Journal of the Korean Physical Society, Vol. 51, No. 3, September 2007, pp. 1166∼1171
Evolution of the Structural and the Optical Properties and the Related Electronic Structure of LiTx Mn2−x O4 (T = Fe and Ni) Thin Films Kwang Joo Kim∗ and Jung Han Lee Department of Physics and Center for Emerging Wireless Transmission Technology, Konkuk University, Seoul 143-701 (Received 13 November 2006) Rechargeable-battery-applicable LiTx Mn2−x O4 (T = Fe or Ni) thin films with a spinel structure were prepared by using a sol-gel method. By Fe doping, cubic LiFex Mn2−x O4 films were produced without any secondary phase for x ≤ 0.9. The lattice constant of the LiFex Mn2−x O4 films was found to decrease with increasing x. The substituting Fe ions take an ionic valence of +3 mostly, as found by using M¨ ossbauer spectroscopy. For LiNix Mn2−x O4 , the cubic structure was maintained at low x, but the tetragonal structure appeared for x ≥ 0.6. Such a cubic-tetragonal phase transition indicates that octahedral Ni3+ (d7 ) ions exist as a low-spin (t2g 6 ,eg 1 ) state in the compound, thus, subject to a Jahn-Teller distortion. By using X-ray photoelectron spectroscopy, both Ni2+ and Ni3+ ions were detected, with a higher Ni2+ density than Ni3+ density. The optical properties of the LiTx Mn2−x O4 films were investigated by using spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer (CT) transitions, O2− (2p) → Mn4+ (3d) for 1.9 (t2g ) and 2.8 ∼ 3.0 eV (eg ) structures and O2− (2p) → Mn3+ (3d) for 2.3 (t2g ) and 3.4 ∼ 3.6 eV (eg ) structures. Also, sharp absorption structures were observed at about 1.6, 1.7 and 1.9 eV and were interpreted as being due to d-d crystal-field (CF) transitions within the octahedral Mn3+ ion. In terms of these CT and CF transitions, the evolution of the optical absorption spectrum of LiMn2 O4 by Fe and Ni doping could be explained, and the related electronic structure parameters were obtained. PACS numbers: 77.84.Bw, 77.55.+f, 71.70.Ch, 78.20.Ci Keywords: Spinel, Lithium, Thin film, Jahn-Teller, Optical properties
I. INTRODUCTION Lithium transition-metal oxides have been under intense research for decades due to their applications for rechargeable lithium batteries. Lithium-manganese oxides have received special attention recently for replacing lithium-cobalt oxides that were first adopted as a cathode material in commercialized batteries but have limitations due to high cost, toxicity, and safety problems. As one of those manganese oxides, lithium manganate (LiMn2 O4 ) has been under considerable research for high-power battery applications [1–6]. LiMn2 O4 exhibits a normal spinel structure with the Li+ ions occupying the tetrahedral sites and a 1:1 mixture of Mn3+ and Mn4+ ions being randomly distributed over the octahedral sites. The Li content can be varied between 0 and 1 and even higher without substantial changes in the crystal structure [6]. The existence of high-spin Mn3+ (t2g 3 ,eg 1 ) ions in the octahedral sites of LiMn2 O4 tends to induce a Jahn-Teller (JT) effect that ∗ E-mail:
is likely to cause structural instability during chargedischarge cycling [5]. If the structural stability is to be improved, it would be worthwhile to investigate the physical properties of manganese-substituted ternary compounds, LiTx Mn2−x O4 (T = metallic element). In the present study, LiTx Mn2−x O4 (T = Fe or Ni) thin films were synthesized using a sol-gel method. LiMn2 O4 films have been prepared using electron-beam evaporation [2,3], sputtering [4] and sol-gel [5,6] methods. Sol-gel growth can overcome the disadvantages of vacuum growth, such as slow deposition rate and high production cost for technological applications. The effects of Fe or Ni substitution on the structural and the optical properties were investigated by using X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). The absorption structures in the optical frequency range observed by using SE were interpreted in terms of chargetransfer (CT) and crystal-field (CF) transitions involving Mn 3d electrons. The variation of electronic structure in LiMn2 O4 by Fe or Ni substitution could be understood based on the optical data.
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Evolution of the Structural and the Optical Properties and· · · – Kwang Joo Kim and Jung Han Lee
Fig. 1. XRD spectra of LiFex Mn2−x O4 films fabricated on Al2 O3 (0001) substrates.
II. EXPERIMENT The Fe- and Ni-substituted LiMn2 O4 films were deposited on Al2 O3 (0001) substrates by using a sol-gel method. For LiMn2 O4 films, the precursor solution was prepared by dissolving (CH3 CO2 )2Mn·4H2 O and CH3 CO2 Li powders together in a mixed solution of 2methoxyethanol and monoethanolamine at 230 ◦ C. The molar ratio Li:Mn in the precursor solution was 1:2 for the LiMn2 O4 films. For Fe and Ni doping, the precursor solution was prepared by adding (FeNO3 )3 ·9H2 O and (CH3 CO2 )2 Ni·4H2 O powders, respectively. The substrate was spin-coated by using the precursor solution at 4000 rpm for 30 sec and then heated at 170 ◦ C for 2 min followed by heating at 300 ◦ C for 5 min after each deposition in order to remove the organic substance. This process was repeated until the desired film thickness had been attained. Then, the precursor films were annealed in air at 600 ◦ C for 8 hr. Prior to the film deposition, the substrates were cleaned by using acetone followed by methanol in an ultrasonic bath. The thicknesses of the films obtained by using the above process were about 1 µm, as determined by imaging the sample section with scanning electron microscopy. The crystalline structure of the samples was monitored by using XRD with Cu Kα radiation. The Fe or Ni compositions (x) in the samples were examined by using energydispersive X-ray spectroscopy. The optical properties of the samples were investigated by SE in the visible-ultraviolet range with a rotatinganalyzer ellipsometer (energy interval of 0.02 eV). Ellipsometry measures the amplitude and the phase of the
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Fig. 2. XRD spectra of LiNix Mn2−x O4 films fabricated on Al2 O3 (0001) substrates.
complex reflectance ratio ρ (= rs /rs ) of the p (parallel) and s (perpendicular) field components of the light beam defined with respect to the plane of incidence of the sample. Then, the dielectric function, ε(= ε1 + iε2 ), of the sample can be obtained from the equation 2 1−ρ 2 2 2 ε = sin φ + sin φ tan φ 1+ρ by assuming an optically flat boundary between sample and air. All the spectra were taken at an angle of incidence (ϕ) of 70◦ and a fixed polarizer angle of 45◦ from the plane of incidence.
III. RESULTS AND DISCUSSION As Figure 1 shows, the XRD spectra of the LiFex Mn2−x O4 films indicate that the cubic structure of LiMn2 O4 is maintained for x ≤ 0.9 without any secondary phase. However, for LiNix Mn2−x O4 , a cubic-totetragonal phase transition was observed for x ≥ 0.6, as marked by ∗ in Figure 2. The lattice constants of the Fe- and the Ni-doped samples estimated from the XRD (400) peak position are exhibited in Figure 3. For the LiFex Mn2−x O4 films, a decrease in the lattice constant with increasing Fe composition is observed. In Figure 4, the conversion electron M¨ossbauer spectrum taken at room temperature for the x = 0.7 sample shows a strong double-line (paramagnetic) pattern near the center and a weak six-line (ferromagnetic) pattern. The isomer shifts of the double-line and the six-line pattern are 0.26 and 0.23 mm/s, respectively, indicating that the Fe ions have
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Journal of the Korean Physical Society, Vol. 51, No. 3, September 2007
Fig. 3. Lattice constants of LiFex Mn2−x O4 and LiNix Mn2−x O4 films evaluated from the XRD (400) peak position.
Fig. 5. XPS spectra of the Ni 2p core levels of LiNi0.4 Mn1.6 O4 films and the results of Doniac-Sunjic lineshape fittings on the spectra. The dot-dashed line represents the background. The open circles and the solid line represent the experimental and the fitted data, respectively.
Fig. 4. Room-temperature conversion electron M¨ ossbauer spectrum for the LiFex Mn2−x O4 (x = 0.7) film.
a valence of +3 mostly. The LiNix Mn2−x O4 films also show a decreasing trend of the lattice constant at low x, similar to that of LiFex Mn2−x O4 , as seen in Figure 3. For x ≥ 0.6, the LiNix Mn2−x O4 samples exhibited a tetragonal structure with c/a = 1.02 and an increased unitcell volume of about 2 % compared to that of LiMn2 O4 . The occurrence of the cubic-to-tetragonal phase transition with increasing Ni composition in LiNix Mn2−x O4 indicates the existence of low-spin Ni3+ (t2g 6 ,eg 1 ) ions in the octahedral sites, inducing a JT distortion in the lattice structure. In Figure 5, Ni 2p core-level spectra, measured by using X-ray photoelectron spectroscopy (XPS), from the LiNi0.4 Mn1.6 O4 film are exhibited. It is seen that the spin-orbit-split 2p3/2 and 2p1/2 peaks are clearly resolved from their respective satellites located at higher binding energies. A shoulder (marked by an arrow) is clearly seen in the experimental XPS data in Figure 5, suggesting the multi-valence nature of Ni ions [7]. The XPS spectra were fitted by using a Doniach-Sunjic (DS) Lorentzian
line shape convoluted with a Gaussian factor and the background due to the inelastic scattering of photoelectrons [8]. The 2p3/2 and the 2p1/2 structures could be well fitted by using two lines, as seen in Figure 5. The binding energy (BE) difference of about 1.8 eV between the two lines supports the assignment of Ni2+ and Ni3+ ions as the main contributors to the structures [7, 9]. Due to a stronger Coulomb attraction, the 2p electrons in the Ni3+ ion have larger BE than those in the Ni2+ ion. There have been reports that the Ni ions substituting in LiNix Mn2−x O4 have ionic valences of both +2 and +3 [10,11]. The intensity of the Ni2+ line is also seen to be stronger than that of the Ni3+ line, indicating that the density of Ni2+ is larger than that of Ni3+ in the sample. The dominance of Ni2+ over Ni3+ in LiNix Mn2−x O4 can explain the absence of the JT distortion at low x. Considering the fact that the Mn ion takes either +3 or +4 as its valence, the substitution of Ni2+ ions at the octahedral sites causes a charge deficiency. Such charge imbalance can be compensated for by an oxidation of Mn3+ into Mn4+ . The evidence for Mn3+ → Mn4+ oxidation can be confirmed by the Mn 2p XPS spectra of the same sample, as shown in Figure 6. The DS fitting on the spectra resulted in a BE difference of about 0.8 eV, in agreement with existing data for Mn3+ and Mn4+ [9,12]. The density of Mn4+ is seen to be larger than that of Mn3+ , indicative of the oxidation, Mn3+ → Mn4+ . The faster
Evolution of the Structural and the Optical Properties and· · · – Kwang Joo Kim and Jung Han Lee
Fig. 6. XPS spectra of the Mn 2p core levels of LiNi0.4 Mn1.6 O4 films and the results of Doniac-Sunjic lineshape fittings on the spectra. The dot-dashed line represents the background. The open circles and the solid line represent the experimental and the fitted data, respectively.
decrease in the Mn3+ density compared to the Mn4+ density by the Ni substitution is likely to reduce the chance of a structural instability during charge-discharge cycling caused by JT-susceptible high-spin Mn3+ ions [13]. In Figure 7, the imaginary parts (ε2 ) of the dielectric functions of LiNix Mn2−x O4 (x = 0.2 and 0.6) films are compared with those of LiMn2 O4 film. Broad absorption structures were observed for LiMn2 O4 at about 1.9, 2.3, 2.8 ∼ 3.0 and 3.4 ∼ 3.6 eV, as marked by ∗. The 1.9and 2.8 ∼ 3.0 eV absorption structures are interpreted as being due to CT transitions involving O2− and octahedral Mn4+ ions, i.e., O2− (2p) → Mn4+ (3d3 ) for the 1.9- (t2g ) and the 2.8 ∼ 3.0 eV (eg ) structures. The t2g band of the Mn4+ ion is half-filled while the higher-lying eg band is empty. The broader nature of the eg energyband width compared to the t2g band is reflected in the broader absorption structure for the transition involving eg states compared to t2g states [14]. On the other hand, the 2.3- and 3.4 ∼ 3.6 eV structures are interpreted as being due to CT transitions, O2− (2p) → Mn3+ (3d4 ) for 2.3- (t2g ) and 3.4 ∼ 3.6 eV (eg ) structures. The stronger Coulomb attraction for the d electrons in Mn4+ ion than for those in Mn3+ ion leads to higher binding energies and resultant lower CT transition energies for Mn4+ than for Mn3+ . From the above CT transition structures, the crystal-field splittings between the eg and the t2g states in LiMn2 O4 are estimated to be about 1.0 and 1.2 eV for octahedral Mn4+ and Mn3+ ions, respectively. Also,
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Fig. 7. Imaginary parts of the dielectric functions of LiNix Mn2−x O4 films measured by using spectroscopic ellipsometry.
the t2g and eg states of the octahedral Mn3+ ion are expected to be located at higher energies than those of the Mn4+ ion by about 0.4 and 0.6 eV, respectively. The d states of substituting Ni2+ and Ni3+ ions are expected to be located below those of the octahedral Mn4+ ion. Thus, the CT transitions involving Ni2+ and Ni3+ ions are likely to be located below 1.9 eV, which is beyond the present SE spectral range. Along with the strong and broad CT structures, weak and sharp absorption structures are clearly resolved at about 1.6, 1.8 and 1.9 eV for LiNix Mn2−x O4 , as indicated by the arrows in Figure 7. These structures are interpreted as being due to CF transitions within octahedral Mn3+ ions. Such interpretation is based on a similarity of the CF spectrum of LiMn2 O4 to that of Mn2 O3 [15], with both compounds containing octahedral Mn3+ ions. These CF transitions overlap with the CT transition at about 1.9 eV, so the reduction in the CT transition strength in LiNix Mn2−x O4 compared to LiMn2 O4 makes the CF structures more discernable. The ground state of the d-d multiplets of the octahedral Mn3+ (d4 ) ion is 5 D with a t2g 3 eg 1 electronic configuration. Under an octahedral crystal field, it is split into 5 Eg and 5 T2g [16]. Thus, the 1.6 eV CF structure is interpreted as being due to the transition between 5 Eg and 5 T2g states, 5 Eg (D) → 5 T2g (D). Also, the 1.8- and 1.9 eV CF structures are interpreted as being due to the transitions to higher-lying 3 Eg (H) and 3 A2g (F) states, respectively, from the 5 Eg ground state, 5 Eg (D) → 3 Eg (H) and 5 Eg (D) → 3 A2g (F).
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Journal of the Korean Physical Society, Vol. 51, No. 3, September 2007
Fig. 8. Imaginary parts of the dielectric functions of LiFex Mn2−x O4 films measured by using spectroscopic ellipsometry.
For tetragonal LiNi0.6 Mn1.4 O4 , the CT absorption strengths are further reduced compared to LiNi0.2 Mn1.8 O4 . The eg -related 2.8 ∼ 3.0- and 3.4 ∼ 3.6 eV structures are reduced more than the t2g -related structures for the tetragonal sample. The JT-induced tetragonal distortion is expected to reduce the overlap between the p(O2− ) and the eg wavefunctions. Such reduced overlap may result in a reduction of the related CT transition strength. In Figure 8, ε2 of LiFex Mn2−x O4 (x = 0.2) is compared with that of LiMn2 O4 . It is seen that the eg -related absorption structures are enhanced while the t2g -related ones at lower energies are reduced. The eg states of octahedral Fe3+ ion are expected to be located between those of Mn3+ and Mn4+ ions. Thus, the increased absorption intensity for LiFex Mn2−x O4 is interpreted as due to the contribution of CT transitions involving octahedral Fe3+ ions, O2− (2p) → Fe3+ (3d5 ). Such CT transitions are known to induce a strong absorption band around 3 eV in Fe3 O4 [17].
IV. CONCLUSIONS With the sol-gel method, normal spinel LiTx Mn2−x O4 (T = Fe and Ni) films could be prepared without any secondary phase by annealing the spin-coated precursor films in air. At low x, a cubic structure was maintained with the lattice constant decreasing with x for
both Fe and Ni doping, and a phase transition from cubic to tetragonal structure was observed for x ≥ 0.6 for Ni doping. The cubic-tetragonal phase transition in LiNix Mn2−x O4 indicates the existence of octahedral Ni3+ ions with a low-spin configuration (t2g 6 , eg 1 ), which is well affected by the JT distortion. Both Ni2+ and Ni3+ ions were found to exist, with a larger Ni2+ density in LiNix Mn2−x O4 . On the other hand, Fe3+ was found to be dominant in LiFex Mn2−x O4 . The optical absorption spectrum of LiMn2 O4 consists of broad CT structures at about 1.9 and 2.8 ∼ 3.0 eV due to O2− → Mn4+ transitions and 2.3 and 3.4 ∼ 3.6 eV due to O2− → Mn3+ transitions. On the other hand, sharp structures observed at about 1.6, 1.8 and 1.9 eV are interpreted as being due to CF transitions in the octahedral Mn3+ ion. The strengths of these absorption structures were reduced by the Ni substitution. The rapid reduction in the CT transition strength involving the eg states of the tetragonal LiNix Mn2−x O4 is attributed to the reduced wavefunction overlap between the eg and the O2− (2p) states due to the JT-induced extension of the lattice constant. By Fe doping, the eg -related absorption structures were enhanced, which is interpreted as being due to the CT transitions involving octahedral Fe3+ ions. From the optical analysis, the crystal-field splittings in the octahedral Mn3+ and Mn4+ ions in LiMn2 O4 are about 1.2 and 1.0 eV, respectively, and the t2g and eg states in the Mn3+ ion are located at higher energies than those in the Mn4+ ion by about 0.4 and 0.6 eV, respectively.
ACKNOWLEDGMENTS This work was supported by Konkuk University in the program year of 2005.
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