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Ind. Eng. Chem. Res. 2008, 47, 7642–7647
Simple Synthesis, Magnetic Properties, and Nonisothermal Decomposition Kinetics of Fe(H2PO4)2 · 2H2O Banjong Boonchom,†,‡ Chanaiporn Danvirutai,*,‡ Sujittra Youngme,‡ and Santi Maensiri§ King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus, 17/1 M. 6 Pha Thiew District, Chumphon, 86160, Thailand, Department of Chemistry, Faculty of Science, Khon Kaen UniVersity, Khon Kaen 40002, Thailand, and Integrated Nanotechnology Research Center (INRC) and Small & Strong Materials Group (SSMG), Department of Physics, Faculty of Science, Khon Kaen UniVersity, Khon Kaen, 40002, Thailand
Fe(H2PO4)2 · 2H2O was synthesized by a simple and cost-effective method using iron metal and phosphoric acid as starting chemicals. The thermal transformation products from the synthesized Fe(H2PO4)2 · 2H2O according to the thermal treatment at 423 and 673 K were found to be FeH2P2O7 and Fe2P4O12, respectively. The activation energies of decomposition reaction of Fe(H2PO4)2 · 2H2O were calculated through the isoconversional methods of Ozawa and Kissinger-Akahira-Sunose. The synthesized Fe(H2PO4)2 · 2H2O and its thermal transformation products were characterized by scanning electron microscopy, X-ray powder diffraction, and FTIR spectroscopy. Room-temperature magnetization results showed a ferromagnetic behavior of the Fe(H2PO4)2 · 2H2O and its thermal transformation products. The hysteresis loops were in the range -10000 Oe < H < +10000 Oe, and the strong specific magnetization values were in the range 32.25-96.28 emu/g at 10 kOe. 1. Introduction The acid inorganic salts, such as hydrogen phosphate, arsenates, sulfates, and so forth, are known to possess optical, electrical, and magnetic properties of practical importance, which are determined by the presence of very strong hydrogen bonds in their crystal lattices.1 The development of electric devices increases the need for new materials, especially in storage and electric energy production areas. Moreover, the recent use of transition-metal phosphates such as LiFePO4 as a positive electrode for Li ion batteries generates a great interest for this class of material.2-7 Several transition-metal phosphates were screened as electrode material such as Nasicon-type phases Li3Fe2(PO4)33,4 or dimetalphosphate LiFePO4.6 Among the acid phosphates, the well-known compounds of the isostructural M(H2PO4)2 · 2H2O (M ) Mn, Co, Ni) series have been the subject of numerous investigations because of interesting ferroand antiferroelectric phase transitions, which occur at different temperatures (they crystallize in monoclinic space group P21/n with Z ) 2).1 Very few studies on the synthesis and crystal structure of Fe(H2PO4)2 · 2H2O have been previously reported. Bagieu-Beucher et al.8 and Koleva and Effenberger9 reported the crystal structure of the synthesized Fe(H2PO4)2 · 2H2O from phosphoric acid and ferrous carbonate or native iron at low temperature with a long time period. These synthesis methods were performed in an argon atmosphere (99.999 wt %) to prevent the oxidation of Fe2+. However, there are no data in the literature about magnetic properties of the dihydrogen phosphate of Fe(H2PO4)2 · 2H2O. Recently, nanocrystalline magnetic materials have been receiving more and more attention because of their novel materials properties, which are significantly different from those of their bulk counterparts.3 For practical applications, it is of particular importance to study the * To whom correspondence should be addressed. Tel.: +66-43202222 ext. 12243. Fax: +66-43-202373. E-mail:
[email protected]. † King Mongkut’s Institute of Technology Ladkrabang. ‡ Department of Chemistry, Khon Kaen University. § Department of Physics, Khon Kaen University.
ferromagnetic properties in materials as they will be potentially useful for ferrofluids and magnetic recording applications. In the present article, we report for the first time the synthesis and magnetic properties of Fe(H2PO4)2 · 2H2O by a simple chemical method using iron metal and phosphoric acid at 313 K with short time consumption (15 min). Thermal stability of Fe(H2PO4)2 · 2H2O was investigated by thermogravimetricdifferential thermal analysis (TG-DTG-DTA). The kinetics analysis of the nonisothermal results for Fe(H2PO4)2 · 2H2O decomposition steps was carried out using the isoconversional methods of Ozawa10 and Kissinger-Akahira-Sunose (KAS).11 The synthesized Fe(H2PO4)2 · 2H2O and its thermal transformation products were characterized by XRD, FTIR, SEM, and VSM techniques. 2. Experimental Section The Fe(H2PO4)2 · 2H2O compound was prepared by adding 2 mL of 70% wt/wt H3PO4 to 0.5585 g of Fe (c) (99.99%, Fluka). The resulting solution was heated on a hot plate at 313 K with continuous stirring until H2(g) was completely evolved (15 min). The nearly dry sample was obtained, and then 10 mL of acetone was added to allow a highly crystalline product to be developed. The prepared solid was filtered by suction pump, washed with acetone, and dried in air. The iron content of Fe(H2PO4)2 · 2H2O compound was determined by being dissolved in 0.0126 M hydrochloric acid using atomic absorption spectrophotometry (Perkin-Elmer, Analyst100). Thermal analysis measurements (thermogravimetry, TG; differential thermogravimetry, DTG; and differential thermal analysis, DTA) were carried out by a Pyris Diamond PerkinElmer apparatus by increasing temperature from 303 to 773 K with calcined R-Al2O3 powder as the standard reference. The experiments were performed in static air, at heating rates of 5, 10, 15, and 20 K min-1. The sample mass was kept 6.0-10.0 mg into alumina crucible without pressing. The structure and crystallite size of the prepared sample and its thermal transformation products were studied by XRD using
10.1021/ie800007j CCC: $40.75 2008 American Chemical Society Published on Web 09/17/2008
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7643
because the second and third steps of eq 1 often overlap.13 This result is also in agreement with those reported in the literature.14,15 To gain the complete dehydration of synthesized Fe(H2PO4)2 · 2H2O, the sample was heated in the furnace at 413 and 673 K for 2 h, and the decomposition products FeH2P2O7 and Fe2P4O12 were obtained, respectively. The specificity of the thermal decomposition was characterized by identifying the bonds to be selectively activated due to energy absorption at vibrational level.27 These bonds were assigned by comparing the calculated wavenumbers with the observed wavenumbers in the FTIR spectra. This breaking of the bond is assimilated with Morse oscillators16 coupled nonlinear17 with the harmonic oscillators of the thermic field. Next, a theoretical treatment developed by Vlase et al.,18 the relation between the maximum temperature peak Tp (DTA) at maximun heating rates (Figure 1c) and the wavenumber of the activated bond, is given as follows: Figure 1. TG-DTG-DTA curves of Fe(H2PO4)2 · 2H2O at heating rate of 5 K min-1 in static air.
a D8 advanced powder diffractometer (Bruker AXS) with Cu KR radiation (λ ) 0.1546 nm). The Scherrer method was used to evaluate the crystalline size.12 The morphologies of the selected resulting samples were examined with scanning electron microscopy using LEO SEM VP1450 after gold coating. The room-temperature FTIR spectra were recorded in the range 4000-370 cm-1 with eight scans on a Perkin-Elmer Spectrum GX FTIR/FT-Raman spectrometer with a resolution of 4 cm-1 using KBr pellets (spectroscopy grade, Merck). The magnetic properties of the prepared sample and its thermal transformation products were examined at 293 K using a vibrating sample magnetometer (VSM 7403, Lake Shore). 3. Results and Discussion 3.1. Thermal Analysis. To confirm the exact amount of water of hydration and also to study the thermal stability of the prepared Fe(H2PO4)2 · 2H2O sample, thermogravimetric analysis (TG-DTG-DTA) was performed. The TG-DTG-DTA curves of Fe(H2PO4)2 · 2H2O are shown in Figure 1. The TG curve shows the weight loss between 303 and 773 K, which involved the dehydration of the coordinated water molecules (2 mol H2O) and an intramolecular dehydration of the protonated phosphate groups (2 mol H2O). A large number of intermediate compounds, such as acid polyphosphate Fe(H2PO4)2, acid condensed phosphate FeH2P2O7, and mixtures of intermediate of both have been registered. The elimination of water was observed in three areas: 350-389, 389-441, and 441-573 K. The corresponding observed weight losses are 15.89, 4.47, and 3.92% by mass, which correspond to 2.54, 0.72, and 0.63 mol of water. Total mass loss is 24.28% (3.89 mol H2O), and iron cyclotetraphosphate Fe2P4O12 is found to be the final product of the thermal decomposition at T > 673 K. Three endothermic effects over the temperature region are shown in the DTA curve, namely at 383, 418, and 480 K. Thermal transformation of Fe(H2PO4)2 · 2H2O relates to the elimination of water of crystallization: -xH2O
-yH2O
T1
T2
Fe(H2PO4)2 · nH2O 98 Fe(H2PO4)2 98 -zH2O 1 FeH2P2O7 98 Fe2P4O12(1) 2 T3
where 0 < (x + y + z) e 4. Subsequently, it was shown that the water of crystallization cannot be usually removed without the destruction of anion,
kb T ) 0.695Tp (2) hc P where kb and h are, respectively, the Boltzmann and Planck constants, and c is the light velocity. Because the breaking of the bond has an anharmonic behavior, the specific activation is possibly also due to more than one quanta, or by a higher harmonic: ωsp ) qωcalc, q ∈ N ) 1, 2, 3..., where ωsp is the assigned spectroscopic number for the bond supposed to break. To corroborate the calculated data with the spectroscopic ones, we drew up the FTIR spectra of the studied compound. According to eq 2, the Tp (DTA) at maximun heating rate (20 K min-1) for the decomposition step is 395.15 K. The calculated harmonic energy (ωcalcd) values of the decomposition step were 1656, 3036, and 3312 cm-1, which correspond to 6, 11, and 12 quanta numbers, respectively. These wavenumbers are close to the water vibration of water of crystallization reported in the literature17 and exhibit a very good agreement with the observed wavenumbers from FTIR spectra for the bonds suggested being broken. This result confirms that the thermal decomposition step corresponds to the elimination of water of crystallization. 3.2. Kinetics Studies. 3.2.1. Calculation of the Activation Energy. Dehydration of crystalline hydrates is a solidstate process of the type19-22 A (solid) f B (solid) + C (gas). The kinetics of such reactions is described by various equations taking into account the special features of their mechanisms. As one of the nonisothermal multiple-scan methods for kinetics studies, isoconversional method is also called a model-free method because no kinetic model was set before the calculation of activation energy. The Ozawa10 and KAS11 methods are two representative ones of model-free, which are convenient to calculate the activation energy. Kinetics analysis was studied only for the major mass loss in the region 350-389 K for the decomposition of Fe(H2PO4)2 · 2H2O, because the other steps are minor mass losses in the short temperature ranges, which are highly sensitive to nonisothermal kinetics analysis errors. Ozawa and KAS equations were used to determine the following activation energies. Ozawa equation: ω)
( )
log β ) log KAS equation:
( )
AER ER - 2.315 - 0.4567 Rg(R) RT
( ) ( ) ( )
(3)
AER ER β ) ln (4) 2 Rg(R) RT T where A (the pre-exponential factor) and E (the activation energy) are the Arrhenius parameters, T is temperature (K), R ln
7644 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 1. r-Τ Data at Different Heating Rates, β (K min-1), for Dehydration of Fe(H2PO4)2 · 2H2O temperature/K R 0.2 0.3 0.4 0.5 0.6 0.7 0.8
β)5 376.29 377.71 379.32 380.97 382.11 384.16 387.04
β ) 10 381.51 383.89 385.10 386.74 388.49 390.47 393.85
β)15 384.77 387.11 388.85 390.61 392.51 394.60 397.75
β ) 20 386.31 389.38 390.90 392.46 395.00 396.87 399.95
is the extent of conversion, and R is the gas constant (8.314 J mol-1 K-1). The Arrhenius parameters, together with the reaction model, are sometimes called the kinetic triplet. g(R) ) ∫R0 [dR/f(R)] is the integral form of the f(R), which is the reaction model that depends on the reaction mechanism. According to the isoconversional method, the basic data of R and T were collected from four TG curves of the second dehydration of Fe(H2PO4)2 · 2H2O at various heating rates and are illustrated in Table 1. According to the above-mentioned equations, the plots of log β versus 1000/T (Ozawa) and ln β/T2 versus 1000/T (KAS) corresponding to different R values can be obtained by a linear regression of least-squares methods, respectively. The Ozawa and KAS analysis results of four TG measurements below 473 K are presented in Figure 2a,b, respectively. The activation energies ER can be calculated from the slopes of the straight lines with better linear correlation coefficient (r2 > 0.99). The slopes change depending on R for the dehydration reaction of Fe(H2PO4)2 · 2H2O. The activation energies are calculated at four different heating rates via the Ozawa and KAS methods in the R range 0.2-0.8 and are shown
Figure 2. Ozawa (a) and KAS (b) analysis of the thermal decomposition step of Fe(H2PO4)2 · 2H2O.
Table 2. Activation Energies (Er) vs Correlation Coefficient (r2) Calculated by the Ozawa and KAS Methods for the Dehydration of Fe(H2PO4)2 · 2H2O Ozawa method
KAS method
R
ER/kJ mol-1
r2
ER/kJ mol-1
r2
0.2 0.3 0.4 0.5 0.6 0.7 0.8 average
155.49 137.57 138.47 139.69 127.43 129.70 129.58 136.85 ( 13
0.9983 0.9992 0.9994 0.9987 0.9998 0.9994 0.9987 0.9977
157.18 138.29 139.21 140.47 127.55 129.90 129.73 137.47 ( 13
0.9982 0.9991 0.9993 0.9985 0.9998 0.9994 0.9986 0.9975
in Table 2. The activation energy values calculated by the KAS method are close to those obtained by the Ozawa method, and therefore the results are credible. The bigger change of the activation energies on R (the biggest difference is about 18 kJ mol-1 when compared with the mean value) leads to the conclusion that the dehydration reaction of Fe(H2PO4)2 · 2H2O could be a multistep mechanism. The water in crystalline hydrate may be considered either as water of crystallization (crystal water) or as coordinated water. The water eliminated at 413 K and below can be considered as water of crystallization, whereas water eliminated at 473 K and above indicates its coordination by the metal atom.21,22 Water molecules eliminated at intermediate temperatures can be coordinately linked water as well as coordinated water. The dehydration temperatures obtained in this work suggest that the water in Fe(H2PO4)2 · 2H2O can be considered as water of crystallization (Figure 1). However, the strengths of binding between water molecules in the crystal lattice are different and, consequently, result in different dehydration temperatures and kinetic parameters. The activation energy for the release of crystal water lies in the range 50-130 kJ mol-1, while the value for the coordinately bounded one is higher than this range.22 The calculated activation energies from the Ozawa and KAS methods for the dehydration reaction (Table 2) suggest that the water molecules are coordinately linked water as well as crystal one. This result is not consistent with the dehydration temperature obtained from DTA-TG experiments, which may be due to the overlapped decomposition steps. 3.3. X-ray Powder Diffraction. The XRD patterns of Fe(H2PO4)2 · 2H2O and the Fe(H2PO4)2 · 2H2O calcinced at 413 and 673 K are shown in Figure 3. All detectable peaks of the Fe(H2PO4)2 · 2H2O and the calcined Fe(H2PO4)2 · 2H2O at 413 and 673 K are indexed as Fe(H2PO4)2 · 2H2O, FeH2P2O7, and
Figure 3. XRD patterns of Fe(H2PO4)2 · 2H2O (a) and Fe(H2PO4)2 · 2H2O calcined at 423 (b) and 673 (c) K.
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7645 Table 3. Average Crystallite Sizes and Lattice Parameters of Fe(H2PO4)2 · 2H2O and Its Transformation Products Calculated from XRD Data compound
method
Fe(H2PO4)2 · 2H2O
a (Å)
b (Å)
c (Å)
β (deg)
PDF # 751444 7.30 9.92 5.34 95.14 this work 7.25(1) 10.10(0) 5.32(0) 95.71(0) difference PDF - this work 0.05 -1.18 0.02 -0.57 FeH2P2O7 (Fe(H2PO4)2 · 2H2O calcined 150 °C) PDF # 200517 this work difference PDF - this work Fe2P4O12 (Fe(H2PO4)2 · 2H2O calcined 400 °C) PDF # 782285 11.94 8.37 9.93 118.74 this work 12.80(0) 8.80(4) 10.56(0) 118.67(4) difference PDF - this work -0.86 -0.43 -0.63 0.07
average crystallite sizes (nm) 28 ( 4 27 ( 6 29 ( 6
Fe2P4O12 structures, which are identified using the standard data of PDF # 751444, PDF # 200517, and PDF # 782285, respectively. These results indicated that the three crystal structures are in monoclinic system with space group P21/n (Z ) 2) for Fe(H2PO4)2 · 2H2O and C2/c (Z ) 4) for Fe2P4O12 (PDF # 200517 for FeH2P2O7 has no data available). The average crystallite sizes and lattice parameters of Fe(H2PO4)2 · 2H2O, FeH2P2O7, and Fe2P4O12 were calculated from X-ray spectra and are tabulated in Table 3. The lattice parameters of Fe(H2PO4)2 · 2H2O and Fe2P4O12 are comparable to those of the standard data of PDF # 751444 and PDF # 782285, respectively. 3.4. FTIR Spectroscopy. The FTIR spectra of the Fe(H2PO4)2 · 2H2O and Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K are shown in Figure 4. The IR spectrum of Fe(H2PO4)2 · 2H2O is very similar to that of M(H2PO4)2 · 2H2O (M ) Mn, Co, Ni) observed by Koleva et al.,1,9 which is assigned relating to the fundamental vibrating units, H2PO4and H2O. The strong bands in the region 1150-950 cm-1 are attributed to the P-O stretching vibrations. The bending OPO vibrations appear in the region 450-600 cm-1. The couple bands at 1250 and 796 cm-1 are assigned to the in-plane and out-of-plane bending modes of P-O-H, respectively. The band around 900 cm-1 could be due to either the γOH or the P-O stretching vibration. The weak band at about 635 cm-1 is due to the water libration. In the O-H stretching mode region, the multiple bands, which are characteristic for the acid salts,23,24 appear as ABC trio bands at about 3130-3180 cm-1 (A band), 2380-2440 cm-1 (B band), and 1680-1860 cm-1 (C band). The νOH stretching modes of HOH in Fe(H2PO4)2 · 2H2O appear at 3140 cm-1 (ν1) and 3320 cm-1 (ν3). The doublet bands at 1630 and 1567 cm-1 contribute both to the C band and to the water bending mode.
The FTIR spectrum of the calcined Fe(H2PO4)2 · 2H2O at 413 K (Figure 3b) is similar to that of FeH2P2O7.25,26 The infrared spectrum can be interpreted on the basis of characteristic
Figure 4. FTIR spectra of Fe(H2PO4)2 · 2H2O (a) and Fe(H2PO4)2 · 2H2O calcined at 423 (b) and 673 (c) K.
Figure 5. SEM micrograms of Fe(H2PO4)2 · 2H2O (a) and Fe(H2PO4)2 · 2H2O calcined at 423 (b) and 673 (c) K.
7646 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
and 673 K, respectively. It is worth noting that the higher value of 96.28 emu/g obtained in the Fe(H2PO4)2 · 2H2O sample was compared with the Ms of bulk Fe3O4 (92 emu/g)30,31 and the reported magnetic properties of nanoparticles are usually smaller than those of the corresponding bulk materials. Gupta and Gupta32 reported that the Ms value of Fe3O4 nanoparticles is in the range 30-50 emu/g. The coercive forces (Hc) obtained on the increasing and decreasing filed sides (blanket) were 488.76 (-488.76), 503.07 (-503.07), and 501.76 (-501.76) Oe for Fe(H2PO4)2 · 2H2O and the Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K, respectively. Ferromagnetic properties of Fe(H2PO4)2 · 2H2O and the Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K were reported for the first time. 4. Conclusions Figure 6. Specific magnetization of the synthesized Fe(H2PO4)2 · 2H2O (a) and Fe(H2PO4)2 · 2H2O calcined at 423 (b) and 673 (c) K as a function of field, measured at 293 K.
vibrations of the PO2 group, P-OH bond, and POP bridge. The P-O stretching modes of the [P2O7]4- anion are known to appear in the 1250-975 cm-1 region.27 The symmetric PO2 stretching vibrations (νsym PO2) for the FeH2P2O7 sample are observed in the range of 1000-1100 cm-1, while the asymmetric stretching vibrations (νasym PO2) are located at 1100 and 1200 cm-1. The asymmetric (νasym POP) and symmetric stretch (νsym POP) bridge vibrations for this sample are observed in the 900-1000 and 400-700 cm-1 regions, respectively. The PO3 deformation, the rocking modes, the POP deformations, and the torsional and external modes are found in the 400-230 cm-1 region. The FTIR spectrum of the calcined Fe(H2PO4)2 · 2H2O at 673 K (Figure 3c) is similar to that of Fe2P4O12.28 The vibrational modes of the P4O124- ion are assigned according to the literature.28,29 One of the most noteworthy features of this spectrum is the presence of strong bands in the ranges 1350-1220, 1150-1100, 1080-950, and 780-400 cm-1. These bands can be assigned to νasOPO-, νsOPO-, νasPOP, and νsPOP vibrations, respectively. The observation of a strong νsPOP band is known to be the most striking feature of cyclotetraphosphate spectra, along with the presence of the νasOPO- band. X-ray diffraction data8 showed that the crystal structure is monoclinic (space group C2/c) with a cyclic structure of the [P4O12]4- anion. This has been confirmed by the FTIR measurements. 3.5. Scanning Electron Microscopy. The morphologies of Fe(H2PO4)2 · 2H2O and Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K are shown in Figure 5. The morphologies of Fe(H2PO4)2 · 2H2O and Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K show coalescence in aggregates of irregularly shaped crystals of different sizes. The corresponding SEM micrographs of the mentioned compounds appear to be highly agglomerated caused primarily by the process of dissolution in precipitation and thermal decomposition reactions. 3.6. Magnetic Properties. The specific magnetization curves of Fe(H2PO4)2 · 2H2O and Fe(H2PO4)2 · 2H2O calcined at 413 and 673 K obtained from room-temperature VSM measurements are shown in Figure 6. This curve is typical for a strong magnetic material and indicates clear hysteresis ferromagnetism in the field range of ∼(10000 Oe; in this range the specific magnetization increases with increasing field and is strong in the field range investigated ((10 kOe). Specific saturated magnetization (Ms) values of 96.28, 32.25, and 37.78 emu/g were observed for Fe(H2PO4)2 · 2H2O and Fe(H2PO4)2 · 2H2O calcined at 413
Fe(H2PO4)2 · 2H2O powder was successfully synthesized by a simple method using Fe metal and phosphoric acid at 313 K with short time consumption (15 min). Kinetic analysis from a nonisothermal TG applying model-fitting method results in a big variation of activation energies on the degree of conversion R in the range 0.2-0.8, which can be assigned to a complex process. The activation energy calculated for the decomposition of Fe(H2PO4)2 · 2H2O by different methods and techniques was found to be consistent. The structures, morphology, and magnetic properties of synthesized powder and its transformation products were investigated. The Fe(H2PO4)2 · 2H2O and its thermal decomposition product show interesting properties, which may be useful in the field of catalysts, wastewater purification systems, ferroelectrics, lithium batteries, ferrofluids, and magnetic recording applications. The presented synthesis method is simple, cost-effective, and environmentally friendly and can be used to prepare other interesting materials. Acknowledgment We thank the Chemistry and Physics Departments, Khon Kaen University, for providing research facilities. This work was financially supported by King Mongkut’s Institute of Technology Ladkrabang and the Center for Innovation in Chemistry: Postgraduate Education and Research Program in Chemistry (PERCH-CIC). Literature Cited (1) Koleva, V.; Mehandjiev, D. Characterization of M(H2PO4)2 · 2H2O (M ) Mn, Co, Ni) and their in situ thermal decomposition by magnetic measurements. Mater. Res. Bull. 2006, 41, 469–477. (2) Huang, H.; Yin, S. C.; Nazar, L. F. Approaching theoretical capacity of LiFePO4 at room temperature at high rates. Electrochem. Solid State Lett. 2001, 4, A170–A172. (3) Masquelier, C.; Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. New cathode materials for rechargeable lithium batteries: The 3-D framework structures Li3Fe2(XO4)3(X ) P, As). J. Solid State Chem. 1998, 135, 228–234. (4) Masquelier, C.; Reale, P.; Wurm, C.; Morcrette, M.; Dupont, L.; Larcher, D. FePO4 · nH2O and Fe4(P2O7)3 · nH2O: Hydrated iron phosphates as 3V positive electrodes in rechargeable lithium batteries. J. Electrochem. Soc. 2002, 149, A1037–A1044. (5) Masquelier, C.; Wurm, C.; Rodriguez-Carvajal, J.; Gaubicher, J.; Nazar, L. F. A powder neutron diffraction investigation of the two rhombohedral NASICON analogues: γ-Na3Fe2(PO4)3 and Li3Fe2(PO4)3. Chem. Mater. 2000, 12, 525–532. (6) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. Effect of structure on the Fe3+/Fe2+ redox couple in iron phosphates. J. Electrochem. Soc. 1997, 144, 1609–1613. (7) Delacourt, C.; Poizot, P.; Morcrette, M.; Tarascon, J.-M.; Masquelier, C. One-step low-temperature route for the preparation of electrochemically active LiMnPO4 powders. Chem. Mater. 2004, 16, 93–99.
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ReceiVed for reView January 2, 2008 ReVised manuscript receiVed July 1, 2008 Accepted July 26, 2008 IE800007J
