Spectroscopic studies of preparation conditions role on the shielding properties of MoO3-doped Na2O–ZnO–P2O5

Glassy samples from the undoped ternary system Na2O–ZnO–P2O5 in combination with other samples from the same system doped with variable amounts of MoO3 (0.25, 0.5, or 1 gm/batch) were successfully melted under ordinary, oxidizing, or reducing conditions followed by annealing. Fourier transform infrared spectrophotometric measurements in combination with optical absorption spectra were recorded before and after doping with variable concentrations of MoO3 in multiple melting media both as prepared or after being subjected to (8 Mrad) gamma irradiation dose. The studied spectral properties of the glasses were correlated with variations with the existing valence state of the Mo ions and the effects of gamma irradiation on the optical spectra and IR vibrational bands together with the role of the condition of melting the batches. The optical spectra of the glasses reveal pronounced UV and visible bands due to trace iron (Fe3+) ions as impurities or the sharing of the low valence states of the added molybdenum ions. The FTIR peaks show condensed spectra of phosphate groups (Q2, Q3), and the effects of dopant are limited because of low percent. Gamma irradiation causes distinct variations in the undoped spectra, but the MoO3-doped glasses show some shielding behavior.


Introduction
Phosphate glasses can be classified as one of the three extensively studied vitreous glassy systems alongside traditional silicate and borate glasses. Phosphate glasses possess extended chemical compositions, and all have the basic building structural PO4 group. They can incorporate high contents of a transition metal or rare earth oxides with the result of obtaining candidates with distinct brilliant colors beside variable optical and electrical properties [1][2][3]. The low chemical durability of phosphate glasses can be overcome by the introduction of multi-valent oxides such as Al2O3, PbO, and Fe2O3 which extended the applications of phosphate glasses including metal-sealing, optical components, to encapsulation of some types of radioactive wastes, and as biomaterials [4][5][6]. The incorporation of the specific transition metal oxides, MoO 3 or WO 3 displays special scientific interest besides the potential applications in various fields of optics [7][8][9].
Molybdenum ions like tungsten ions can be introduced in glasses in four different possible oxidation states, namely Mo 3+ (d 3 ), Mo 4+ (d 2 ), Mo 5+ (d 1 ), or Mo 6+ (d o ) or a mixture of some of these states [10][11][12][13]. Parke et al. [10][11][12][13][14] reported the presence of Mo 3+ ions in phosphate glassy matrix showing absorption bands at nearly 360 and 460 nm in their optical spectra in addition to other absorption bands within the spectral ranges 360-380 and 710-780 nm assigned to the presence of Mo 5+ ions, while Mo 4+ ions showed an absorption band at about 550 nm. It has been recognized that the presence of a high level of MoO 3 or WO 3 produces a photochromic effect [15][16][17]. Structural studies of these specific glasses indicated the formation of WO 6 clusters and these highly polarizable clusters are responsible for the nonlinear and photochromic properties. However, glasses containing such transition metal ions often exhibit an intense and broad absorption band in the visible and near-IR regions which is attributed to the reduction of these transition metals to lower valencies during melting at high temperatures [7]. It is evident that redox effects during melting without controlled procedure produce absorption bands that limit the application of such glasses containing MoO 3 or WO 3 as optical candidates [9].
In this study, glasses were prepared based on the system Na 2 O-ZnO-P 2 O 5 with increasing added MoO 3 as dopant oxide (0.25, 0.5, 1%) with variable conditions of melting including normal melting under the atmospheric condition without any addition. Two other varieties of glasses were prepared through the addition of either oxidizing agent (NaNO 3 ) or of reducing agent (sugar). The study includes the investigation of optical and FT-infrared absorption spectra of the collectively prepared glasses within the three conditions of melting. The same spectral measurements were repeated after subjecting the glasses to a gamma dose of 8 Mrad (8 × 10 4 Gy). This specific dose of gamma irradiation has been selected because earlier studies by the sharing of one of the authors showed that the intensities of generated defects in glasses increased with progressive increase in the dose of radiation until reaching about 6 Mrad,{18-20}. After that, a saturation of the induced defects was assumed to be reached. A further study in this work was carried out of measuring the thermal expansion properties of some selected glasses. These collective studies are expected to give more insight into the states of molybdenum ions on the three conditions of melting. Also, the work is intended to justify the effects of gamma irradiation on the two spectral properties of the studied glasses.

Experimental details
Preparation of the glasses Glasses from the system P 2 O 5 -Na 2 O-ZnO with varying additions of MoO 3 were prepared from chemically laboratory pure materials. They include sodium dihydrogen orthophosphate (NaH 2 PO 4 ) and ZnO. The chemicals used are of a local company for the sodium dihydrogen phosphate (ADWIC, Egypt, with 0.1% Fe as an impurity, while zinc oxide supplied by Sigma-Aldrich Co with Fe 0.001%., molybdenum oxide (MoO 3 ) from Alpha Chemika (India) with Fe 0.002% impurities. The dopant oxide MoO 3 was added over the weight of batch as such with the percents (0.25, 0.5, 1%). The weighed batches were melted in covered porcelain crucibles in a SiC electric furnace (Vecstar, UK) at a temperature of 1100 ± 20 °C for 90 min with rotating the melts at intervals of 30 min to promote complete mixing and acceptable homogeneity. The melts were poured into warmed stainless steel molds with the required dimensions. The prepared samples were transferred immediately to an annealing muffle furnace regulated at 280 °C. After 1 h, the muffle was switched off and left to cool with the samples inside to room temperature at a rate of 30 °C /hour.

Optical absorption measurements
The optical (UV-visible) absorption spectra before and after gamma irradiation were measured at room temperature in the range from 200 to 2500 nm using a recording spectrophotometer (type Shimadzu UV-3600, UV-VIS-NIR Spectrophotometer, Japan). Polished samples of equal thickness (2 mm ± 0.1 mm) were measured, before and after gamma irradiation. The samples were measured twice to confirm the accuracy of the absorption peaks.

Infrared absorption measurements
The infrared absorption spectra of the glasses were measured at room temperature in the range 4000-400 cm −1 by (Nicolet 6700 FT-IR, USA) infrared spectrophotometer, using the KBr technique disc. Two milligrams of powdered samples was mixed with 200 mg of KBr, and the mixture was subjected to a load of 5 tons/cm 2 to produce clear homogenous discs. The infrared absorption spectra were measured immediately at room temperature after preparing the desired discs to avoid moisture attack, with a resolution of 2 cm −1 . At least two IR spectra for each sample were recorded. Infrared spectra were corrected for the dark current noises and background using the two-point baseline corrections. After correction, the IR spectra were analyzed using the deconvolution method to identify the various hidden or overlapped peaks.

Irradiation facility
An Indian 60 Co gamma cell (2000 Ci) was used as a gammaray source with a dose rate of 1.5 Gy/s (150 rad/s) at a temperature of 30˚C. Each glass sample was subjected to a total dose of (8 Mrad = 8 × 10 4 Gy). Figure (1) illustrates the X-ray diffraction experimental data of the studied glasses. XRD pattern reveals the absence of any sharp peaks indicating and confirming the amorphous nature of the prepared glasses.

Optical absorption
Optical (UV-Visible) absorption spectra of the glasses melted under normal atmospheric conditions before and after gamma irradiation The optical spectra of the three MoO 3 -doped samples show similar behavior, revealing strong UV absorption with some shift extension to a higher wavelength than that for the undoped glass, with three peaks at 345 nm and succeeded by a broad visible band centered at 780-800 nm and with two small curvature at 1890 and 2340 nm. The gamma-irradiated undoped glass reveals a slight change of the absorbance combined with the generation of an induced broad visible band centered at 580 nm. The irradiated MoO 3 -doped glasses reveal a slight decrease in the UV spectrum but remain unchanged and parallel in the remaining spectrum.
The high content MoO 3 (1%) shows a small distinct increase in both the UV and visible absorption with parallel behavior.

Optical (UV-Visible) absorption spectra of the glasses melted under oxidizing conditions (NaNO 3 )
The optical spectra of the glasses melted under oxidizing conditions (added NaNO 3 ) shown in Fig. (3) reveal very similar spectral absorption to that identified in the optical spectra shown in Fig. (2) of the glasses melted under normal conditions.

Optical absorption spectra of undoped and MoO 3 -doped glasses melted with reducing agent
Figure (4) illustrates the optical spectra of the studied glasses melted in reducing conditions. The undoped glass of the basic composition Na 2 O-ZnO-P 2 O 5 reveals strong ultraviolet absorption extending from 300 to about 400 nm with a distinct peak at 345 nm and without any further absorption to the end of measurement at 2400 nm. The spectrum of the glass containing the lowest MoO 3 content (0.25%) reveals a strong UV absorption with a peak at 345 nm and followed by a medium peak at 357 nm and succeeded by broadband extending from about 600 to 1100 nm with a broad peak centered at 812 nm.
The second MoO 3 -doped glass (0.5%) shows a strong UV absorption with a peak at 300 nm followed by two peaks at 375 and 444 nm and succeeded by a broad visible-near-IR band centered at 815 nm. The third MoO 3 -doped sample (1%) shows a spectrum consisting of a strong UV-near-visible absorption from 200 to 500 nm with multi-split peaks at and followed by a centered visible peak at about 440 nm and succeeded by a strong visible-near IR broadband extending from about 530 nm to 1200 nm and with multi-split broad peaks at 716, 780, and 855 nm.    Figure (9) illustrates the FTIR of two selected glasses consisting of the base host glass and the highest MoO 3 -doped sample after gamma irradiation; the composition of the derived IR data with that obtained before irradiation indicates that gamma irradiation slightly reduces the intensities of the IR vibrational bands in the undoped base glass. On the other hand, the MoO 3 -doped glass shows maintenance of the IR bands due to the shielding effect of molybdenum ions, and the IR spectrum did not show any distinct variation after gamma irradiation.

Discussion
Interpretation of the identified strong UV absorption in the optical spectrum of the undoped Na 2 O-ZnO-P 2 O 5 glass.
Experimental optical results indicate the identification of distinct UV absorption with three peaks extending to 345 nm. This specific strong UV absorption can be related to being due to absorption of trace iron (mainly Fe 3+ ions) present as impurities within the chemicals used for the preparation of the host glass. This postulation is based primarily on the published review article on charge transfer spectra in glasses introduced by prof Duffy [18]. He classified different UV absorption generally identified in various undoped glasses and referred that some transition metal ions (e.g., Fe 3+ , Cr 6+ ) produce high intense UV absorption even if present in the ppm-level. This type of absorption involves the transfer of an electron from the glass network to the metal ion, and normally, it is highly intense and nominated as a charge transfer type. The same assignment has been adopted by Ehrt et al. in their publications [19,20] and her recent review article [21]. ElBatal et al. [12,[22][23][24][25] have reconfirmed that the UV absorption bands identified in various phosphate glasses are correlated with trace ferric ions impurities within the chemicals used for their preparation. We accept the previous postulation regarding the origin of the UV absorption identified in the host undoped ternary phosphate glass to trace iron impurities ( Fe 3+ ions).

Interpretation of the optical spectra of MoO 3 -doped glasses under varying melting conditions.
Careful inspection of Figs. 2, 3 and 4 indicates that the optical spectra of MoO 3 -doped glasses melted under ordinary conditions (Fig. 2) are very similar to those melted under oxidizing conditions (Fig. 3) and quite different than the optical spectra of glasses melted under reducing condition (Fig. 4). The first two Figs. 2 and 3 have common and similar UV-visible spectra consisting of extended UV spectra than that identified for the undoped glass beside the appearance of three broad curvatures centered at 840, 1840, and 2320 nm. The mentioned spectral features identified in visible broadband due to pentavalent Mo 5+ ions have been previously given by several authors [6-8, 15, 16, 20, 21].

Interpretation of the FT infrared spectra of the studied glasses
The experimental FTIR spectra appear condensed within the mid-region (400-1500 cm −1 ), and the understanding and explanation of the detailed IR results are based on the following basis [24][25][26][27][28][29][30]. The formed IR vibrational bands are virtually depending on the constitutional chemical composition and specifically on the glass-forming oxide percent which forms the fundamental building groups in the glass network, and hence, it can be recognized that: a-The host glass is composed of main P 2 O 5 with 70% as a network forming oxide with phosphate groups (mainly Q 2 and Q 3 types) and with modifier oxide Na 2 O with 24%, and the rest is ZnO with 6%. ZnO is known to be conditional oxide which can act as modifier oxide or former with ZnO 4 with the necessary oxygen ions available from neighboring alkali oxide. viii-The near IR broad band centered at about 3450 cm −1 can be related to vibrations of water, OH, POH.
The observed limited changes between IR spectra melted under different conditions are explained as follows: a-The distinct decrease in the intensities of the bands at about 1640,2857,2921 cm −1 and the near IR broadband centered at 3439 cm −1 upon increasing MoO 3 (Fig. 4) can be related to the chemical stability in the glasses upon adding the TM ions (Mo 6+ ). This stability effect reduces the ability of the MoO 3 -doped glasses to absorb water or moisture in the powder form and thus the cited bands are known to be assigned to vibrations of water, OH, or POH. The previous results are concerned with glasses melted under the ordinary atmospheric condition where the molybdenum ions are expected to be existing in both hexavalent and pentavalent states. b-The glasses melted under oxidizing conditions (Fig. 5) reveal only the decrease in the mid-bands extending from about 900 -1350 cm −1 where molybdenum ion concentration reaches (1% MoO 3 ). These results can be related to the suggested depolymerization effect caused by the Mo ions through changing of the metaphosphate groups. Similar behavior has been identified by some authors about the action of copper ions [28] or iron ions [29] on phosphate glasses. Also, the distinct appearance of the far-IR bands may be related to the addition of NaNO 3 which seems to affect the ability of the powder grains to absorb water or moisture. Figure (7) shows that the melting of the glasses under reducing conditions causes the distinct decrease in the main IR vibrational bands within the mid-region 600-1350 cm −1 . It can be suggested that during the melting process with the reducing agent, the redox reaction between the molybdenum ions and the reducing agent affects to some extent the structural building units leading to the decrease in their intensities.

Effect of gamma irradiation on glass
Perfect crystalline materials are realized to be unaffected upon being subjected to ionizing gamma irradiation. On the other hand, glasses are accepted to be non-periodic solids and exhibit intrinsic defects or pre-existing defects such as non-bridging oxygen, vacancies, and impurities, and upon gamma irradiation, some physical and chemical properties are assumed to show changes. These formedinduced changes or defects can be followed by many tools such as measuring their optical or E.S.R spectra after gamma irradiation. During gamma irradiation, pairs of electrons and positive holes are generated and thus induced defects are expected to be formed including ionization, radiolysis, and photochemical reactions [14,20,32].
Interpretation of gamma irradiation on the optical spectra and FTIR spectra of the studied glasses.
Gamma irradiation on the undoped glass causes a slight increase in the UV absorption besides the generation of an induced visible band centered at 580 nm appeared in base sample. These specific responses can be interpreted as follows: (a) The host undoped glass is a ternary phosphate glass (70 P 2 O 5 -15 Na 2 O-ZnO15 mol%) and generally, phosphate glasses are favoring the lower valencies of the transition metal ions [4,6,9,16]. Hence, the trace unavoidable iron impurities present in the glass contains a measurable percent of ferrous (Fe 2+ ) ions besides some few ferric (Fe 3+ ) ions. This assumption is confirmed by the appearance of curvature centered at 1070 nm which is accepted to be the characteristic position for the absorption due to ferrous ions [9]. Upon gamma irradiation, some of the ferrous ions react or capture liberated positive holes and the net result is the formation of additional ferric ions (Fe 3+ ) or (Fe 2+ ) + and the result is the observed increase in the intensity of the UV absorption in the vicinity of the characteristic position of the absorption of the ferric ions (200-310 nm) [9]. (b) The generation of an induced visible band upon gamma irradiation of the undoped glass can be related to the formation of phosphorus oxygen hole center (POHC) through the assumption of the effect of generated hole center on the phosphate network itself. The same assumption has been accepted by various authors [13,14,20].
The MoO 3 -doped glasses reveal quite different responses toward gamma irradiation. The UV absorption shows a slight decrease in intensity and the visible absorption remains unchanged with parallel behavior. The last-mentioned results can be explained by assuming that the liberated electrons and positive holes are faced with the MoO 3 -doped glasses by two different transition metal ions (traces of iron ions and dopants of molybdenum ions) and it is obvious that a competition of reactions between the two TM ions and the generated pairs of electrons and positive holes. The identified optical results of the MoO 3 -doped glasses reveal that molybdenum ions capture positive holes or shield their effects in increasing the UV absorption or the formation of an induced visible positive hole. It can be assumed that molybdenum ions shield or retard the effect of generated positive holes. Several authors have agreed that molybdenum ions (and tungsten ions) are possessing the capability of reducing the effect of gamma irradiation [10,31,32]. This specific behavior can be related to the relatively heavy mass of the Mo ions which retard or block the free passages of electrons or position holes during the irradiation process and also to photochemical reaction as discussed before.
Regarding the almost maintenance of the FTIR results after gamma irradiation, it is assumed that previous studies [40 → 43] indicate that the structural building units generally remain unaffected by irradiation except by the identification of some change in the band angles and/or band lengths leading to slight changes in the intensities of some of the vibrational bands.

Conclusion
Glasses from the system P 2 O 5 70%-Na 2 O 24%-ZnO 6% consisting of undoped and doped with varying MoO 3 contents (0.25-0.50-1%) were prepared under three varying conditions (normal-atmospheric-oxidizing-reducing). Characterization of the prepared glasses includes collective optical and FT infrared absorption spectra before and after gamma radiation with a specified dose (8 Mrad). The spectral results indicate that the two melting conditions of the normal atmospheric and oxidizing conditions produce similar optical spectra while the reducing condition initiates the low valence state of pentavalent molybdenum. FTIR spectra reveal distinct vibrational bands due to (Q 2 , Q 3 ) structural phosphate groups. The molybdenum ions as dopants cause no distinct variations in the IR spectra due to their low percent to affect the structural phosphate glasses Gamma irradiation affects the spectrum of the undoped glass through photochemical reactions with trace impurities, and the generation of an induced POHC. The MoO 3 -doped glasses reveals shielding behavior toward gamma irradiation and the spectral curves show stability.
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