Effects of Ce+Gd on the structural features and thermal stability of iron-boron-phosphate glasses

Materials Chemistry and Physics 201 (2017) 170e179

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Effects of CeþGd on the structural features and thermal stability of iron-boron-phosphate glasses Fu Wang a, *, Zhiwei Fang a, b, Hong Wang a, Qilong Liao a, **, Hanzhen Zhu a a b

School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China School of National Defense Science and Technology, Southwest University of Science and Technology, Mianyang 621010, PR China

h i g h l i g h t s  Solubility limit of Ce þ Gd in the IBP glass is about 8 mol%.  Incorporation of Ce þ Gd increases the fraction of Q0 and Q1, but decreases Q2 groups.  Density and Tg increase with the addition of Ce þ Gd in the glass formation range.  Incorporation of Ce þ Gd decreases the thermal stability of the IBP glasses.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2017 Received in revised form 19 August 2017 Accepted 21 August 2017 Available online 23 August 2017

In this paper, effect of Ce þ Gd incorporation on the structural features and properties of iron borophosphate (IBP) glasses was investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and differential scanning calorimetric (DSC) measurements. The results show that the solubility limit of Ce þ Gd in the IBP glass with the composition of 36Fe2O3-10B2O354P2O5 is about 8 mol%. The structural networks of the Ce þ Gd containing IBP glasses consist predominantly of orthophosphate, Q0, groups and pyrophosphate, Q1, groups, along with [BO4] units and a small amount of metaphosphate, Q2, groups. The Ce þ Gd incorporation increases the density and Tg but decreases thermal stability of the IBP glass. However, the thermal stability of the Ce þ Gd doped IBP glasses increases with Ce þ Gd until the contents of Ce þ Gd nearly reach the limit solubility. Moreover, the incorporation of Ce þ Gd offers depolymerizing effect on the glass networks, which increases the fraction of Q0 and Q1 groups, and decreases the fraction of Q2 groups. The conclusions provide researchers with useful information to understand the roles of Ce þ Gd in iron-boron-phosphate based glasses for high-level nuclear waste immobilization. © 2017 Elsevier B.V. All rights reserved.

Keywords: Inorganic compounds Glasses Fourier transform infrared spectroscopy Raman spectroscopy and scattering

1. Introduction Borophosphate based glasses have been received widely studies due to their unique features for a variety of applications [1e8]. These materials show low dispersion, low refractive indices and improved chemical durability compared with phosphate or borate glasses [3,5e8]. For instance, this kind of glasses is of interest for photoinduced optical applications including nonlinear optical glasses [9,10], and it has been approved that doping by transition

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (Q. Liao). http://dx.doi.org/10.1016/j.matchemphys.2017.08.060 0254-0584/© 2017 Elsevier B.V. All rights reserved.

metals may be a promising way for increase of the nonlinear optical coefficients [9]. Recently, iron doped borophosphate (IBP) based glasses have been suggested to a potential application in immobilization of high-level radioactive wastes (HLWs) containing actinides such as Pu or Am [6,11e14]. That is because IBP based glasses possess acceptable chemical durability and more importantly, boron provides the final waste forms with high ‘‘thermal’’ neutron absorption cross sections [11e13,15]. These effects of B2O3 are significantly useful for immobilizing HLWs [9,12,16]. The investigations on IBP based glasses for immobilization of HLWs are mainly focused on the solubility of the elements coming from HLWs [6,13,17e21]. It has been revealed that the solubility of Hf which is a surrogate for nuclide Pu4þ is about 2 mol% in IBP glasses [13]. The solubility of some metal oxides in IBP glasses has been investigated by our group [6,18e22]. The results show that

F. Wang et al. / Materials Chemistry and Physics 201 (2017) 170e179

homogeneous glass phase is still detected for this system glasses containing 6 mol% La2O3 [6], CeO2 [18] or Gd2O3 [21], 3 mol% ZrO2 [20], 1 wt.% Cr2O3 [19], or 30 wt.% of a simulated nuclear waste [14]. Rare earth elements, which commonly are constituent parts of nuclear wastes, have been extensively investigated in glasses because these elemental ions usually play an important role in the structure and properties of the resultant glass waste forms. The roles of rare earth ions are strongly affected by the local structures at the rare earth sites [13,19,21]. The knowledge of such information is useful for designing glass waste forms [14]. Moreover, IBP glass itself also exhibits diversified structural features due to the coexistence of glass forming oxides P2O5 and B2O3 [22e24]. Cerium (Ce) and gadolinium (Gd) are the two of common elements in nuclear wastes, occurring both as a fission product and a fuel cladding component [18,21,26]. Previous investigations are mainly focused on the influences of the two elements independently on glasses [18,25,27]. It is known that Ce and Gd are usually surrogates for tetravalent and trivalent actinide elements such as nuclides Pu4þ/Pu3þ [25,27,28], and the simultaneous influences of Ce and Gd on glasses are the more reliable implication for immobilization of nuclear wastes containing Pu4þ/Pu3þ [29]. In addition, it is well known that the phase of glass waste form is metastable, which can spontaneously crystallize to a crystalline state. The stability and crystallization tendency of a glass mainly depend on compositions and storage temperatures. Once uncontrolled crystallization occurs, structural stability, thermal stability and chemical durability of glass waste forms would dramatically deteriorate. However, possible micro-crystals observed usually arises from various processes including the excess loading of an element at a concentration that exceeds the solubility limit of corresponding base glass used. Therefore, the investigations on the structure and thermal stability changes induced by composition variations are significantly important to estimate the comprehensive performances of glass waste forms. In this publication, we continue our study to the investigations of the structural features and thermal stability of IBP glasses containing both Ce and Gd. The properties and structure of the IBP glasses containing different amounts of Ce þ Gd were studied in detail to understand the variations of structure and thermal stability in IBP glass matrix due to Ce þ Gd. 2. Experimental 2.1. Sample preparation In our study, Ce and Gd are introduced by CeO2 and Gd2O3 respectively. Gd which has high thermal neutron capture crosssection is usually incorporated into the final glass waste forms to minimize the likelihood of criticality during storage period [29,30], although the concentration of Gd in nuclear wastes is lower than Ce [26]. Therefore, in this experiment, the molar ratio of Ce: Gd is constantly 1: 1 for the implication of real nuclear wastes immobilization. Glasses with the compositions of x(Ce þ Gd)-(100-x)(36Fe2O3-10B2O3-54P2O5), where x ¼ 0, 2, 4, 6, 8 and 10, were synthesized by a traditional melt-quenching method. Batches that can produce 50 g of glass melts were prepared by mixing Fe2O3, (NH4)H2PO4, H3BO3, CeO2 and Gd2O3 powders (AR) according to the designed compositions. According to literature data [11] and our experience, the volatilization loss of Fe2O3, CeO2 and Gd2O3 during melting is negligible, the amounts of H3BO3 and (NH4)H2PO4 used in the batches include an additional 10 mol% and 2 mol% of the required amount in order to compensate volatilization loss of B2O3 and P2O5 respectively. The thoroughly mixed powders were melted in a high temperature furnace (KBF1400, Instrument Co., Ltd. Nanjing Nanda, China) at

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1200  C for 2e3 h in air. Subsequently, the melted liquids were poured onto a preheated stainless steel plate. Finally, the quenched glasses were annealed at 450  C for 1 h and cooled to room temperature for 500 min to eliminate the inner stress. The samples are marked by x value. For example, the 2 mol% Ce þ Gd containing 36Fe2O3e10B2O3e54P2O5 sample is marked as “x ¼ 2 mol%”. The compositions with respect to each constituent of the as-prepared samples are listed in Table 1. 2.2. Measurements X-Ray diffraction (XRD) patterns were used to detect the potential crystals in glasses and were obtained on a X'Pert PRO diffractometer (PANalytical Company, Holland), using Cu Ka radiation (l ¼ 0.15406 nm), in angular range of 2q ¼ 3e80 . Differential scanning calorimetric (DSC) was used to investigate glass transition temperature (Tg), onset temperature of glass crystallization (Tr) of the glass samples. The differential scanning calorimetric (DSC) measurements were performed on a SDT Q600 instrument (TA, USA) in the temperature range of 300e1000  C in an air flux at the heating rate of 20  C$min1, and about 20 mg of each sample with particle size < 75 mm was used for all measurements. The characteristic temperatures were determined with precision ±1  C using the microprocessor of thermal analyzer. The density (r) of each sample (g$cm3) was determined by Archimedes Principle at room temperature by using distilled water as the liquid medium. 2.3. Vibrational spectra The structure of the samples was characterized by a Spectrum One Infrared Spectrometer (PerkinElmer Instrument Co. USA) and a Renishaw InVia Raman spectrophotometer (UK). Fourier transform infrared (FTIR) spectra were measured in the wavenumber range of 400e2000 cm1 by standard KBr pellet technique at room temperature. The pellets for testing were prepared by mixing and grinding about 2 mg of powder sample with about 200 mg of anhydrous KBr, then compressing the mixtures. Raman spectra were measured in the wavenumber range of 400e1500 cm1 with Raman shifts at 1e2 cm1 spectral resolution at room temperature. The spectra were recorded in back-scattering geometry under excitation with Ar ion laser radiation (514.5 nm) at a power of 5 mW. The spectral slit width is 1.5 cm1 and the total integration time is 100 s. 3. Results and discussion 3.1. Phase and density analysis XRD patterns of the samples are shown in Fig. 1. From Fig. 1, it is shown that the as-cast and annealed IBP glasses containing no more than 8 mol% Ce þ Gd are fully amorphous. For the sample

Table 1 Chemical compositions of the as-prepared samples. Samples

x x x x x x

¼ ¼ ¼ ¼ ¼ ¼

0 mol% 2 mol% 4 mol% 6 mol% 8 mol% 10 mol%

Molar compositions (mol%) P2O5

Fe2O3

B2O3

Gd

Ce

53.99 52.92 51.84 50.76 49.68 48.60

36.00 35.28 34.56 33.84 33.12 32.40

10.10 9.86 9.69 9.48 9.29 9.12

0.00 1.01 2.00 3.00 3.99 5.00

0.00 1.00 2.01 2.99 4.01 5.01

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Fig. 1. XRD patterns of the samples.

containing 10 mol% Ce þ Gd, several weak peaks representing to monazite crystalline phase (PDF No. 32-0199, space group: Monoclinic, P21/n(14)) are observed in the XRD pattern. This indicates that the solubility limit of Ce þ Gd in the IBP glass is about 8 mol%. When the content of Ce þ Gd is up to 10 mol%, monazite crystalline phase forms in the structure of the IBP glass.

As can be seen from Fig. 2, the density of the samples increases, from 2.979 g cm3 for the sample with x ¼ 0 mol% to 3.28 g cm3 for the sample with x ¼ 10 mol%, with the increase of molar percentage of Ce þ Gd. This phenomenon is mainly ascribed to the higher atomic weight of Ce and Gd compared with Fe, P and B [18,21]. Moreover, the doped Ce þ Gd are mainly located at the interstitial

Fig. 2. Density of the samples containing different amounts of Ce þ Gd.

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173

Fig. 3. FTIR spectra of the Ce þ Gd containing IBP compounds in the wavenumber range of 400e2000 cm1.

sites among glass network groups [21,24], which strengthens the structure of the samples and also results in the increase of density. When monazite crystalline phase appears for the x ¼ 10 mol% sample, the density sharply increases from 3.19 g cm3 for the sample with x ¼ 8 mol% to 3.28 g cm3. This is because the monazite crystal formed shows much more compact structure compared with the base IBP glass. 3.2. FTIR spectra To better understand the structure modifications induced by Ce þ Gd for the studied samples, FTIR and Raman spectra are measured. Fig. 3 shows FTIR spectra of the prepared IBP glass containing different amounts of Ce þ Gd. The spectra are characterized by two peaks around 547 cm1 and 640 cm1, two quite weak peaks around ~760 cm1 and ~1470 cm1, a weak peak at ~850 cm1, a broad overlapping absorption band in the range of 890e1370 cm1 and an obvious peak at ~1635 cm1. According to literature data and reasonable analysis, the FTIR spectra are interpreted as follows, as listed in Table 2. The absorption band around 547 cm1 is assigned

to the bending vibration modes of O-P-O bonds in (P2O7)4-, Q1, groups [31,32]. The peak at 640 cm1 principally corresponds to the stretching vibrations of Fe(Ce, Gd)-O-P bonds [18,21,33]. The absorption band at ~760 cm1 is induced by the bending vibrations of B-O-B bonds in [BO4] units [34]. The weak peak at ~850 cm1 is characteristic of the symmetric modes of P-O-P bonds in Q1 groups [35,36]. Moreover, the P-O-B linkages in the structure of borophosphate glass are also suggested by a broad absorption band at about 800e890 cm1 [37]. When glass forming oxide P2O5 exists together with B2O3 in compositions, phosphate and borate groups will have a broad overlapping absorption band located in the range of 890e1370 cm1 in IR spectra [6,21,31,38e40]. The deconvolution of the broad peak is carried out. The deconvolution in gausslorentzian bands of the spectra for x ¼ 2 mol%, 6 mol% and 10 mol % samples is shown in Fig. 4. The deconvolution data and the specific assignments of these bands are listed in Table 3. The weak absorption band at ~1470 cm1 has been attributed to the asymmetric stretching of (PO2)þ in [PO3], Q2, units [41]. The absorption band at ~1635 cm1 is attributed to the H-O bonds in the water introduced by damp during the test process [32,42].

Table 2 Assignments of the absorption bands in the FTIR spectra for the samples containing different amounts of Ce þ Gd. No.

Wave numbers 1

Assignments of bands

References 1

1 2 3 4

547 cm 640 cm1 ~760 cm1 800-890 cm1

Bending modes of O-P-O in [P2O7], Q , groups Stretching vibrations of Fe(Ce, Gd)-O-P bonds Bending vibrations of B-O-B bonds in [BO4] groups Symmetric modes of P-O-P bonds in Q1 groups P-O-B linkages

5

890-1370 cm1

6 7

~1470 cm1 ~1635 cm1

Stretching vibrations of B-O bond in [BO4] groups Stretching vibrations of [PO4], Q0, groups Asymmetric stretching of (PO3)- in Q1 groups Asymmetric stretching of (PO2)þ in [PO3], Q2, groups H-O bonds

[31,32] [19,21,33] [34] [35,36] [37] [6,21,31,38e40]

[41] [32,42]

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conversion of other phosphate groups, usually metaphosphate Q2 groups, into Q1 groups. Moreover, FeeOeP bonds form to replace part of PeOeP bonds in iron phosphate glass [21,25]. When CeO2 and Gd2O3 are incorporated into the glasses, the formation of the Fe(Ce, Gd)eOeP bonds also attributes to the increasing intensity of the peak around 640 cm1. Especially, the formed monazite crystalline phase which contains Ce(Gd)eOeP bonds in its structure makes the peak significantly increase in intensity [43,44]. In Table 3, the dimension of a relative area belonging to one structural group is proportional to the quantity of the corresponding group in samples [18]. Thus, another interesting feature in the FTIR spectra is that the broad overlapping absorption band in the range of 890e1370 cm1 increases in intensity with the increasing content of Ce þ Gd, as also can be seen in Fig. 3. With the increase of Ce þ Gd, the studied glass will be depolymerized because of the incorporation of the glass modifying oxides CeO2 and Gd2O3, which leads to a conversion of other phosphate groups into Q0 and Q1 groups, thus the two phosphate groups increase in quantity (Table 3). Monazite crystalline phase which contains orthophosphate groups forms in the sample x ¼ 10 mol%, the quantity of orthophosphate Q0 groups highly increases. In addition, the incorporation of CeO2 and Gd2O3 increases the molar ratio of O/P in the glass phase, which also leads to the conversion of other phosphate groups into Q0 and Q1 groups. We attribute the increasing intensity of the broad overlapping absorption band in the range of 890 cm1 to 1370 cm1 to these reasons. It is worth noting that the FTIR spectra suggest that the fundamental structural changes in the studied glasses' networks which consist predominantly of Q0 groups and Q1 groups, along with [BO4] units and a small amount of Q2 groups, do not produce when CeO2 and Gd2O3 are incorporated. 3.3. Raman spectra

Fig. 4. Deconvolution of fragments of FTIR spectra of the samples containing (a) 2 mol %, (b) 6 mol% and (c) 10 mol% Ce þ Gd.

Some changes are observed in the FTIR spectra with the increasing content of Ce þ Gd. The first noticeable change occurs at the peaks at 547 and 640 cm1. The intensity of the two peaks increases with Ce þ Gd. The increase in intensity of the peak at 547 cm1 indicates that the CeO2 and Gd2O3 loading induces the

Fig. 5 shows Raman spectra of the samples in the wavenumber range of 400e1500 cm1. The main spectral bands locate at ~441 cm1, 650 cm1 and 756 cm1 in the low frequency range, and 800-1400 cm1 in the high frequency range. According to previous literature [45e47], the Raman scattering efficiency of borate groups is much lower than that of phosphate groups in borophosphate glasses. And the molar ratio of B/P is very low in our study. Therefore, the obtained Raman spectra show no clear evidence of vibrations related to borate groups. We made the following band assignments for our Raman spectra (Table 4): The ~ 441 cm1 band is induced by OePeO bending mode of Q0 groups [48,49]. The 650 cm1 band is characteristic of the overlapping vibrations involving iron oxygen polyhedral and Q1 groups [6,21,50]. The weak band at 756 cm1 is assigned to the symmetric stretching vibration of P-O-P bridging oxygen bonds in Q1 groups [42]. In the high frequency range, the broad band centered at 800-1400 cm1 includes a number of overlapping groups. Generally, the origin of the shape of this broad band relates to the distribution of Q0, Q1 and Q2 phosphate groups [6,21]. In order to precise the distribution of Q0, Q1 and Q2 phosphate groups, the gauss-lorentzian function is used to deconvoluate the broad band. The deconvolution in gauss-lorentzian bands of the spectra for x ¼ 2 mol%, 6 mol% and 10 mol% samples are shown in Fig. 6 and the deconvolution data of all samples are listed in Table 5. In Table 5, the dimension of a relative area belonging to one group is also proportional to the quantity of the corresponding group in samples [21]. A weak band at ~962 cm1 is assigned to the stretching vibration of Q0 ortho-groups [49,51]. The predominant bands at 1038-1191 cm1 are related to the vibration of Q1 pyro-groups [52,53]. The band at ~1223 cm1 is assigned to antisymmetric stretching vibration of P-O-P non-

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Table 3 Band centers C (±1 cm1), the relative area A (errors, <1.5%) and bands assignments for FTIR spectra in wavenumbers range of 700 cm1 to 1800 cm1. x ¼ 0 mol%

x ¼ 2 mol%

x ¼ 4 mol%

x ¼ 6 mol%

x ¼ 8 mol%

x ¼ 10 mol%

C

A

C

A

C

A

C

A

C

A

C

A

752 843

1.11 8.57

755 844

1.38 10.81

753 843

1.36 9.95

758 847

1.12 9.39

760 853

0.78 10.26

758 844

1.40 10.66

986

23.74

999

26.38

994

27.73

990

28.99

992

29.71

993

32.63

1195 1455 1630

38.92 26.08 1.58

1210 1461 1628

45.66 14.36 2.42

1196 1465 1630

46.60 10.35 4.23

1199 1466 1632

46.83 10.02 3.54

1188 1456 1633

47.44 9.65 2.16

1211 1458 1631

41.83 11.28 2.20

units

B-O-B in [BO4] Sym. P-O-P in Q1 P-O-B linkages B-O in [BO4] Stretch Q0 (PO3)- in Q1 (PO2)þ in Q2 H-O bonds

Fig. 5. Raman spectra of the Ce þ Gd containing IBP compounds in the wavenumber range of 400e1500 cm1.

bridging oxygen in Q2 groups [49,51]. Location of stretching modes in Q1 groups depends on the number of bridging/nonbridging oxygen in the structure of glass network. The most predominant band around 1040 cm1 is induced by vibrations of the end of Q1 groups, non-bridging oxygen symmetry stretching vibration of P-O-P bonds [52,53]. The strong bands in the range of 1101e1190 cm1 may also be induced by antisymmetric stretching vibration of P-O-P non-bridging oxygen [53], symmetric stretching of (PO2)þ in Q2 groups [21]. In general, the incorporation of CeO2 and Gd2O3 in the glass increases the

fraction of Q1 groups and decreases the fraction of middle (PO3)groups, i.e. an increase of CeO2 and Gd2O3 offers depolymerizing effect on the glass network, which is consistent with FTIR data. That is because CeO2 and Gd2O3 is a glass modifying oxide, firstly, the CeO2 and Gd2O3 loading leads to the conversion of Q2 groups to Q1 groups, as can be deduced by FTIR analysis. Thus, the peak at ~1223 cm1 which has been considered as Q2 units decreases in intensity and finally disappears for the sample containing up to 4 mol% of Ce þ Gd. Furthermore, it is worth noting that the intensity of the peak at ~962 cm1 which has been assigned to

Table 4 Assignments of Raman spectra for the samples containing different amounts of Ce þ Gd. Wave numbers

Assignments of bands

References

~441 cm1 650 cm1 756 cm1 800-1400 cm1

OePeO bending mode of (PO4)3e, Q0, groups Vibrations involving iron oxygen polyhedral and Q1 groups Symmetric stretching vibration of P-O-P bonds in Q1 groups Relates to Q2, Q1 and Q0 phosphate groups

[48,49] [6,21,50] [42] [6,21]

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F. Wang et al. / Materials Chemistry and Physics 201 (2017) 170e179

3.4. DSC analysis In order to investigate the influence of Ce þ Gd on thermal stability of the IBP glasses, the DSC curves of the samples are measured. Fig. 7 shows the DSC curves of the IBP glasses containing different amounts of Ce þ Gd. In general, the DSC curves have one endothermic peak and one exothermic peak. The endothermic peak represents heat absorption induced by structure relaxation that the glass occurs. Such a peak represents the glass transition phenomenon and the onset temperature of the endothermic peak corresponds to the glass transition temperature (Tg) [19]. The exothermic peak represents that crystallization in the structure of glasses occurs at the peak temperature (Tp), and the corresponding onset temperature at the exothermic peak is the crystallization onset temperature (Tr) [17,21]. It is observed that Tg varies from approximately 498  C for the pristine IBP glass to 530  C for the sample containing 8 mol% Ce þ Gd (Fig. 8 and Table 6). Tg is characteristic of structural relaxation taking place in the glass network and hence strongly depends on the structural units. The increase of Tg can be attributed to the significantly larger ion radius of Ce and Gd than that of Fe2þ/Fe3þ [18,21]. The existence of larger Ce and Gd ions in glass structure leads to the more difficulty of glassy structure modification. On the other way, the replacement of PeOeP bonds by MeOeP (M ¼ Fe, Ce and Gd) bonds (Table 2) also is attributed to the increase of Tg [18]. The DSC curves also show that the incorporation of Ce þ Gd decreases Tr and Tp of the IBP glass. That is because Ce and Gd as glass modifying oxides make the IBP glass crystallize more easily due to the high coordination number, free oxygen supply and depolymerization function of Ce and Gd in the glass structure. However, with the increase of Ce þ Gd, Tr and Tp slightly increase for the formation of MeOeP (M ¼ Fe, Ce and Gd) bonds. When the content of Ce þ Gd reaches the limit solubility of 8 mol%, Tr and Tp begin to decrease because of the possible overload of Ce and Gd at the interstitial sites among glass network groups [21]. For the sample x ¼ 10 mol%, Tr and Tp decrease more sharply because monazite crystalline phase forms (Fig. 1). It is mentioned here that the absence of obvious endothermic peak for the sample containing 10 mol% Ce þ Gd suggests its inertness to respond to the structural modification process in the measurement condition. Evaluation of thermal stability of glass materials has been widely discussed through Hruby's method [17,21,54]. According to the Hruby's theory [54], the greater the differences between Tg and Tr, the more stable the glass is. Therefore, the increase of (Tr-Tg) value indicates the increase of thermal stability of the glass materials. As shown in Fig. 8, the values of (Tr-Tg) for the samples are dependent on the content of Ce þ Gd in the measurement condition. The results do show that doping by Ce þ Gd decreases the thermal stability of the IBP glass. However, with the increase of Ce þ Gd, the thermal stability of the Ce þ Gd doped IBP glasses becomes better until the content of Ce þ Gd nearly reaches the limit solubility. 4. Conclusions Fig. 6. Deconvolution of fragments of Raman spectra of the samples containing (a) 2 mol%, (b) 6 mol% and (c) 10 mol% Ce þ Gd.

Q0 groups significantly increases when the content of Ce þ Gd is 10 mol% because of the formation of monazite crystalline phase. All in all, as follows from FTIR and Raman data, at low Ce and Gd contents, the doped ions are positioned in the voids of boron(iron)-phosphorus-oxygen networks as network-modifiers, while high amounts of Ce and Gd depolymerize phosphate-based networks and form polyanions.

The influence of Ce þ Gd incorporation on the structural features and thermal property of the IBP glass has been investigated. The IBP glass is capable to incorporate Ce þ Gd introduced as CeO2 and Gd2O3 in amounts of about 8 mol% without devitrification. When the content of Ce þ Gd is up to 10 mol%, monazite crystalline phase forms. The FTIR and Raman spectral studies reveal that the structural networks of the Ce þ Gd containing IBP glasses consist predominantly of orthophosphate, Q0, groups and pyrophosphate, Q1, groups, along with [BO4] units and a small amount of metaphosphate, Q2, groups. The addition of Ce þ Gd increases the fraction of Q0 and Q1 groups and decreases the fraction of Q2

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177

Table 5 Band centers C (±1 cm1), the relative area A (errors, <1.5%) and bands assignments for Raman spectra in wavenumbers range of 800 cm1 to 1400 cm1. x ¼ 0 mol%

x ¼ 2 mol%

x ¼ 4 mol%

x ¼ 6 mol%

x ¼ 8 mol%

x ¼ 10 mol%

C

A

C

A

C

A

C

A

C

A

C

A

905 1040 1121

5.63 66.45 18.97

913 1038 1105

1.95 49.82 47.46

936 1039 1101

1.96 44.89 53.15

938 1041 1103

1.47 42.55 55.98

940.8 1038 1097

1.49 39.86 58.65

912 1043 1191

8.64 73.50 17.86

1216

8.95

1223

0.76

e

e

e

e

e

e

e

e

Fig. 7. DSC curves of the samples containing different amounts of Ce þ Gd. the curves are offset for clarity only.

Phosphate units

Stretch, Q0 groups Sym stretch, Q1 vs(PO3) in Q1 Sym stretch Q2 Asym stretch, Q2

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F. Wang et al. / Materials Chemistry and Physics 201 (2017) 170e179

Fig. 8. Variation tendencies of Tg and Tr-Tg with the content of Ce þ Gd.

Table 6 DSC parameters of the prepared samples. Samples

x ¼ 0 mol%

x ¼ 2 mol%

x ¼ 4 mol%

x ¼ 6 mol%

x ¼ 8 mol%

x ¼ 10 mol%

Tg ± 1 ( C) Tr ± 1 ( C) Tp Tr-Tg ± 1 ( C)

498 835 867 337

503 772 804 269

511 784 815 273

527 811 863 284

530 795 817 265

e 774 796 e

groups. Moreover, the incorporation of Ce þ Gd leads to increase of the density and Tg, but decreases the thermal stability of the IBP glass. However, the thermal stability of the Ce þ Gd containing IBP glasses increases with the increase of Ce þ Gd until their contents nearly reach the limit solubility of 8 mol%. Acknowledgments The funds supported by the National Natural Science Foundation of China (51702268), the Southwest University of Science and Technology (13zx7130) and the 973 program (2012CB722707) are greatly acknowledged and appreciated. References [1] P.S. Anantha, K. Hariharan, Mater. Chem. Phys. 89 (2005) 428e437. [2] R.S. Muralidhara, C.R. Kesavulu, J.L. Rao, R.V. Anavekar, R.P.S. Chakradhar, J. Phys. Chem. Solids 71 (2010) 1651e1655. [3] F.H. ElBatal, S. Ibrahim, A.M. Abdelghany, J. Mol. Struct. 1030 (2012) 107e112. [4] A. Saitoh, G. Tricot, P. Rajbhandari, S. Anan, H. Takebe, Mater. Chem. Phys. 150 (2015) 648e656. [5] M. Li, A. Verena-Mudring, Cryst. Growth Des. 16 (2016) 2441e2458. [6] F. Wang, Q.L. Liao, Y.Y. Dai, H.Z. Zhu, Mater. Chem. Phys. 166 (2015) 215e222. [7] P.S. Anantha, K. Hariharan, Mater. Chem. Phys. 89 (2005) 428e437. [8] A. Saranti, I. Koutselas, M.A. Karakassides, J. Non-Cryst. Solids 352 (2006) 390e398. [9] T. Satyanarayana, I.V. Kityk, Y. Gandhi, V. Ravikumar, W. Kuznik, M. Piasecki, M.A. Valente, N. Veeraiah, J. Alloy. Compd. 500 (2010) 9e15.

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