Solid State Communications 146 (2008) 273–277www.elsevier.com/locate/ssc

Mossbauer analysis of Fe ion state in lithium iron phosphate glasses andtheir glass-ceramics with olivine-type LiFePO4 crystals

K. Hirosea, T. Honmaa, Y. Doib, Y. Hinatsub, T. Komatsua,∗

a Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka 940-2188, Japanb Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

Received 8 November 2007; received in revised form 12 February 2008; accepted 14 February 2008 by C. LacroixAvailable online 4 March 2008

Abstract

The valence and local coordination state of Fe ions in 26Li2O–43FeO–5Nb2O5–26P2O5 (mol%) glasses and glass-ceramics consisting ofLiFePO4 crystals (a potential candidate as cathode materials for rechargeable lithium ion batteries) are studied from Fe57 Mossbauer effectspectrum measurements. The main valence of Fe ions in the precursor and crystallized glasses is found to be Fe2+ (more than 80%). Fe2+

ions in the precursor glasses give the values of the isomer shift δ = 1.16–1.19 mm/s and the quadrupole splitting ∆ = 2.26–2.40 mm/s,indicating the presence of FeO6 octahedra. Olivine-type LiFePO4 crystals in the crystallized glasses show the values of δ = 1.22 mm/s and∆ = 2.94–2.96 mm/s, demonstrating that Fe2+ sites in LiFePO4 crystals are highly distorted octahedra. The presence of Fe3+ ions withoctahedral and/or tetrahedral local structures are suggested in the precursor glasses. It is proposed that similarities in the valence and localcoordination state of Fe2+ in the precursor glasses and LiFePO4 crystals would be one of the reasons why LiFePO4 crystals are easily formedthrough the crystallization of lithium iron phosphate glasses.c© 2008 Elsevier Ltd. All rights reserved.

PACS: 61.43.Fs; 81.05.Kf; 81.05.Pj

Keywords: A. Disorderd systems; B. Crystal growth; C. Crystal structure and symmetry; E. Mossbauer effect

1. Introduction

Lithium iron phosphate LiFePO4 with an olivine structurehas been proposed to be a potential candidate for use ascathode materials for the next generation of rechargeablelithium ion batteries [1]. LiFePO4 cathode materials have ahigh theoretical capacity of 170 mAh/g, are environmentallybenign, thermally stable in the fully charged state, and havelow raw material costs. Numerous studies reported so farsuggest that the key points to achieve a high performanceas cathode materials are to control or design particle size,morphology, and interface between LiFePO4 crystal particles.LiFePO4 is commonly synthesized via solid-state reactions, co-precipitation, hydrothermal methods, and so on.

Very recently, Hirose et al. [2] proposed new routes for thefabrication of LiFePO4 crystals. One is the crystallization of

∗ Corresponding author. Tel.: +81 258 47 9313; fax: +81 258 47 9300.E-mail address: komatsu@mst.nagaokaut.ac.jp (T. Komatsu).

0038-1098/$ - see front matter c© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2008.02.013

precursor LiFePO4 glasses using a conventional heat treatmentin an electric furnace and the other is the patterning ofhighly oriented LiFePO4 crystal lines on the glass surfaceby irradiation with a continuous wave Nd:YAG laser at awavelength of λ = 1064 nm. It is of importance to clarifythe crystallization mechanism of lithium iron phosphate glassesfor the fabrication of LiFePO4 crystals with high performance.The valence of iron in LiFePO4 crystals is ferrous Fe2+. On theother hand, usually Fe ions exist as the valence states of Fe2+

and ferric Fe3+ ions in oxide glasses prepared in a conventionalmelt quenching method in air. It is, therefore, of importanceto clarify the Fe valence state in the precursor and crystallizedglasses for an in-depth understanding of the crystallizationmechanism in lithium iron phosphate glasses and to designcrystallized glasses with high performance.

The purpose of this study is to clarify the valence and localcoordination state of Fe ions in lithium iron phosphate glassesand glass-ceramics consisting of LiFePO4 crystals by usingFe57 Mossbauer effect spectroscopy. The Fe57 Mossbauer effect

274 K. Hirose et al. / Solid State Communications 146 (2008) 273–277

is known as a structural sensitive technique, and it has beenapplied on a great number of Fe-based crystals and amorphousmaterials by numerous researchers. For instance, the valencestate and local coordination environments of Fe ions in ironphosphate glasses [3–8] and the crystallization behaviors of Fe-based crystals in oxide glasses [9–12] have been studied fromFe57 Mossbauer effect measurements.

2. Experimental

Glasses with the composition of 26Li2O–43FeO–5Nb2O5–26P2O5 (mol%) were prepared using a conventional melt-quenching method. Commercial powders of reagent gradeLi2CO3, FeC2O4:2H2O, Nb2O5, and NH4H2PO4 were mixedand heated at 300 ◦C for 10 h in nitrogen atmosphere, andthen the mixtures were melted at 1200 ◦C for 15 min. Twodifferent melting atmospheres were applied: in Glass A, themixtures were melted in air using an alumina crucible. In GlassB, the mixtures were melted in a reducing atmosphere usingtwo alumina crucibles and carbon particles (double crucibletechnique) [2]. The melts were poured onto an iron plate andpressed to a thickness of 1–1.5 mm by another iron plate. Theglass transition, Tg , and crystallization peak, Tp, temperatureswere determined using differential scanning calorimetry (DSC)at a heating rate of 10 K/min. The glasses were heat treatedat around Tp in air, and the crystalline phases present in thecrystallized samples were identified by X-ray diffraction (XRD)analyses (Cu Kα radiation) at room temperature.

The Mossbauer effect measurements were conducted onpowder samples (∼120 mg) spread on an aluminum foil(∼2 cm × 2 cm) at room temperature using an equipmentof VT-6000 (Laboratory Equipment Co.). A radioactive 57Coin Pd matrix was used as the γ -ray source, and the velocitycalibration was obtained from the 6-line hyperfine spectra ofiron foil. Mossbauer spectra obtained were analyzed (fitted) byusing the following functions for singlet and doublet peaks.

fsin glet =σ(Γ/2)2

(x − δ)2 + (Γ/2)2 (1)

fdoublet =σ(Γ/2)2

(x − δ + ∆/2)2 + (Γ/2)2

+σ(Γ/2)2

(x − δ − ∆/2)2 + (Γ/2)2 (2)

where δ is the isomer shift, ∆ is the quadrupole splitting, σ isthe absorption relative intensity (negative), and Γ is the halfwidth of peak.

3. Results and discussion

As reported in the previous paper [2], it is difficult toget clear glasses for the sample with the composition of25Li2O–50FeO–25P2O5 corresponding to that of the LiFePO4crystalline phase, but it is found that the glass forming abilityof lithium iron phosphate is largely improved by the addition ofNb2O5. Therefore 26Li2O–43FeO–5Nb2O5–26P2O5 (mol%)glasses were prepared. The values of the glass transition and

Fig. 1. Powder XRD patterns at room temperature for the precursor Glass Aand crystallized samples obtained by heat treatments at 580 and 700 ◦C for30 min in air.

crystallization peak temperatures were found to be Tg =

460 ◦C and Tp = 583 ◦C for Glass A and Tg = 439 ◦C andTp = 560 ◦C for Glass B [2].

The XRD patterns for the crystallized Glass A obtainedby heat treatments at 580 and 700 ◦C for 30 min inair are shown in Fig. 1, where powdered (pulverized aftercrystallization) samples were used. It is seen that LiFePO4crystals (space group of Pnmb, JCPDS: 40–1499) are formedthrough crystallization at 580 ◦C. In the sample crystallizedat 700 ◦C, formations of Nasicon-like Li3Fe2(PO4)3 andLiNb3O8 crystals are detected. The XRD patterns for thecrystallized Glass B obtained by heat treatments at 560 and700 ◦C for 30 min in air are shown in Fig. 2. It is confirmedthat LiFePO4 crystals were formed in the sample crystallized at560 ◦C. In the sample crystallized at 700 ◦C, the formationsof Li3Fe2(PO4)3 and LiNb3O8 crystals are detected, but theamount of the Li3Fe2(PO4)3 crystalline phase would be smallcompared with the case of Glass A. These results demonstratethat olivine-type LiFePO4 crystals are mainly formed in bothGlass A and Glass B at least by heat treatments at crystallizationpeak temperatures.

The Mossbauer spectrum at room temperature for theprecursor sample of Glass A is shown in Fig. 3, in which thebest fitting curves are indicated. It is clear that the Mossbauerspectrum consists of the overlapping of two doublets peaks. Theestimated Mossbauer parameters of isomer shift δ, quadrupolesplitting ∆, half width of peak Γ , and absorption area intensityratio S are summarized in Table 1. The doublet peak with largevalues of δ = 1.19 mm/s and ∆ = 2.26 mm/s is assigned toFe2+ ions with an 3d6 electronic configuration. On the otherhand, the doublet with small values of δ = 0.32 mm/s and∆ = 0.42 mm/s is assigned to Fe3+ ions with an 3d5 electronicconfiguration. The Mossbauer spectrum at room temperaturefor the crystallized (580 ◦C, 30 min) Glass A is shown in Fig. 4,

K. Hirose et al. / Solid State Communications 146 (2008) 273–277 275

Table 1Mossbauer parameters obtained by fittings for the precursor and crystallized glasses of 26Li2O–43FeO–5Nb2O5–26P2O5 (mol%)

Sample Site δ (mm/s) ∆ (mm/s) Γ (mm/s) S

Glass APrecursor Fe2+ 1.19 2.26 0.59 0.80

Fe3+ 0.32 0.92 0.42 0.20Heat-treated Fe2+ 1.22 2.94 0.31 0.75580 ◦C, 30 min, Air Fe3+ 0.46 0.72 0.53 0.25Glass BPrecursor Fe2+ 1.16 2.40 0.63 0.87

Fe3+ 0.24 – 0.42 0.13Heat-treated Fe2+ 1.22 2.96 0.31 0.93560 ◦C, 30 min, Air Fe3+ 0.42 0.66 0.46 0.07

δ, ∆, Γ , and S are isomer shift, quadrupole splitting, half width of peak, and absorption area intensity ratio between Fe2+ and Fe3+ peaks, respectively.

Fig. 2. Powder XRD patterns at room temperature for the precursor Glass Band crystallized samples obtained by heat treatments at 560 and 700 ◦C for30 min in air.

Fig. 3. Mossbauer spectrum at room temperature for the Glass prepared by amelt-quenched method in air.

and the estimated Mossbauer parameters are given in Table 1.It is seen that, in particular, the ∆ value increases largely due tothe crystallization, reaching to the value of ∆ = 2.94 mm/s.

Fig. 4. Mossbauer spectrum at room temperature for the crystallized sampleobtained by a heat treatment at 580 ◦C for 30 min. The precursor Glass A wasprepared by a melt-quenched method in air.

In the olivine-type LiFePO4 crystalline phase with anorthorhombic structure, it is understood that Fe2+ ions arepresent in the form of FeO6 distorted octahedral, with the Fe-Odistances ranging from 0.208 to 0.22 nm [13]. The Mossbauerspectra for LiFePO4 crystals have been measured by someresearchers [13–15], and the values of δ = 1.20–1.22 mm/s and∆ = 2.95–2.98 mm/s have been reported. As seen in Table 1,the crystallized Glass A shows similar large values of δ and ∆,demonstrating that certainly olivine-type LiFePO4 crystals aresynthesized even in the crystallization of lithium iron phosphateglass. It should be pointed out that the precursor Glass A hasa small ∆ value compared with the crystallized sample. Thiswould be interpreted as the site symmetry of Fe2+ ions in theprecursor Glass A being more symmetric than average becauseof the randomness and flexibility in the glass structure.

The local coordination state of Fe ions in iron phosphateglasses has been studied extensively from Mossbauer spectra sofar, and the following features have been recognized [3–8]. ForFe2+ ions, δ values below 1 mm/s are associated to tetrahedralcoordination, Fe2+(Td), and those above 1 mm/s are relatedto octahedral coordination, Fe2+(Oh). For Fe3+ ions, althoughthe exact coordination number, i.e., Fe3+(Td) and Fe3+(Oh),is less clear from the isomer shift, the following criterion hasbeen proposed; Fe3+(Td) < 0.3 ∼ 0.4 mm/s < Fe3+(Oh). Itis, therefore, concluded that the value of δ = 1.19 mm/s for

276 K. Hirose et al. / Solid State Communications 146 (2008) 273–277

Fe2+ ions in the precursor Glass A indicates the coordinationstate of Fe2+(Oh). Fe2+ ions in phosphate glasses have beenwidely concluded to be in octahedral coordinations [3–8]. Thepresent study demonstrates that the coordination states of Fe2+

ions in the precursor Glass A and olivine-type LiFePO4 are thesame. For Fe3+ ions in the precursor Glass A, the value ofδ = 0.32 mm/s was obtained. This value suggests that Fe3+

ions might be present in both coordination states of Fe3+(Td)

and Fe3+(Oh). It should be pointed out that the isomer shiftfor Fe3+ ions increases due to the crystallization as shown inTable 1, i.e., from δ = 0.32 to δ = 0.46 mm/s. The valueof δ = 0.46 mm/s suggests strongly the coordination state ofFe3+(Oh), indicating the change in the coordination number ofFe3+ ions during the crystallization.

The Γ value of Fe2+ ions in the precursor Glass A decreaseslargely due to the crystallization, i.e., from Γ = 0.59 to Γ =

0.31 mm/s, as shown in Table 1. This result indicates that thedegree of the randomness of Fe2+ ion sites decreases due to theformation of LiFePO4 crystals. On the other hand, the Γ valueof Fe3+ ions increases due to the crystallization, implying thepresence of different local coordination environments such asFe3+ ions in the residual glassy phase, the interface betweenthe glassy phase and LiFePO4 crystals, and incorporations intoLiFePO4 crystals.

The absorption area intensity ratio of Fe2+ ions in theprecursor Glass A was found to be S = 0.80. Although somediscussions on the direct determination of the Fe2+/Fe3+ ratioby Mossbauer spectrum analyses have been made [16], thepresent study indicates that the main valence of Fe ions in theprecursor Glass A is ferrous Fe2+ and suggests that the fractionof Fe2+ ions, i.e., R(Fe2+) = Fe2+/(Fe2+

+ Fe3+), would beclose to R(Fe2+) = 0.80. In the previous paper [2], the fractionof Fe2+ ions in the precursor Glass A was determined to beR(Fe2+) = 0.79 by using a cerium redox titration method, thusbeing almost the same as that (S = 0.8) estimated from theMossbauer spectrum. As seen in Table 1, the absorption areaintensity ratio of Fe2+ ions decreases due to the crystallization,i.e., from S = 0.80 to S = 0.75, suggesting that some Fe2+

ions in the precursor Glass A are oxidized to Fe3+ ions duringthe crystallization in air.

The Mossbauer spectra at room temperature for theprecursor Glass B and crystallized sample are shown in Figs. 5and 6, and the estimated Mossbauer parameters are summarizedin Table 1. The absorption area intensity ratio of Fe2+ ions inthe precursor Glass B was estimated to be S = 0.87, being thesame as that (R(Fe2+) = 0.87) determined from a chemicalanalysis [2]. In the preparation of Glass B, the mixtures weremelted in a reducing atmosphere using carbon particles, andthus it is expected that the fraction of Fe2+ ions would belarge compared with Glass A prepared in air. Indeed, as seen inTable 1, the absorption area intensity ratio of Fe2+ ions in theprecursor Glass B, i.e., S = 0.87, is larger than that (S = 0.80)in the precursor Glass A. In Glass B, the fraction of Fe2+ ions isconsidered to increase due to the crystallization, i.e., from S =

0.87 to S = 0.93. This behavior is opposite in comparison withthe case of Glass A. For understanding the valence change ofFe ions during the crystallization in Li2O–FeO–Nb2O5–P2O5

Fig. 5. Mossbauer spectrum at room temperature for the Glass B prepared bya melt-quenched method in a reducing atmosphere.

Fig. 6. Mossbauer spectrum at room temperature for the crystallized sampleobtained by a heat treatment at 560 ◦C for 30 min. The precursor Glass B wasprepared by a melt-quenched method in a reducing atmosphere.

glasses, more detailed studies would be needed. The valueof δ = 1.16 mm/s for Fe2+ ions in the precursor Glass Bindicates the coordination state of Fe2+(Oh). On the other hand,the value of δ = 0.24 mm/s for Fe3+ ions strongly suggeststhe presence of the tetrahedral coordination state Fe3+(Td). Ithas been proposed that Fe3+ ions in phosphate glasses withlow (<2 mol%) Fe2O3 contents are present in the tetrahedralcoordination state [3].

It was clarified from Mossbauer spectrum measurementsthat the main valence of Fe ions in lithium iron phosphateglasses is Fe2+ and the local coordination state of Fe2+ isthe FeO6 octahedron. These features of Fe2+ ions in lithiumiron phosphate glasses are consistent with those in olivine-type LiFePO4 crystals. And similarities in the valence andlocal coordination state of Fe2+ ions in the base glasses andLiFePO4 crystals would be one of the reasons why LiFePO4crystals are easily formed through the crystallization of lithiumiron phosphate glasses prepared in the present study. Asshown in Figs. 1 and 2, the Li3Fe2(PO4)3 crystalline phaseis crystallized after the formation of LiFePO4 crystals. Itshould be pointed out that the valence of Fe ions in theLi3Fe2(PO4)3 crystalline phase is trivalent Fe3+. The following

K. Hirose et al. / Solid State Communications 146 (2008) 273–277 277

crystallization mechanism is, therefore, considered in theglasses prepared in this study. That is, firstly LiFePO4 crystalsare formed in the glasses with large amounts of Fe2+ ions,i.e., R(Fe2+) = 0.80–0.87, and then Li3Fe2(PO4)3 crystalsare precipitated in the remaining glassy phase with Fe3+ ions.As the crystallization proceeds faster in heat treatments athigh temperatures, and, therefore, it would also be necessaryto crystallize the glasses at high temperatures in a reducingatmosphere in order to depress the oxidation of Fe2+ into Fe3+.In this study, we added an amount of 5 mol% Nb2O5 in order topromote the glass formation, but this oxide is not a constituentcomponent in LiFePO4. And thus, it is of great importance toreduce the amount of Nb2O5 in the glass preparation.

4. Conclusions

The valence and local coordination state of Fe ions in26Li2O–43FeO–5Nb2O5–26P2O5 (mol%) glasses and glass-ceramics consisting of LiFePO4 crystals were studied fromFe57 Mossbauer effect spectra. It was clarified that the mainvalence of Fe ions in the precursor and crystallized glasses wasFe2+. The values of the isomer shift δ = 1.16–1.19 mm/s andthe quadrupole splitting ∆ = 2.26–2.40 mm/s were obtainedfor Fe2+ ions in the precursor glasses, indicating the presenceof FeO6 octahedra. Olivine-type LiFePO4 formed through thecrystallization of the precursor glasses showed the values of δ =

1.22 mm/s and ∆ = 2.94–2.96 mm/s, demonstrating that Fe2+

sites in LiFePO4 crystals are highly distorted octahedra. It wasproposed that similarities in the valence and local coordinationstate of Fe2+ in the precursor glass and LiFePO4 crystals wouldbe one of the reasons why LiFePO4 crystals are easily formedthrough the crystallization of lithium iron phosphate glasses.

Acknowledgements

This work was supported from Grant-in-Aid for ScientificResearch from the Ministry of Education, Science, Sports,Culture and Technology, Japan.

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