Synthesis, Characterization and Thermal Behavior of HYP2O7·3H2O Electrical Properties of HYP2O7

The synthesis of the diphosphate HYP 2 O 7 ·3H 2 O was made via soft chemistry route from evaporation of aqueous solution at room temperature. The obtained compound, was characterized by means of X-ray diffraction (XRD) and infrared spectroscopy (IR). The results showed a high purity phase. IR spectrum of this diphosphate revealed usual signals related to P 2 O 7 diphosphate group and water molecules. The thermal decomposition of the synthesized product by DTA / TG proceeded through four stages leading to the formation of the Y 2 P 4 O 13 as a nal product. On the other hand, its decomposition by CRTA took place in three stages leading to the formation of the anhydrous diphosphate HYP 2 O 7 as a nal product. X-ray powder diffraction and infrared spectroscopy were used to identify these materials. Furthermore the electrical properties of the HYP 2 O 7 were investigated through impedance complex analysis. Modest conductivity has been observed in this material at relatively medium temperature range. Activation energy of 0.67 and 1.44 eV, was deduced from the corresponding Arrhenius plot. The optical band gap of the title compound is calculated and found to be 2.71 eV.


Introduction
Much scienti c disciplines are now concerned with phosphor compounds and / or ionic conductors.
Several types of host materials for rare earths are studied: oxides, sulphates and phosphates. Their applications are many and varied.
In this context, rare earth phosphates have been the subject of much research in order to investigate their electrical and optical properties. In the lighting eld, uorescent lamps have been manufactured using lanthanum phosphate doped with Ce 3+ and Tb 3+ ions (LaPO 4 : Ce 3+ , Tb 3+ ) [1]. The compound CsPrP 4 O 12 was used in scintillators [2]. Glassy phosphates are also used as laser materials such as NaPO 3 , Al(PO 3 ) 3 doped with Nd 3+ [3][4] ions, Y(PO 3 ) 3 doped with Yb 3+ ions [5]. They are also used in medicine, as optical tracers or in the treatment of cancer by target molecule [6].
The present work is part of the search for new multifunctional rare earth phosphates with electrical and optical properties of interest for industrial applications.
In this paper, we describe synthesis, characterization spectroscopic properties and thermal decomposition of HYP 2 O 7 ·3H 2 O. The anhydrous product HYP 2 O 7 was also prepared and investigated by impedance complex analysis.
X-ray powder diffraction Powder X-ray diffraction pattern was recorded, at room temperature, in the 2θ range of 10-60° by Panalytical X'Pert PRO MPD diffractometer.

Spectroscopic technics
The functional vibrations groups are examined through Fourier Transform Infrared spectral analysis at room temperature in the range 400-4000 cm −1 using the NICOLET IR 200 FT-IR infrared spectrometer. The optical absorption is studied at room temperature with a Perkin Elmer Lambda 11 UV/Vis spectrophotometer in the range of 200-400 nm. The experiments by controlled rate of thermal analysis (CRTA) were carried out with 50 mg samples weighed into a fused silica cell which was placed into a refrigerated furnace constructed in house and operating in the -30 -600°C temperature range. Once the equilibrium temperature was reached, the pressure above the sample was lowered using vacuum pumping system from 1 bar to 5•10 -3 mbar. During the CRTA experiment, where the decomposition leaded to the production of vapor, the vapor pressure was measured by a Pirani gauge placed in proximity of the sample. The pressure signal produced by the Pirani gauge was sent to the furnace heating controller. The heating of the sample then took place in such a way as to keep constant at a preset value the vapor pressure generated by the sample. The use of a diaphragm, placed between the Pirani gauge and the vacuum system, permitted an increase of the residual pressure (5 mbar) above the sample without changing the rate of vapor elimination.

Impedance Spectroscopy
Electrical conductivity measurements were performed with a Hewlett-Packard 4192 A impedance analyzer.

Results And Discussion X ray Diffraction
The X-ray patterns of the obtained crystals are identical to those of the acidic gadolinium diphosphate trihydrate HGdP 2 O 7 ·3H 2 O type II [7][8][9]. So the prepared compound is identi ed as HYP 2 O 7 ·3H 2 O isostructural with the last compound.
Its cell parameters were calculated on the basis of its powder diffractogram starting from the cell parameters of HGdP 2 O 7 ·3H 2 O. The indexed diffractogram and the cell parameters obtained are given in Table 1.

IR Absorption Spectroscopy
The infrared spectrum of diphosphate HYP 2 O 7 ·3H 2 O is shown in Fig.1 In the TG curve the weight loss can be divided into four areas: 27-91 °C, 91-204 °C, 204-485°C, and 485-796°C. The TG weight loss in the rst stage (6.6 %) would correspond to the removal of two water mlecules (%th = 11.32 %).It is related to the endothermic peak at 79 °C. The second one occurs between 91 and 204 °C and is accompanied by an endothermic peak at 112 °C. The corresponding water loss 9.9 % is close to the theoretical value calculated for the loss of two crystallization water molecules. The third and the fourth stages would correspond to the departure of 0.5 water molecule per formula unit (%exp =2.2 %, %th = 2.83). They are accompanied by a large thermal effect.
So, the total weight loss in 27-796 °C temperature range (19.9 %) would correspond to the loss of the three crystallization water molecules and of the half constitution water molecule, which is in agreement with the calculated value 19. 81 %.
The product obtained at the end of the thermolysis has a complex IR spectrum (Fig.3). It particularly shows a wide band between 960 and 1320 cm -1 and two bands of low intensity between 800 and 750 cm -1 . The analysis of its powder X-ray diffractogram (Fig.4) shows that it is a well crystallized product.
So we can say that the thermal effect appearing between 685 and 900°C is an exothermal one peaking at 802°C and corresponding to the crystallisation of the decomposition product. According to the decomposition equation the nal product would be of stoichiometry (Y 2 O 3 ,2P 2 O 5 ):

2[HYP 2 O 7 ·3H 2 O] (s) → 7 H 2 O(g) + (Y 2 O 3 ,2P 2 O 5 ) (s)
The comparison of the obtained patterns with those found in the literature showed that they have no correspondence in the database. So the decomposition product may correspond to a new salt of formula Y 2 P 4 O 13 . It should be noted that in a previous work [10] we have studied the thermal decomposition of HGdP 2 O 7 . 2H 2 O. NH 3 and we have reported the formation of a gadolinium tetraphosphate Gd 2 P 4 O 13 identi ed by X-ray diffraction (00-035-0078 JCPDS le). This salt has been so far reported as equilibrium phase in the gadolinium phosphate system Gd 2 O 3 -P 2 O 5 and shown to be a de ned congruent fusion compound [11]. It seems that the gadolinium tetraphosphate and the new obtained one are not isostructural.
To better specify the in uence of water vapor on the decomposition stages of ytterium acid diphosphate trihydrate, we undertook this study using thermal analysis at controlled transformation rate (CRTA). The IR spectrum and the X-ray patterns of the CRTA residue are shown, respectively, in Figs. 6 and 7.
The IR spectrum of the obtained product (Fig.6) shows the persistence of the characteristic bands of the diphosphate anion and the decrease of the O-H intensity band.
The correspending X-ray patterns (Fig.7) are found to be identical to those given of the anhydrous diphosphate HGdP 2 O 7 [13].
So, we can conclude that the vapor phase existing under the sample during its decomposition is constituted only by water vapor. This permit to a rm that the sample decomposition took place at a constant rate. In this condition, the weight loss at such step is proportional to the time. Thus, the rst and the second steps would be related to the removal of two and half water molecules respectively.
The IR spectra of the products isolated at 98 and 330°C (Fig.9) show both characteristic bands of the diphosphate group and those of the crystallization water. The corresponding X-ray Patterns (Fig.8) show the formation of well crystallized diphosphates. According to the CRTA results, these diphosphates would be the monohydrate and the hemihydrate respectively. It seems that their structures are similar to those of anhydrous salt considering the similarity between the corresponding RX patterns and those of the anhydrous product.
The comparison of our results with those found for HGdP 2 O 7 ·3H 2 O [12] show that the decomposition scheme of the two salts are different in spite of their isotypy. Indeed, it was found that the rst decomposition step in HGdP 2 O 7 ·3H 2 O by CRTA under 5 mbar water vapor correspond to the removal of only one water molecule. This rst water molecule leaved the salt without changing its structural arrangement because it was loosely bounded [7]. A dehydrate isostructural with the initial trihydrate salt was then obtained [13]. This difference between the two CRTA results shows that the water molecules of crystallization are bound differently in the two diphosphate crystal lattices, despite being isostructural.

Optical study
The UV-Vis electronic spectrum of the studied compound, HYP 2 O 7 ·3H 2 O are reported respectively in Figs.
10.a-b. Fig.10.a illustrating the absorption spectrum show three absorption bands: an intense one observed at 290 nm and two other large ones with maxima located respectively at 470 and 531 nm.
Consequently, the bands observed on the absorption spectrum of this compound show an energy transfer between Y-Y or Y-O.
The band gap between HOMO (the ability to provide electron) and LUMO orbital (the ability to accept electron) was determined using Tauc method [14] (Fig.10b). The value determined is 2.71 eV. As well as, it has the character of a semiconductor with a wide band gap suggesting applications in optoelectronics.

Electrical properties
The electrical properties of HYP 2 O 7 were investigated through impedance complex analysis.
In Fig.11 are shown some Nyquist plots of the anhydrous compound HYP 2 O 7 at different temperatures. The Arrhenius diagram is illustrated in gure 12. It is formed by two linear curves with a meeting point located at Tr = 874 K. Such a break is generally due to a crystal structure transition. However, an X-ray diffraction study carried out on a sample calcined at a temperature slightly above the breaking point temperature (Tr), shows that no transition of crystal structure has taken place. Furthermore, no thermal accident was observed on the differential thermal analysis curve of HYP 2 O 7 between 773 and 923 K. The activation energy was determined in the two intervals. We give in Table II the average activation energy value for T<Tr and T>Tr as well as the conductance value of this anhydrous phosphate.