky data (not shown), which can be related to poorer conductivity than for the carbon-doped material.We then measured the flat band potential at different pH values in the stability range of the material by linear sweep voltammetry under chopped illumination.This method is usually more reliable than the Mott-Schottky method. [50]he amplitude of the anodic and cathodic currents are comparable (Figure S26A), which is characteristic of an ambipolar behavior of the semiconductor.E FB decreases linearly with increasing pH values (Figure S26B), with a slope of 23 mV pH -1 , which deviates from the Nernstian behavior met in oxides, as observed for semiconductor oxides and other oxyhalides, [51] which could be related to different free energies of proton adsorption and desorption. [52]E FB at pH 7.4 is evaluated at 0.7 V vs. RHE, respectively.As an estimation of the CBM and VBM positions, we follow the indication of ambipolarity to state that the flat band potential is in the middle of the band gap.With this assumption, the CBM and VBM positions are reasonably consistent with those evaluated for C-doped Zn 4 Si 2 O 7 Cl 2 and with calculations based on Mulliken electronegativities. [46,47] This confirms that C-doping does not modify in depth the band structure and that the band positions are suitable for water splitting near neutral pH for undoped and C-doped Zn 4 Si 2 O 7 Cl 2 .
Overall, Zn 4 Si 2 O 7 Cl 2 is a semiconductor that combines adequate VB and CB positioning versus water oxidation/reduction half-reactions, long term stability in water, mobility of charge carriers, ability to efficiently separate charge carriers thanks to very different carrier mobilities and to a strong built-in electric field [44] linked to the polarity of the structure and the particle anisotropy.
These features are promising for photoelectrochemical (PEC) water splitting and add to the recent interest of heteroleptic environments for water-splitting. [14,53,54]lthough oxyhalides show promises in the photocatalytic degradation of organic dyes and water splitting, [14,55,56] very little has been reported on their PEC properties.As an ambipolar material, undoped Zn 4 Si 2 O 7 Cl 2 may operate in both water reduction and oxidation half-reactions.We focused on the H 2 evolution cathodic side, for which efficient oxide-based materials are scarce.Photoelectrochemical measurements were then performed in a 0.1 M Na 2 SO 4 electrolyte under a 280W Xenon lamp (Figure 4C).Linear sweep voltammetry (LSV) for the undoped material shows larger cathodic current under light, in agreement with photocatalytic reduction of water.Chronoamperometry at a bias of -0.05 V/RHE show a decrease in the cathodic current over the first 0.5 h, which cannot be related to any structural evolution, as EDS, XRD, scanning electron microscopy (SEM) and TEM (Figures 4G-I and S27, S28) confirm the excellent stability of the nanowires with no compositional, morphological or structural evolution after H 2 evolution.After 0.5 h, the photocurrent reaches a steady state with no significant decrease even after several hours under chopped on/off light (Figure 4D).The photocurrent is 23±0.7 A cm -2 under light during chronoamperometry evaluted from three measurements.Water photoelectrolysis at a bias of 0.2 V vs RHE (Figure 4E) for three hours under the Xe lamp results in the evolution of 3.7 mol (or 1.85 mol cm -2 electrode ) of H 2 quantified by gas chromatography, thus corresponding to a Faradaic efficiency of (85±14)% (Figures 4C, S30).The incident photon-to-current conversion efficiency (IPCE) measured at different monochromatic wavelengths in visibleUV region shows highest conversion efficiency of ca.2% at 350 nm (Figure 4F), in agreement with the 3.3 eV band gap and diffuse reflectance UVvisible spectroscopy (Figure 3B).
C-doped Zn 4 Si 2 O 7 Cl 2 shows the behavior expected for an ntype semiconductor: enhanced activity in the anodic range for oxygen evolution (Figures S30 and S31).The ability to tune the semiconductor type by doping in oxychlorides brings further opportunities to tune electrochemical and transport properties in the future.

Conclusions
We have described the first zinc-silicon oxyhalide, Zn 4 Si 2 O 7 Cl 2 as high aspect ratio nanowires.It crystallizes in a unique polar structure built on tetrahedral units that originate from the molten salt reaction medium.This 3D framework stands out from common layered metal oxychlorides, by providing high thermal and chemical stability.The ambipolar behavior we detected and the strong electric field built in the polar structure provides suitable conditions for photoelectrochemical H 2 production from water.Our synthesis approach relies on a low valence non oxidic silicon precursor and on molten salts as liquid reaction media and reagents.This method allows triggering the crystallization of complex compounds at low temperature and then isolating metastable solids, which would be prone to decomposition in harsher conditions.We demonstrate that the approach is suitable for controlled doping and then adjustment of the nature of major charge carriers.This last result emphasizes our ability to extensively tune the composition and electronic structure of oxyhalides by synthesis in molten salts.This paves the way to an entirely new family of mixed anion solids.

Entry for the Table of Contents
Nanowires of the new oxychloride Zn 4 Si 2 O 7 Cl 2 crystallize from molten salts as a polar three-dimensional tetrahedral framework.They exhibit strongly anisotropic charge carrier mobility, adjustable electrical properties, and photoelectrochemical properties for H 2 production from water.

Figure S9
Reconstructed reciprocal space sections S-16

Table S1
Combined PEDT data sets to obtain a single data set used for the structure solution step S-16

Figure S10
Electrostatic potential map obtained from ab initio structure solution using single nanowires PEDT datasets S-17

Table S2
Crystallographic details of data reduction and dynamical refinement S-18

Table S3
Structural parameters of the refinement based on PEDT S-18

29 Si
MAS solid-state nuclear magnetic resonance NMR (Figure 2B) with two main signals typical of SiO 4 tetrahedra at approx.-70 and -81 ppm in a 20:80 ratio, which are ascribed to Si(3) on one hand, Si(1) and Si(2) on the other hand according to density functional theory DFT calculations (Figure 2B, S-8).The static 35 Cl solid-state NMR spectrum is also reasonably fitted with the DFT calculated parameters of the 3 chlorine positions (Figure S12).Electron energy loss spectroscopy (EELS) highlights Si-L 2,3 and Si-L 1 edges (Figure 2C) at ca. 100 and 150 eV, which are characteristic of Si 4+ in tetrahedra

Figure 2 .
Figure 2. X-ray diffraction, scattering and spectroscopic evidences of the Zn4Si2O7Cl2 structure.(A) Rietveld refinement of the SXRD pattern of Zn4Si2O7Cl2 ( = 0.412831 Å).The red crosses, black, and blue lines, and green tick bars represent the observed, calculated, and difference patterns, and Bragg positions, respectively.As inset is the crystallite shape deduced from the refinement of the anisotropic size broadening.(B) Experimental 29 Si MAS solid-state NMR spectrum of the Zn4Si2O7Cl2 nanowires and the corresponding spectrum calculated by DFT with the structure from refined PEDT data.(C) EELS spectrum showing the characteristic Si-L and O-K edges, respectively (Inset: enlarged view of O-K edge).(D) Pair distribution function (PDF) analysis of the nanowires (experimental = red, fitting = black, difference = blue) taking into account the presence of amorphous silicon (*).
8 eV emission (450 nm, Figure S21) with a lifetime value of about 12 µs.UVvisible diffuse reflectance measurements (Figure 3B) indicate a band gap of 3.3 eV.DFT calculations were used to get deeper insights into the electronic structure.A full geometry relaxation (Table S8) yields little changes in the experimentally determined structural parameters.The band structure (zoom in Figure 3C and overall view in Figure S22) indicates a direct band gap of 3.3 eV consistent with diffuse reflectance.

Figure 3 .
Figure 3. Optical and electronic properties.(A) Emission spectra at 10 K and room temperature.(B) Kubelka-Munk function obtained from UV-visible diffuse reflectance measurements.(C) Zoom in the band structure of Zn4Si2O7Cl2 with the valence band maximum (VBM) and conduction band minimum (CBM) highlighted and showing a direct band gap of 3.28 eV at the  point.The effective masses of electrons and holes calculated along the different directions are indicated.(D) Projected Density of States (PDOS) for Zn4Si2O7Cl2 on each type of element within the unit cell.The inset represents the total DOS.The Fermi level is set to 0 eV.

Figure 4 .
Figure 4. Photoelectrocatalytic water splitting with Zn4Si2O7Cl2 nanowires.(A) Mott-Schottly plot recorded at 100 k Hz in a 0.1 M Na2SO4 electrolyte at pH 7.4.(B) Scheme of the band structure versus potentials of water redox couples at pH 7.4.(C) Linear sweep voltammetry of a Zn4Si2O7Cl2 photoelectrode at a scan rate of 10 mV s -1 in 0.1 M Na2SO4).(D) Chopped-light chronoamperometric measurement of a Zn4Si2O7Cl2 photoelectrode biased at -0.05 V/RHE (shutter time = 1 min for each plateau, 0.1 M Na2SO4, 280 W Xe-lamp). (E) Electrode surface normalized evolution of H2 vs. time at a bias of -0.2 V/RHE (0.1 M Na2SO4, 280 W Xelamp). (F) Incident photon to current conversion efficiency (IPCE) of Zn4Si2O7Cl2 nanowires biased at -0.2 V/RHE and under different monochromatic wavelengths in 0.1 M Na2SO4.(G) Photoelectrode used in water splitting experiments.(H) Top-view SEM images of the electrode before and (I) after the H2 evolution experiment.

Table S3 .
Figure S19 TEM images of Zn 4 Si 2 O 7 Cl 2 nanowires exposed to different atmospheres S-28 Figure S20 TEM images of Zn 4 Si 2 O 7 Cl 2 nanowires exposed to different aqueous solutions Structural parameters of the refinement based on PEDT.Site occupancy are all equal to 1.
* All Si Uiso were constrained to be identical.** All O Uiso were constrained to be identical.*** fixed parameter

Table S5 .
Structural Parameters for Zn 4 Si 2 O 7 Cl 2 deduced from the Rietveld refinement of the SXRD pattern, starting from the model obtained from PEDT.Isotropic temperature factors (B iso ) were constrained to be equal for the same chemical species.*The x coordinate of O3 was not refined because it serves as floating for the A2mm space group.The atomic positions deduced from Rietveld refinement lead to slightly more distorted ZnO 3 Cl and SiO 4 tetrahedra than from PEDT, which is attributed to structural disorder and fluctuations in atomic positions from one crystallite to another.Zn 4 Si 2 O 7 Cl 2 Space group:  2   a = 5.184172(12) Å, b = 17.7897(2)Å , c = 20.7771(3)Å, V = 1916.16(3)Å 3 , Z = 8

Table S8 .
Atomic positions of Zn 4 Si 2 O 7 Cl 2 structure after full geometry optimization leading to the unit cell parameters: a=5.2648Å b=18.0190Å c=20.9947Å in space group A2mm.