It was known that P₂O₅ consists of a three dimensional (3-D) network of trigonally connected tetrahedral PO₄ units with a P = O unit [1] [2] [3] [4] . Depending on the number of bridging oxygen atoms in Qⁿ species (n = 3, 2, 1, 0), the phosphate tetrahedral units can be described by containing high concentration of Q³ species in glasses of lower modifier content [2] [3] [4] [5] . Addition of modifier oxide degrades the 3-D network through generation of nonbridging oxygens (NBO). Continuous increasing of fraction of NBO with increasing modifier oxide was interpreted in terms of an increasing long-chain PO₂ groups in the glass [2] [3] [6] [7] [8] [9] [10] . This mechanism causes a structural transition from a 3-D network of interconnected PO₄ groups to a 1-D chain structure of intermediate-range order. This denotes a degradation and transformation of a fully polymerized neutral unit (Q³) of extremely high shielded structure to new phosphate units (Q² − Q⁰) of higher NBO concentration. Specifically, Q² is based on chains and rings and it has one negative charge. Q¹ means two tetrahedral units connected by a corner. Finally, Q⁰ is referred to isolated and less shielded phosphate tetrahedral units with three negative charges.

The structure of phosphate glasses strongly depends on specific structural factor namely R which represents modifier (M) to phosphor (P) ratio in the glass composition [10] [11] [12] . The metaphosphate (Q³) glasses have relatively low R values (<1). The pyrophosphate glasses have intermediate R (~1). This composition is consisting of Q¹as the major structural units. The orthophosphate glasses typically have the highest R (>1) and contain isolated Q⁰ units.

Incorporation of metal oxide or alkali oxide in phosphate glasses [10] [13] [14] [15] is commonly consumed in breaking down of the P-O-P linkages and formation of P-OM (M: metal) bonds. The prevalence of particular Qⁿ unit depends on the nature and concentration of the cations present in the glass [15] [16] .

Many of previous studies were carried out on phosphate glasses modified by alkali or alkaline earth oxides in the region of ultra, meta and pyrophosphate [2] [3] [17] [18] [19] . On the other hand, limited reports on modification by Ag₂O, particularly in pyro- and orthophosphate regions, have been considered before. Therefore, this work is devoted to shed light on structural role of Ag₂O in enriched modified glasses. In addition, we make use of the advantage of the high resolution modern NMR spectroscopy to report on anomalous behavior of metaphosphate glass, which to our knowledge, has not been reported before.

Binary silver phosphate glasses in the system, xAg₂O∙(100 − x)P₂O₅ where 40 ≤ x ≤ 62.5 mol% have been prepared by mixing AgNO₃, and NH₄H₂PO₄ as starting raw materials. NH₄H₂PO₄ was heated in a porcelain crucible at 300˚C for 30 min in order to evaporate ammonia and water. Then AgNO₃ was added and the mixture was melted at temperature between 900˚C and 1100˚C, depending on the composition. The melt was swirled severally to attain homogeneity. The glass samples were obtained by pouring the melt onto flat plate. The glasses were kept in a desiccator until required.

X-ray diffraction (XRD) was used to examine the amorphous nature of the samples. The patterns were obtained by using a Burker D8 Advance powder XRD instrument. It is fitted with a Vantech super speed position sensitive detector and a Cu Ka X-ray tube with a Gobel mirror. Measurements were made over the range 4˚ to 70˚ in 2θ scale.

The internal micro structural features of glass samples were investigated using a transmission electron microscope TEM model JEOL-JEM-2100, Japan equipped (EDP) with an electron diffraction pattern unit. TEM investigations were performed at an electron acceleration voltage of 200 Kv.

Infrared spectra of the glasses were measured by a Mattson 5000 FTIR spectrometer in the range 400 and 2000 cm⁻¹. The measurements were carried out on powdered samples which is mixed with KBr (1 wt%), and then compressing the mixtures to form pellets for measurements.

The distribution of Qⁿ of phosphate unit obtained from ³¹P NMR spectra of selected glasses (x = 40, 50, and 62.5 mol%). The investigations were carried out using JEOL RESONANCE GSX-500 High Resolution Solid state MAS NMR spectrometer. The spectra were obtained at high external magnetic field (11.747T), at a frequency of 160.47 MHz and spinning rate of 40 MHz.

X-ray diffraction patterns of glasses in xAg₂O∙(100−x)P₂O₅ system are shown in Figure 1 and Figure 2. Broad pattern characterizes vitreous nature of the invest-

tigated glass is obviously observed between 2q = 18˚ - 38˚. This amorphous nature is dominated in glasses of 40, 45, 50 and 55 mol% Ag₂O, Figure 1. On the other hand, significant changes were observed in Ag₂O rich glasses (>55 mol% Ag₂O). XRD patterns which are shown in Figure 2 contain sharp diffraction peaks superimposed on the wide hump of the glassy network. Specifically, X-ray diffraction patterns of the sample containing 57.5, 60 and 62.5 mol% Ag₂O are characterized by presence of the intense sharp peak centered at 2θ = 31.9˚. Presence of these diffraction lines may indicate formation of more ordered species that characterize metaphosphate (Ag₂PO₃), pyrophosphate Ag₄P₂O₇ and orthophosphate Ag₃PO₄ units distributed in the main phosphate network [14] [15] [16] . Ag₄P₂O₇ is the most dominant phase found particularly in pyrophosphate glass network containing 57.5 mol% Ag₂O. X-ray diffraction data obtained for the resulting glass were found to be fitted with those reported for Ag₄P₂O₇ [JCPDS Cards File No. 11-0637]. Increasing intensity of the sharp peak with increasing Ag₂O may reflect an important feature for the studied glasses. This feature could be attributed to formation of (Ag₄P₂O₇) pyrophosphate and Ag₃PO₄ orthophosphate crystalline phases [JCPDS Cards File No. 89-7399]. The principal peak at 31.9˚ (triplet) for reflections (211), (112) and (300) is the main diffraction pattern.

Additional evidence for the formation of an Ag₄P₂O₇ and Ag₃PO₄ crystalline phases in silver rich glasses was also found by examination through TEM and EDP (Figure 3). It appears from EDP that there is a certain degree of crystallinity in glass of 62.5 mol% Ag₂O. The feature appeared in EDP is in agreement with that obtained from XRD of the same glass, since sharp single diffraction is clearly evidenced from both XRD and EDP patterns, Figure 3(a). The remain glass network seems to be in amorphous state as represented by EDP (b), Figure 3.

Formation of high concentration of non-bridging oxygen (NBO) in silver rich glasses are considered as the main reason for building crystallized phosphate species as resolved by both XRD and TEM. When the glass network is constructed from symmetric type of units such as orthophosphate (in 62.5 Ag₂O glass), symmetric and ordered phase enriched with NBO species are also constructed [11] [16] . This consideration is clearly verified from both XRD and EDP of glass containing 62.5 mol% Ag₂O, since one resolved XRD peak (Figure 2) and single diffraction of electron patterns (Figure 3(a)) are clearly evidenced to represent Q⁰ in orthophosphate Ag₃PO₄ crystalline units.

The movement from a high cross-linked Q³ structure, to chain-like Q² network, to depolymerized Q¹/Q⁰ glasses with increasing Ag₂O concentration can be monitored by a variety of other spectroscopic probes [22] - [27] . In pure vitreous P₂O₅ and in glass of high contents of P₂O₅, the basic structure entities are PO₄ tetrahedra forming predominantly three-dimensional network (3D) with Q³ type of structural units [7] [9] . Addition of modifier, part of 3D-network would depolymerize and 2D chain structure is simultaneously formed (Q² structural units). NMR Data of present glasses indicated that, the fraction of 3D network structure decreases quickly and chain structure created by Q² units dominates to the network of the glass containing 50 mol% Ag₂O. More addition of Ag₂O at expense of P₂O₅ the two dimensional chain structure decreases and ring like structure containing mainly Q¹ and Q⁰ units are formed. Based on this information, the following observations can be extracted from FTIR spectra upon increasing Ag₂O content. From Figure 6 it can observe that:

1. The intensity and area of the band centered at about 1290 cm⁻¹, attributed to the absorption of P = O and (PO₂) in Q² chains [10] are both decreased quickly with increasing Ag₂O content. This leads that the long chain is reduced, and thus the structure of phosphate glasses is composed from shorter chain of Q¹ and/or isolated tetrahedral (Q⁰) structural unit [9] [27] [29] .

2. The wide humplies in the region between 800 - 1200 cm⁻¹ becomes narrower and shifted to higher wavenumber. The intensity and band area are both changed with increasing Ag₂O concentration. For the last composition, its centering at (1140 cm⁻¹) indicates increasing number of Q⁰ and Q¹ units [28] [30] .

3. The area and intensity of the broad band in the region 620 - 830 cm⁻¹ showed decreasing trends and its center (ca 680 cm⁻¹) becomes narrower. It is referred to the stretching vibration of oxygen atoms in P-O-P bridges [29] [30] . Decreasing area of this band leads to decreasing concentration of bridging oxygen linked between two phosphorus ions. This leads to increasing NBO in phosphate network.

Both NMR and FTIR data imply mainly considerable fragments or depolymerization of the phosphate network with an increase of Ag₂O content. This is in harmony with the work presented by Hoppe [31] , since it was concluded that the network depolymerization is the dominant principle in modified phosphate glasses.

We consider that Ag₂O as a glass modifier enters the glass by breaking up the P-O-P bonds and may introduce coordination defects along with NBO atoms in the studied glasses. This leads that the long chain is lowered, and thus the structure of P₂O₅ glasses would involve shorter chain of Q¹ and/or isolated tetrahedral (Q⁰) structural unit [26] [30] [31] .

The areas of absorbance spectra of the “nonbridging” in Q² stretching mode (Ca 1300 cm⁻¹) decreases upon increasing Ag₂O contents reaching its minimum value at 62.5 mol% Ag₂O. This decreasing behavior may lead to progressive depolymerization of the phosphate network, as well as the increase in the average bond length of P-non-bridging oxygens presented by the band between 950 and 1200 cm⁻¹. Figure 7 presents a correlation between different Qⁿ upon Ag₂O addition. This behavior showed some few increment in Q² species with increasing Ag₂O concentration then it abruptly decreases. A reverse behavior is seen in which it can be realized that the total NBO concentration in Q¹ and Q⁰ increases at the expense of Q² structural species. The above observation is greatly correlated with that derived from both NMR and XRD results which are discussed above, since a decrease in the average p-bond character of the P-O-P upon increasing Ag₂O concentration is additionally confirmed.

Different phosphate glasses in meta, pyro and orthophosphate compositions have been studied by distinguished techniques. The structure of silver phosphate is slightly deviated from the alkali phosphate. This is considered due to the

difference in field strength between silver and alkali ion. X-ray and electron diffraction pattern of metaphosphate composition revealed the amorphous nature of investigated network. Pyro and orthophosphates are documented to contain some crystallized species which are correlated with increasing NBO concentration. Using the advantage of the more recent NMR spectroscopy (11.74 T), the anomalous feature found in silver metaphosphate composition has been explored for first time. Regarding this unique feature, no previous studies have been reported.³¹PNMR spectra of the studied silver metaphosphate composition showed three splitting peaks of the same unit of phosphate species (Q¹) which was not evidenced in alkali metaphosphate. The diverse of spectra is diminished with increasing Ag₂Oto P₂O₅ ratio and well resolved specific peaks characterized Q¹ and Q⁰ are obtained. FTIR analyzed data have demonstrated the inverse relation between quantity of NBO in Q² and in other lower species (Q¹ + Q⁰). NBO species reach their highest concentration in glass of 62.5 mol% Ag₂O. The concentration of NBO in this situation is high enough to construct Ag₃PO₄ apatite units which promote the biomedical application of the glass under investigation.

El-Damrawi, G., Hassan, A., Doweidar, H. and Shaboub, A. (2017) Structural Studies on Ag₂O-P₂O₅ Glasses. New Journal of Glass and Ceramics, 7, 77-89. https://doi.org/10.4236/njgc.2017.73007