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Jahn–Teller transition in Al3+ doped LiMn2O4 spinel
Solid State Communications 126 (2003) 169–174 www.elsevier.com/locate/ssc
Jahn –Teller transition in Al3þ doped LiMn2O4 spinel Doretta Capsonia, Marcella Binia, Gaetano Chiodellia, Vincenzo Massarottia,*, Piercarlo Mustarellia,1, Laura Linatib, Maria Cristina Mozzatic, Carlo B. Azzonic a
Dipartimento di Chimica Fisica “Mario Rolla”, Universita` di Pavia and IENI-CNR, Sezione di Pavia, viale Taramelli 16, 27100 Pavia, Italy b Centro Grandi Strumenti, Universita` di Pavia, Via Bassi 21, I-27100 Pavia, Italy c INFM—Dipartimento di Fisica “Alessandro Volta”, Universita` di Pavia, via Bassi 6, I-27100 Pavia, Italy Received 15 January 2003; accepted 7 February 2003 by P. Wachter
Abstract Al-doped lithium manganese spinels, with starting composition Li1.02AlxMn1.982xO4 ð0:00 , x # 0:06Þ; are investigated to determine the influence of the Al3þ doping on the Jahn – Teller (J– T) cooperative transition temperature TJ – T : X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al3þ ions in the spinel phase. It is observed that Al3þ may distribute between the two cationic sublattices. The J – T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x . 0:04 TJ – T it increases with increasing x; as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior. q 2003 Elsevier Science Ltd. All rights reserved. PACS: 76.30; 78.70.C; 71.70.E Keywords: E. Electron paramagnetic resonance; E. X-ray scattering; E. Nuclear magnetic resonance; D. Charge distribution models
1. Introduction A lot of recent studies was devoted to the investigation of LiMn2O4 spinel doped by different cations [1 – 9]. In the pure LiMn2O4 a Jahn – Teller (J – T) distortion occurs near room temperature (rt) [10,11], evidenced by a cubic to orthorhombic reversible hysteretic transition at about 280 K on cooling. This effect, due to tetragonal cooperative elongation of the Mn – O apical bonds in Mn3þO6 octahedra, strongly affects the stability of the material in its electrochemical applications. It was demonstrated that small amounts of cation doping (#3%) could inhibit the transition * Corresponding author. Fax: þ 39-382-507575. E-mail address: [email protected] (V. Massarotti). 1 Also with INFM, Pavia, Italy.
or significantly shift the transition temperature TJ – T below rt. Among the possible trivalent non-transition doping ions, Al3þ like Ga3þ is potentially interesting to prepare materials suitable for electrochemical applications, e.g. for rechargeable lithium batteries [7,12]. In this paper, we analyze Li – Mn spinel doped with Al3þ with starting composition Li1.02AlxMn1.982xO4 ð0:00 , x # 0:06Þ; to verify the possibility to prevent the occurrence of the J– T transition with very low amount of dopant. X-ray powder diffraction (XRPD), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), conductivity and static magnetic susceptibility measurements allowed us to test the location on both tetrahedral and octahedral sites and the homogeneous dilution of the Al3þ ions and to detect the behavior of the J – T transition as a function of the temperature and of the Mn valence state.
0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00129-7
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2. Experimental 2.1. Materials and samples preparation A set of Al-doped samples was prepared by reacting MnO2 with Al(NO3)3·9H2O and LiOH·H2O in the amount required to obtain the composition Li1.02AlxMn1.982xO4 with 0:00 , x # 0:06 following the thermal treatment previously reported [2]. An additional sample with x ¼ 0:20 was also prepared to try out the formation of other Li compounds, as LiAlO2 (see below in Section 3). 2.2. Apparatus and procedures The instrumentation, experimental conditions and computation procedures for XRPD, EPR, NMR, static magnetic susceptibility and conductivity measurements were already reported in detail [2]. Some crystallographic models considering different cationic distribution on both tetrahedral and octahedral sites were used to perform Rietveld refinements using both FULLPROF [13] and TOPAS [14] softwares. NMR 27Al spectra were acquired at the frequency of 104.2 MHz with a 4 mm CP-MAS probe (Bruker). The spectra were averaged over 24,000 acquisitions. A single pulse sequence (pulse width 6 ms) was used. We did not see significant differences between the spectra in static conditions and those acquired under MAS rotation. A recycle time of 0.2 s was employed. The spectra were referenced to Al(NO3)3 saturated solution.
3. Results 3.1. XRPD Fig. 1 shows the diffraction patterns of the doped samples in comparison with the pure ðx ¼ 0:00Þ sample: for x # 0:06 only the cubic spinel phase can be observed, independent of the dopant content. A greater line broadening with respect to the pure sample is observed in doped ˚ can samples for which a crystallite size of about 750(150) A be evaluated. This value, related to the peculiar synthesis from nitrates, is at least 1/3 of the crystallite size value estimated for the pure sample. By increasing x; small amount of LiAlO2 begins to be detected as well evidenced in the inset of Fig. 1 for x ¼ 0:20: For the samples with x # 0:06; the value we obtained through Rietveld refinement [13,14] of the lattice parameter a is sensibly lower than the pure spinel one ða ¼ 8:2419ð1Þ and results independent of x (Table 1) and of the AÞ crystallographic model. The results pertinent to the most reliable model of cation substitution constrained on both lattice sites are reported in Table 1 together with the discrepancy factors [13]. The results indicate that Al3þ
Table 1 x ˚) a (A xt xo Rwp Rp S
0.00 8.2419(1)
15.3 11.3 1.16
0.04
0.06
8.2334(1) 0.009(3) 0.031(3) 15.7 11.5 1.15
8.2337(1) 0.015(3) 0.045(3) 15.2 11.3 1.15
octahedral substitution is preferred with respect to the tetrahedral one with about 3:1 ratio. A further refinement test on the basis of a partial inversion model (Mn on Li and Al on Mn site) gave much greater discrepancy factors (Rwp ¼ 18:6; Rp ¼ 13:6; S ¼ 1:41 for x ¼ 0:06 sample) and was not considered. To check the presence of the J– T cubic to orthorhombic transition [10,11], diffraction patterns were taken at different temperatures. For the x ¼ 0:04 sample the observation of 222 and 440 line profiles as a function of T shows at 223 K a splitting into three peaks of the 440 cubic line, suggesting the onset of the transition, as previously explained [2] (Fig. 2(a)). For the x ¼ 0:06 sample (Fig. 2(b)) this split occurs at 243 K. As expected, the 222 line does not split in this temperature range (Fig. 2(a) and (b)). In both cases, TJ – T is remarkably lower with respect to the known value for the pure sample [15,16]. 3.2. DC conductivity In Fig. 3(a) and (b) the DC conductivity (s) curves of the x ¼ 0:04 and 0.06 samples, respectively, are reported vs. T: The step in conductivity observed at about 285 K in the undoped sample, related to the J – T transition [11], results no longer present in the doped samples. However, for the x ¼ 0:04 sample, a gradual slope change in s curve is observed between 280 and 245 K on cooling, while on heating it is observed in the whole temperature range. A hysteretic cycle is observed that seems to complete only at lower temperatures below the observable range. For the x ¼ 0:06 sample a gradual slope change is better observed in the whole temperature range on cooling than on heating and the hysteretic behavior characteristic of the x ¼ 0:04 sample is less evident. 3.3. EPR and magnetic susceptibility measurements Fig. 4(a) and (b) shows the thermal behavior of the EPR signal of x ¼ 0:04 and 0.06 samples, respectively. The EPR spectra consist of a broad signal ðDB < 0:3 TÞ centered at g < 2 (signal A), very similar to that of pure spinel [1]. For x ¼ 0:04; a weak signal with a resonant field at 0.2– 0.25 T (signal B) is detectable for T , 263 K (see inset), and another very week signal with DB ø 0:02 T centered at g ¼ 1:994 (signal C) is present over the whole T range. These
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Fig. 1. A selected angular region of the X-ray diffraction patterns for the x ¼ 0:00; 0.04, 0.06 and 0.20 samples. The inset shows the main reflections of LiAlO2, marked by stars, in the x ¼ 0:20 sample.
signals, already observed in samples doped by transition cations (Ni, Co, Cr, Ti) [1] and by Ga3þ and Mg2þ ions [2, 17] were attributed to the spinel phase (signal A), to clusters of Mn4þ ions in low symmetry sites, due to inhomogeneous distribution of doping ions in the spinel matrix (signal B), and to traces of the Li2MnO3 phase (signal C). The signal B depends on the different synthesis processes (as thermal
Fig. 2. Peak positions of the 222 (circle) and 440 (square) reflections as a function of T on cooling (filled symbols) and on heating (empty symbols) for (a) the x ¼ 0:04 and (b) 0.06 samples.
treatment) but does not depend on the specific dopant [1,2, 17]. The signal C allows to estimate the weight percent of Li2MnO3 phase in the considered sample x ¼ 0:04; as shown in a previous work [15]. This impurity amount is estimated less than 0.01% and does not influence significantly the Al/Mn ratio in the spinel phase. Fig. 5 shows the reciprocal magnetic susceptibilities of x ¼ 0:04; 0.06 and of the pure spinel samples. Both field
Fig. 3. logðsÞ as a function of T of the (a) x ¼ 0:04 and (b) 0.06 samples. Arrows indicate the cooling and heating process.
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Fig. 5. Reciprocal magnetic susceptibility of pure and doped samples. FC and ZFC curves trend is shown.
0.06 samples. The spectrum of the sample x ¼ 0:06 is characterized by a very large manifold (thousands of ppm) of spinning sidebands, similar to those observed in the 7Li NMR spectra. The isotropic part of the spectrum is made of two contributions at , 0 and ,70 ppm (marked with stars),
Fig. 4. EPR spectra at different temperatures (163, 213, 243, 263, 293 and 473 K) of (a) x ¼ 0:04 and (b) x ¼ 0:06 samples. The weak B component at 243 K is indicated in the inset for the x ¼ 0:04 sample.
cooling (FC) and zero field cooling (ZFC) curves at 0.1 T are reported. 3.4. NMR The 7Li spectra of x ¼ 0:04 and 0.06 samples, very similar, are constituted by a large manifold of spinning sidebands, due to a partial removal by MAS rotation of the anisotropic part of dipolar interaction among the Li nuclei and the Mn unpaired electrons [18,19]. The spectra (not shown) are constituted by a single isotropic peak at about 520 ppm, which is attributed to Li in the 8a tetrahedral position. A weak shoulder of the main peak is also observed which can be related to an isotropic feature near 850 ppm due to lithium in the octahedral 16d site [2,19]. Fig. 6 shows the 27Al NMR spectra of the x ¼ 0:04 and
Fig. 6. 27Al NMR spectra of the x ¼ 0:04 (lower) and 0.06 (upper) samples. The peaks at ,0 and ,70 ppm due to sixfold and fourfold coordinated aluminum are marked by stars.
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that can be attributed to sixfold and fourfold coordinated aluminum, respectively [20]. The information is also semiquantitative, and by a simple Gaussian deconvolution we can estimate a distribution of 85 ^ 5% for AlO6 and 15 ^ 5% for AlO4. The uncertainty is due: (i) to the spectrum noise, and (ii) to the fact that the lines are still partially broadened by second order quadrupolar interaction, which limits the accuracy of a fit performed with symmetric functions. The spectrum of x ¼ 0:04 sample shows an anisotropic broadening strongly reduced with respect to the x ¼ 0:06 sample and only few spinning sidebands observed under MAS rotation. However, for what concerns the aluminum coordination, a careful analysis of the central line of the spectrum shows that only a small fraction (,5%) of the Al nuclei is in tetrahedral coordination, while the remaining part is octahedrally coordinated.
4. Discussion and conclusions The rt EPR spectra show substantially signal A. In Fig. 4(a) and (b) it is observed that the signal narrows and increase in amplitude with increasing T without changing the resonance field: this behavior is typical of the signal of the pure spinel phase [1]. At T , 263 K; in the x ¼ 0:04 sample the signal B appears, while in the x ¼ 0:06 one, only the signal A is evident in all the investigated temperature range as observed in pure spinel. This fact suggests that the x ¼ 0:04 sample could be affected by grain boundary disorder [1] at low temperature. NMR measurements on 27Al probe gives sufficiently precise and direct estimation of the presence of Al3þ cations in tetrahedral site resulting in a higher content (15%) of tetrahedral Al3þ in the x ¼ 0:06 sample with respect to the x ¼ 0:04 one (5%). On the other hand, also XRPD data refinement may suggest to select a substitutional model, reliable and useful to explain the experimental data, in agreement with NMR results: Al3þ may substitute on both cationic sites with preferred occupation of the 16d site. No evidence for Mn substitution on the Li site is gained in the present samples, differently from Ga-doped samples [2]. For Al-doped samples, low temperature XRPD results clearly show that the TJ – T of the x ¼ 0:04 sample (223 K) is lower than the x ¼ 0:06 sample one (243 K), suggesting that the Mn3þ relative amounts, responsible for the J – T transition, increase with x: This influence of doping upon the J– T transition is different with respect to the other trivalent and bivalent [1,2,17] ions doped samples for which TJ – T decreases with x: Besides, the NMR results suggest a very low Li inversion degree. So, the charge distribution models proposed in the previous works [1,2,17] are no longer applicable and a different charge distribution model
173
could be proposed: 3þ þ 3þ 3þ 4þ ðLiþ 12xt Alxt Þtetr ½Liy Alxo Mn123y2xo þ2xt Mn1þ2y22xt octa
ð1Þ
with y ¼ 0:02 in these samples. In this model no Li inversion is taken into account; rather, an amount xt is lost and is not substituted on the octahedral site. The partial Li loss in the spinel phase and the partial occupation of Al on the 8a site can be justified by the increasing quantity of LiAlO2 phase observed with XRPD on samples with Al content up to x ¼ 0:20 (Fig. 1). The excess positive charge in the tetrahedral sublattice ð2xt Þ is compensated by the equal deficit in the octahedral sublattice related to the increase in lMn3þl and the decrease in lMn4þl. Substituting in Eq. (1) xt ¼ 0:009 and 0.002, according to NMR results, and the related xo ¼ x 2 2xt values, for x ¼ 0:06 and 0.04 samples, respectively, the changes in Mn3þ and Mn4þ fractions agrees with a lMn3þl increase with Al3þ addition thus favoring the J – T transition as proved by the increase in TJ – T for x . 0:04: In agreement with the proposed model, higher x values of the x ¼ 0:06 sample with respect to the 0.04 one (Fig. 5) may be also related to the higher lMn3þl amount, being S ¼ 2 for Mn3þ and S ¼ 3=2 for Mn4þ. In addition, a comparison with the conductivity data near rt of the pure spinel [1] and Ga doped [2] samples shows the greatest s value for the Al-doped samples, where the charge carrier content available for the hopping process [15] should be the highest according to model (1). The substantially constant value of the lattice parameter for the x ¼ 0:04 and 0.06 samples agrees with the model (1), if the effective ionic radii by Shannon [21] are taken into account with the pertinent values for the different site coordination (for Mn3þ the high spin configuration has been considered). However, the best comparison with the experimental a values is obtained if a slight increase in Al3þ (8a) and of Al3þ (16d) radii up to ˚ , respectively, is allowed. Such increase 0.45 and 0.60 A seems consistent with both the size of the pertinent interstices and with the M – O Pauling electronegativity difference. As already observed, the cooperative J – T transition is inhibited and the spinel lattice maintains undistorted cubic structure when the ratio r ¼ lMn4þ l=lMn3þ l is $1.18 [1,2, 17]. This value cannot be reached for Al substituted samples because of the decrease in r with increasing x; as evidenced in this work for 0:00 , x # 0:06 : according to model (1), r takes the values 1.11 and 1.14 for x ¼ 0:06 and 0.04 sample, respectively. Even greater Al substitution cannot enable us to decrease TJ – T or reach the inhibition of J – T distortion because Al3þ progressively occupies the tetrahedral sites and, at the same time, the LiAlO2 phase grows. However, for x ¼ 0:06; a very high homogeneity of the cation distribution is obtained, and the properties of this material make it suitable for electrochemical applications down to 243 K.
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Acknowledgements This work has been partially supported by ‘Consorzio per i Sistemi a Grande Interfase’ (CSGI) and IENI-CNR Department of Pavia.
References [1] D. Capsoni, M. Bini, G. Chiodelli, V. Massarotti, C.B. Azzoni, M.C. Mozzati, A. Comin, Phys. Chem. Chem. Phys. 3 (2001) 2162. [2] D. Capsoni, M. Bini, G. Chiodelli, P. Mustarelli, V. Massarotti, C.B. Azzoni, M.C. Mozzati, L. Linati, J. Phys. Chem. B 106 (2002) 7432. [3] H. Berg, J.O. Thomas, W. Liu, G.C. Farrington, Solid State Ionics 112 (1998) 165. [4] B. Ammundsen, D.J. Jones, J. Roziere, F. Villain, J. Phys. Chem. B 102 (1998) 7939. [5] K. Oikawa, T. Kamiyama, F. Izumi, D. Nakazato, H. Ikuta, M. Wakihara, J. Solid State Chem. 146 (1999) 322. [6] P. Aitchison, B. Ammundsen, D.J. Jones, G. Burns, J. Roziere, J. Mater. Chem. 9 (1999) 3125. [7] A. Antonini, C. Bellitto, M. Pasquali, G. Pistoia, J. Electrochem. Soc. 145 (1998) 2726.
[8] H. Shigemura, M. Tabuchi, H. Kobayashi, H. Sakaebe, A. Hirano, H. Kageyama, J. Mater. Chem. 12 (2002) 1882. [9] S.-T. Myung, S. Komaba, N. Kumagai, J. Electrochem. Soc. 148 (2001) A482. [10] J. Rodriguez Carvajal, G. Rousse, C. Masquelier, M. Hervieu, Phys. Rev. Lett. 81 (1998) 4660. [11] V. Massarotti, D. Capsoni, M. Bini, P. Scardi, M. Leoni, V. Baron, H. Berg, J. Appl. Crystallogr. 32 (1999) 1186. [12] E. Iguchi, Y. Tokuda, H. Nakatsugawa, F. Munakata, J. Appl. Phys. 91 (2002) 2149. [13] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [14] R.W. Cheary, A.A. Coelho, J. Appl. Crystallogr. 25 (1992) 109. [15] V. Massarotti, D. Capsoni, M. Bini, G. Chiodelli, C.B. Azzoni, A. Paleari, J. Solid State Chem. 131 (1997) 94. [16] V. Massarotti, D. Capsoni, M. Bini, G. Chiodelli, C.B. Azzoni, M.C. Mozzati, A. Paleari, J. Solid State Chem. 147 (1999) 509. [17] D. Capsoni, M. Bini, G. Chiodelli, V. Massarotti, M.C. Mozzati, C.B. Azzoni, Solid State Commun. 125 (2003) 179. [18] P. Mustarelli, V. Massarotti, M. Bini, D. Capsoni, Phys. Rev. B 55 (1997) 10218. [19] Y.J. Lee, F. Wang, C.P. Grey, J. Am. Chem. Soc. 120 (1998) 12601. [20] G. Engelhardt, D. Michel, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. [21] R.D. Shannon, Acta Crystallogr. A32 (1976) 751.
Jahn –Teller transition in Al3þ doped LiMn2O4 spinel Doretta Capsonia, Marcella Binia, Gaetano Chiodellia, Vincenzo Massarottia,*, Piercarlo Mustarellia,1, Laura Linatib, Maria Cristina Mozzatic, Carlo B. Azzonic a
Dipartimento di Chimica Fisica “Mario Rolla”, Universita` di Pavia and IENI-CNR, Sezione di Pavia, viale Taramelli 16, 27100 Pavia, Italy b Centro Grandi Strumenti, Universita` di Pavia, Via Bassi 21, I-27100 Pavia, Italy c INFM—Dipartimento di Fisica “Alessandro Volta”, Universita` di Pavia, via Bassi 6, I-27100 Pavia, Italy Received 15 January 2003; accepted 7 February 2003 by P. Wachter
Abstract Al-doped lithium manganese spinels, with starting composition Li1.02AlxMn1.982xO4 ð0:00 , x # 0:06Þ; are investigated to determine the influence of the Al3þ doping on the Jahn – Teller (J– T) cooperative transition temperature TJ – T : X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al3þ ions in the spinel phase. It is observed that Al3þ may distribute between the two cationic sublattices. The J – T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x . 0:04 TJ – T it increases with increasing x; as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior. q 2003 Elsevier Science Ltd. All rights reserved. PACS: 76.30; 78.70.C; 71.70.E Keywords: E. Electron paramagnetic resonance; E. X-ray scattering; E. Nuclear magnetic resonance; D. Charge distribution models
1. Introduction A lot of recent studies was devoted to the investigation of LiMn2O4 spinel doped by different cations [1 – 9]. In the pure LiMn2O4 a Jahn – Teller (J – T) distortion occurs near room temperature (rt) [10,11], evidenced by a cubic to orthorhombic reversible hysteretic transition at about 280 K on cooling. This effect, due to tetragonal cooperative elongation of the Mn – O apical bonds in Mn3þO6 octahedra, strongly affects the stability of the material in its electrochemical applications. It was demonstrated that small amounts of cation doping (#3%) could inhibit the transition * Corresponding author. Fax: þ 39-382-507575. E-mail address: [email protected] (V. Massarotti). 1 Also with INFM, Pavia, Italy.
or significantly shift the transition temperature TJ – T below rt. Among the possible trivalent non-transition doping ions, Al3þ like Ga3þ is potentially interesting to prepare materials suitable for electrochemical applications, e.g. for rechargeable lithium batteries [7,12]. In this paper, we analyze Li – Mn spinel doped with Al3þ with starting composition Li1.02AlxMn1.982xO4 ð0:00 , x # 0:06Þ; to verify the possibility to prevent the occurrence of the J– T transition with very low amount of dopant. X-ray powder diffraction (XRPD), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), conductivity and static magnetic susceptibility measurements allowed us to test the location on both tetrahedral and octahedral sites and the homogeneous dilution of the Al3þ ions and to detect the behavior of the J – T transition as a function of the temperature and of the Mn valence state.
0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00129-7
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2. Experimental 2.1. Materials and samples preparation A set of Al-doped samples was prepared by reacting MnO2 with Al(NO3)3·9H2O and LiOH·H2O in the amount required to obtain the composition Li1.02AlxMn1.982xO4 with 0:00 , x # 0:06 following the thermal treatment previously reported [2]. An additional sample with x ¼ 0:20 was also prepared to try out the formation of other Li compounds, as LiAlO2 (see below in Section 3). 2.2. Apparatus and procedures The instrumentation, experimental conditions and computation procedures for XRPD, EPR, NMR, static magnetic susceptibility and conductivity measurements were already reported in detail [2]. Some crystallographic models considering different cationic distribution on both tetrahedral and octahedral sites were used to perform Rietveld refinements using both FULLPROF [13] and TOPAS [14] softwares. NMR 27Al spectra were acquired at the frequency of 104.2 MHz with a 4 mm CP-MAS probe (Bruker). The spectra were averaged over 24,000 acquisitions. A single pulse sequence (pulse width 6 ms) was used. We did not see significant differences between the spectra in static conditions and those acquired under MAS rotation. A recycle time of 0.2 s was employed. The spectra were referenced to Al(NO3)3 saturated solution.
3. Results 3.1. XRPD Fig. 1 shows the diffraction patterns of the doped samples in comparison with the pure ðx ¼ 0:00Þ sample: for x # 0:06 only the cubic spinel phase can be observed, independent of the dopant content. A greater line broadening with respect to the pure sample is observed in doped ˚ can samples for which a crystallite size of about 750(150) A be evaluated. This value, related to the peculiar synthesis from nitrates, is at least 1/3 of the crystallite size value estimated for the pure sample. By increasing x; small amount of LiAlO2 begins to be detected as well evidenced in the inset of Fig. 1 for x ¼ 0:20: For the samples with x # 0:06; the value we obtained through Rietveld refinement [13,14] of the lattice parameter a is sensibly lower than the pure spinel one ða ¼ 8:2419ð1Þ and results independent of x (Table 1) and of the AÞ crystallographic model. The results pertinent to the most reliable model of cation substitution constrained on both lattice sites are reported in Table 1 together with the discrepancy factors [13]. The results indicate that Al3þ
Table 1 x ˚) a (A xt xo Rwp Rp S
0.00 8.2419(1)
15.3 11.3 1.16
0.04
0.06
8.2334(1) 0.009(3) 0.031(3) 15.7 11.5 1.15
8.2337(1) 0.015(3) 0.045(3) 15.2 11.3 1.15
octahedral substitution is preferred with respect to the tetrahedral one with about 3:1 ratio. A further refinement test on the basis of a partial inversion model (Mn on Li and Al on Mn site) gave much greater discrepancy factors (Rwp ¼ 18:6; Rp ¼ 13:6; S ¼ 1:41 for x ¼ 0:06 sample) and was not considered. To check the presence of the J– T cubic to orthorhombic transition [10,11], diffraction patterns were taken at different temperatures. For the x ¼ 0:04 sample the observation of 222 and 440 line profiles as a function of T shows at 223 K a splitting into three peaks of the 440 cubic line, suggesting the onset of the transition, as previously explained [2] (Fig. 2(a)). For the x ¼ 0:06 sample (Fig. 2(b)) this split occurs at 243 K. As expected, the 222 line does not split in this temperature range (Fig. 2(a) and (b)). In both cases, TJ – T is remarkably lower with respect to the known value for the pure sample [15,16]. 3.2. DC conductivity In Fig. 3(a) and (b) the DC conductivity (s) curves of the x ¼ 0:04 and 0.06 samples, respectively, are reported vs. T: The step in conductivity observed at about 285 K in the undoped sample, related to the J – T transition [11], results no longer present in the doped samples. However, for the x ¼ 0:04 sample, a gradual slope change in s curve is observed between 280 and 245 K on cooling, while on heating it is observed in the whole temperature range. A hysteretic cycle is observed that seems to complete only at lower temperatures below the observable range. For the x ¼ 0:06 sample a gradual slope change is better observed in the whole temperature range on cooling than on heating and the hysteretic behavior characteristic of the x ¼ 0:04 sample is less evident. 3.3. EPR and magnetic susceptibility measurements Fig. 4(a) and (b) shows the thermal behavior of the EPR signal of x ¼ 0:04 and 0.06 samples, respectively. The EPR spectra consist of a broad signal ðDB < 0:3 TÞ centered at g < 2 (signal A), very similar to that of pure spinel [1]. For x ¼ 0:04; a weak signal with a resonant field at 0.2– 0.25 T (signal B) is detectable for T , 263 K (see inset), and another very week signal with DB ø 0:02 T centered at g ¼ 1:994 (signal C) is present over the whole T range. These
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Fig. 1. A selected angular region of the X-ray diffraction patterns for the x ¼ 0:00; 0.04, 0.06 and 0.20 samples. The inset shows the main reflections of LiAlO2, marked by stars, in the x ¼ 0:20 sample.
signals, already observed in samples doped by transition cations (Ni, Co, Cr, Ti) [1] and by Ga3þ and Mg2þ ions [2, 17] were attributed to the spinel phase (signal A), to clusters of Mn4þ ions in low symmetry sites, due to inhomogeneous distribution of doping ions in the spinel matrix (signal B), and to traces of the Li2MnO3 phase (signal C). The signal B depends on the different synthesis processes (as thermal
Fig. 2. Peak positions of the 222 (circle) and 440 (square) reflections as a function of T on cooling (filled symbols) and on heating (empty symbols) for (a) the x ¼ 0:04 and (b) 0.06 samples.
treatment) but does not depend on the specific dopant [1,2, 17]. The signal C allows to estimate the weight percent of Li2MnO3 phase in the considered sample x ¼ 0:04; as shown in a previous work [15]. This impurity amount is estimated less than 0.01% and does not influence significantly the Al/Mn ratio in the spinel phase. Fig. 5 shows the reciprocal magnetic susceptibilities of x ¼ 0:04; 0.06 and of the pure spinel samples. Both field
Fig. 3. logðsÞ as a function of T of the (a) x ¼ 0:04 and (b) 0.06 samples. Arrows indicate the cooling and heating process.
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Fig. 5. Reciprocal magnetic susceptibility of pure and doped samples. FC and ZFC curves trend is shown.
0.06 samples. The spectrum of the sample x ¼ 0:06 is characterized by a very large manifold (thousands of ppm) of spinning sidebands, similar to those observed in the 7Li NMR spectra. The isotropic part of the spectrum is made of two contributions at , 0 and ,70 ppm (marked with stars),
Fig. 4. EPR spectra at different temperatures (163, 213, 243, 263, 293 and 473 K) of (a) x ¼ 0:04 and (b) x ¼ 0:06 samples. The weak B component at 243 K is indicated in the inset for the x ¼ 0:04 sample.
cooling (FC) and zero field cooling (ZFC) curves at 0.1 T are reported. 3.4. NMR The 7Li spectra of x ¼ 0:04 and 0.06 samples, very similar, are constituted by a large manifold of spinning sidebands, due to a partial removal by MAS rotation of the anisotropic part of dipolar interaction among the Li nuclei and the Mn unpaired electrons [18,19]. The spectra (not shown) are constituted by a single isotropic peak at about 520 ppm, which is attributed to Li in the 8a tetrahedral position. A weak shoulder of the main peak is also observed which can be related to an isotropic feature near 850 ppm due to lithium in the octahedral 16d site [2,19]. Fig. 6 shows the 27Al NMR spectra of the x ¼ 0:04 and
Fig. 6. 27Al NMR spectra of the x ¼ 0:04 (lower) and 0.06 (upper) samples. The peaks at ,0 and ,70 ppm due to sixfold and fourfold coordinated aluminum are marked by stars.
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that can be attributed to sixfold and fourfold coordinated aluminum, respectively [20]. The information is also semiquantitative, and by a simple Gaussian deconvolution we can estimate a distribution of 85 ^ 5% for AlO6 and 15 ^ 5% for AlO4. The uncertainty is due: (i) to the spectrum noise, and (ii) to the fact that the lines are still partially broadened by second order quadrupolar interaction, which limits the accuracy of a fit performed with symmetric functions. The spectrum of x ¼ 0:04 sample shows an anisotropic broadening strongly reduced with respect to the x ¼ 0:06 sample and only few spinning sidebands observed under MAS rotation. However, for what concerns the aluminum coordination, a careful analysis of the central line of the spectrum shows that only a small fraction (,5%) of the Al nuclei is in tetrahedral coordination, while the remaining part is octahedrally coordinated.
4. Discussion and conclusions The rt EPR spectra show substantially signal A. In Fig. 4(a) and (b) it is observed that the signal narrows and increase in amplitude with increasing T without changing the resonance field: this behavior is typical of the signal of the pure spinel phase [1]. At T , 263 K; in the x ¼ 0:04 sample the signal B appears, while in the x ¼ 0:06 one, only the signal A is evident in all the investigated temperature range as observed in pure spinel. This fact suggests that the x ¼ 0:04 sample could be affected by grain boundary disorder [1] at low temperature. NMR measurements on 27Al probe gives sufficiently precise and direct estimation of the presence of Al3þ cations in tetrahedral site resulting in a higher content (15%) of tetrahedral Al3þ in the x ¼ 0:06 sample with respect to the x ¼ 0:04 one (5%). On the other hand, also XRPD data refinement may suggest to select a substitutional model, reliable and useful to explain the experimental data, in agreement with NMR results: Al3þ may substitute on both cationic sites with preferred occupation of the 16d site. No evidence for Mn substitution on the Li site is gained in the present samples, differently from Ga-doped samples [2]. For Al-doped samples, low temperature XRPD results clearly show that the TJ – T of the x ¼ 0:04 sample (223 K) is lower than the x ¼ 0:06 sample one (243 K), suggesting that the Mn3þ relative amounts, responsible for the J – T transition, increase with x: This influence of doping upon the J– T transition is different with respect to the other trivalent and bivalent [1,2,17] ions doped samples for which TJ – T decreases with x: Besides, the NMR results suggest a very low Li inversion degree. So, the charge distribution models proposed in the previous works [1,2,17] are no longer applicable and a different charge distribution model
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could be proposed: 3þ þ 3þ 3þ 4þ ðLiþ 12xt Alxt Þtetr ½Liy Alxo Mn123y2xo þ2xt Mn1þ2y22xt octa
ð1Þ
with y ¼ 0:02 in these samples. In this model no Li inversion is taken into account; rather, an amount xt is lost and is not substituted on the octahedral site. The partial Li loss in the spinel phase and the partial occupation of Al on the 8a site can be justified by the increasing quantity of LiAlO2 phase observed with XRPD on samples with Al content up to x ¼ 0:20 (Fig. 1). The excess positive charge in the tetrahedral sublattice ð2xt Þ is compensated by the equal deficit in the octahedral sublattice related to the increase in lMn3þl and the decrease in lMn4þl. Substituting in Eq. (1) xt ¼ 0:009 and 0.002, according to NMR results, and the related xo ¼ x 2 2xt values, for x ¼ 0:06 and 0.04 samples, respectively, the changes in Mn3þ and Mn4þ fractions agrees with a lMn3þl increase with Al3þ addition thus favoring the J – T transition as proved by the increase in TJ – T for x . 0:04: In agreement with the proposed model, higher x values of the x ¼ 0:06 sample with respect to the 0.04 one (Fig. 5) may be also related to the higher lMn3þl amount, being S ¼ 2 for Mn3þ and S ¼ 3=2 for Mn4þ. In addition, a comparison with the conductivity data near rt of the pure spinel [1] and Ga doped [2] samples shows the greatest s value for the Al-doped samples, where the charge carrier content available for the hopping process [15] should be the highest according to model (1). The substantially constant value of the lattice parameter for the x ¼ 0:04 and 0.06 samples agrees with the model (1), if the effective ionic radii by Shannon [21] are taken into account with the pertinent values for the different site coordination (for Mn3þ the high spin configuration has been considered). However, the best comparison with the experimental a values is obtained if a slight increase in Al3þ (8a) and of Al3þ (16d) radii up to ˚ , respectively, is allowed. Such increase 0.45 and 0.60 A seems consistent with both the size of the pertinent interstices and with the M – O Pauling electronegativity difference. As already observed, the cooperative J – T transition is inhibited and the spinel lattice maintains undistorted cubic structure when the ratio r ¼ lMn4þ l=lMn3þ l is $1.18 [1,2, 17]. This value cannot be reached for Al substituted samples because of the decrease in r with increasing x; as evidenced in this work for 0:00 , x # 0:06 : according to model (1), r takes the values 1.11 and 1.14 for x ¼ 0:06 and 0.04 sample, respectively. Even greater Al substitution cannot enable us to decrease TJ – T or reach the inhibition of J – T distortion because Al3þ progressively occupies the tetrahedral sites and, at the same time, the LiAlO2 phase grows. However, for x ¼ 0:06; a very high homogeneity of the cation distribution is obtained, and the properties of this material make it suitable for electrochemical applications down to 243 K.
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Acknowledgements This work has been partially supported by ‘Consorzio per i Sistemi a Grande Interfase’ (CSGI) and IENI-CNR Department of Pavia.
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