Synthesis, Vibrational Spectroscopy and Optical Study of Bi-Tetrabutylphosphonium Hexachlorostannate

: The single crystals of bi-tetrabutylphosphonium hexachlorostannate were grown by solvent evaporated method and characterized by X-ray powder diffraction, Raman spectroscopy and optical study. The X-ray powder diffraction indicates that it was crystallized in the monoclinic system, with C2/c space group. Besides, temperature-controlled X-ray diffraction show that the [P(C 4 H 9 ) 4 ] 2 SnCl 6 compound indicate the presence of two phase transitions detected at 388 and 403 K. The evolution of Raman line shifts, “ν” , and the half-width, “Δν” , versus temperature show some singularities associated with the transitions, suggesting the important role of the anionic parts ((SnCl 6 ) 2- ). Besides, the results of the Raman study confirms the conclusion drawn from the X-ray powder diffraction measurements that the phase transition are located near 388K and 403K. Furthermore, an optical absorption measurement confirms the semiconductor nature with a band gap equal to 3.46eV.

The field of organic-inorganic hybrid materials has been growing intensively during the past 20 years and is certainly nowadays one of the major fields of research and unambiguously one of the most exciting. Moreover, the need for always better materials for complex devices with improved properties and multifunctional responses is a perpetual demand, which often can be fulfills by the use of hybrid materials.
In addition to that, these materials are both fundamentally and technologically important because of their structural versatility and interesting/unique properties that result from the incorporation of both inorganic and organic components in a single crystal lattice. [1,2] Particularly, hybrid organic-inorganic compounds based on halogenometallates such as Sb, Sn, and Bi…. [3,4] have been widely explored due to their interesting physical properties and their application in new technology such as the manufacture of devices as well as their ability to present phase transitions. In fact, much spate of interest has been devoted to the development of a new multifunctional generation of organic-inorganic structures with the general formula A 2 SnCl 6 (A = organic cation) displaying new interesting properties. In the case of tin halide systems, some interest have been widely studied as semiconducting components [5] in organic-inorganic hybrid semiconductors with potential applications in display and storage technologies because of their stable exciton and superior carrier mobility [6]. In fact, the Sn (II) represents a potential class of materials with unusual structural archetypes. In the present study, we report on the investigation of the phase transitions in bitetrabutylphosphonium hexachlorostannate as a function of temperature that is based on X-ray powder diffraction and Raman scattering. Thus, optical investigation was carried out. compounds [7,8]. It was prepared in two steps. Firstly, SnCl 2 (98%; SIGMA ALDRICH) dissolved in concentrated HCl. Secondly, the obtained solution was added in molar ratio of 1:1 to [P(C 4 H 9 ) 4 ]Cl (Purity 97% ; FLUKA). After few days and at room temperature a good quality of transparent single crystal was obtained. It's were washed by absolute ethanol and dried in vacuum desiccators.

Characterization
The phase purity and homogeneity of the as-prepared powder was first checked by recording X-ray powder diffraction (XRPD) pattern at room temperature on a PANalytical θ /θ Bragg-Brentano, Empyrean diffractometer (Cu Kα 1+2 radiations) equipped with the PIXcel 1D detector. The XRD pattern was recorded in the scattering angle range, [5-60°] with a 0.0131 step size, for a total acquisition time of 10 h. The Rietveld refinement of the XRD profile was performed using the Fullprof program [9].
Raman scattering spectra was collected with a T64000 Raman spectrometer (Horiba-JobinYvon), using the 514.53 nm radiation of an Ar/Kr laser as excitation. All measurements were done under microscope using x50LF objective in back scattering geometry on transparent single crystal using the parallel polarization, and collected in the wavenumber range 100 -500 cm -1 . The crystals were set in a Linkam heating stage for measurements as a function of temperature up to 433 K. In order to quantitatively analyze the evolution of Raman bands versus temperature, each spectrum were analyzed by means Labspec software based on Gaussian and Lorentzian shapes.
The optical properties were measured at room temperature, using a UV-3101PC scanning spectrophotometer that has a dual-beam monochromator, covering a spectrum from 200 nm to 800 nm and using two sources: (i) xenon lamp for the UV-visible domain and (ii) halogen lamp for the infrared range. This technique makes it possible to determine the absorbance (A) and the reflectance (R). In order to learn more information about the mechanism of phase transition in [P(C 4 H 9 ) 4 ] 2 SnCl 6 , we have undertaken the evolution of the X-ray powder diffraction in the temperature range from 293K to 413K. X-rays diffraction at selected temperatures of the sample is reported in Fig. 2. An overview of this figure, the positions and intensities of the peaks in the XRD patterns measured at 393 K and 413 K were greatly changed. Particularly, at 393K, we observe a large decrease in the intensities for all reflections. When the temperature attends 413K, we note a decrease in the intensities of some peaks ("A" and "B")

Temperature dependence of X-ray diffraction
while the emergence of novel peaks for higher angles ("C"). This suggests that the approximate structures of the high-temperature and low-temperature phases are so different.
These changes correspond to the thermal phase transitions detected using DSC. So, all phase transitions that were detected in the DSC [10], could be observed by a change in the crystal structure in the HT-XRPD diagram

Temperature evolution of the Raman spectra of the [P(C 4 H 9 ) 4 ] 2 SnCl 6 crystals
In order to get more about on the phase transitions mechanisms in [P(C 4 H 9 ) 4 ] 2 SnCl 6 the temperature evolution of the Raman bands has been investigated in the 293K-433K range.
The tentative assignments at room temperature for a majority of the vibrational bands using previously reported data in the literature on similar compounds are: NH 3 (CH 2 ) 3 NH 4 SnCl 6 [11], [NH 3 (CH 2 ) 5 NH 3 ]SnCl 6 [12], NH 3 (CH 2 ) 3 NH 3 SnCl 6 [12], NH 3 (CH 2 ) 4 NH 3 SnCl 6 [12],  Fig.3 shows the temperature dependent Raman spectra of the [P(C 4 H 9 ) 4 ] 2 SnCl 6 . A deconvolution of all spectra was necessary to follow the temperature dependence of the wavenumber and half maximum. This deconvolution was carried out by means of Labspec software with a combination of two Lorentzian and Gaussian functions. An example of the deconvolution of spectrum recorded at 303K in the frequency range 100 -500 cm -1 is shown in fig. 4.
The positions and width at half maximum for selected lines, obtained in this range where the anionic part is found are shown in fig.5 and fig.6, respectively. Significant changes are observed in the vicinity of 388K and 403K where the phase transitions have been detected, which is in good agreement with DSC and electrical measurements [10]. These observations are indicative of strong contributions of the anionic parts to the mechanism of phase transitions.

Optical absorbance spectra
At room temperature, the experimental UV-Vis absorption spectrum of [P(C 4 H 9 ) 4 ] 2 SnCl 6 compound in the region 200-800 nm is shown in Fig.7. It is clearly seen from this figure that the absorption coefficient tends to decrease exponentially as the wavelength increases. This is a typical behavior for numerous semiconductors and can be due to many reasons, such as internal electric fields within the crystal, deformation of lattice due to strain caused by imperfection and inelastic scattering of charge carriers by phonons [14].
The absorption spectrum showed three distinct peaks located at 238, 287 and 386 nm. The

Optical band gap.
The optical band gap is an important factor in several fields of science including photovoltaics, solar cells, lasers, photoluminescence, or diodes. [15,16] Where (h ) is the energy of the incident photons, A is a constant characteristic of the material, Eg is the optical energy gap,  is the absorption coefficient, and n is an empirical exponent, which characterizes the type of optical transition during the absorption process.
The transition is called direct, if the extremities of VB (valence band) and CB (conduction band) lie at the same K-space, however, the transition is called indirect, if the transition is possible only with phonon assisted. Thus, the value of (n) may be 1/2 and 2, corresponding to the allowed direct and indirect [20]. The energy of the incident photon is calculated using the following equation: [21] 1240 After plotting (αhν) 2 and (αhν) 1/2 in function of the photon energy (hν), it was found that the most suitable and adequate plot was when n=2. Eg was found herein equal to 3.46 eV. The value of the gap energy show that this material can be classified as semiconductor and it's smaller than Cs 2 SnCl 6 's gap energy value. [22]

Urbach energy:
The Urbach energy characterizes the disorder of a material and corresponds to transitions between extended states of the valence band and localized states of the conduction band.
From the variation of the absorption coefficient, it is possible to deduce the disorder in the compound, given by this equation: [23] Where α 0 is a constant and E U is an energy, which is interpreted as the width of the tail of localized states in the forbidden band gap, ν is the frequency of radiation, and h is Planck's constant. Fig.9 shows the theoretical adjustment of the plot ln (α) as function of energy (hν).
The Urbach parameter was calculated by the inverse of the slope to the curve. The value of this energy was equal to 0.27eV, which is weaker than Cs 2 SnCl 6 's Urbach energy [22]. This explains the good crystal quality of this material and confirms the result found by XRD [24].

The extinction coefficient
The extinction coefficient (K) is linked to the absorption coefficient by the following equation: Where λ is the wavelength of the incident photons. Fig.10 represents the variation of k in function of λ for the incident electromagnetic radiation. This low value confirms the good transparency of our material. [25]

Conclusion
In summary, the [P(C 4 H 9 ) 4 ] 2 SnCl 6 organic-inorganic compound has been successfully synthesized by the Solvent evaporation method. X-ray diffraction shows its monoclinic structure with C2/c space group. The Temperature dependence of X-ray diffraction suggests that the approximate structures of the high and low-temperature phases are not the same, which indicate the existence of two phase transitions at 388k and 403k. Besides, the Raman spectroscopic study clearly shows spectacular temperature changes of the internal vibrations modes of the anion, these changes confirm the important role played by the anionic part in the phase transitions mechanism. UV-Vis spectroscopy was investigated to decide certain main parameters, for example, the band gap energy and the Urbach energy. The optical properties show that the entitled compound has a semiconducting character for which the gap energy is 3.46eV.             Temperature evolution of the Raman spectra in the 100-500 cm-1 frequency range.

Figure 4
Deconvolution of the Raman spectrum at T=303 K in the 100-500 cm-1 spectral range.

Figure 5
Temperature dependence of the position of the hexachlorostannate anions bands.

Figure 6
Temperature dependence of the half-widths of the hexachlorostannate anions bands Optical absorption spectrum of the [P(C4H9)4]2SnCl6 compound.

Figure 10
Dispersion curve of extinction coe cient k as a function of wavelength.