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Optical, structural and thermal properties of sodium metaphosphate glasses containing Bi2O3 with interactions of gamma rays
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 171 (2017) 454–460
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Optical, structural and thermal properties of sodium metaphosphate glasses containing Bi2O3 with interactions of gamma rays M.A. Marzouk ⁎, F.H. ElBatal, K.M. ElBadry, H.A. ElBatal Glass Research Department, National Research Centre, 33 EL Bohouth St. (Former EL Tahrir St.), Dokki, P.O. 12622, Giza, Egypt
a r t i c l e
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Article history: Received 26 October 2015 Received in revised form 17 August 2016 Accepted 18 August 2016 Available online 23 August 2016 Keywords: Sodium metaphosphate glass Bi2O3, Optical–FTIR Thermal properties Gamma irradiation
a b s t r a c t Sodium metaphosphate glasses with successive increasing added Bi2O3 contents (5–40%) were prepared to improve their chemical stability and increase their optical and thermal properties through the additional building BiO6 and BiO3 units. The optical spectrum of the base metaphosphate glass reveals strong UV absorption due to the presence of trace iron (Fe3+) ions present as impurities. Glasses containing additional 5, 7.5 and 10% Bi2O3 show further band around 406 nm which can be related to absorption of Bi3+ ions. With increasing the Bi2O3 content, this near visible band is observed to disappear indicating peculiar behavior needing further work. Gamma irradiation causes only minor changes in the position of the strong UV peaks but an obvious induced visible broad band centered at 452–460 nm in the base and Bi2O3 containing glasses. This induced band is related to the generation of phosphorus oxygen hole center or non bridging oxygen hole center as revealed by various authors. FTIR results reveal characteristic vibrational bands due to phosphate groups and with the addition of Bi2O3, some interference of Bi\\O vibrational units are expected. Gamma irradiation causes limited changes in the IR spectra due to suggested shielding effect of the heavy metal oxide Bi2O3. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phosphate glasses are interesting vitreous system that has gained great attention in the past and in recent applications. In the 1950s, interest in amorphous alkali phosphates was stimulated by their use in a variety of industrial applications, including sequestering agents for hard water treatments and dispersants for clay processing pigment manufacturing [1]. Owing to the outstanding optical properties of phosphate glasses including higher transparency for ultraviolet light especially when additional multivalent oxides are formulated in their compositions, they are introduced and recommended as laser glasses [2]. Also, the high thermal expansion coefficients, and low softening temperatures of phosphate glasses make them suitable for hermetic seals [3]. Biocompatible phosphate glasses and glass-ceramics have been recommended for medical applications [4] and lithium phosphate glasses [5] have fast ion conductivity that makes them useful as solid state electrolytes. The application extends to explore lead iron phosphate glass to be a host matrix for radioactive wastes because of possessing promising chemical durability, radiation shielding beside lower melting temperature [6]. Glasses containing heavy metal Bi2O3 have wide and potential applications in the field of optical glasses with high refractive indices, low melting temperature, semiconducting glasses and also as a shielding candidate for gamma irradiation [7–9]. The ability of Bi2O3 like PbO to ⁎ Corresponding author. E-mail address: [email protected] (M.A. Marzouk).
http://dx.doi.org/10.1016/j.saa.2016.08.038 1386-1425/© 2016 Elsevier B.V. All rights reserved.
share or behave partly as glass forming oxide comes from the high polarizability of Bi3+ and Pb2+ cations and their constitutional asymmetric polyhedra which inhibit crystallization and thus possess wide range of glass formation beside being less toxic than PbO [10]. The aim of the present work is to study by combined optical and FT infrared absorption spectroscopic measurements detailed spectral properties of sodium metaphosphate glasses containing increasing Bi2O3 contents before and after gamma irradiation. It is expected that these collective spectral studies will throw some insight on the effect of Bi2O3 on the phosphate network with the introduction of additional Bi\\O units and to deduce the induced defects generated during the irradiation process through the addition of heavy metal oxide (Bi2O3) expected to cause shielding behavior. A further aim includes the study of the thermal expansion properties of the studied glasses to find out their possible suitability to serve as sealing glass candidate. 2. Experimental and methods 2.1. Preparation of the glasses Glasses of the basic sodium metaphosphate composition (50 mol% Na2O-50 mol% P2O5) to which successive added Bi2O3 contents (5% → 40%) were prepared. Laboratory chemicals of NaH2PO4 and Bi2O3 were used to prepare the glasses. All weighed batches were melted in alumina crucibles at 1100 °C and melting time was extended to 1 h with stirring of the melts at intervals to promote complete mixing and homogeneity. The melts were cast into warmed stainless steel
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molds of the required dimensions. The prepared glassy samples were immediately transferred to a muffle furnace regulated at 250 °C for annealing to remove any stresses or strain formed during the rapid cooling of the melts. The annealing muffle was left to cool to room temperature after 1 h with the samples inside with a cooling rate of 20 °C/h. 2.2. Optical absorption spectral measurements The optical (UV–visible) absorption spectra of polished glass samples (2 mm + 0.1 mm) were recorded at room temperature before and after gamma irradiation by a recording spectrophotometer (type T80 PG Instruments, England) in the range 190–1000 nm. 2.3. FT infrared absorption measurements FTIR absorption spectra of the glasses were measured at room temperature in the range 400–4000 cm−1 by a Fourier transform infrared spectrometer (type Nicolet is 10, USA) using the KBr disc technique. The IR measurements were immediately done after preparing the discs to avoid moisture attack. The same measurements were repeated after subjecting the glass powder to a gamma dose of 8 M rad (8 × 104 Gy). 2.4. Gamma irradiation facility An Indian 60Co gamma cell (2000Ci) was used as a gamma ray source with a dose rate of 1.5 Gy/S (150 rad/s) at a temperature of 30 °C. The glasses were subjected to a total dose of 8 M rad (8 × 104 Gy). This specific amount of radiation dose was chosen based on previous studies on irradiation of glasses which revealed that distinct optical spectral variations were observed until the gamma dose reached 6 M rad after which saturation effect was reached stopping the creation of more induced defects [11,12]. 2.5. Thermal expansion investigations Thermal expansion properties of base sodium metaphosphate glass together with samples containing varying Bi2O3 contents were measured using a computerized dilatometer (type NETZSCH DIL 402 PC 5, Germany). All the measurements were made within the temperature range from room temperature up to the dilatometeric softening temperature (Tg) with a heating rate of 5 °C/min. The cylindrical glass samples with the dimension (25 × 6 mm) were used and at least two measurements were made on each glass with reproducibility (±0.01%).
Fig. 1. UV–visible absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses before gamma irradiation.
peaks are identified at 240 and 310 nm and with no further visible absorption. The glass containing 40% Bi2O3 shows a strong UV absorption with two peaks at 240 and 298 nm and two small peaks 380 nm and 420 nm and with no further bands in the rest of the visible region. Fig. 2 illustrates the optical spectra of the studied glasses after gamma irradiation with a dose of 8 M rad. The base glass containing no Bi2O3 reveals strong UV absorption with two peaks at 240 and 296 nm and with a distinct induced visible band at 452 nm. All the Bi2O3-containing glasses exhibit the same UV absorption with two distinct peaks at about 240 and 296 nm and together with a further broad visible band centered at about 460 nm. 3.2. FT infrared absorption spectra of the glasses Fig. 3 illustrates the FTIR spectra of all the studied glasses before irradiation. The IR spectral features of the base sodium metaphosphate glass can be summarized as follows: (a) The far-IR absorption reveals distinct and sharp connected five peaks at about 430, 457, 485, 518 and 550 cm−1. (b) The appearance of a medium broad band with two peaks at 732 and 769 cm−1. (c) A very broad and strong absorption extending from about 800 to 1600 cm−1 is observed with seven peaks at about 873, 968, 998, 1014, 1089, 1205 and 1275 cm−1.
3. Results 3.1. Optical absorption spectra before and after gamma irradiation Fig. 1 illustrates the optical spectrum of the base sodium metaphosphate glass together with the spectra of samples of the same composition and containing successive Bi2O3 additions (5–40% Bi2O3). The base glass exhibits a strong and broad UV absorption band with two distinct peaks at 240 and 300 nm and with no further visible absorption to the end of measurement at 1100 nm. On introducing 5% Bi2O3, the UV spectrum remains strong revealing two peaks at 228 and 282 nm and with the appearance of a new visible peak at 403 nm. The spectrum of the glass containing 7.5% Bi2O3 reveals strong UV absorption with two bands at 306 nm and a lower band at 384 nm followed by a distinct visible band at 430 nm. The sample containing 10% Bi2O3, reveals a strong UV absorption with two distinct peaks at 240 and 305 nm very similar to that of the base glass but with a small peak at 394 nm. The samples with 12.5% or 15% Bi2O3 show the same spectrum consisting of a strong UV absorption the same as the spectrum of the base glass but the positions of the two
Fig. 2. UV–visible absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses after 8 MR gamma irradiation.
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1105 and 1285 cm−1. A final medium band is observed at 1620 cm−1. (b) The glass containing 5% Bi2O3 reveals a deconvoluted IR spectrum consisting of (i) the far IR absorption shows three peaks at 413, 523 and 580 cm−1. (ii) A new connected band is identified at 760 cm−1. (iii) A very broad connected spectrum extends from 800 to 1650 cm−1 and revealing seven successive peaks at 880, 985, 1109, 1217, 1291, 1440 and 1615 cm−1. (c) The glass containing 15% Bi2O3 also shows connected IR spectrum. (i) The far-IR spectrum reveals four peaks at 405, 480, 552 and 610 cm− 1.(ii) A connecting peak is identified at 708 cm−1. (iii) The very broad mid spectrum shows five peaks at 870, 992, 1133, 1408 and 1623 cm−1.
Careful inspection of these IR curves indicates some distinguished features on the introduction of increasing Bi2O3 content which can be summarized as follows: (i) The clear observation of at least four peaks in the range 400– 650 cm−1. (ii) The appearance of a new peak in the spectral range 720– 730 cm−1. (iii) The growth or increase of the intensities of the composite peaks within the spectral range between 800–1400 cm−1.
3.4. Infrared absorption spectra after gamma irradiation Fig. 3. FTIR absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses before gamma irradiation.
Fig. 4 reveals the FTIR spectra of the studied glasses after gamma irradiation (8 M rad = 8 × 104 Gy). Carful inspection and comparison of
(d) Two small peaks are identified at about 1616 and 1630 cm−1. (e) Three small peaks are observed at 2360, 2850 and 2923 cm−1. (f) A broad medium near IR band is observed with three distinct peaks at 3413, 3450, 3548 cm−1. Fig. 3 also indicates that the introduction of successive Bi2O3 contents 5, 15 and 40% reveals the following variations: (1) The first far-IR peaks are connected to appear as two composite bands with several peaks losing the sharpness observed in the spectrum of the undoped sample. (2) The medium peaks at 732 and 769 cm−1 are changed to a single distinct medium broad band centered at 752 cm−1 which decreases in intensity at higher Bi2O3 contents (12.5%, 20%). (3) The middle peaks extending from about 800 to about 1600 cm−1 grow in intensity and become compacted or gathered in a single very broad band with a peak at about 1100 cm−1.
3.3. Deconvoluted FTIR spectra Due to the obvious composite and broad IR spectra of the studied samples especially within the region 400–1700 cm−1, it is found necessary to apply a deconvolution method to find out and differentiate between overlapped vibrational modes which result from the vibrations of both phosphate groups and Bi\\O groups in the specified region. The deconvoluted spectra of the samples (Fig. 3) can be summarized as follows: (a) The base sodium phosphate glass reveals in the far-IR region three peaks at 406, 436 and 535 cm−1. The mid region shows a very broad band extending from about 780 to 1450 cm−1 and the deconvolution spectrum reveals four peaks at 870, 968,
Fig. 4. FTIR absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses after 8 MR gamma irradiation.
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the FTIR spectra of the base undoped sodium phosphate before and after irradiation indicates that: (a) The deconvoluted far-IR peaks at 437 and 533 cm−1 are shifted to 487 and 560 cm−1 respectively. (b) The main mid IR absorption extending from about 800 to 1400 cm−1 show more splitted peaks at 871, 996, 1097, 1144, 1274 and 1351 cm−1 beside a separate band at 1635 cm−1. The glasses containing Bi2O3 reveal an additional band around 700 cm−1 beside increase of the intensity of the very broad absorption composite band.
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3.6. Thermal expansion results Fig. 6(a & b) illustrates the measured thermal expansion data of the studied glasses. The thermal expansion measurements data can be summarized as follows: (a) The thermal expansion coefficients at 100 °C–200 °C and 300 °C are observed to progressively decrease with the increase of Bi2O3 content. (b) The transformation and softening temperatures are found to increase with the introduction of 5, 7.5 or 10% Bi2O3 and then decrease or reach constancy with further Bi2O3 contents.
3.5. Infrared absorption spectra of crystalline raw materials 4. Discussion Fig. 5 illustrates the FTIR spectra of the crystalline chemical materials used for the preparation of the studied glasses, namely crystalline sodium metaphosphate (NaH2PO4) and bismuth oxide (Bi2O3). The IR spectrum of crystalline sodium metaphosphate (Fig. 5a) is observed to consist of the following spectral features: (a) The far-IR absorption reveals two strong and very sharp peaks at 435and 539 cm−1 and with an attached peak at about 605 cm−1. (b) A very broad and distinct absorption extending from about 906 to 1268 cm−1 is observed and revealing five peaks at 906, 979, 1087, 1168 and 1268 cm−1. (c) A distinct medium band is identified at 1670 cm−1. (d) The rest of the spectrum shows numerous small peaks extending from about 2379 to 3532 cm−1. The absorption spectrum of the crystalline Bi2O3 (Fig. 5b) reveals the following characteristics (a) Two very sharp distinct far-IR peaks at 478 and 516 cm−1 are observed. (b) A very broad medium absorption extending from about 871 to 1639 cm−1 is observed and revealing four peaks at 871, 1018, 1396 and 1639 cm−1. (c) A broad band is identified with numerous peaks at about 2410, 2917, 3228, 3394, and 3548 cm−1.
Fig. 5. FTIR absorption spectra of raw materials (a) crystalline sodium metaphosphate (NaH2PO4) and (b) crystalline bismuth oxide (Bi2O3).
4.1. Interpretation of the optical UV–visible absorption spectra of the studied glasses The spectrum of the base host sodium metaphosphate is observed to exhibit only a strong broad UV absorption band with two distinct peaks at 240 and 300 nm. Many glass scientists [13,14] had identified strong UV absorption in numerous undoped commercial glasses and the origin of this strong UV absorption was related to the presence of unavoidable trace impurities and specifically to trace ferric ions contaminated within the raw materials used for the preparation of such commercial glasses. Duffy and Ingram [15] and Duffy [16] have agreed to this assumption and they have classified various charge transfer UV absorption bands usually observed in various undoped glasses and have concluded that charge transfer absorption of some ions (e.g. Fe3+, Cr6+) to originate through electron transfer mechanism even if these ions are present in traces within the ppm level. The electron transfer mechanism involves the transition of an electron from the orbital of a coordinating oxygen atom to an orbital of the metal ion. Ehrt et al. [17,18] and ElBatal et al. [19,20] have identified similar strong UV absorption in various phosphate, borosilicate and borate glasses and they have reached to the same attribution. They have further claimed for the need of ultrapure materials to improve the transmission of special optical glasses for recent applications. Based on previous considerations, the two strong UV absorption peaks identified in the spectrum of undoped base sodium metaphosphate glass can be attributed to the presence of unavoidable traces of ferric ions (Fe3+) present as impurities within the chemicals used for the preparation of such glass and this same strong UV absorption is observed also to be extended and identified in the optical spectra of the Bi2O3-containing glasses. The two glasses containing additional Bi2O3 5% and 10% Bi2O3 reveal a further distinct visible band at 403–430 nm surplus to the strong UV peaks previously identified in the base glass. Parke and Webb [21] previously observed an ultraviolet- near visible peak when traces of Bi3 + ions were added to borate and phosphate glasses and this peak was related to the transition 1S0 → 3P1 of Bi3 + ions [22]. The appearance of an extra band in the visible at 403– 420 nm with the addition of 5 or 10% Bi2O3 can be related to the contribution of absorptions from Bi3 + ions according to the previous considerations. Duffy [16] has assumed that certain other metal ions including Ce3+, Tb3+, U4+ as well as d10S2 ions (such as Pb2+ and Bi3+) absorb radiation through electronic transitions involving orbitals essentially of the metal ion only, and the name “Rydberg” has been suggested for such spectra to distinguish them from the common charge transfer spectra (e.g. Fe3+ and Cr6+). Successive studies on borate and silicate glasses containing high Bi2O3 content (~60–90%) have reached the conclusion that the UV-near visible bands in the range 350–450 nm observed in high bismuth borate and high bismuth silicate glasses are correlated with absorption from Bi3+ ions [23–29].
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Fig. 6. (a) Thermal expansion undoped and Bi2O3 doped sodium phosphate glasses, (b) extended thermal expansion undoped and Bi2O3 doped sodium phosphate glasses.
The present optical results reveal a peculiar behavior through the disappearance of the near UV–visible band generated upon the addition 5 mol% or 10 mol% Bi2O3 by further increasing the added Bi2O3 to reach 40 mol%. This behavior is observed for the first time from the studied N10–40% Bi2O3 within host sodium metaphosphate glass. Binary bismuth borate or bismuth silicate glass containing high Bi2O3 from 60 mol% up to 90 mol% show extended UV–visible absorption bands within the range 400–850 nm as evidenced in previous publications [23–27]. Some suggested assumptions can thus be introduced regarding this peculiar behavior: concerning the disappearance of the extra band at 403–430 nm on increasing Bi2O3 content N10 mol% until 40 mol%. (a) It is assumed that the Bi3+ ions might undergo reduction in the host phosphate glass to Bi2+ or to metallic aggregates of Bi+. It is known that 3d transition metal ions reveal mostly the optical spectra of their lower valence or the octahedral coordination in sodium metaphosphate host glass [19]. But this assumption is faced by the first appearance of absorption due to Bi3+ ions at relatively low concentration (5 → 10%). (b) It is suggested that bismuth ions are involved in new structural
arrangement or exhibit different structural units as for example the transformation of [BiO6] to [BiO3] and this assumption is supported by the present obvious distinct change in the FTIR spectra when the concentration of Bi2O3 exceeds 10 mol%. (c) A further suggestion is the formation of nonbridging oxygens with Bi2O3 content higher than 10 mol% together with the formation of Bi\\O\\P linkages. (d) A recent contribution of some of the authors [30] on the effects of Bi2O3 on optical FTIR and thermal expansion of SrO–B2O3 glasses has reached to the conclusion that increasing the Bi2O3 content to 15 mol% reveals obvious changes in the IR spectra due to suggested formation of both BiO6 and BiO3 groups.
The previous suggestions need further studies through different techniques to specify the reason for this peculiar behavior. 4.2. Interpretation of the FT infrared absorption spectra To realize and interpret the detailed FTIR spectra of the studied glasses, the following basic parameters are introduced [1,12,31–34]:
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(a) The chemical composition of the basic glass is sodium metaphosphate (50 mol% Na2O, 50 mol% P2O5) together with samples containing additional Bi2O3 (5, →40%). (b) It is expected that the base glass structure consists of metaphosphate groups (PO2) in accordance with the chemical composition. The following vibrational modes are expected: νs POP, να POP, νs PO2, ν (OPOP) deformation or bending modes. (c) The numerous sharp far-IR peaks at 430 → 550 cm−1 can be related to O\\P\\O bending or lattice mode vibrations together with possible contribution of alkali cation vibrations (Na+). (d) The medium band with peaks at 732 and 764 cm−1 can be related to symmetric stretching of P\\O\\P bonding. (e) The peaks at 873 and 768 cm− 1 may be linked to asymmetric stretching of P\\O\\P groups linked to metaphosphate chains. (f) The peaks at 998, 1014, 1089 cm−1 may be related to asymmetric stretching of PO2-terminal groups. (g) The peaks at 1205 and 1275 cm−1 can be correlated with asymmetric stretching of PO2 groups, stretching of doubly bonded oxygen with phosphorus vibrations (P_O). (h) The rest peaks at 1616, 1630, 2360, 2850, 2923, 3413, 3450 and 3548 cm−1 can be related to, OH, water, POH vibrations.
do contain pre-existing intrinsic defects such as vacancies, non-bridging oxygens, impurities, etc. These intrinsic defects are obviously formed through the traditional fast cooling step during the preparation of glasses. It is accepted that [37,38] the exposure of most glasses to high energy radiation (e.g. x-ray, γ-ray, UV light) produces generally induced absorption bands in the UV–visible part of the spectrum. This is due to the creation of induced defects centers as a result of capturing of liberated pairs of electrons and positive holes during the irradiation process. However, some authors [34,37–39] have assumed that glasses containing heavy metal oxides (PbO, Bi2O3, WO3, MoO3) show shielding behavior towards gamma irradiation. Also, the presence of some transition metal ions (e.g. Cu2+, V5+) are observed to show some retardation of the effect of successive gamma irradiation. The studied glasses including the base glass and Bi2O3 containing samples reveal an induced visible band at 460 nm which can be related to either to the formation of phosphorus oxygen hole center (POHC) or to nonbridging oxygen hole center (NBOHC) as previously suggested by Bishay [37] and Friebele [38].
The support of the previous interpretation can be derived from the IR spectra of crystalline sodium ultraphosphate (Fig. 5a) which are greatly similar to that of vitreous sodium metaphosphate and their bands are similarly attributed [33,34]. The observed changes in the FTIR spectra upon the addition of progressive Bi2O3 can thus be interpreted by introducing the following suggestions:
Glass, like most other solids; generally expands on heating. The dimensional changes which occur with temperature are very important for sealing purposes and the ability of the glass to survive thermal shock or cycling [40–42]. Normally, expansion is described as to be due to the increasing amplitude of atomic vibrations of the constituents and thermal expansion is determined by anharmonic lattice vibrations. The expansion of a glass is accepted to depend mainly on the internal network structure, the arrangement of the constituting building units and the bonding strength to each other. Extended thermal expansion coefficient studies of lead silicate, lead borate, ternary alkali/alkaline earth silicate, ternary alkali alkaline earth borate, binary Bi2O3·B2O3 and ternary Bi2O3·B2O3·PbO glasses have reached to important parameters on the relation of the type and composition of the glasses to their thermal expansion coefficients [43–46] and they can be summarized as follows;
(a) The changes can be correlated with the formation of nonbridging oxygens by the introduction N 10 mol% Bi2O3 in the neighborhood of [BiO6] groups. (b) The increase of Bi2O3 added to 10 mol% or more reaching 40 mol% causes the formation of additional structural forming [BiO3] groups with bridging oxygen. (c) A similar result is observed upon the addition of progressive Bi2O3 within the host strontium borate glass [30]. (d) The attribution of the IR bands related to Bi\\O groups can be understood and realized by referring to the IR spectrum of crystalline Bi2O3 (Fig. 5b) in which [BiO6] units are the main constituent [23,35,36]. Table 1 summarizes the FTIR peak and their assignments. 4.3. Interpretation of the effect of gamma irradiation on the optical and FTIR spectra Before discussing the effect of gamma irradiation, it is considered that glasses deviate from the ideal network structure of real solids and
4.4. Interpretation of the thermal expansion data
(i) The increase of heavy metal oxide (PbO) in lead silicate and lead borate glasses increases the thermal expansion coefficient due to the increase of Pb2+ ions in network modifying positions which decreases the connectivity of the glass structure. (ii) The relative differing effects of the divalent or trivalent oxides will depend on the bond strength of the cation to oxygen and the ability of some oxides such as MgO, ZnO to form additional structural units in the glass (MgO4, ZnO4). (iii) In lead borate glasses, there are two factors affecting the thermal expansion coefficient. The first factor is the forms in which the lead ions are studied in glass structure as modifier or former. The second factor is the sharing of PbO in the transformation of
Table 1 FT-infrared absorption peaks and their assignments. Peak wavenumber cm−1
Assignment
References
(a) For base sodium metaphosphate 460–485 500–536 725–765 870–900 1060–1160 1260–1280 1360 1630–1640
Bending vibrations of (O\ \P\ \O) units δ(PO2) modes of (PO− 2 ) chain groups O_P\ \O bending vibrations or harmonic of P\ \O\ \P bending vibrations \O\ \P) Attributed to symmetric stretching vibrations of P\ \O\ \P rings νs (P\ \O\ \P) Attributed to asymmetric stretching vibrations of P\ \O\ \P groups linked with metaphosphate chains νas (P\ − − Asymmetric stretching vibrations of PO2 groups νas (PO2 ) modes and νas (PO3) terminal Asymmetric stretching of doubly bonded oxygen vibrations νas (P_O) modes Harmonic of the previous mode OH bending vibrations, P\ \O\ \H bridges
[31–34] [31–34] [31–34] [31–34] [31–34] [31–34] [31–34] [31–34]
(b) For Bi\ \O vibrations 460–478 840
Bi\ \O bending vibrations or distorted BiO6 octahedra Stretching vibrations of Bi\ \O (BiO3 units)
[35], [36] [35], [36]
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some BO3 to BO4 groups until certain percent. The presence of PbO as modifier increases the thermal expansion coefficient while the formation of tetrahedral borate (BO4) groups decreases the coefficient. (iv) In alkali silicate glasses, the increase of alkali oxide increases the thermal expansion coefficient and the different effects of various alkali earth oxides will be dependent on the bond strength between the cation and oxygen. The present experimental thermal expansion data can thus be understood and explained as follows: (a) The increase of Bi2O3 percent to about 10 mol% increases the thermal expansion coefficient because bismuth ions (Bi3+) are situated mostly in modifying positions (BiO6) causing some loosening in the glass structure. (b) After certain Bi2O3 percent, the coefficient of thermal expansion is observed to be constant or slightly decreases because bismuth ions are assumed to form new structural building (BiO3) units giving compactness to glass network.
5. Conclusions Combined optical, FTIR spectral investigations were carried out on undoped and Bi2O3 (5 → 40 mol%) containing sodium metaphosphate glasses before and after gamma irradiation with a dose of 8 M rad (8 × 104 Gy). Optical absorption spectra indicate the appearance of two strong UV absorption peaks in the spectra of undoped and also the spectra of Bi2O3 containing glasses which are related to trace Fe3+ ions present as unavoidable impurities. The Bi2O3 glasses exhibit an additional band at 403–430 nm which is related with absorption from Bi3 + ions but on further increase of Bi2O3 content beyond 10 mol%, this band disappears. This peculiar behavior is attributed to possible alternation in the structure by the formation of additional BiO3 groups, formation of Bi\\O\\P linkages or nonbridging oxygens and the authors have suggested that further work is needed to confirm these suggestions. Gamma irradiation shows the maintenance of the UV bands while a new induced visible band at 460 nm is generated in the base glass and Bi2O3-containing samples which is related to the formation of phosphorus oxygen hole center (POHC) or nonbridging oxygen hole center (NB OHC). Infrared absorption spectra show characteristic bands due mainly to metaphosphate groups besides the sharing of Bi\\O or Bi\\O\\P vibrations upon the introduction of Bi2O3. The glasses containing Bi2O3 reveal obvious shielding upon gamma irradiation and the FTIR show minor effect due to the presence of heavy metal ions (Bi3+). Additional thermal expansion measurements show the decrease of thermal expansion coefficient with the increase of Bi2O3 5 → 10 mol% and then the expansion
coefficient reveals constancy due to suggested structural changes as suggested before in the explanation of optical and FTIR spectra.
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R.K. Brow, J. Non-Cryst. Solids 263&269 (2000) 1–28. J.H. Campbell, T.I. Suratwala, J. Non-Cryst.Solids 263&264 (2000) 318. R.K. Brow, L. Kovacic, R.E. Loethman, Ceram. Trans. 70 (1996) 177. A.M. Monem, H.A. ElBatal, E.M. khalil, M.A. Azooz, Y.M. hamdy, J. Mater. Sci. Mater. Med. 19 (3) (2008) 1097–1108. J. Fu, J. Mater. Sci. 33 (1998) 1549. C.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, D.K. Shuh, P.O. Allen, M.I. Saboungi, D. Haeffner, J-Non-Cryst, Solids 263 (264) (2000) 146–154. W.H. Dumbaugh, J.C. Lapp, J. Am. Ceram. Soc. 75 (1992) 2315. F.H. ElBatal, M.A. Azooz, F.M. EzzElDin, Phys.Chem.Glasses 43 (2002) 260. Y.B. Dimitriev, Phys. Chem. Glasses 43 (2003) 380. L. Baia, R. Stefan, J. Popp, S. Simon, W. Kiefer, J. Non-Cryst. Solids 324 (2003) 109. F.H. ElBatal, M.A. Azooz, S.Y. Marzouk, Phys. Chem. Glasses Eur. J. Glass Sci. Technol. B 47 (2006) 588. F.H. ElBatal, M.A. Ouis, R.M. Morsi, S.Y. Marzouk, J. Non-Cryst. Solids 356 (2010) 46. G.H. Sigel, R.J. Ginther, Glass Technol. 9 (1968) 66. L. Cook, K.H. Mader, J. Am. Ceram. Soc. 65 (1982) 609. J.A. Duffy, M.D. Ingram, Phys. Chem. Glasses 15 (1974) 34. J.A. Duffy, Phys. Chem. Glasses 38 (1997) 289. U. Natura, D. Ehrt, Glastech. Ber. Glass Sci. Technol. 72 (9) (1999) 295. D. Moncke, D. Ehrt, Opt. Mater. 25 (2004) 425. S.Y. Marzouk, F.H. ElBatal, Nucl. Instr. Meth. Phys. Res. B 248 (2006) 90. F.H. ElBatal, A.M. Abdelghany, R.L. Elwan, J. Mol. Struct. 1000 (2011) 103. S. Parke, R.S. Webb, J. Phys. Chem. Solids 34 (1973) 85. J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids, 21 (1976) 373. F.H. ElBatal, Nucl. Instr. Meth. Phys. Res. B 254 (2007) 243. F.H. ElBatal, S.Y. Marzouk, N. Nady, S.M. Desouky, Physica B 391 (2007) 88. F.H. ElBatal, S.Y. Marzouk, N. Nada, S.M. Desouky, Phil. Mag 90 (6) (2010) 675. F.H. ElBatal, M.A. Marzouk, A.M. Abdelghany, J. Mater. Sci. 46 (2011) 5140. M.A. Marzouk, A.M. Abdelghany, H.A. ElBatal, Phil. Mag. 93 (19) (2013) 2465–2484. O. Sanz, E. Hazo-Poenatwaski, J. Gonzalo, J.M. Ftrnandez Navarro, J. Non-Cryst. Solids 352 (2006) 761. F.H. ElBatal, A.M. Abdelghany, H.A. ElBatal, Spectrochim. Acta 122 (2014) 461–468. A.H. Hammad, M.A. Marzouk, H.A. ElBatal, SILICON 8 (2016) 123–131. A. Abdel Kader, A.H. Higazy, M.M. El Kholy, J. Mater. Sci. Mater. Electron. 2 (1991) 157. P. Znasik, M. Jamnicky, J. Non. Cryst.Solids 146 (1992) 74. Y.M. Moustafa, K. El-Egili, J. Non-Cryst. Solids 240 (1998) 144. F.H. ElBatal, J. Mater. Sci. 43 (2008) 1070. H. Doweidar, Y.B. Saddeek, J. Non-Cryst. Solids 355 (2009) 348–354. N. Vedeanu, O. Cozar, R. Stanescu, I. Cozar, I. Ardelean, J. Mol. Struct. 1044 (2013) 323–327. A. Bishay, J. Non-Cryst. Solids 3 (1970) 54. E.J. Friebele, in: D.R. Uhlmann, N.J. Kreidl (Eds.), Optical Properties of Glass, American Ceramic Society, Westerville, OH, U.S.A. 1991, pp. 203–262. H.A. ElBatal, A.M. Abdelghany, I.S. Ali, J. Non. Cryst.Solids 358 (2012) 820–825. D.G. Hollaway, The Physical Properties of Glass, Wykeham publications Ltd., London, 1973 36–41. H. Rawson, Properties and Applications of Glasses, Elsevier, Amsterdam, 1980 69. J.E. Shelby, Introduction to glass science and technology, The Royal Society of Chemistry, 2nd Edition 2005, p. 154. N.A. Ghoneim, K.M. Elbadry, Thermochim. Acta 60 (1983) 253–263. N.A. Ghoneim, A.F. Abbas, Thermochim. Acta 66 (1983) 91–103. F.M. Ezz ElDin, H.A. Elbatal, J. Non-Cryst. Solids 152 (1983) 195–200. H.A. ElBatal, N.A. Ghoneim, F.M. Ezz ElDin, Nucl. Instr. Meth. Meth. Phys. Res. (B) 88 (1994) 411–417.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Optical, structural and thermal properties of sodium metaphosphate glasses containing Bi2O3 with interactions of gamma rays M.A. Marzouk ⁎, F.H. ElBatal, K.M. ElBadry, H.A. ElBatal Glass Research Department, National Research Centre, 33 EL Bohouth St. (Former EL Tahrir St.), Dokki, P.O. 12622, Giza, Egypt
a r t i c l e
i n f o
Article history: Received 26 October 2015 Received in revised form 17 August 2016 Accepted 18 August 2016 Available online 23 August 2016 Keywords: Sodium metaphosphate glass Bi2O3, Optical–FTIR Thermal properties Gamma irradiation
a b s t r a c t Sodium metaphosphate glasses with successive increasing added Bi2O3 contents (5–40%) were prepared to improve their chemical stability and increase their optical and thermal properties through the additional building BiO6 and BiO3 units. The optical spectrum of the base metaphosphate glass reveals strong UV absorption due to the presence of trace iron (Fe3+) ions present as impurities. Glasses containing additional 5, 7.5 and 10% Bi2O3 show further band around 406 nm which can be related to absorption of Bi3+ ions. With increasing the Bi2O3 content, this near visible band is observed to disappear indicating peculiar behavior needing further work. Gamma irradiation causes only minor changes in the position of the strong UV peaks but an obvious induced visible broad band centered at 452–460 nm in the base and Bi2O3 containing glasses. This induced band is related to the generation of phosphorus oxygen hole center or non bridging oxygen hole center as revealed by various authors. FTIR results reveal characteristic vibrational bands due to phosphate groups and with the addition of Bi2O3, some interference of Bi\\O vibrational units are expected. Gamma irradiation causes limited changes in the IR spectra due to suggested shielding effect of the heavy metal oxide Bi2O3. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phosphate glasses are interesting vitreous system that has gained great attention in the past and in recent applications. In the 1950s, interest in amorphous alkali phosphates was stimulated by their use in a variety of industrial applications, including sequestering agents for hard water treatments and dispersants for clay processing pigment manufacturing [1]. Owing to the outstanding optical properties of phosphate glasses including higher transparency for ultraviolet light especially when additional multivalent oxides are formulated in their compositions, they are introduced and recommended as laser glasses [2]. Also, the high thermal expansion coefficients, and low softening temperatures of phosphate glasses make them suitable for hermetic seals [3]. Biocompatible phosphate glasses and glass-ceramics have been recommended for medical applications [4] and lithium phosphate glasses [5] have fast ion conductivity that makes them useful as solid state electrolytes. The application extends to explore lead iron phosphate glass to be a host matrix for radioactive wastes because of possessing promising chemical durability, radiation shielding beside lower melting temperature [6]. Glasses containing heavy metal Bi2O3 have wide and potential applications in the field of optical glasses with high refractive indices, low melting temperature, semiconducting glasses and also as a shielding candidate for gamma irradiation [7–9]. The ability of Bi2O3 like PbO to ⁎ Corresponding author. E-mail address: [email protected] (M.A. Marzouk).
http://dx.doi.org/10.1016/j.saa.2016.08.038 1386-1425/© 2016 Elsevier B.V. All rights reserved.
share or behave partly as glass forming oxide comes from the high polarizability of Bi3+ and Pb2+ cations and their constitutional asymmetric polyhedra which inhibit crystallization and thus possess wide range of glass formation beside being less toxic than PbO [10]. The aim of the present work is to study by combined optical and FT infrared absorption spectroscopic measurements detailed spectral properties of sodium metaphosphate glasses containing increasing Bi2O3 contents before and after gamma irradiation. It is expected that these collective spectral studies will throw some insight on the effect of Bi2O3 on the phosphate network with the introduction of additional Bi\\O units and to deduce the induced defects generated during the irradiation process through the addition of heavy metal oxide (Bi2O3) expected to cause shielding behavior. A further aim includes the study of the thermal expansion properties of the studied glasses to find out their possible suitability to serve as sealing glass candidate. 2. Experimental and methods 2.1. Preparation of the glasses Glasses of the basic sodium metaphosphate composition (50 mol% Na2O-50 mol% P2O5) to which successive added Bi2O3 contents (5% → 40%) were prepared. Laboratory chemicals of NaH2PO4 and Bi2O3 were used to prepare the glasses. All weighed batches were melted in alumina crucibles at 1100 °C and melting time was extended to 1 h with stirring of the melts at intervals to promote complete mixing and homogeneity. The melts were cast into warmed stainless steel
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molds of the required dimensions. The prepared glassy samples were immediately transferred to a muffle furnace regulated at 250 °C for annealing to remove any stresses or strain formed during the rapid cooling of the melts. The annealing muffle was left to cool to room temperature after 1 h with the samples inside with a cooling rate of 20 °C/h. 2.2. Optical absorption spectral measurements The optical (UV–visible) absorption spectra of polished glass samples (2 mm + 0.1 mm) were recorded at room temperature before and after gamma irradiation by a recording spectrophotometer (type T80 PG Instruments, England) in the range 190–1000 nm. 2.3. FT infrared absorption measurements FTIR absorption spectra of the glasses were measured at room temperature in the range 400–4000 cm−1 by a Fourier transform infrared spectrometer (type Nicolet is 10, USA) using the KBr disc technique. The IR measurements were immediately done after preparing the discs to avoid moisture attack. The same measurements were repeated after subjecting the glass powder to a gamma dose of 8 M rad (8 × 104 Gy). 2.4. Gamma irradiation facility An Indian 60Co gamma cell (2000Ci) was used as a gamma ray source with a dose rate of 1.5 Gy/S (150 rad/s) at a temperature of 30 °C. The glasses were subjected to a total dose of 8 M rad (8 × 104 Gy). This specific amount of radiation dose was chosen based on previous studies on irradiation of glasses which revealed that distinct optical spectral variations were observed until the gamma dose reached 6 M rad after which saturation effect was reached stopping the creation of more induced defects [11,12]. 2.5. Thermal expansion investigations Thermal expansion properties of base sodium metaphosphate glass together with samples containing varying Bi2O3 contents were measured using a computerized dilatometer (type NETZSCH DIL 402 PC 5, Germany). All the measurements were made within the temperature range from room temperature up to the dilatometeric softening temperature (Tg) with a heating rate of 5 °C/min. The cylindrical glass samples with the dimension (25 × 6 mm) were used and at least two measurements were made on each glass with reproducibility (±0.01%).
Fig. 1. UV–visible absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses before gamma irradiation.
peaks are identified at 240 and 310 nm and with no further visible absorption. The glass containing 40% Bi2O3 shows a strong UV absorption with two peaks at 240 and 298 nm and two small peaks 380 nm and 420 nm and with no further bands in the rest of the visible region. Fig. 2 illustrates the optical spectra of the studied glasses after gamma irradiation with a dose of 8 M rad. The base glass containing no Bi2O3 reveals strong UV absorption with two peaks at 240 and 296 nm and with a distinct induced visible band at 452 nm. All the Bi2O3-containing glasses exhibit the same UV absorption with two distinct peaks at about 240 and 296 nm and together with a further broad visible band centered at about 460 nm. 3.2. FT infrared absorption spectra of the glasses Fig. 3 illustrates the FTIR spectra of all the studied glasses before irradiation. The IR spectral features of the base sodium metaphosphate glass can be summarized as follows: (a) The far-IR absorption reveals distinct and sharp connected five peaks at about 430, 457, 485, 518 and 550 cm−1. (b) The appearance of a medium broad band with two peaks at 732 and 769 cm−1. (c) A very broad and strong absorption extending from about 800 to 1600 cm−1 is observed with seven peaks at about 873, 968, 998, 1014, 1089, 1205 and 1275 cm−1.
3. Results 3.1. Optical absorption spectra before and after gamma irradiation Fig. 1 illustrates the optical spectrum of the base sodium metaphosphate glass together with the spectra of samples of the same composition and containing successive Bi2O3 additions (5–40% Bi2O3). The base glass exhibits a strong and broad UV absorption band with two distinct peaks at 240 and 300 nm and with no further visible absorption to the end of measurement at 1100 nm. On introducing 5% Bi2O3, the UV spectrum remains strong revealing two peaks at 228 and 282 nm and with the appearance of a new visible peak at 403 nm. The spectrum of the glass containing 7.5% Bi2O3 reveals strong UV absorption with two bands at 306 nm and a lower band at 384 nm followed by a distinct visible band at 430 nm. The sample containing 10% Bi2O3, reveals a strong UV absorption with two distinct peaks at 240 and 305 nm very similar to that of the base glass but with a small peak at 394 nm. The samples with 12.5% or 15% Bi2O3 show the same spectrum consisting of a strong UV absorption the same as the spectrum of the base glass but the positions of the two
Fig. 2. UV–visible absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses after 8 MR gamma irradiation.
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1105 and 1285 cm−1. A final medium band is observed at 1620 cm−1. (b) The glass containing 5% Bi2O3 reveals a deconvoluted IR spectrum consisting of (i) the far IR absorption shows three peaks at 413, 523 and 580 cm−1. (ii) A new connected band is identified at 760 cm−1. (iii) A very broad connected spectrum extends from 800 to 1650 cm−1 and revealing seven successive peaks at 880, 985, 1109, 1217, 1291, 1440 and 1615 cm−1. (c) The glass containing 15% Bi2O3 also shows connected IR spectrum. (i) The far-IR spectrum reveals four peaks at 405, 480, 552 and 610 cm− 1.(ii) A connecting peak is identified at 708 cm−1. (iii) The very broad mid spectrum shows five peaks at 870, 992, 1133, 1408 and 1623 cm−1.
Careful inspection of these IR curves indicates some distinguished features on the introduction of increasing Bi2O3 content which can be summarized as follows: (i) The clear observation of at least four peaks in the range 400– 650 cm−1. (ii) The appearance of a new peak in the spectral range 720– 730 cm−1. (iii) The growth or increase of the intensities of the composite peaks within the spectral range between 800–1400 cm−1.
3.4. Infrared absorption spectra after gamma irradiation Fig. 3. FTIR absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses before gamma irradiation.
Fig. 4 reveals the FTIR spectra of the studied glasses after gamma irradiation (8 M rad = 8 × 104 Gy). Carful inspection and comparison of
(d) Two small peaks are identified at about 1616 and 1630 cm−1. (e) Three small peaks are observed at 2360, 2850 and 2923 cm−1. (f) A broad medium near IR band is observed with three distinct peaks at 3413, 3450, 3548 cm−1. Fig. 3 also indicates that the introduction of successive Bi2O3 contents 5, 15 and 40% reveals the following variations: (1) The first far-IR peaks are connected to appear as two composite bands with several peaks losing the sharpness observed in the spectrum of the undoped sample. (2) The medium peaks at 732 and 769 cm−1 are changed to a single distinct medium broad band centered at 752 cm−1 which decreases in intensity at higher Bi2O3 contents (12.5%, 20%). (3) The middle peaks extending from about 800 to about 1600 cm−1 grow in intensity and become compacted or gathered in a single very broad band with a peak at about 1100 cm−1.
3.3. Deconvoluted FTIR spectra Due to the obvious composite and broad IR spectra of the studied samples especially within the region 400–1700 cm−1, it is found necessary to apply a deconvolution method to find out and differentiate between overlapped vibrational modes which result from the vibrations of both phosphate groups and Bi\\O groups in the specified region. The deconvoluted spectra of the samples (Fig. 3) can be summarized as follows: (a) The base sodium phosphate glass reveals in the far-IR region three peaks at 406, 436 and 535 cm−1. The mid region shows a very broad band extending from about 780 to 1450 cm−1 and the deconvolution spectrum reveals four peaks at 870, 968,
Fig. 4. FTIR absorption spectra of undoped and Bi2O3 doped sodium phosphate glasses after 8 MR gamma irradiation.
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the FTIR spectra of the base undoped sodium phosphate before and after irradiation indicates that: (a) The deconvoluted far-IR peaks at 437 and 533 cm−1 are shifted to 487 and 560 cm−1 respectively. (b) The main mid IR absorption extending from about 800 to 1400 cm−1 show more splitted peaks at 871, 996, 1097, 1144, 1274 and 1351 cm−1 beside a separate band at 1635 cm−1. The glasses containing Bi2O3 reveal an additional band around 700 cm−1 beside increase of the intensity of the very broad absorption composite band.
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3.6. Thermal expansion results Fig. 6(a & b) illustrates the measured thermal expansion data of the studied glasses. The thermal expansion measurements data can be summarized as follows: (a) The thermal expansion coefficients at 100 °C–200 °C and 300 °C are observed to progressively decrease with the increase of Bi2O3 content. (b) The transformation and softening temperatures are found to increase with the introduction of 5, 7.5 or 10% Bi2O3 and then decrease or reach constancy with further Bi2O3 contents.
3.5. Infrared absorption spectra of crystalline raw materials 4. Discussion Fig. 5 illustrates the FTIR spectra of the crystalline chemical materials used for the preparation of the studied glasses, namely crystalline sodium metaphosphate (NaH2PO4) and bismuth oxide (Bi2O3). The IR spectrum of crystalline sodium metaphosphate (Fig. 5a) is observed to consist of the following spectral features: (a) The far-IR absorption reveals two strong and very sharp peaks at 435and 539 cm−1 and with an attached peak at about 605 cm−1. (b) A very broad and distinct absorption extending from about 906 to 1268 cm−1 is observed and revealing five peaks at 906, 979, 1087, 1168 and 1268 cm−1. (c) A distinct medium band is identified at 1670 cm−1. (d) The rest of the spectrum shows numerous small peaks extending from about 2379 to 3532 cm−1. The absorption spectrum of the crystalline Bi2O3 (Fig. 5b) reveals the following characteristics (a) Two very sharp distinct far-IR peaks at 478 and 516 cm−1 are observed. (b) A very broad medium absorption extending from about 871 to 1639 cm−1 is observed and revealing four peaks at 871, 1018, 1396 and 1639 cm−1. (c) A broad band is identified with numerous peaks at about 2410, 2917, 3228, 3394, and 3548 cm−1.
Fig. 5. FTIR absorption spectra of raw materials (a) crystalline sodium metaphosphate (NaH2PO4) and (b) crystalline bismuth oxide (Bi2O3).
4.1. Interpretation of the optical UV–visible absorption spectra of the studied glasses The spectrum of the base host sodium metaphosphate is observed to exhibit only a strong broad UV absorption band with two distinct peaks at 240 and 300 nm. Many glass scientists [13,14] had identified strong UV absorption in numerous undoped commercial glasses and the origin of this strong UV absorption was related to the presence of unavoidable trace impurities and specifically to trace ferric ions contaminated within the raw materials used for the preparation of such commercial glasses. Duffy and Ingram [15] and Duffy [16] have agreed to this assumption and they have classified various charge transfer UV absorption bands usually observed in various undoped glasses and have concluded that charge transfer absorption of some ions (e.g. Fe3+, Cr6+) to originate through electron transfer mechanism even if these ions are present in traces within the ppm level. The electron transfer mechanism involves the transition of an electron from the orbital of a coordinating oxygen atom to an orbital of the metal ion. Ehrt et al. [17,18] and ElBatal et al. [19,20] have identified similar strong UV absorption in various phosphate, borosilicate and borate glasses and they have reached to the same attribution. They have further claimed for the need of ultrapure materials to improve the transmission of special optical glasses for recent applications. Based on previous considerations, the two strong UV absorption peaks identified in the spectrum of undoped base sodium metaphosphate glass can be attributed to the presence of unavoidable traces of ferric ions (Fe3+) present as impurities within the chemicals used for the preparation of such glass and this same strong UV absorption is observed also to be extended and identified in the optical spectra of the Bi2O3-containing glasses. The two glasses containing additional Bi2O3 5% and 10% Bi2O3 reveal a further distinct visible band at 403–430 nm surplus to the strong UV peaks previously identified in the base glass. Parke and Webb [21] previously observed an ultraviolet- near visible peak when traces of Bi3 + ions were added to borate and phosphate glasses and this peak was related to the transition 1S0 → 3P1 of Bi3 + ions [22]. The appearance of an extra band in the visible at 403– 420 nm with the addition of 5 or 10% Bi2O3 can be related to the contribution of absorptions from Bi3 + ions according to the previous considerations. Duffy [16] has assumed that certain other metal ions including Ce3+, Tb3+, U4+ as well as d10S2 ions (such as Pb2+ and Bi3+) absorb radiation through electronic transitions involving orbitals essentially of the metal ion only, and the name “Rydberg” has been suggested for such spectra to distinguish them from the common charge transfer spectra (e.g. Fe3+ and Cr6+). Successive studies on borate and silicate glasses containing high Bi2O3 content (~60–90%) have reached the conclusion that the UV-near visible bands in the range 350–450 nm observed in high bismuth borate and high bismuth silicate glasses are correlated with absorption from Bi3+ ions [23–29].
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Fig. 6. (a) Thermal expansion undoped and Bi2O3 doped sodium phosphate glasses, (b) extended thermal expansion undoped and Bi2O3 doped sodium phosphate glasses.
The present optical results reveal a peculiar behavior through the disappearance of the near UV–visible band generated upon the addition 5 mol% or 10 mol% Bi2O3 by further increasing the added Bi2O3 to reach 40 mol%. This behavior is observed for the first time from the studied N10–40% Bi2O3 within host sodium metaphosphate glass. Binary bismuth borate or bismuth silicate glass containing high Bi2O3 from 60 mol% up to 90 mol% show extended UV–visible absorption bands within the range 400–850 nm as evidenced in previous publications [23–27]. Some suggested assumptions can thus be introduced regarding this peculiar behavior: concerning the disappearance of the extra band at 403–430 nm on increasing Bi2O3 content N10 mol% until 40 mol%. (a) It is assumed that the Bi3+ ions might undergo reduction in the host phosphate glass to Bi2+ or to metallic aggregates of Bi+. It is known that 3d transition metal ions reveal mostly the optical spectra of their lower valence or the octahedral coordination in sodium metaphosphate host glass [19]. But this assumption is faced by the first appearance of absorption due to Bi3+ ions at relatively low concentration (5 → 10%). (b) It is suggested that bismuth ions are involved in new structural
arrangement or exhibit different structural units as for example the transformation of [BiO6] to [BiO3] and this assumption is supported by the present obvious distinct change in the FTIR spectra when the concentration of Bi2O3 exceeds 10 mol%. (c) A further suggestion is the formation of nonbridging oxygens with Bi2O3 content higher than 10 mol% together with the formation of Bi\\O\\P linkages. (d) A recent contribution of some of the authors [30] on the effects of Bi2O3 on optical FTIR and thermal expansion of SrO–B2O3 glasses has reached to the conclusion that increasing the Bi2O3 content to 15 mol% reveals obvious changes in the IR spectra due to suggested formation of both BiO6 and BiO3 groups.
The previous suggestions need further studies through different techniques to specify the reason for this peculiar behavior. 4.2. Interpretation of the FT infrared absorption spectra To realize and interpret the detailed FTIR spectra of the studied glasses, the following basic parameters are introduced [1,12,31–34]:
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(a) The chemical composition of the basic glass is sodium metaphosphate (50 mol% Na2O, 50 mol% P2O5) together with samples containing additional Bi2O3 (5, →40%). (b) It is expected that the base glass structure consists of metaphosphate groups (PO2) in accordance with the chemical composition. The following vibrational modes are expected: νs POP, να POP, νs PO2, ν (OPOP) deformation or bending modes. (c) The numerous sharp far-IR peaks at 430 → 550 cm−1 can be related to O\\P\\O bending or lattice mode vibrations together with possible contribution of alkali cation vibrations (Na+). (d) The medium band with peaks at 732 and 764 cm−1 can be related to symmetric stretching of P\\O\\P bonding. (e) The peaks at 873 and 768 cm− 1 may be linked to asymmetric stretching of P\\O\\P groups linked to metaphosphate chains. (f) The peaks at 998, 1014, 1089 cm−1 may be related to asymmetric stretching of PO2-terminal groups. (g) The peaks at 1205 and 1275 cm−1 can be correlated with asymmetric stretching of PO2 groups, stretching of doubly bonded oxygen with phosphorus vibrations (P_O). (h) The rest peaks at 1616, 1630, 2360, 2850, 2923, 3413, 3450 and 3548 cm−1 can be related to, OH, water, POH vibrations.
do contain pre-existing intrinsic defects such as vacancies, non-bridging oxygens, impurities, etc. These intrinsic defects are obviously formed through the traditional fast cooling step during the preparation of glasses. It is accepted that [37,38] the exposure of most glasses to high energy radiation (e.g. x-ray, γ-ray, UV light) produces generally induced absorption bands in the UV–visible part of the spectrum. This is due to the creation of induced defects centers as a result of capturing of liberated pairs of electrons and positive holes during the irradiation process. However, some authors [34,37–39] have assumed that glasses containing heavy metal oxides (PbO, Bi2O3, WO3, MoO3) show shielding behavior towards gamma irradiation. Also, the presence of some transition metal ions (e.g. Cu2+, V5+) are observed to show some retardation of the effect of successive gamma irradiation. The studied glasses including the base glass and Bi2O3 containing samples reveal an induced visible band at 460 nm which can be related to either to the formation of phosphorus oxygen hole center (POHC) or to nonbridging oxygen hole center (NBOHC) as previously suggested by Bishay [37] and Friebele [38].
The support of the previous interpretation can be derived from the IR spectra of crystalline sodium ultraphosphate (Fig. 5a) which are greatly similar to that of vitreous sodium metaphosphate and their bands are similarly attributed [33,34]. The observed changes in the FTIR spectra upon the addition of progressive Bi2O3 can thus be interpreted by introducing the following suggestions:
Glass, like most other solids; generally expands on heating. The dimensional changes which occur with temperature are very important for sealing purposes and the ability of the glass to survive thermal shock or cycling [40–42]. Normally, expansion is described as to be due to the increasing amplitude of atomic vibrations of the constituents and thermal expansion is determined by anharmonic lattice vibrations. The expansion of a glass is accepted to depend mainly on the internal network structure, the arrangement of the constituting building units and the bonding strength to each other. Extended thermal expansion coefficient studies of lead silicate, lead borate, ternary alkali/alkaline earth silicate, ternary alkali alkaline earth borate, binary Bi2O3·B2O3 and ternary Bi2O3·B2O3·PbO glasses have reached to important parameters on the relation of the type and composition of the glasses to their thermal expansion coefficients [43–46] and they can be summarized as follows;
(a) The changes can be correlated with the formation of nonbridging oxygens by the introduction N 10 mol% Bi2O3 in the neighborhood of [BiO6] groups. (b) The increase of Bi2O3 added to 10 mol% or more reaching 40 mol% causes the formation of additional structural forming [BiO3] groups with bridging oxygen. (c) A similar result is observed upon the addition of progressive Bi2O3 within the host strontium borate glass [30]. (d) The attribution of the IR bands related to Bi\\O groups can be understood and realized by referring to the IR spectrum of crystalline Bi2O3 (Fig. 5b) in which [BiO6] units are the main constituent [23,35,36]. Table 1 summarizes the FTIR peak and their assignments. 4.3. Interpretation of the effect of gamma irradiation on the optical and FTIR spectra Before discussing the effect of gamma irradiation, it is considered that glasses deviate from the ideal network structure of real solids and
4.4. Interpretation of the thermal expansion data
(i) The increase of heavy metal oxide (PbO) in lead silicate and lead borate glasses increases the thermal expansion coefficient due to the increase of Pb2+ ions in network modifying positions which decreases the connectivity of the glass structure. (ii) The relative differing effects of the divalent or trivalent oxides will depend on the bond strength of the cation to oxygen and the ability of some oxides such as MgO, ZnO to form additional structural units in the glass (MgO4, ZnO4). (iii) In lead borate glasses, there are two factors affecting the thermal expansion coefficient. The first factor is the forms in which the lead ions are studied in glass structure as modifier or former. The second factor is the sharing of PbO in the transformation of
Table 1 FT-infrared absorption peaks and their assignments. Peak wavenumber cm−1
Assignment
References
(a) For base sodium metaphosphate 460–485 500–536 725–765 870–900 1060–1160 1260–1280 1360 1630–1640
Bending vibrations of (O\ \P\ \O) units δ(PO2) modes of (PO− 2 ) chain groups O_P\ \O bending vibrations or harmonic of P\ \O\ \P bending vibrations \O\ \P) Attributed to symmetric stretching vibrations of P\ \O\ \P rings νs (P\ \O\ \P) Attributed to asymmetric stretching vibrations of P\ \O\ \P groups linked with metaphosphate chains νas (P\ − − Asymmetric stretching vibrations of PO2 groups νas (PO2 ) modes and νas (PO3) terminal Asymmetric stretching of doubly bonded oxygen vibrations νas (P_O) modes Harmonic of the previous mode OH bending vibrations, P\ \O\ \H bridges
[31–34] [31–34] [31–34] [31–34] [31–34] [31–34] [31–34] [31–34]
(b) For Bi\ \O vibrations 460–478 840
Bi\ \O bending vibrations or distorted BiO6 octahedra Stretching vibrations of Bi\ \O (BiO3 units)
[35], [36] [35], [36]
460
M.A. Marzouk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 171 (2017) 454–460
some BO3 to BO4 groups until certain percent. The presence of PbO as modifier increases the thermal expansion coefficient while the formation of tetrahedral borate (BO4) groups decreases the coefficient. (iv) In alkali silicate glasses, the increase of alkali oxide increases the thermal expansion coefficient and the different effects of various alkali earth oxides will be dependent on the bond strength between the cation and oxygen. The present experimental thermal expansion data can thus be understood and explained as follows: (a) The increase of Bi2O3 percent to about 10 mol% increases the thermal expansion coefficient because bismuth ions (Bi3+) are situated mostly in modifying positions (BiO6) causing some loosening in the glass structure. (b) After certain Bi2O3 percent, the coefficient of thermal expansion is observed to be constant or slightly decreases because bismuth ions are assumed to form new structural building (BiO3) units giving compactness to glass network.
5. Conclusions Combined optical, FTIR spectral investigations were carried out on undoped and Bi2O3 (5 → 40 mol%) containing sodium metaphosphate glasses before and after gamma irradiation with a dose of 8 M rad (8 × 104 Gy). Optical absorption spectra indicate the appearance of two strong UV absorption peaks in the spectra of undoped and also the spectra of Bi2O3 containing glasses which are related to trace Fe3+ ions present as unavoidable impurities. The Bi2O3 glasses exhibit an additional band at 403–430 nm which is related with absorption from Bi3 + ions but on further increase of Bi2O3 content beyond 10 mol%, this band disappears. This peculiar behavior is attributed to possible alternation in the structure by the formation of additional BiO3 groups, formation of Bi\\O\\P linkages or nonbridging oxygens and the authors have suggested that further work is needed to confirm these suggestions. Gamma irradiation shows the maintenance of the UV bands while a new induced visible band at 460 nm is generated in the base glass and Bi2O3-containing samples which is related to the formation of phosphorus oxygen hole center (POHC) or nonbridging oxygen hole center (NB OHC). Infrared absorption spectra show characteristic bands due mainly to metaphosphate groups besides the sharing of Bi\\O or Bi\\O\\P vibrations upon the introduction of Bi2O3. The glasses containing Bi2O3 reveal obvious shielding upon gamma irradiation and the FTIR show minor effect due to the presence of heavy metal ions (Bi3+). Additional thermal expansion measurements show the decrease of thermal expansion coefficient with the increase of Bi2O3 5 → 10 mol% and then the expansion
coefficient reveals constancy due to suggested structural changes as suggested before in the explanation of optical and FTIR spectra.
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