Journal of Molecular Structure 1076 (2014) 592–599

Contents lists available at ScienceDirect

Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Influence of rare earth addition on the thermal and structural stabilityof CaOAFe2O3AP2O5 glasses

http://dx.doi.org/10.1016/j.molstruc.2014.08.0320022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Analytical and Testing Center, Southwest Universityof Science and Technology, Mianyang 621010, PR China. Tel.: +86 0816 6089507.

E-mail address: XFLiang@swust.edu.cn (X. Liang).1 Co-first authors.

Haijian Li a,b, Xiaofeng Liang a,b,⇑,1, Cuiling Wang a,b, Huijun Yu a,b, Zhen Li b, Shiyuan Yang b

a Analytical and Testing Center, Southwest University of Science and Technology, Mianyang 621010, PR Chinab State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, PR China

h i g h l i g h t s

� The thermal and structural stability of RE2O3ACaOAFe2O3AP2O5 were studied.� Thermal stability of glass increased with the increase of cationic field strength.� Release rate of rare earth ions decreased with the increase of its field strength.� Metaphosphate chains by the cross-links of calcium ions were easier to hydrolyze.� The REAOAP bonds have more chemical resistance by producing non-bridging oxygen.

a r t i c l e i n f o

Article history:Received 1 July 2014Received in revised form 29 July 2014Accepted 18 August 2014Available online 26 August 2014

Keywords:Phosphate glassRare earthThermal stabilityFTIR spectraRaman spectra

a b s t r a c t

The thermal and structural stability of calcium iron phosphate glasses doped with rare earth oxides hasbeen studied by investigating differential scanning calorimetry, the concentration of modified ions inleachate, Fourier transform infrared spectra, and Raman spectra. The results showed that thermal stabil-ity of the rare earth phosphate glasses increased and the release rate of rare earth ions in leachatedecreased with the increase of cationic field strength. It related to the known structural units and char-acteristic bands of these glasses detected by infrared and Raman spectra. Metaphosphate chains by thecross-links of calcium ions and PAOAP linkages were easier to hydrolyze, which converted to orthophos-phate units after corrosion. The formation of more chemically resistant REAOAP bonds with the additionof rare earth leaded to an increased quantity of nonbridging oxygen, which caused consequent variationsin the chemical durability of rare earth glasses.

� 2014 Elsevier B.V. All rights reserved.

Introduction

In radioactive nuclear glasses, the rare earths (RE) are present aswaste forms in the glass matrix. For example, gadolinium with lownon-radiative decay rate and high thermal neutron capture cross-section may be added to the final actinide-containing glasses tominimize the likelihood of criticality during the storage period[1,2]. On the other hand, rare earth elements are also present as sur-rogates for the heaviest transuranic elements occurring in HighLevel Waste (HLW) such as Cm and Am [3,4]. They have cation radiir similar to the one of rare earth ions (for instance in sixfold coordi-nation: r(Am3+) = 1.01 Å, r(Cm3+) = 0.98 Å and r(Nd3+) = 0.995 Å

[5]). Wang et al. [6,7] have reported the corrosion behavior of sodalime silicate glasses containing about 1.0 mol% rare earths (Y2O3,La2O3, CeO2, Nd2O3 and Gd2O3), and they pointed out that the forma-tion of a hydrated layer depleted the dissolution rate of rare earthions.

Phosphate glasses have been investigated for immobilization inhigh-level waste for over 30 years [8]. The glass has lower meltingtemperatures and higher waste loading capacities than borosilicateglass. However, phosphate glasses have relatively low chemicaldurability. Several studies have shown that the chemical durabilityof phosphate glasses can be improved by the addition of variousoxides such as calcium oxide and, especially, ferric oxide [8–10].It has been suggested that Fe3+ cations can enter in the glass net-work with four-fold coordination and form stable PAOAFe cova-lent bonds and divalent cations like Ca2+ can form PAOACacovalent bonds and serve as ionic crosslinks between the nonbrid-ging oxygens of two different phosphate chains, and accordinglyincrease chemical durability [10–12].

H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599 593

Corrosion mechanism of glass has received attention for a longtime [13,14]. It is known that an ionic exchange takes placebetween mobile ions such as alkali earth ions and H+, and thewater molecules diffuse into the glass, reacting with PAOAP andPAOAM (where M = metal cation), forming PAOH groups [10,15].Hamilton and Pantano [16] found that the aqueous corrosionbehavior of glasses was related to the concentration of nonbridgingoxygen (NBO) sites in the glass network structure. Vibrationalspectroscopy has been employed to investigate the structure ofglasses and specifically identification of the main structural units,which are connected through nonbridging oxygen (NBO) andbridging oxygen (BO) [17]. Therefore, vibrational spectroscopy isa valuable tool for studying the changes of glass structure beforeand after aqueous corrosion.

Palavit et al. [18] examined the chemical reactions betweenP2O5AZnOAH2O ultraphosphate glasses and water by 31P nuclearmagnetic resonance, and the microstructure of the devitrified glassconsisted of crystalline Zn2P4O12. Banerjee et al. [19] studied thestructural aspects of caesium borosilicate glasses with varyingamounts of CaO before and after leaching by using infrared spec-troscopy. The reduction of the peak intensity at SiAOH and BAOHbonds showed that the chemical durability of the glasses wasimproved significantly as CaO was increased at the expense ofB2O3 content in the glass. Molières et al. [20] showed that the roleof lanthanum in the alteration kinetics of borosilicate glass at dif-ferent degrees of leaching progress was investigated by Ramanspectroscopy. After leaching, the Raman spectrum of the glassexhibited the presence of highly depolymerized entities.

The chemical durability of phosphate glasses was investigatedby the dissolution rate of ions in previous studies [10,15]. However,little systematic work in such glasses with rare earth addition hasbeen done to identify the structural units before and after corro-sion, to follow the structural changes and to correlate thesechanges with the corrosion mechanisms of glass. Therefore, theaim of this paper is to focus on the effect of rare earth oxides onthe structural stability of CaOAFe2O3AP2O5 glasses before and afteraqueous corrosion by FTIR spectra and Raman spectra, and thethermal stability of the glass was investigated by differential scan-ning calorimetry (DSC).

Experiment

Preparation of glass samples

The glass system of xRE2O3–(100 � x)(12CaOA20Fe2O3A68P2O5)(x = 10 mol%) with RE = Y, La, Nd, Sm and Gd were prepared using astandard melt-quench technique. After thorough mixing the pow-ders were introduced in corundum crucibles, in order to preventthe excess boiling and consequent spillage, water and ammonia inammonium phosphate monobasic were removed initially by pre-heating it at 220 �C for about 2 h and then the electric furnace wasraised to 1250 �C (heating rate was 10 �C min�1), and samples weremelted at 1250 �C for 3 h. The melts were then poured into pre-heated steel molds, and moved quickly to an annealing furnace,annealed at 475 �C for 2 h and cooled down to room temperaturemore than 12 h. Samples for property measurements were groundto give fine powder.

Chemical durability tests

The chemical durability of glasses was determined according toProduct Consistency Test B (PCT–B) in ASTM C1285-02, using 3 g ofglass powder (100 to +200 mesh) in 30 ml deionized water. Testswere carried out in duplicate at 90 �C for 7 days. The rare earth ele-ments in the leachate were measured by inductively coupled

plasma mass spectrometry (ICP-MS) using the Agilent 7700x (Agi-lent, U.S.A), and the calcium and iron elements were measured byinductively coupled plasma (ICP) emission spectroscopy using theiCAP 6500 (Thermo Fisher, U.S.A). Solution pH was measured witha calibrated KEDIDA CT-6023 pH meter.

The normalized elemental mass release rate, ri, was calculatedfrom the equation:

ri ¼Ci

f iðA=VÞt

where ri is the normalized elemental mass release (g/m2/min) ofelement i, Ci the concentration of element i in solution (lg/ml org/m3), fi the mass fraction of element i in the glass (unitless) and tthe time duration of test in minutes (min). A/V is the ratio of thesample surface area to volume of leachate (m�1), a value of1800 m�1 was used. All tests were made in duplicate and the errorin the average reported herein was estimated at ±15%.

Structural investigation

X-ray diffraction (XRD) analysis was performed on samplesemploying a X-ray diffractometer (PANalytical X’Pert PRO, TheNetherlands). The 2h scans were made between 5� and 80� withstep width of 0.03� and utilized Cu Ka radiation (k = 1.5405 Å).Preparation of the samples used a simple top pack loading methodfor an acquired smooth surface [21].

The density, D, of each glass was measured at room temperatureusing the Archimedes method with water as an immersing liquid.The sample weights varied between 3 and 4 g, and the measureddensities were reproducible within 0.01 g cm�3. The molar volume(Vm) was calculated using the relation Vm =

P(xiMi)/D, where xi is

the molar fraction and Mi is the total molecular weight of thecomponent.

The glass transition temperature (Tg) and first crystallizationtemperature (Tr) were measured on DSC by utilizing a SDT Q600instrument (TA, USA) in a flowing air atmosphere at a heating rateof 20 �C min�1. The temperature scanned over a range from roomtemperature to 800 �C and the estimated error in Tg and Tr were±2 �C.

The infrared spectra of the samples were measured from 400 to2000 cm�1 using a Spectrum One FTIR spectrometer (Perkin Elmer,USA) and the KBr standard pellet method. Glass pellets were pre-pared by mixing about 2 mg powder with 200 mg dried KBr pow-der and compressing the resulting mixture in an evacuated die. Theaccuracy of this technique is estimated to be ±4 cm�1.

Because the majority of the bands in the infrared spectra arebroad and asymmetric, presenting also some shoulders, a deconvo-lution of the experimental spectra was necessary. This was madewith ORIGIN 7.5 program (Originlab Corp.) using a Gaussian typefunction and allowed us a better identification of all the bandswhich appear in these spectra and their assignments. The propor-tion of particular structures corresponding to different vibrationmodes, was calculated from the areas of the fitted Gaussian bandsdivided by the total area of all bands. The two parameters of eachband (peak frequency and relative areas) were allowed to be vari-able during the iterations.

Raman spectra at 400–1600 cm�1 were collected from glasspowders using the InVia Raman Microscope (Renishaw, U.K.) atroom temperature. The Raman spectra were measured by excita-tion light of 514.5 nm light from an argon ion laser. The spectralresolution was about 1–2 cm�1 and the wavenumber accuracywas 0.2 cm�1. Six multiple measurements per sample were doneto check for the potential micron-range heterogeneity and theeffects of sample orientation; both have not been found, as itwould be expected for a nearly homogeneous glass sample.

Table 2The results of leachate analyses performed in deionized water at 90 �C for 7 days.

Glass composition Initial pH Final pH Concentration of ions inleachates (lg mL�1)

[RE] [Ca] [Fe]

RE0 5.60 1.85 0 15.368 10.946Y10 5.60 3.16 0.056 13.878 11.275La10 5.60 2.03 0.715 8.120 6.240Nd10 5.60 2.09 0.657 17.393 12.178Sm10 5.60 2.12 0.551 14.748 13.802Gd10 5.60 2.34 0.524 14.517 12.526

594 H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599

Results and discussion

XRD, DSC and density measurements

The crystallization tendency of phosphate glasses adding rareearth elements is related to the high amount of rare earth ions thatact as nucleating agent in the calcium iron phosphate glass. In ourprevious paper, the investigation on structural aspects of ironphosphate glasses containing neodymium oxide prepared with tra-ditional melt-quenching methods was carried out by XRD, infraredand Raman spectra [21]. It showed that excess neodymium oxidewill induce the crystallization of the phosphate glass, which wasdisadvantageous for the vitrification of high-level radioactivewastes and was often assumed to reduce the chemical durability[22]. X-ray diffraction analysis indicated that base glass forundoped rare earths (RE0) and doped 10 mol% rare earth glasseswere amorphous before and after corrosion. Hence, these glassescould contain abundant the surrogates of transuranic elementsand remained amorphous after corrosion.

Table 1 showed that the density increase from 2.82 to 3.33 g/cm3 as the atomic number of the rare earth elements increase. Thisincrease is primarily due to the increased atomic weights of therare earth cation. The molar volume decrease slightly as the atomicweight of the rare earth elements increase. It is mainly attributedto that the higher molecular weight of rare earth decrease the totalnumber of oxygen atoms [23]. This decrease in molar volume indi-cates that the phosphate glass network becomes more compact asthe atomic weight of the rare earth elements increase.

Cationic field strength (CFS) of the rare earth elements isdefined as CFS = Z/r2, where Z is the valence of the correspondingelements and r is its ionic radius. Ionic radius at the given valencestate depend upon the coordination number, which was reportedby Shannon [5]. Table 1 shows the CFS values of the rare earth ele-ments, which are inversely proportional to the ionic radii square ofthe elements. It causes consequent variations in the various prop-erties, such as greater hardness, greater thermal characterizationand elastic modulus, especially, higher chemical durability [24,25].

The thermal characteristics of all samples, which are the glasstransition (Tg) and the first crystallization peak (Tr), are presentedin Table 1. The Tg values are improved by rare earth addition, whichis mainly due to the increase of cross-linking density and bondingstrength of the structure owing to the incorporation of the mixedoxides. Several studies have shown that the changes of Tg valuesare consistent with the density of the studied glasses [26,27]. How-ever, Table 1 doesn’t show that the increase of Tg is attributed tothe changes of density.

On the other way, the difference between the glass transitiontemperature (Tg) and the onset crystallization temperature (Tr),DT = Tr � Tg, has been frequently used as a rough estimate of glassformation ability or glass thermal stability [26]. The base glassdemonstrates good glass-forming tendency or thermal stabilitywith the temperature difference DT � 127 �C, which is larger thanthe rare earth glass. The rare earth glass exhibits lower exothermicbehavior than the base glass, which is consistent with lower ther-

Table 1The density, mole volume and DSC parameters of the studied glasses and cationicfield strength (CFS) of the corresponding rare-earth element.

Physical parameters RE0 Y10 La10 Nd10 Sm10 Gd10

CFS (�2) 0 3.703 2.817 3.105 3.269 3.410D (g/cm3) 2.82 3.01 3.22 3.26 3.29 3.33Vm (cm3/mol) 47.94 47.92 47.90 47.64 47.58 47.42Tg (�C) 570 574 576 584 578 574Tr (�C) 697 669 663 681 666 666Tr � Tg (�C) 127 95 87 97 88 92

mal stability. Table 1 shows that the DT values are seen to increasewith the increase of CFS (except Nd-doped glass). Lofaj et al. [25]showed that Nd-doped silicon glass had higher Tg and thermal sta-bility than other rare earth glasses. Therefore, it is then deducedthat larger cation filed strength increased structural rigidity, andso increasing thermal stability.

Leachate analyses

The initial pH values of the deionized water before corrosion was5.60. Table 2 shows that the final pH values of all samples decreases.This is attributed to the dissolution of phosphorus from the glass,which formed phosphoric acid [28]. In addition, the decrease ofpH values for the base glass (from 5.60 to 1.85) is larger than theone for the rare earth glass. Yu et al. [27] thought that the changeof pH was related to the chemical durability of the glass network.Therefore, rare earth elements addition can improve the chemicaldurability of glasses, and the reduction in pH values is consistentwith the concentration of rare earth ions in leachates.

The effect of CFS on the concentration of different rare earth ionsin leachate is shown in Fig. 1. The concentration of rare earth ions isseen to decrease with the increase of CFS. In other words, the smal-ler ionic radii of rare earth with the same coordination number, thelarger cationic field strength (CFS), and so the better the chemicalstability of the rare earth glass will be. Similar behavior wasobserved by Yin et al. [29]. The normalized elemental mass releaserate of the studied glasses in deionized water at 90 �C for 7 days isshown in Fig. 2. After a 7-day test period at 90 �C, the normalizedelemental mass release rate of lanthanum ions (rLa) with the highestconcentration La-doped glass was as low as�2.19 � 10�7 g/m2/minfor iron phosphate glass waste forms [30]. Moreover, the release

Fig. 1. The effect of cationic field strength (CFS) on the concentration of differentrare earth ions in leachate.

Fig. 2. The normalized elemental mass release rate of the studied glasses indeionized water at 90 �C for 7 days.

H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599 595

rate of rare earth ions in leachate decrease with the increase ofcationic field strength.

Small amounts, usually <20 lg mL�1 Ca and Fe were in leachatefrom the samples containing CaO and Fe2O3 (Table 2). Compared tothe base glass, the normalized release rate of calcium ions by rareearth addition decrease obviously in Fig. 2 (except Nd-dopedglass). Brow [31] found that the CaAOAP bond is weaker thanthe FeAOAP bond. Leachate analyses show that the normalizedrelease rate of calcium ions in leachate is higher than iron ions.In addition, divalent cations Ca2+ act as a glass modifier, whichcould serve as (PAO� Ca2+ �OAP) ionic cross-links, they are easierto hydrolyze [10,32]. The presence of rare earth (Y, La, Sm and Gd)reduces the release of Ca ions, but the introduction of Nd increasesthe release of Ca. It may be due to that Nd element in the phos-phate glass melts exists the form of Nd4+ ions. The decrease ofthe normalized release rate of Ca ions could be explained by thepresence of rare earth which probably increases the compactnessand connection of the glass structure.

Fig. 3. FTIR spectra of different rare earth phosphate glasses (a) before corrosionand (b) after corrosion.

Infrared spectroscopy studies

FTIR spectroscopy of different rare earth phosphate glassesbefore and after corrosion is given in Fig. 3(a) and (b). In the baseglass, the main features of FTIR spectra are eight bands at �1618,�1256, �1061, �920, �777, �698, �553 and �487 cm�1, respec-tively (Fig. 3(a)). The weak band at �1618 cm�1 reflected the bend-ing vibrations of PAOH bonds [33]. The new band at �1734 cm�1

in the Y-doped glass may be assigned to water-bending mode[33,34]. The band at �1256 cm�1 is assigned to the asymmetricstretching vibration of the P@O bonds, similar band at�1235 cm�1 was observed in Nd-doped glass [34,35]. The bandposition has an obvious red shift with the addition of rare earth,the new band at �1162 cm�1 is assigned to the terminal (PO2)�

groups of Q2 units [36]. It is then deduced that the rare earth ionsact as a glass modifier to the breaking of the P@O bonds (Q3) andmay be to form REAOAP bonds (Q1).

The strong absorption bands at�1061,�920 and�777 cm�1 cancorrespond to the symmetric stretching vibrations of (PO4)3� tetra-hedra (PO� ionic group) in Q0 units [35], symmetric stretchingmodes of PAOAP bonds [37] and asymmetric modes of the PAOAPlinkages in Q1 units [35,37]. The another weak absorption band at�698 cm�1 is assigned to the asymmetric vibration of PAOAP link-ages, similar bands was observed in Fe2O3APbOAP2O5 glasses byDoweidar et al. [37]. The weak shoulder peak at �553 cm�1 is

assigned to the bending mode of OAPAO in Q1 units, while thelow-frequency absorption band at �487 cm�1 may be assigned toharmonics of bending vibration of O@PAO linkages in Q3 units[35,37]. The two bands are merged into one band at�527 cm�1 with

Fig. 4. Deconvoluted FTIR spectra of the base glass after corrosion using a Gaussian-type function.

596 H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599

the addition of rare earth, which is assigned to the symmetricstretching vibrations of (PO4)3� in Q0 units [37]. It suggests that rareearth addition strengthens the crosslinking of the glass network bycreating more NBOs.

Compared to the FTIR spectra of phosphate glasses before corro-sion, Fig. 3(b) shows that the O@PAO linkages at �484 cm�1 andthe PAOAP linkages at �698 cm�1 in the base glass disappears ordecreases in intensity, the (PO4)3� of Q0 units at �503 cm�1

appears after corrosion. Bunker et al. [10] showed that the corro-sion process depended on chain hydrolysis, and hydroxyl ionsattacked the long phosphate chains, to released orthophosphateanions. Brow [31] studied alkaline corrosion resistant propertiesof calcium iron phosphate glasses by infrared spectroscopy. Itshowed that the samples formed (PO4)3� and (P2O7)4� surface spe-cies after corrosion.

In the rare earth glasses, the bands at�1162 cm�1 shifts to lowerfrequency after corrosion, which is due to that the Ca ions act as aglass modifier, which breaks up the PAOAP linkages creating NBOsby forming (PAO�Ca2+ �OAP) ionic cross-links [10,11,37]. It leads tothat the Q2 units are easier to hydrolyze [10]. The special bands at�1115 cm�1 in Sm-doped glass before corrosion may be is assignedto the (PO4)3� of Q0 units and (PO2)� of Q2 units, it shifts to the lowfrequency bands at �1095 cm�1 ((PO4)3� of Q0 units) after corro-sion. Compared to the base glass before and after corrosion, thespectrum shapes of rare earth glasses do not show a clear change.It is attributed to the replacement of easy hydrated PAOAP bondsby more chemically resistant REAOAP bonds with the addition ofrare earth, in agreement with the chemical durability analyses.

The quantitative analyses of FTIR spectra by using the deconvo-lution method can determine the relative concentration of struc-tural units and thus, to analyze the modified role of rare earth

Table 3Deconvolution parameters (the band centers and the relative area) and the bands assignm

Range frequency (cm�1) Assignments RE0

B (%)

462�475 O@PAO linkages (Q3) 3.57515�526 (PO4)3�

sym stretch (Q0) 5.81579�592 MAOAP bonds 2.84712�727 (PAOAP)sym stretch in rings 3.38769�782 (PAOAP)asym stretch in linkages 2.96904�920 (PAOAP)sym stretch (Q1) 15.001028�1041 (PO3)2�

sym stretch (Q1) 16.831139�1152 (PO2)�asym stretch (Q2) 12.331256�1263 (P@O)sym stretch 15.001368�1396 (P@O)sym stretch 12.701581�1632 PAOH bonds 9.56

oxides. The deconvoluted FTIR spectra of the base glass after corro-sion are shown in Fig. 4. The new band at �1375 cm�1 is assignedto the asymmetric stretching vibration of the P@O bonds [35]. Thenew bands at �1033 and �720 cm�1 can correspond to the sym-metric stretching vibration of (PO3)2� and the symmetric stretch-ing vibration of PAOAP rings in Q1 units [35–37]. While that at�584 cm�1 may be assigned to the symmetric stretching vibrationof MAOAP bonds [37].

Bingham and Hand [38] showed that rings or chains linkedPAOAP bonds were susceptible to hydrolyze in corrosion. Table 3shows that PAOAP linkages decrease slightly in the base glass andthe rare earth (La, Y) phosphate glasses after corrosion, and thePAOAP rings of the base glass were easier to hydrolyze than therare earth (La, Y) phosphate glasses. It may be due to that rareearth ions breaks up the PAOAP rings by forming the REAOAPbonds [31,37]. Too many polyphosphate chains are released intosolution, which leads to the solution pH to drop, this hydrolysiscontinues, the glass continues to dissolve and a lower pH of thesolution will accelerate the ion exchange process as well as thedirect hydrolysis of PAOAP rings [37,39]. Therefore, the lower pHof the base glass in leachate analyses will release more phosphoricacid into solution.

Hydrolysis and leaching usually act simultaneously in glass cor-rosion. Hydrolysis according to the following reaction: PAOAP + H2

O ? 2PAOH, it can rupture directly the PAOAP to destroy the mainnetwork structure of glass [10,15]. Leaching according to the follow-ing reaction: MAOAP + H2O ? PAOH + MAOH, the exchangebetween hydrogen and glass network modifiers weakens the struc-tural stability of phosphate glass [15,40].

The decline of MAOAP bonds in the base glass is larger than therare earth glasses after corrosion, and the PAOH bonds have rela-tively higher fraction. It is then deduced that chemical durabilityof phosphate glasses was improved with the addition of rare earth.Several studies have shown that RE3+ acts as network modifiers,producing more nonbridging oxygen as its cationic field strengthincreases, these NBOs are favorable sites for slowing down the cor-rosion mechanism [41,42]. Therefore, Y-doped phosphate glasscould produce more NBOs by the formation of the REAOAP bondsthan La-doped glass, which leads to the best corrosion resistancewith the Y-doped phosphate glass.

Raman spectroscopy

Raman spectroscopy is able to reveal vibrational mechanismsinside materials in different phases. One advantage of thisapproach is non-destructive and non-contact, which assures theintegrity of the material and permits long distance detection[43,44].

The Raman spectra, in the frequency region 400–1600 cm�1, ofdifferent rare earth phosphate glasses before corrosion (B) andafter corrosion (A) are shown in Fig. 5. The Raman spectra of the

ents of rare earth phosphate glasses before corrosion (B) and after corrosion (A).

La10 Y10

A (%) B (%) A (%) B (%) A (%)

2.46 2.74 2.81 2.21 2.666.42 5.60 7.96 6.53 6.252.54 2.30 2.13 2.64 2.562.83 1.81 2.23 2.40 2.562.07 3.34 3.01 2.72 2.56

13.10 17.18 15.94 14.79 13.9716.83 17.62 17.03 15.01 17.7612.87 18.78 15.99 13.23 14.1817.64 13.68 12.24 10.11 15.7210.67 12.11 14.39 19.50 11.3712.59 4.84 6.11 9.44 10.18

Fig. 5. Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) from the samples: RE0 (a), La10 (b), Nd10 (c), Sm10 (d), Gd10 (e)and Y10 (f).

H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599 597

base glass before corrosion, Fig. 5a, shows that the two prominentbands at 1065 and 1205 cm�1 can correspond to the symmetricstretching mode of (PO3)2� bonds in Q1 units and symmetricstretching mode of ‘strained’ (PO2)� (strained structural units, pos-sibly three- or four-membered rings) in Q2 units, respectively[34,36]. The ‘strained’ (PO2)� bonds after corrosion shifts slightlyto lower frequency and decrease obviously in intensity, in agree-

ment with infrared spectroscopy studies. Silva et al. [45] showedthat divalent cations Ca2+ acted as a network modifier, which couldserve as (PAO� Ca2+ �OAP) ionic cross-links in the metaphosphateglass. Therefore, the metaphosphate chains exhibited lower resis-tance to aqueous corrosion. The higher frequency band at�1295 cm�1 is assigned to the P@O symmetric stretching [34].The band at �1130 cm�1 is assigned to phosphorus-oxygen

Table 4Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A).

Glass composition PAOAP bonds (PO4)3� groups (PO3)2� groups (PO2)� group

B A B A B A B A

La10 716 726 944 960 1089 1077 1148 1140Nd10 708 716 933 924 1085 1075 1163 1151Sm10 736 715 943 944 1097 1076 1166 1152Gd10 713 716 934 954 1098 1079 1160 1164Y10 719 722 949 951 1090 1078 1167 1152

598 H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599

stretching modes in Q1 phosphate chain terminator units formedby the scission of phosphate chains [46].

It is observed that the bands at �524 and �706 cm�1 is dimin-ished after corrosion tests, which can correspond to (P2O7)4� groupsof Q1 units and PAOAP bonds of Q2 units, respectively [46,47]. Thenew bands at �600, �787, �884 cm�1 and a weak band at�961 cm�1 can correspond to the symmetric stretching mode ofPAOAP bonds in Q2 units [45], the symmetric stretching mode ofPAOAP bonds in Q1 units [26,45,47], the asymmetric stretchingmode of PAOAP bonds and nonbridging (PO4)3� oxygen ions in Q0

units [48]. The appearance of these bands, especially, isolated(PO4)3� groups, exhibits the depolymerization of the base glass net-work after corrosion.

In Fig. 5e, the most notable change with rare earth addition is theincrease of (PO3)2� bonds in frequency, and ‘strained’ (PO2)� shiftsmarkedly the lower-frequency band at �1160 cm�1, which isassigned to the symmetric stretching mode of (PO2)� groups in Q2

units [45]. The change of frequency is attributed to structural rear-rangements in the main phosphate network due to the replacing ofPAOAP and P@O (Q3) by REAOAP bonds (Q1) [34]. The red shift ofthe (PO3)2� bonds, resulting from the formation of the REAOAPbonds, is related to an increased quantity of NBOs.

Peak positions and assignments of Raman spectra of differentrare earth phosphate glasses before and after corrosion are shownin Table 4. The (PO4)3� groups shift mostly to higher frequenciesand increase in intensity after corrosion. It may be due to a pene-tration process of intact water molecular towards glass network,which leads to the breaking of the PAOAP bonds and the formationof (PO4)3� groups [39,40]. The (PO3)2� groups of Q1 units shift tolower frequencies, which may be attributed to that hydration ofthe glass network results in the breakage of PAOAP linkages andrings in Q1 units in aqueous solution. The (PO2)� groups shiftsmostly to lower frequencies and decrease in intensity after corro-sion. Moreover, the (PO2)� for La-doped phosphate glass after cor-rosion closes to phosphorus-oxygen bond for the RE0 glass inFig. 5a. It indicates that the ions exchange in corrosion will destroythe cross-link structure, which causes the dissociation of phos-phate chains.

Conclusions

The thermal and structural stability of CaOAFe2O3AP2O5 glassesdoped with rare earth oxides (Y2O3, La2O3, Nd2O3, Sm2O3, Gd2O3) indeionized water was investigated. X-ray diffraction analysis indi-cated that the base glass and the glasses doped 10 mol% rare earthremained amorphous before and after corrosion. DSC resultsshowed that thermal stability of the rare earth phosphate glassesincreased with the increase of CFS. Leachate analyses showed thatthe release rate of rare earth ions in leachate decreased with theincrease of its CFS and rare earth addition reduced the release ofCa ions. The decrease of pH for the base glass was larger than therare earth glasses by the formation of phosphoric acid. Both Ramanand IR spectra indicated that metaphosphate chains were easier tohydrolyze, which converted to orthophosphate units after corro-

sion. It was attributed to that Ca2+ ions could serve as (PAO�

Ca2+ �OAP) ionic cross-links. The decline of MAOAP bonds in thebase glass is larger than the rare earth glasses after corrosion,and the PAOH bonds have relatively higher fraction. The formationof more chemically resistant REAOAP bonds with the addition ofrare earth leaded to an increased quantity of nonbridging oxygen,which causes consequent variations in the chemical durability ofrare earth glasses.

Acknowledgments

This work was supported by the Science Foundation of South-west University of Science and Technology (11zx7157), the Scien-tific Research Fund of SiChuan Provincial Education Department(14ZA0105 and 14ZD1122) and Postgraduate Innovation Fund Pro-ject by Southwest University of Science and Technology(14ycxjj0018).

References

[1] Y.H. Zhang, A. Navrosky, H. Li, L.Y. Li, L.L. Davis, D.M. Strachan, J. Non-Cryst.Solids 296 (2001) 93–101.

[2] G.M. Tao, H.T. Guo, L. Feng, M. Lu, W. Wei, B. Peng, J. Am. Ceram. Soc. 92 (2009)2226–2229.

[3] P. Loiseau, D. Caurant, J. Nucl. Mater. 402 (2010) 38–54.[4] G.K. Liu, V.V. Zhorin, M.R. Antonio, S.T. Li, C.W. Williams, L. Soderholm, J. Chem.

Phys. 112 (2000) 1489.[5] R.D. Shannon, Acta Crystallogr., Sect. A: Found. Crystallogr. 32 (1976) 751–767.[6] M.T. Wang, M. Li, J.S. Cheng, F. He, Z.G. Liu, Y.H. Hu, J. Nucl. Mater. 433 (2013)

287–296.[7] M.T. Wang, J.S. Cheng, Z.G. Liu, P.J. Tian, M. Li, J. Nucl. Mater. 400 (2010) 107–

111.[8] B.C. Sales, L.A. Boatner, Science 266 (1984) 45–48.[9] B. Kumar, S. Lin, J. Am. Ceram. Soc. 74 (1991) 226–228.

[10] B.C. Bunker, G.W. Arnold, J.A. Wilder, J. Non-Cryst. Solids 64 (1984) 291–316.[11] J. Clement, G. Avila, M. Navarro, S. Martinez, M.P. Ginebra, J.A. Planell, Key Eng.

Mater. 192 (2000) 621–624.[12] Y.M. Moustafa, J. Phys. D: Appl. Phys. 32 (1999) 2278–2286.[13] D.M. Sanders, L.L. Hench, J. Am. Ceram. Soc. 56 (1973) 373–377.[14] D.E. Clark, M.F. Dilmore, E.C. Ethridge, L.L. Hench, J. Am. Ceram. Soc. 59 (1976)

62–65.[15] X.F. Meng, Q.T. Zhang, C.H. Lu, Z.Z. Xu, J. Rare Earths 25 (2007) 125–128.[16] J.P. Hamilton, C.G. Pantano, J. Non-Cryst. Solids 222 (1997) 167–174.[17] R.K. Brow, J. Non-Cryst. Solids 263&264 (2000) 1–28.[18] G. Palavit, C. Mercier, L. Montagne, M. Drache, J. Am. Ceram. Soc. 81 (1998)

1521–1524.[19] D. Banerjee, V. Sudarsan, P.K. Wattal, D. Das, Phys. Chem. Glasses 54 (2013)

95–100.[20] E. Molières, F. Angeli, P. Jollivet, S. Gin, T. Charpentier, O. Majérus, P. Barboux,

D.D. Ligny, O. Spalla, Int. J. Appl. Glass Sci. 4 (2013) 383–394.[21] H.J. Li, X.F. Liang, C.L. Wang, H.J. Yu, Z. Li, S.Y. Yang, J. Mol. Struct. 1067 (2014)

154–159.[22] W.H. Huang, D.E. Day, C.S. Ray, C.W. Kim, A. Mogus-Milankovic, J. Nucl. Mater.

327 (2004) 46–57.[23] S. Rani, S. Sanghi, Anshu, A. Agarwal, N. Kishore, V.P. Seth, J. Phys. Chem. Solids

69 (2008) 1855–1860.[24] J. Marchi, D.S. Morais, J. Schneider, J.C. Bressiani, A.H.A. Bressiani, J. Non-Cryst.

Solids 351 (2005) 863–868.[25] F. Lofaj, R. Satet, M.J. Hoffmanna, A.R. de Arellano López, J. Eur. Ceram. Soc. 24

(2004) 3377–3385.[26] B. Qian, X.F. Liang, C.L. Wang, S.Y. Yang, J. Nucl. Mater. 443 (2013) 140–144.[27] X.Y. Yu, D.E. Day, G.J. Long, R.K. Brow, J. Non-Cryst. Solids 215 (1997) 21–31.[28] S.T. Reisa, M. Karabulut, D.E. Day, J. Non-Cryst. Solids 292 (2001) 150–157.[29] H. Yin, C. Zhang, L. Liu, G. Chen, B. Tang, J. Wuhan Univ. Technol. Mater. Sci. Ed.

24 (4) (2009) 640–643.

H. Li et al. / Journal of Molecular Structure 1076 (2014) 592–599 599

[30] D.E. Day, Z. Wu, C.S. Ray, P. Hrma, J. Non-Cryst. Solids 241 (1998) 1–12.[31] R.K. Brow, in: Proc. 17th Int. Congress on Glass, 5 (1995) pp. 25–30.[32] A.M. Abdelghany, H.A. ElBatal, F.M. EzzElDin, Ceram. Int. 38 (2012) 1105–

1113.[33] F.H. ElBatal, S. Ibrahim, A.M. Abdelghany, J. Mol. Struct. 1030 (2012) 107–112.[34] Y.M. Lai, X.F. Liang, S.Y. Yang, J.X. Wang, L.H. Cao, B. Dai, J. Mol. Struct. 992

(2011) 84–88.[35] D.A. Magdas, O. Cozar, V. Chis, I. Ardelean, N. Vedeanu, Vib. Spectrosc. 48

(2008) 251–254.[36] N.A. Ghoneima, A.M. Abdelghany, S.M. Abo-Naf, F.A. Moustafa, Kh.M. ElBadry,

J. Mol. Struct. 1035 (2013) 209–217.[37] H. Doweidar, Y.M. Moustafa, K. El-Egili, I. Abbas, Vib. Spectrosc. 37 (2005) 91–

96.[38] P.A. Bingham, R.J. Hand, J. Hazard. Mater. B119 (2005) 125–133.[39] G.N. Greaves, S.J. Gurman, L.F. Gladden, C.A. Spence, P. Cox, B.C. Sales, L.A.

Boatner, R.N. Jenkins, Philos. Mag. B 58 (1988) 271–283.

[40] J.X. Yu, Q.Y. Jian, W.F. Yuan, B. Gu, F. Ji, W. Huang, Appl. Surf. Sci. 292 (2014)267–277.

[41] N.J. Clayden, S. Esposito, A. Aronne, P. Oernice, J. Non-Cryst. Solids 258 (1999)11–19.

[42] J.S. Wu, J.F. Stebbins, J. Non-Cryst. Solids 362 (2013) 73–81.[43] L. Lyon, C. Keating, A. Fox, B. Baker, L. He, S. Nicewarner, S. Mulvancy, M. Natan,

Anal. Chem. 70 (1998) 341R–361R.[44] H.Y. Mi, S. Mikael, T. Allen, K. Sridharan, D. Butt, J.P. Blanchard, Z.Q. Ma, J. Nucl.

Mater. 445 (2014) 7–11.[45] A.M.B. Silva, R.N. Correia, J.M.M. Oliveira, M.H.V. Fernandes, J. Eur. Ceram. Soc.

30 (2010) 1253–1258.[46] R.K. Brow, D.R. Tallant, S.T. Myers, C.C. Phifer, J. Non-Cryst. Solids 191 (1995)

45–55.[47] Y.M. Lai, X.F. Liang, S.Y. Yang, J.X. Wang, B.T. Zhang, J. Mol. Struct. 1013 (2012)

134–137.[48] A. Chahine, M. Et-tabirou, J.L. Pascal, Mater. Lett. 58 (2004) 2776–2780.

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具