Synthesis, Characterization and Crystal Structure of Gallosilicate

Perchlorate enclathrated sodalite with gallosilicate host framework has been synthesized under low temperature (100 C) hydrothermal technique and characterized by X-ray powder diffraction, IR, Raman spectroscopy, UV DRS, SEM, MAS NMR spectroscopy and thermogravimetry. The structural features were investigated by IR, MAS NMR spectroscopy of Si and Na nuclei and by Reitveld refinement of X-ray powder diffraction data. IR and Raman spectroscopy confirms formation of framework and presence of ClO4 group. The tetrahedral framework T-atoms are completely ordered and the chloride atoms are located at the centre of the sodalite cages. The crystal structure of this new sodalite was refined in the space group P 4 3n with a = 9.15788 Å, V = 768.5 Å, Z =1, Rwp = 0.0611, Rp = 0.1079 and Si-O-Ga is 144.036. The surface area of single entity with stoichiometry Na8[GaSiO4]6(ClO4)2 was found to be 8.385x10 cm/g. Si MAS NMR study confirms complete ordering of Si and Ga in the gallosilicate framework. Thermo gravimetric analysis has provided information on the extent of perchlorate entrapment, stability within the cage and decomposition properties.


Introduction
The cubic framework structure of sodalites with general composition Na 8 [T 1 T 2 O 4 ] 6 X 2 is formed by a space filling array of truncated octahedral cages each built up of tetrahedral TO 4 units. The framework T-atoms are usually Si and Al but even others like Ga or Ge can be introduced during synthesis . According to the host guest interactions the cage filling ions and the type of framework T-atoms are both responsible for the chemical and physical properties of a certain sodalite species. Mean while many sodalites have been synthesized with large number of element combinations with compositions Na 8 [AlSiO 4 ] 6 X 2 , where X is a monovalent guest anion, X= Cl -, Br -, ClO 4 -, is formed by TO 4 tetrahedra as elementary building unit (Pauling, 1930;Lons et al., 1967). The gallosilicate framework is made of regularly alternating tetrahedrally coordinated Ga and Si atoms, which are connected through oxygen atoms. The framework of sodalite is highly flexible and can accommodate its degree of expansion due to enclatharated guest species of different size. Depending on the composition sodalite posses photochromic, cathodochromic, ion conducting properties and as high density optical data storage materials. Sodalites with special guest anion can be used for various technical applications like pigments, ultramarine, graphical and digital storage system, luminescence and catalytic activity (Hassan et al., 1984;Schipper et al., 1972;Van Doorn et al., 1972;Mclaughan et al., 1970;Bolwijn et al., 1972;Chang, 1974 (Piontkovskaya et al., 1970, Suzuki et al., 1985, McCusker et al., 1986, Newsam et al., 1987, Bu, Xianhui et al., 1998Gesing et al., 2000, Buhl et al., 2006, Murshed et al., 2007 Sodalites are expected to be an interesting model system for studying simple principle of perchlorate storage in small cavities of framework structure (Weitkamp et al., 1995), we extend our experimental work here on synthesis, characterization and structure of perchlorate encapsulated sodalite with gallosilicate framework. Substitution of framework by gallium is of special interest to study interactions of encapsulated ClO 4 guest anion with a host framework of certain composition. The aim of the present paper is to establish synthesis and crystal structure of Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite as a new model compound for long time in a solid matrix.

Synthesis at low temperature
Apart from several methods (Barrer et al., 1970;Veit et al., 1991;Buhl et al., 1991;Fleet, 1989) low temperature hydrothermal technique has been employed for the synthesis of Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite . The synthesis was carried out in a single step without precursors in Teflon autoclaves at a temperature of 100 o C and autogenous pressure for reaction time of seven days. The stiochiometric amount of gallium oxide (source of gallium), silicic anhydride (source of silicon) along with NaOH (as a mineralizing agent) and sodium salt of perchlorate were taken in a Teflon autoclave. After the reaction period, the product was washed with deionized water and dried overnight at 100 o C to remove weakly adsorbed surface water. The product obtained was characterized by IR, Raman, MAS NMR spectroscopy, UV DRS, TGA/DTA and SEM.

Crystallography
The characterization of Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite was performed by X-ray powder diffraction method using Rietveld refinement GSAS program. The X-ray powder diffraction data were collected, using diffractometer operating in θ-θ geometry (5-80 o 2θ, step width 0.017 o 2θ, sample time 1s per step), using CuKα radiation for 5<2θ<90 o . The crystallographic data and experimental conditions are given in Table 1.

Spectroscopy
An infrared spectrum in the region 350-4000 cm -1 was measured as KBr pellets on a computer interfaced Bruker FTIR spectrophotometer. The Raman spectrum was collected at room temperature using Nicolet Almega XR dispersive Raman Spectrophotometer (Thermo Electron Corporation) with 780 nm Laser. The sample was mixed with KBr and pressed into a disk which was rotated during excitation to minimize heating effect (Kiefer et al., 1971).
Further, structural characterization was also performed by solid state MAS NMR technique. The use of high magnetic field, combined with spatial-averaging sample reorientations NMR technique like magic angle spinning (MAS) is used to obtain detailed structural information on solid lattice. This work concentrates on the application of 29 Si and 23 Na MAS NMR. The MAS NMR spectra were recorded on a Bruker solid state MAS NMR Spectrometer DSX 300. The 29 Si MAS NMR spectrum was recorded at 59.62 MHz with 5 μsec pulse duration, 15 sec pulse delay and a spinning rate of 5 KHz. Upto 3447 scans were accumulated at a rotation frequency in a 5 mm probe (Tetramethylsilane as a internal standard). The 23 Na MAS NMR spectrum was recorded at 79.39 MHz with 40.5 μsec pulse duration, 1 sec pulse delay and a spinning rate of 5 KHz. Upto 429 scans were accumulated at a rotation frequency in a 5 mm probe (sodium chloride is used as a internal standard). The crystal morphology and EDAX of ClO 4 sodalites were carried out using scanning electron microscopy on a JEOL JEM-6360A model equipped with JEOL JEC_560 auto carbon coater SEM.

Thermogravimetry
The thermal stability of the Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite was studied by TGA/DTA using Metlor Toledo instrument at a heating rate 10 0 C/min, in the temperature range, room temperature to 900 0 C.

IR and Raman spectroscopy
The IR spectrum of Na 8 [GaSiO 4 ] 6 (ClO4) 2 sodalite, obtained in the mid infrared and far-infrared region is shown in In addition, the asymmetric stretching vibration ν 3 of ClO 4 at 1116.82 cm -1 is clearly visible in the infrared spectrum. This strong absorption peak confirms the enclathration of ClO 4 ion in the gallosilicate sodalite cage.
The Raman spectrum is shown in Figure 2. The framework vibrations are weak in the Raman and hence, ν 1 ,ν 2 and ν 4 vibrations can be clearly seen. The Raman spectrum shows bands at 227.56 cm -1 , and 442.84 cm -1 (T-O deformation mode), 631.80 cm -1 (symmetric Ga-O-Si vibration) and 931.56 cm -1 can also be classified as framework bands. In addition a deformation band at 1111.56 cm -1 can be assigned to ClO 4 anion.

Photo physical properties
The photo-absorption of the photocatalyst depends on the mobility of electron-hole pairs, which determines the probability of electrons and holes to reach reaction sites on the surface of the photocatalyst. Figure 3 shows the diffuse reflection spectrum of the Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite. This shows absorption in the UV region at 248 nm (The band gap of Eg = 5 eV, by the formula Eg = hν/λ ) which indicates that Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 have the ability to respond to the wavelength of ultra violet region.

Structure refinement
The crystallographic data and experimental conditions for the structure refinement of gallosilicate perchlorate sodalite are given in Table 1. The refined positional, displacement and occupancy parameters are given in Table 2. The X-ray powder pattern of sodalite synthesized with an ideal composition Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 is shown in Figure 4 together with the final Reitveld difference plot. The cubic lattice parameter, a = 9.15788 Å was refined for the gallosilicate sodium perchlorate sodalite. The final residuals for the pattern and the structure factor are Rwp = 0.0611, Rp = 0.1079. The refinement was performed on arranging chloride at the centre and the four oxygen at 24i position of the space group P 4 3n. Selected geometrical data are given in Table 3. The gallosilicate framework consists of a long-range disordered arrangement of the gallium and silicon atoms. The average bond length between the tetrahedrally coordinated atoms and oxygen atoms of 1.70202 Å (   Further, the surface area of single entity of Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 was found to be 8.385x10 -15 cm 2 /g. The surface area of single entity is successfully calculated by using equation (Overman, 1965), b = (M/dL) 2/3 , where, b -surface area, Mformula weight, d -density and L -Avogadro's number.

MAS NMR Spectroscopy
In order to confirm the framework structure and Na interaction with other atoms, 29 Si and 23 Na MAS NMR is studied. The 29 Si MAS NMR spectrum of the reaction product Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite consists of a sharp single resonance line at δiso = -82.93 ppm for Si(OGa) 4 units (Figure 6a). This indicates Si/Ga ratio of 1.0 and confirms the alternate Si, Ga ordering of the framework in the synthesized sodalite (Engelhardt et al., 1989. The 23 Na MAS NMR spectrum for the ClO 4 -GaSi-SOD is also shown in Figure 6b. The 23 Na MAS NMR spectrum shows a quadrupole pattern depending more or less upon the perchlorate cage contents of the Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite. 23 Na MAS NMR shows quadrupole pattern with two well resolved peaks with δiso= -6.135 ppm and δiso = -33.608 ppm. Due to the 3/2 spin, the 23 Na nucleus exerts a quadrupole moment, interacting with local electric field gradient. This causes line broadening as well as specific line shapes of the spectrum. The line width and the isotropic chemical shift position are proportional to the square of the quadrupole interaction constant.
The above results confirm that, the sodium cations are located above the centre of the six-ring windows of the cages and are co-ordinated with three oxygen atoms and anions in the sodalite cage. Calculated Na-O1 and Na-O2 distances show that chloride atom at the centre of the sodalite cage.

Thermal analysis
The weight temperature behavior of the perchlorate gallosilicate sodalite is studied in detail. Figure 7 shows the TGA/DTA of perchlorate gallosilicate sodalite. An endothermic behavior in the temperature range of 600-700 o C, is due to the decomposition of encapsulated salt molecules. The thermal analysis shows high thermal stability of sodalite framework after 700 o C. The Na 8 [GaSiO 4 ] 6 (ClO4) 2 sodalite onset decomposition temperature ~600 o C, where as the solid NaClO 4 decomposes nearly at 400 o C. This behavior is similar to aluminosilicate perchlorate sodalite (Weller et al., 1991) which shows onset decomposition temperature ~620 o C. The weight loss indicates full occupancy of anion sites in the sodalite cages by ClO 4 -anions and negligible amount of water. Gallosilicate perchlorate sodalite can be converted to chloride sodalite via heating in air at 700 o C for 4 hrs. This provides an indirect route to the chloride sodalite.

Crystal morphology
SEM study were carried out to provide information about the particle morphology and the macroscopic crystal growth mechanism, as well as to determine the distribution of different cages (with or without anion) through out the lattice. Figure 8 shows the scanning electron microscopic pictures of the synthesized sodalite loaded with sodium perchlorate.
SEM (Figure 8) shows balanced crystal size with cubic morphology. The surfaces of these crystals are smoother than those of the silica sodalite crystals. Atomic proportions in the same product were determined using energy dispersive X-ray Spectroscopy (EDX). EDX analysis has detected that Ga, Si and Na are present in the crystal with respective compositions.

Conclusions
Perchlorate anions have been successfully encapsulated within gallosilicate sodalite. The Na 8 [GaSiO 4 ] 6 (ClO 4 ) 2 sodalite can be synthesized directly without any precursors at 100 o C with reaction period of seven days. The perchlorate sodalite crystallizes in a space group P 4 3n with a unit cell parameter a = 9.15788 Å. The bond distances and bond angles are considerably modified comparative to other gallosilicate sodalites. Present study shows that perchlorate ion decomposes between 620 -700 o C in gallosilicate sodalite and the sodalite is stable there after. The perchlorate sodalite can respond to wavelength of ultra violet region and the surface area of single entity of the sodalite is 8.385x10 -15 cm 2 /g. SEM study supports the cubic structure of gallosilicate perchlorate sodalite.