(FeNaP)2O9 Glasses Doped with Different TM Ions

In an attempt to check the effect of the transition metal ions on the glass network, the following chemical formula was used to prepare some of Iron sodium phosphate glasses doped with different transition metal (TM) ions. 50 mol% P2O5 + 20 mol% Na2O + 15 mol% Fe2O3 + 15 mol% MnOm, where MnOm represents V2O5, MnO, Co3O4, CuO and ZnO. Mossbauer Effect (ME) spectroscopic analysis showed that, the iron ions are located in two different oxidation states Fe 3+ and Fe 2+ . A fraction of Fe 3+ ions act as glass network formers (GNFs), whereas the other in addition to all Fe 2+ ions act as glass network modifiers (GNMs). The electrical properties for all samples have been measured and then studied as a function of the TMIs atomic number. The D.C. conductivity showed a gradual decreasing with increasing the atomic number of TMIs, while the A.C. conductivity has a similar behavior as predicted by CBH model.


Introduction
Phosphate glasses technologically considered as an important materials, because they have generally higher thermal expansion characteristic and lower transition temperatures comparing with silicate and borate glasses. Oxide glasses such as boron oxide, lead oxide and silicon oxide are characterized by a hard stability and a chemical durability. Also, phosphate glasses were found to be electronically conductors or semiconductors, due to the addition of transition metal oxides . Nowadays Oxide glasses containing large amounts of transition-metal ions exhibit interesting electrical properties, since their behaviors are strongly influenced by the presence of their ions in more than one valence state [1]. These states appeared in the glass networks as a result of the redox processes occurring during melting process. The majority of the experimental measurements have carried out on vanadate glasses [2]. Oxide glasses containing high percentages of iron [3] and copper [4], [5] have a great interesting because copper ions serve not only as a shopping center, but they also contribute in the ionic conductivity. Iron phosphate glasses were found to behave as a typical semiconductor and exhibit unusually a good chemical durability. This work aims to show the effect of the different transition metal ions on both structural and electrical properties of the phosphate glasses.

Experimental
Glasses of the composition 50 mol% P 2  Each batch was grounded and mixed well by using agate mortar and then transferred to porcelain crucible. The crucible of all batches inserted, at the same time, for two hours in an electric muffle furnace which had reach fixed temperature about 1000°C. Melts were stirred from time to time to get a complete homogeneity for melts. After duration of time, they were poured between two precooled stainlesssteel plates. Mossbauer Effect spectroscopy have been performed for all samples at room temperature using 20 mci 57 Co radioactive source in Rh matrix. For AC conductivity measurements, each sample was cut in the form of a square plane of area in the order of 1cm 2 and of a constant thickness. All Ac measurements carried out by using RLC bridge (Stanford model SR 720 RLC meter) in the temperature range from 303K up to 417 K and at four fixed different frequencies (0.12, 1, 10 and100 kHz).

ME Spectra
Mossbauer Effect spectra give very useful information about hyper fine structure of the solid compounds. ME parameters carried out a good description for type, length and energy of chemical bonds within the solid internal structure. For all samples ME spectra and their treatment by a computer analysis software have been displayed separately in Figures  1, 2, 3, 4 and 5. All these spectra showed clearly that, each sample exhibits three different paramagnetic doublets, without participation of any crystalline phases. The thing which indicates the formation of pure and good glassy/amorphous phase in all samples. As a result to computer treatment, for ME spectra, some valuable Quantities was obtained. Such Quantities called ME parameters and defined as (i) the quadrupole splitting Q.S, (ii) the isomer shift I.S, (iii) spectra Line width W and finally (iv) the relative area. These parameters are very important since the Q.S reflects the local environment for iron ions symmetrical/asymmetrical, while the I.S helps in determining both oxidation states and coordination of iron ions in the structure. Generally the values of both Q.S. and I.S, for all samples, have revealed that iron ions have two different oxidation states Fe 3+ and Fe 2+ . Such that some of Fe 3+ ions have tetrahedral configuration and shared in the glass network formation, whereas the rest in addition to all Fe 2+ ions have an octahedral configuration and occupied some interstitial spaces throughout the glass network. In other words, a fraction of Fe 3+ ions acts as glass network former (GNF), whereas the other in addition to all Fe 2+ ions act as glass network modifiers (GNMs). Figure 6a showed that the Q.S of Fe 3+ O 4 increased as the TM atomic number increased, such behavior refers to an increase in the asymmetry (Q.S ≠ 0) of the local environment around the Fe atom within the chain layers of the phosphate glass.

Electric Conductivity
Electric properties are very helpful for check any precise changes in the internal structure of the solid. Therefore, the electrical conductivity has been measured as a function of both temperature and frequency.
is the d.c conductivity, . , a.c conductivity, is a pre-exponential factor, A is a weakly temperature dependent constant, is the angular frequency, T is the absolute temperature and s is the exponent factor. " Figures 14, 15 for all samples. The measurements carried in the temperature range from 300K up to 417K at the three fixed frequencies 1, 10 and 100 kHz. These figures showed that for each sample the electric conductivity has increased regularly with increasing of the absolute temperature. This may be attributed to the thermally activated process from different localized states in the energy gap [8]. On the other hand, it is easy to observe that, the electric conductivity displays a weak dependence on the frequency at high temperature, whereas at low temperature it depends strongly on the frequency     There are some basic models that have been proposed for describe the ac conduction mechanism in amorphous solids, like glasses. These models can be distinguished using the dependence of the exponent factor (s) on the frequency and temperature or on both of them. Therefore, the Exponent factor (s) is very important parameter to recognize the type of ac conduction mechanisms. According to relation (1), at a fixed temperature, both and . will have the same behavior with the frequency changing. Therefore, the following relation can be used to determine the value of the exponent factor s, for each sample, By plotting Ln σ (T , ω) against Ln ω at fixed temperatures the exponent factor (s) will be equal to the slope of the segment straight line, while the extrapolated segment line can be used to calculate the D.C. conductivity at the same temperature. Figures 19, 20,  For all samples, the measured conductivity as a function of the frequency at temperature T = 373 K, have been fitted to the relations 1, 2 and 3 to achieved the D.C. conductivity at this temp., as shown in "Figures from 14 to 18", where dots refer to the experimental data and the solid lines refer to the theoretical one. " Figure 24" shows the calculated d.c. conductivity as a function of the atomic number of the TMIs. The D.C. conductivity decreased approximately linearly as the atomic number of the introduced TMIs increased. Like, this behavior may be attributed to the decreasing of the polaron radius with the increasing of the atomic number of TMIs [8][9][10] The exponent factor (s) has been calculated and then plotted as a function of temperature as in " Figures 25, 26, 27, 28 and 29". These figures show that (s) decreasing gradually with increasing the temperature. Such behavior resample to the mechanism predicted by the correlated barrier hopping C.B.H. model [9]. This model proposed that, the charge carrier transfer by thermal activated over the barriers between two sites, each of individual potential well. If these sites are neighbor to each other their potential wells will overlap.

Conclusion
Some Iron sodium phosphate glasses doped with different transition metal ions have been prepared by fast cooling method. The prepared glasses based on the following chemical formula: 50 mol% P 2 O 5 + 20 mol% Na 2  Mossbauer spectra showed that all studied samples are in a pure glassy state and are of pure paramagnetic character. The increase of the transition metal cause a reduction of Fe (III) to Fe (II). Electric Conductivities of all samples are in the semiconductors range. Whereas, the conductivity behavior is inversely with the atomic number. The AC conduction mechanism was found to belong to the correlated barrier hopping (CBH) model.