Optical, photoluminescence, and E.S.R spectral analysis of manganese ions in phosphate glasses melted under various conditions and impact of gamma irradiation

Glasses from NaCa fluorophosphate doped with varying MnO2 dopant (0.25, 0.50, 1.0%) and melted under different conditions were prepared by normal melting annealing technique. Collective optical, FTIR, photoluminescence, and E.S.R spectral measurements for the various MnO2-doped glasses were done. Optical spectrum of the undoped host fluorophosphate glass reveals only distinct UV absorption which is correlated to contaminated trace iron (Fe3+) ions present as impurities. The MnO2-doped glasses show the same UV absorption beside two additional bands, a small one at 410 nm which is assumed to originate from (Mn2+) ions of spin-forbidden nature and a broad visible and distinct band centered at 500 nm representing the absorption of trivalent manganese (Mn3+) ions of spin-allowed nature. The effects of varying melting conditions reveal obvious changes in the intensities of the two visible bands. FTIR spectra reveal extended vibrational peaks due to phosphate (PO4) groups with sharing of some fluorophosphate (PO3F) groups and the introduction of increasing percent of the dopant that causes minor variations in the FTIR spectra. Photoluminescence spectra reveal a broad emission band centered at about 635 nm and the intensity of the emission band increases with the increase of MnO2 content and is more distinct in glass melted under reducing conditions than that melted at normal condition. E.S.R spectral measurements were carried out to confirm the optical results indicating the existence of measurable divalent Mn2+ ions which increases in reducing conditions. The effects of gamma irradiation (6Mrad dose) on the studied spectral properties were evaluated.


Introduction
Phosphate glasses are considered to be a special system from the three widely studied vitreous glasses beside silicates and borates. They exhibit wide limiting compositions based on the tetrahedral PO 4 units within all glasses holding a double bond P = O to compensate for the penta-valency of the phosphorus [1][2][3][4]. Alkali phosphates are highly soluble in water but lead iron phosphate glasses are chemically superior than borosilicate glasses and therefore recommended for burial of some radioactive wastes [4]. The interesting physical properties of some phosphate glasses reveal anomalies especially in the (MgO-P 2 O 5 ) or (ZnO-P 2 O 5 ) systems which are related to changes in the coordination number of the respective cation (Mg 2? , Zn 2? ) [3,5,6]. The introduction of any of the transition metal or rare-earth ions, fluorides, or heavy metal oxides widens the suitability of the modified phosphate glasses to extend to metal sealing, special optical components, agriglass fertilizer, or as bioactive candidates [5,[7][8][9][10].
Manganese as one of the 3d transition metals is recognized to be able to occur in glasses as both divalent (Mn 2? ) and trivalent (Mn 3? ) ions [11,12]. The ratio of each valence depends on the glass type and its chemical composition and condition of melting [11,12]. The trivalent manganese ions impart pinkish purple color to glass and exhibit a broad visible band at 450-540 nm which is assumed to be correlated with a spin-allowed 5 E g ? 5 T 2g transition of octahedral symmetry of trivalent manganese [13][14][15]. On the other hand, the divalent manganese (Mn 2? ) ions exhibit four spin-forbidden weak peaks which can be quietly identified upon increasing the concentration of manganese ion to (8-10%). It is recognized that optical spectral measurements cannot easily distinguished the difference between the absorption peaks due to both the two valences of manganese ions because the spectrum of Mn 3? ions easily obscures or overlaps the absorption due to the weak transitions of spin-forbidden peaks of Mn 2? ions [16,17].
Therefore, photoluminescence spectroscopy is assumed to be an efficient analytical tool to allow the study of a wide concentration range without interference between Mn 3? and Mn 2? ions due to selectivity towards the divalent Mn 2? ions [15][16][17].
Numerous glass researchers [16][17][18][19][20] have studied the optical and PL spectra of manganese ions in different glass systems and have distinguished the tetrahedrally coordinated and octahedrally coordinated Mn 2? ions by their specific and characteristic positions of the PL emission maximum in the fluorescence spectra.
The present study aims to investigate collective optical, photoluminescence, and FTIR spectral analysis of some prepared host NaCa fluorophosphate glass containing increasing dopant of manganese ions melted under normal, oxidizing, or reducing conditions. The combined spectral analysis is expected to throw more insight about the appearance or abundance of the two valence states of manganese in the studied host glass and the effects of the concentration of Mn ions and condition of melting on the derived spectral results, together with the effects of gamma irradiation (6 Mrad) on the studied spectral properties. A further confirmative study through E.S.R measurements from some selected glasses was followed out and compared with optical results.

Experimental details 2.1 Preparation of the glasses
The studied glasses were prepared from laboratory chemicals including sodium dihydrogen orthophosphate (NaH 2 PO 4 ), CaF 2 , and MnO 2 . The batches were melted in covered porcelain crucibles at 900°C for 60 min including rotating the melts at intervals. The melting was made in a SiC heated furnace (Vecstar, UK). Different additional reagents were added to the batch to induce different conditions of melting including sodium nitrate, sugar, starch, or fine charcoal). The prepared glasses were poured into slightly warmed stainless-steel molds and the samples were immediately transferred to an annealing muffle regulated at 280°C. The muffle was switched off after 1 h with the samples inside and left to cool to room temperature at a rate of 30°C/hour.

Measurements techniques
Polished glass samples of equal thickness (2 mm ± 0.1 were measured for their UV-visible absorption spectra using a recording spectrophotometer (Type JASCO-V-630, Japan) covering the range from 200-1100 nm.
Photoluminescence measurements were carried out at room temperature under the excitation wavelength of 410 nm in spectrophotometer (type JASCO-FP 6500, Japan) equipped with a Xenon arc lamp 150 W as the light source. The scan speed is 500 nm/ min with a step interval of 0.5 nm and slit band width 5 nm for both excitation and emission monochromators. Long pass cut on filter was placed on the emission side to avoid second-order light from the monochromator.
FTIR spectral measurements were recorded at room temperature in the wavenumber range 400-4000 cm -1 by a spectrometer (FTIR-4600 Jasco corp., Japan) using the KBr disc technique. Mixtures of 1:100 mg glass powder to KBr were mixed and then subjected to a pressure of 5 tons/cm 2 to produce clear discs. At least two measurements were made for each sample. Infrared spectra were corrected for the dark current noises and back ground using the twopoint line correction.
Electron spin resonance measurements were carried out at room temperature using a Bruker EMX spectrometer operating in the X-band frequency (9.677 GHz) with field modulation 100 kHz. The microwave power used was 3.196 mW. A powdered glass specimen of 0.3 g was taken, for each sample, in a quartz tube.
An Indian 60 Co gamma cell was used for irradiation facility with a dose of 6 Mrad for the studied glasses.

Optical absorption spectra of undoped
host glass before and after gamma irradiation   and broad UV absorption extending from 200 to about 350 nm with two distinct peaks at 240 and 300 nm with the first peak higher in intensity than the other peak. The spectrum reveals also a broad visible band extending from 400 to 600 nm centered at about 492 nm. (b) The two glasses melted with starch or sugar show the same strong UV absorption with lower intensities relative to that under normal condition. The rest of the spectral curves show similar data with starch but with sugar, the intensity of the broad band decreases. The glass melted with charcoal shows much lower intensity for UV absorption and the peaks are not clearly identified and the remaining spectrum is similar to that under normal melting but with nearer intensity. Figure 4 illustrates the optical spectra of the 0.5% MnO 2 -doped glasses melted at the same four melting conditions. The optical spectra are identified to consist of the distinct strong UV absorption with three peaks (240, 250, 304 nm) and the broad visible band centered at 502 nm. Figure 5 shows the optical spectra of the glasses containing dopant 1.0% MnO 2 . The spectral curves are nearer to each other revealing strong UV absorption with 3 distinct peaks at 242, 300, and 322 nm (a small band cited before the cut off) and followed by a broad visible band centered at 504 nm. Figure 6 shows the optical spectra of the same studied glasses containing 0.25 MnO 2 after gamma irradiation (6 Mrad). The optical spectra reveal repetitive curves showing extended UV absorption from 200 to about 350 nm with distinct peaks at 242, 300, and 346 nm and followed by a very broad band extending from about 420 to 650 nm with a peak at 532 nm.
After subjecting to irradiation by gamma rays, the optical spectral curves at Fig. 7 show the extension of the strong UV absorption to about 350 nm with three distinct peaks at 242, 300, and 346 nm and followed by the broad visible band centered at 526 nm. Figure 8 illustrates the optical spectra of the doped glasses 1% MnO 2 after gamma irradiation. The same strong UV absorption with three peaks at 240, 304,  The IR spectral details appear repetitive in which the derived IR curves seem parallel and keeping all the detailed vibrational peaks within their positions either for the undoped or MnO 2 -doped samples. It is to be mentioned that the intensities of the IR spectral curves are observed to increase with the increase of MnO 2 content and remain parallel.

Photoluminescence spectra
The emission spectra of Mn-doped glasses prepared in different environments with different MnO 2 contents are shown in Fig. 11. The emission intensity scales are normalized to the highest intensity scale of Fig. 11c, to reveal the changes of the emission intensity with the MnO 2 concentration. The emission spectra show a broad emission band centered at about 620 nm. It is clear that the emission intensity increases with the increase of the MnO 2 concentration from 0.25 to 1.0% with red shift of the emission band from about 615 nm for 0.25% MnO 2 content to 618 nm for 0.5% and 623 nm for 1.0% MnO 2 content. The emission intensity of the prepared glasses at different melting conditions reveals different results for each MnO 2 content. At 0.25% MnO 2 concentration, the emission intensities of the samples prepared under reducing conditions are lower than that of the sample prepared at normal atmosphere. At 0.5% and 1.0% MnO 2 concentrations, the emission intensities of the samples prepared under reducing conditions (sugar and starch) are higher than that of the other samples. Figure 12 shows the emission spectra of the MnO 2doped glasses after irradiation with gamma rays (6 Mrad). It is clear that the intensities of the emitted bands are lower than the emission intensities before irradiation.

Interpretation of the distinct UV absorption in the undoped NaCa fluorophosphate glasses
The experimental optical spectral results revealed strong and wide UV absorption spectra with two peaks in the spectrum of the undoped fluorophosphate glass and extended to appear within the spectra of MnO 2 -doped glasses with various melting conditions. Sigel and Ginther [21] previously identified distinct UV absorption in commercial silicate glasses and referred to their relation to trace impurities and specifically to traces of iron present in the raw materials during the preparation of the glasses.
A group of glass scientists from Jena University [22][23][24] have for extended times searching for specific optical phosphate, fluorophosphate, and borosilicate glasses to be suitable for distinct recent optical applications. They have studied various glasses doped with TM ions, rare-earth ions, and have reached the conclusion that traces of iron impurities decrease the transmission properties of the various glasses even if present within the ppm level and have recommended the need for ultrapure chemicals to produce special optical glasses suitable for the preparation of lenses, glass lasers, and other optical candidates. They have summarized their extended studies in successive recent review articles [25][26][27][28] ( Table 1).
Duffy [29] classified and specified that the strong UV absorption due to traces of iron impurities is due to their charge transfer electron origin and can be easily identified even when the impurity is within the ppm level. Based on previous extended considerations, we assume that the identified strong UV absorption in the optical spectra of the undoped NaCa fluorophosphate glass and extended to all the MnO 2 -doped glasses spectra is originating from traces of ferric ions within the chemicals used for the preparations of the studied glasses.  whole optical results can be understood and explained on the following basis:

Interpretation of the optical spectra of the MnO 2 -doped glasses
(i) The combined appearance of the distinct and broad UV absorption with three peaks in the spectra can be related to the same origin through the presence of unavoidable traces ferric ions present as impurities within the chemicals used for the preparation of the glasses. (ii) The two additional variations in the optical spectra due to the MnO 2 -doped glasses are attributed to the absorption of manganese ions as referred by the previous published studies on manganese-doped glasses. It is agreed by numerous authors that Mn 3? ions belong to d 4 configuration and exhibit a single broad band centered at 480-540 nm due to octahedral coordinated Mn 3? ions while Mn 2? ions belong to d 5 configuration and generally exhibit spin-forbidden weak peaks with molar extinction coefficients of these transitions around 250 times lower than that for the spin-allowed transitions of Mn 3? ion [11,12,15]. As a result, even traces of Mn 3? ions can mask the appearance of absorption due to Mn 2? ions except when the concentration of manganese ions reaches (8-10%) [11,12,15]. Therefore, we conclude that the identified peak at 410 nm is related to Mn 2? ions identified under reducing conditions and the broad band centered at 500 nm is correlated with Mn 3? ions.

Interpretation of the FTIR spectra
The realization and interpretation of the resultant FTIR spectra are based on the following points [15,[30][31][32][33][34][35]: (i) It is accepted that FTIR spectra reflect the vibrational bands which represent the structural building groups or to be finger-prints of them. (ii) The resultant FTIR spectra are evidently depended to the chemical constituents of the glass and specifically to the glass-forming oxides. (iii) The studied host glass composition consists of P 2 O 5 with main constituent forming structural building (PO 4 ) groups. The other constituents include Na 2 O as solely modifier oxide, CaF 2 which is assumed to be able to form few CaF 2 or be composed with phosphate groups as (PO 3 F) and Ca 2? ions are considered as charge compensators or residence as modifiers.

Interpretation of the photoluminescence spectra
It is recognized that Mn 2? ions emit one band resulted from the transition from the excited state 4 T 1 (G) to the 6 A 1 (S) ground state. The energy of the 4 T 1 (G) state depends strongly on the Mn 2? ions-ligands interactions, which result in variable emission wavelengths of the emitted band. Octahedrally coordinated Mn 2? ions perform a red emission band at about 615 nm due to strong crystal field and in this case the transition is spin-forbidden transition, while the tetrahedrally coordinated Mn 2? ions exhibit green emission, due to weak crystal field and the transition is spin-allowed transition [18,20,36]. These assumptions can also be confirmed from Tanabe-Sugano diagram in which Mn 2? belongs to d 5 configuration [37]. So, the red emission of the glass samples at about 615 nm is due to 4 T 1 (G) ? 6 A 1 (S) transition, which indicates the octahedral coordination of Mn 2? ions for the prepared glasses. The broad emission bands reveal the broad distribution of Mn 2? ions sites. The red shift of the emission bands with the increase in the MnO 2 content can be interpreted by exchange interactions between the pairs of Mn 2? ions [38] and could also be due to site distribution of Mn 2? ions in the vicinity of ligand fields [39]. The large decrease in emission intensity after irradiation with gamma rays indicates the conversion of Mn 2? ions into Mn 3? ions which is confirmed also by the increase of the absorption band at 532 nm which is characteristic for Mn 3? ions.

Interpretation of the E.S.R spectral data
The E.S.R spectral details can be understood and explained as follows: The successive gradient six small signals or six-line pattern identified at low concentration-doped MnO 2 glass can be related to several reasons including the small contents of the divalent manganese ions or any related coordination which are able to identify specific signals. Also, it can be assumed to be related to the dipole interactions between the paramagnetic ions coupled by magnetic dipole and the exchange interactions according to the obvious line extension or broadening as suggested by some authors [40,41]. A similar result has been identified by Konidakis [16] on E.S.R data of mixed strontium-manganese metaphosphate glasses. These authors have assumed that PL spectra are typical for octahedral coordination of the Mn 2? ions. Our PL results are similar to those findings.
The high concentrated MnO 2 content (1.0%) reveals distinct and typical signals for Mn 2? ions and the addition of manganese ions in the form of MnO 2 seems to support the PL signals and their stability in the phosphate host glasses.

Interpretation the action of gamma
irradiation on the studied glasses.
Radiation processes that take place in glasses are the same that occurs in crystals. It is accepted that multivalent Mn can easily trap electrons or positive holes when irradiated. From the absorption spectra, induced optical spectra are due to the formation of (Mn 2? ) ? -HC. The very small optical absorbed at 400 nm arises from the consumption of Mn 2? by photooxidation [17].

Conclusion
Undoped Optical spectral data indicate the appearance of strong UV absorption in the undoped glass and with two additional bands in the spectra of MnO 2 -doped samples. The first small peak at 410 nm is assumed to originate from spin-forbidden Mn 2? ions, while the characteristic broad band centered around 500 nm originated from Mn 3? ions in characteristic octahedral coordination. The intensities of the absorption bands are identified to change with the concentration of MnO 2 and also with the type of melting condition.
FTIR spectra are identified to extend within the mid-region 400-1500 cm -1 revealing the vibrational peaks due mainly to PO 4 groups besides the sharing of (PO 3 F) groups. The observed limited IR spectral changes with concentration of MnO 2 dopant or with condition of MnO 2 dopant or with condition of melting have been discussed and evaluated.
Photoluminescence spectral results indicate the appearance of PL spectra with limited variations with the percent of MnO 2 or condition of melting.
Gamma irradiation is identified to cause some variations of the measured properties and are evaluated.

Author contribution
All authors of this work sharing each other as a team group, where all of them did his best effort to manage this work.

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). We did not receive any funding at all.

Data availability
The data of this work are available for any person after publication.

Declarations
Conflict of interest The authors declare that they have no conflict of interest.
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