Thermal analysis, X-ray diffraction, spectroscopy studies and magnetic properties of the new compound Tl 2 HAsO 4 .Te(OH) 6 .

: The Tl 2 HAsO 4 .Te(OH) 6 (TlAsTe) compound crystallizes in the triclinic system P1 with unit cell parameters: a= 7.100(10) Å, b= 7.281(13) Å, c= 8.383(11) Å, α= 76.91(1)°, β= 87.16(1)°, γ= 66.96(2)°, Z= 2 and V= 388.19(1) Å 3 . This new structure can be described as a lamellar one with the atomic arrangement being built by planes of Te(OH) 6 octahedra alterning with planes of arsenate tetrahedra. Raman and infrared spectra recorded at room temperature confirm the presence of As O 43− and Te O 66− groups and characterize the hydrogen bonds present in the crystal lattice. Differential scanning calorimerty (DSC) shows the presence of three-phase transitions at 396 K, 408 K and 430 K present in the title compound. Typical thermal analyses, such as differential thermal analysis (DTA) and thermogravimetry (TG-DTG) show that the decomposition of this material starts at about T= 445 K. Magnetization curve of Tl 2 HAsO 4 ·Te(OH) 6 substance have revealed a diamagnetic response overall temperature range By comparison with the Tl 2 SeO 4 Te(OH) 6 (TlSeTe) material that exhibits three endothermic peaks at 373, 395 and 437 K. The first peak at 373 K was attributed to a structural phase transition. Whereas, the second peak, at about 395 K, was assimilated to a ferroelectric paraelectric phase transition. The third peak detected at 437 K was assigned to ionic protonic conduction type [10]. We can estimate that the first and the second peaks can be assimilated to an order-disorder phase transitions. The third peak detected at T= 430 K may be of ionic protonic conduction type. The calculated transition enthalpies, for the first transition at T 1 = 408 K and for the third anomaly at T 3 = 430 K are respectively ΔH 1 = 937.26 Jg -1 and ΔH 3 = 400.38 Jg -1 [1, 2, 10].


INTRODUCTION:
A large number of telluric salts with general formula M2AO4.Te(OH)6, where M is a monovalent cations and (A = As, S, Se…), undergo several structural phase transitions and interesting physical properties [1][2][3][4][5][6]. In order to expand upon the emergent chemistry of template tellurate a new compound using the arsenate group HAsO 4 2− has been synthesized. Thus, the presence of the hydrogen atom in this ionic group can differentiate this new tellurate to other previously studied and give new insights on the addition compound based on tellurate. Therefore, there is a great interest to create and study the new family of arsenates tellurates.
In this paper, we present and discuss the results of our investigation concerning the (TlAsTe) material by X-ray diffraction, IR and Raman studies, thermal analysis (DSC, DTA, DTG and TG) and magnetic properties.

EXPERIMENTAL:
Colorless and transparent single crystals of Tl2HAsO4.Te(OH)6 were prepared by slow evaporation at room temperature from a mixture solution of telluric acid H6TeO6, thallium carbonate Tl2CO3 and arsenic acid H3AsO4 in the stoechiometric ratio reaction.
The formula of this material was determined by chemical analysis and confirmed by structural refinement. The structure was solved by using the SUPERFLIP program [7] and completed by the Fourier synthesis performed using SHELXL-97 program [8]. The structure graphics were created by the Diamond program [9]. The masses of samples used in TG, DTG and DTA measurements were 9.035 mg. Differential scanning calorimetry (DSC) measurements were carried out by means of a Mettler Toledo DSC model DSC821 with samples placed inside platinum crucibles, at a heating rate of 5K/min. Infrared absorption spectra of suspension of crystalline in KBr have been recorded using Jasco-FT-IR-420 spectrophotometer in the (4000-500) cm -1 frequency range. Raman spectra of polycrystalline samples sealed in glass tubes have been recorded on a Labrama HR 800 instrument using 632.81 nm radiations from a physics argon ion laser.
The details of data collection and refinement for the title compound are summarized in Table 1. The final positions and equivalent isotropic atomic displacement parameters for the new compound are given in Table 2 and Table 3.

Calorimetric study:
A typical result of the calorimetric study of the Tl2HAsO4.Te(OH)6 compound is presented in Figure 1. The superposition of TG-DTA and DTG curves are showing in Figure 2. According to these figures, the (TlAsTe) material presents three-phase transitions at T= 408 K, with a shoulder at T= 396 K, and an endothermic peak at T= 430 K, also we can deduce that the decomposition of this material starts at about T= 445 K. In fact, the first mass loss of 4.5 % may be corresponding to the release of two water molecules per chemical formula. Thus, in the temperature range 400 K -500 K the telluric acid Te(OH)6 decomposes to disengage the water molecule and give the orthotelluric acid H2TeO4 [6,30]. By comparison with the Tl2SeO4Te(OH)6 (TlSeTe) material that exhibits three endothermic peaks at 373, 395 and 437 K. The first peak at 373 K was attributed to a structural phase transition. Whereas, the second peak, at about 395 K, was assimilated to a ferroelectric paraelectric phase transition. The third peak detected at 437 K was assigned to ionic protonic conduction type [10]. We can estimate that the first and the second peaks can be assimilated to an orderdisorder phase transitions. The third peak detected at T= 430 K may be of ionic protonic conduction type. The calculated transition enthalpies, for the first transition at T1= 408 K and for the third anomaly at T3= 430 K are respectively ΔH1= 937.26 Jg -1 and ΔH3= 400.38 Jg -1 [1,2,10].

Vibrational studies:
In the present investigation, IR and Raman spectroscopic studies of Tl2HAsO4.Te(OH)6 have been performed and analyzed in order to confirm the presence and the independence of the two anions (As O 4 3− and TeO 6 6− ). These two analyses give more importance to hydrogen bonds in the new crystal lattice. Raman and infrared spectra of Tl2HAsO4.Te(OH)6, at room temperature, are shown in Figure 7 and Figure 8, respectively. The observed Raman and IR bands are given in Table 6.

Interpretation of Raman spectrum:
According to the literature of the addition compounds based on tellurate and materials stable due to the hydrogen bonds, we could interpret the different peaks observed in Figure 7. The intense and narrow peak at 634 cm -1 is assigned to ν1(TeO6) [16], while in the others tellurates alkaline, this vibration mode appears at peak with frequency superior than 634 cm -1 . Thus, in the Rb2HAsO4.Te(OH)6 ,the ν1(TeO6) observed at 673 cm -1 [6]. The shoulder detected at 606 cm -1 is attributed to ν2(TeO6) [17,18]. However, the peak at 396 cm -1 is assimilated to ν4(TeO6) [18]. In addition the peak detected at 375 cm -1 is attributed to ν5(TeO6) [17][18][19]. Whereas the band which appears at 290 cm -1 is attributed to ν6(TeO6) [16]. Vibrational analysis for the isolated AsO 4 3− anion with point group Td leads to four modes: A1(ν1), E(ν2), 2F2(ν3 and ν4). The ν1 mode is the totally symmetric stretching vibrational mode of the AsO 4 3− anion, ν2 is the doubly degenerate bending mode, ν3 and ν4 are the triply degenerate stretching mode and bending mode respectively. In the free AsO 4 3− anion, they are found at 837, 349, 878, and 463 cm -1 , respectively. Based on this interpretation and examining the Raman spectra, we can note that A1 band may shift to different wave numbers and the doubly degenerate E and triply degenerate F modes may give rise to several new A1, B1, and/or E vibrations [20,21]. In fact, we can assimilate the band which appears at 721 cm -1 to ν1(AsO4). While the three peaks observed at 797 cm -1 , 806 cm -1 and 823 cm -1 are relating to the triply degenerate bending and stretching mode ν3(AsO4) [20,21]. The two regions (300-350) cm -1 and (400-450) cm -1 are assigned respectively to doubling degenerate bending mode ν2(AsO4) and ν4(AsO4) [22]. Thus, the peaks observed at 320 cm -1 and 341 cm -1 are attributed to ν2(AsO4). Whilst the band appears at 410 cm -1 is assimilated to ν4(AsO4).

Interpretation of IR spectrum:
In order to confirm the results got by the Raman spectra and to gain more information on the strong O-H…O hydrogen bond in this new structure ,we have undertaken the IR study, at room temperature ,in the frequency range (4000 -500) cm -1 .
The  (As-O…H) bending vibration mode appears in the region between 1222 cm -1 and 1312 cm -1 [15,23,24]. While the peaks detected at 1567 cm -1 and 1689 cm -1 are assigned to the presence of the As-O…H free and the presence of strong hydrogen bonds [25].
The peaks observed in the region being from 2300 to 2836 cm -1 may be assigned to the asymmetric and symmetric stretching vibrations of the O-H group of strong hydrogen bond [23,26,27].

MAGNETIC PROPERTIES.
The crystals of Tl2HAsO4·Te(OH)6 substances are massive enough to reach a magnetic response that could be higher than the signal detection for VSM option (>10 -6 emu). Therefore, we have crushed the crystal with agate mortar and pestle in order to obtain a powder specimen. Flattened monocrystal had mounted on a quartz paddle-shaped sample holder and fixed with vacuum grease and PTFE tape. Powder materials had compressed inside a polypropylene powder holder that fit in a brass through-shaped sample holder. These materials have been supplied by Quantum Design Company [28]. Previously, the mass of all samples have been accurate determinate by a Sartorius balance (model CP225D).
Specific sequences have design in order to carry out the magnetic characterization at each composition: isothermal magnetization curves (for example, M(H,T=300 K).seq), thermal variation of magnetization at fixed magnetic field (e.g., M(H=1kOe,T).seq. For diamagnetic substance a single-valued function between both magnitudes is observed and at regular temperatures and magnetic fields is given by the linear relation: Diamagnetic substance: Tl 2 HAsO 4 ·Te(OH) 6 .
A thorough study, as an example for a diamagnetic material, was carried out over the Tl2AsO4·Te(OH)6 sample, where there is not a magnetic ion on its composition. It is worth noting that this sample have been mounted on paddle-shape sample holder which produces a lower level of noise in the measurement, will lead more accurate determination on magnetization (Table 7). Table 7. Mass susceptibilities at T=300 K for Tl2HAsO4.Te(OH)6 substance.
The Figure 9 shows the variation of  m with temperature derived from temperature-dependence of  at applied magnetic field of T 14 0  H  . A remarkable peak is observed around 50 K that can ascribe to oxygen trapped by the Teflon tape used to wrap the sample [29]. In this way, above this temperature  m is almost constant; whereas below it, paramagnetic oxygen contamination produces a slight increase on the value of  m measured. Paramagnetic contribution, from oxygen migrating from the PTFE tape, is even clearly visible if an observer compares the isothermal magnetization curves at 300 and 2 K ( Figure 10). The difference between both curves has roughly the shape of a Brillouin function, which is a feature of paramagnetic substances at low temperature and/or high applied magnetic field and which should be produced by the presence of the paramagnetic oxygen.