Weak ferromagnetism of LiMnPO4

Journal of Physics and Chemistry of Solids 65 (2004) 1773–1777 www.elsevier.com/locate/jpcs

Weak ferromagnetism of LiMnPO4 D. Arcˇona,b,*, A. Zorkoa, P. Cevca, R. Dominkoc, M. Belec, J. Jamnikc, Z. Jaglicˇic´d, I. Golosovskye a

Jozˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia c National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia d Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia e St Petersburg Nuclear Physics Institute, RAS, 188350 Gatchina, Leningrad District, Russia

b

Received 24 March 2004; revised 7 June 2004; accepted 7 June 2004

Abstract Structural and magnetic properties of the novel materials for lithium batteries LiFePO4 and LiMnPO4 were studied by X-ray diffraction, SQUID magnetometry and EPR spectroscopy. LiMnPO4 has an olivine-type structure with a Mn-ion square lattice in the b–c plane. The occupation factors for Li and those oxygen atoms, which bridge Mn ions in the b–c plane showed noticeable deviation from the stoichiometry. In addition, the oxygen atoms, which are in the same layer as Li ions, exhibit a remarkable mean-square displacement in LiMnPO4 but not in LiFePO4. The olivine structure suggests quasi-two-dimensional (quasi-2D) antiferromagnetic structure of Mn(II) ions (SZ5/2) with sizable interlayer exchange interactions. Magnetization measurements clearly revealed a transition to a weak ferromagnetic state below TNZ45 K. On the other hand we find that LiFePO4 orders antiferromagnetically below 50 K. The difference in the magnetic properties of LiMnPO4 and LiFePO4 reflect the differences in the electronic states between these two compounds and may be very important for the electrochemical inactivity of LiMnPO4. EPR measurements also suggest that at temperatures above TN the low-energy magnetic excitations in LiMnPO4 are characteristic for the quasi-2D magnetic structure with the soliton excitation energy ESZ139 K. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; C. X-ray diffraction; C. Electron paramagnetic resonance

Search for new materials that could be used as electrode materials for lithium batteries triggered intensive research of the structures containing both first row transition metal cations and polyanions [1]. Prominent examples of this class of materials are LiMPO4 and LiMBO3 (MZFe, Mn, Co) [2–4]. In spite of similar structure, phospho-olivines (i.e. LiMPO4 where MZFe, Mn,.) have different electrochemical activity. Namely, the electrochemical reversibility in LiFePO4 is rather high [2,5] while LiMnPO4 is almost electrochemically inactive toward lithium extraction [3,6]. At the moment, the reasons for lower electrochemical activity of LiMnPO4 are not completely clear [6] and call for additional experiments. * Corresponding author. Tel.: C386-1-477-34-92; fax: C386-1-42632-69. E-mail address: [email protected] (D. Arcˇon). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.002

Another important factor is the magnetism in these compounds [7–9]. Magnetic properties are determined by the electronic states and may thus reflect the potential differences within the LiMPO4 family. Therefore they are indirectly important for the electrochemical properties as well. For instance, the magnetic properties of b-LiMnBO3 are characteristic of a one-dimensional Heisenberg antiferromagnet and orders into a very unusual chiral ground state below 28.5 K [10]. It is thus the purpose of this work to shed some additional light on the structural and magnetic properties of LiMnPO4 and compare them with those of LiFePO4. We emphasize that the focus of this work was on the samples as prepared for the electrochemical studies with all possible defects, size effects, etc. The magnetic properties of LiMnPO4 are a result of the particular Mn-ion layer-like arrangement with presumably dominant exchange intralayer Mn–Mn

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Fig. 1. (a) Simulation of the X-ray diffraction pattern of powdered LiMnPO4. The proposed crystal structure of LiMnPO4 is shown in (b).

exchange interaction. Interlayer exchange interaction at low temperatures lead to a three-dimensional (3D) magnetic ordering below TN. Structurally the LiFePO4 is rather similar. The magnetic properties are not the same though. While the collinear antiferromagnetic ordering is rather robust in LiFePO4 in LiMnPO4 various defects lead to the occurrence of weak ferromagnetism. Thus, when comparing the electrochemistry of these two materials the difference in the magnetic properties should be taken into account as well. At temperatures just above TN we suggest that the magnetic excitations are those found in the two-dimensional (2D) magnetic systems of ‘classical’ spins [11,12]. The samples were prepared by a standard sol–gel method. Equimolar amounts of lithium phosphate (Li3PO4, Aldrich 33,889-3), phosphoric(V) acid (H3PO4, Aldrich 31,027-1), manganese(II) acetate tetrahydrate (C4 H6MnO 4$4H 2O, Fluka, 63537) and citric acid (C6H8O7$H2O, Kemika, 0319506) were dissolved separately in water. Clear solutions were mixed together and dried at 60 8C for 24 h. After thorough grinding with a mortar and pestle, the obtained material was fired in inert (argon) or reductive (5 wt% of hydrogen in argon) atmosphere at 900 8C for 10 h [13]. X-ray powder diffraction (XRD) patterns were measured on a Philips diffractometer PW 1710 using Cu Ka radiation in 0.028 2q steps from 15 to 708. The magnetization was measured using a Quantum Design SQUID magnetometer, equipped with a 50 kOe superconducting magnet. Continuous wave EPR

measurements were performed on a Bruker E580 spectrometer and a Varian dual resonator with a reference sample in the second resonator to account for the small changes in the Q factor during the measurements. Structures of LiMnPO4 and LiFePO4 were studied at room temperature with X-ray diffraction. LiMnPO4 crystallizes in olivine-type crystal structure with orthogonal space group Pnma and its structure is isomorphous to the one reported for the LiFePO4 [14]. The patterns of LiMnPO4 and LiFePO4 are thus similar and therefore only the pattern of Mn-compound is reported in Fig. 1. The profile refinement by the FullProf program gives for LiMnPO4 the lattice ˚. constants aZ10.4447(6), bZ6.1018(3), cZ4.7431(3) A Because of low statistics and impurities the factors describing the quality of the agreement, namely, profile factor and weighted profile factor were of 11.0 and 14.5%, respectively. The main impurity has been identified to be Li3PO4, which is not magnetic. The results of the refinement are given in Table 1. We took into consideration the absorption as the refined addition to the general Debye– Waller factor. Therefore in the Table 1 the relative Debye– Waller factors are given as well. The deviation of the occupancy factors from the unity for Li and O3 oxygen sites, clearly indicate the presence of local disorder in the Li coordination sphere that is consistent with the results of Ref. [15]. In other words, the sample has some oxygen vacancies, which are restricted only to the O3 oxygen sites. Within the experimental error we do not find any oxygen vacancy at O1 and O2 sites. Such oxygen

Table 1 Atomic coordinates, the relative Debye–Waller factors and the occupancy factors in LiMnPO4

Li Mn P O1 O2 O3

x

y

z

B

N

0 0.2814(2) 0.0967(4) 0.0980(9) 0.4452(11) 0.1546(7)

0 0.2500(0) 0.2500(0) 0.2500(0) 0.2500(0) 0.0408(13)

0 0.9704(7) 0.4048(12) 0.7271(21) 0.2262(17) 0.2556(14)

1 1 1 0.4(3) 3.0(3) 0.4(5)

1.21(2) 1.00(1) 0.92(1) 1.06(1) 1.01(1) 0.83(1)

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vacancies could have a dramatic impact on the magnetic properties of the sample, namely a vacancy can locally change the Mn environment, its valence state as well as the exchange pathway. Interestingly we find that the same type of oxygen vacancy exists also in LiFePO4. The two samples are thus in this respect rather similar. Another unexpected result is abnormal Debye–Waller factor for O2 oxygen. The refinement of anisotropic factors shows that oxygen atoms O2 vibrate preferably in the b–c plane (Fig. 1b). A comparative structural study of LiFePO4 shows on the other hand that all oxygen atoms have the similar mean-square displacements. Since O2 oxygen and Li sites are practically in the same plane (b–c) it is possible that large atomic vibrations of oxygen atoms O2 may be important factor for the LiC conductivity and the difference in the electrochemical activity of LiFePO4 and LiMnPO4. Further neutron diffraction studies are necessary to make more firm conclusions about the atomic displacements. The Mn-ions form a 2D square lattice in the b–c plane ˚ (Fig. 1b). with the nearest neighbor Mn–Mn distance 3.92 A The interlayer Mn–Mn distance along the crystal a-axis is ˚ . We stress out that much larger and corresponds to 5.62 A the shortest Mn–Mn distance in LiMnPO4 is still significantly larger than the one in LiMnBO3, [10] where it ˚ . This should result in weaker Mn– corresponds to 3.147 A Mn exchange interactions in the former system. The olivine LiMnPO4 crystal structure thus implies a quasi-2D packing of Mn ions with important interlayer exchange. We note, however, that the Mn ions are not all at the same level of the b–c plane as the Mn layer is buckled leading to a tilt of the individual MnO octahedra in different directions. Magnetic properties of LiMnPO 4 powder were first studied by the SQUID magnetometer. The static magnetic susceptibility c0 follows the Curie–Weiss law c0 Z C=ðT K qÞ between room temperature and 70 K (Fig. 2). The Curie temperature is qZK87%2 K indicating antiferromagnetic correlations between the Mn spins. Curie constant CZ4.23(1) emuK/mol corresponds to the effective magnetic moment meffZ5.82(2)mB and is close to the value expected for Mn(II) spins (SZ5/2). We find it somewhat interesting that the temperature dependence of the static spin susceptibility does not show a typical 2D behavior [16], i.e. exhibiting a broad peak in the temperature dependence. Instead it rather follows a type of the behavior expected for the 3D system implying the importance of interlayer exchange interactions. Below 70 K significant deviations from the Curie–Weiss law can be seen as the magnetic susceptibility becomes enhanced. At around 45 K a dramatic increase of the magnetic moment is observed indicating a transition to a magnetically ordered phase. We note, however, that the saturated magnetic moment (c0w0.145 emu/mol at TZ2 K) is rather small suggesting a weak ferromagnetic ground state. Further evidence for the magnetic ordering comes from the ac-susceptibility c 0 ZdM/dH measurements (insert to Fig. 2a), which clearly show a peak at TNZ43 K.

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Fig. 2. The temperature dependence of the: (a) magnetic susceptibility and (b) inverse magnetic susceptibility in powdered LiMnPO4. The applied magnetic field was 1000 Oe. In the inset we show the ac-susceptibility in the vicinity of the magnetic transition.

We would like to stress out that the peak position is frequency independent, i.e. it is precisely the same at nZ 1 Hz as well as at nZ100 Hz, excluding some spin-glass or superparamagnetic-like behavior. The low temperature phase thus possesses a 3D long-range weak ferromagnetic ordered state below TNZ43 K. We note, however, that the magnitude of the spontaneous magnetic moment and the Neel temperature slightly vary from sample to sample. The origin of this variation will be discussed later. In addition, we performed also hysteresis curve measurements below and above TN (not shown here) [17]. The magnetic field was varied between K50 and C50 kOe. We managed to measure a significant hysteresis in the magnetization curves at 2 and 30 K with a coercive field of about 1000 Oe and the remanent magnetization w60 emu/mol, which corresponds to the saturated moment of about m SZ0.01m B/Mn. Above T N the hysteresis disappears and we observe only a straight paramagneticlike behavior. Magnetic properties of the LiFePO4 sample are not so dramatic. The magnetic susceptibility follows a Curie– Weiss behavior with a Curie constant CZ4.28(1) emuK/mol and a Curie temperature qZK115%1 K. The susceptibility clearly shows a peak characteristic for an antiferromagnetic ordering at TN Z50 K. Details of the temperature

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D. Arcˇon et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1773–1777

dependence of the magnetic susceptibility of LiFePO4 will be published elsewhere. Magnetic transition in LiMnPO4 is reflected also in the temperature dependence of the EPR signal. The EPR signal has a Lorentzian lineshape over the entire temperature range. This result is somehow surprising as in a 2D lattice formed by the Mn-ions one would expect deviations from the Lorentzian lineshape as a result of the spin-diffusion [18]. At this stage we can exclude the importance of spin diffusion as the primary source for the EPR line broadening. The EPR peak-to-peak linewidth is at room temperature 298 Oe. We note that this linewidth is by an order of magnitude larger than the one measured in perovskite layered Mn square-lattice antiferromagnets [19]. A possible reason for this could be enhanced magnetic anisotropy. The temperature dependence of the EPR signal intensity, which is in the paramagnetic phase directly proportional to the static spin susceptibility, is shown in Fig. 3. At high temperatures the intensity of the signal follows the one measured by SQUID magnetometer (Fig. 2a). However, on cooling below around 50 K the EPR signal starts to disappear quite rapidly and nearly vanishes in an interval of few K. This proves that the magnetic transition observed in SQUID measurements is indeed intrinsic and it is not related to some non-identified impurities. We were not able to detect antiferromagnetic resonance in powdered LiMnPO4 below TN. Information about the spin dynamics is given by the temperature dependence of the EPR signal linewidth (Fig. 4). The EPR signal linewidth monotonically increases with decreasing temperature reaching a maximum at TN. Though the single crystal data is needed to clearly discuss the dominant relaxation mechanisms, we can give here at least a tentative analysis. On cooling the EPR linewidth increases monotonically and on approaching TN from above the EPR linewidth enormously increases. Such an increase

Fig. 4. (a) The temperature dependence of the peak-to-peak X-band EPR linewidth in powdered LiMnPO4. (b) Logarithmic versus reciprocal presentation of the peak-to-peak EPR linewidth. The solid line represents a fit to Eq. (1).

could be a signature of the critical fluctuations although it is difficult to believe that they could survive even at that high temperatures as TZ2TN. Instead we base our linewidth analysis on the structural two-dimensionality of the Mn network. Namely, in 2D antiferromagnetic square-lattice one can expect that the nonlinear soliton-like excitations will determine the EPR linewidth [19,20]. If this is the case the EPR linewidth should vary as DHpp Z A C B expðES =kTÞ;

(1)

where ES is the soliton energy and A is the temperature independent linewidth contribution. A fit of the EPR linewidth to Eq. (1) with parameters ESZ139%1 K, BZ23.3%0.3 Oe and AZDHpp(N)Z280%1 Oe is shown in Fig. 3b. The deviations from Eq. (1) can be seen only in the close vicinity of TN, where one expects that the 3D critical fluctuations will become important and the linewidth should then obey a power law   T K TN Kp DHpp f : (2) TN

Fig. 3. The temperature dependence of the integrated EPR signal intensity in powdered LiMnPO4. Please note a sharp drop of the EPR signal at TNZ 46 K (inset).

Though the number of experimental points is not sufficient for a reliable determination of the critical exponent p, we estimated that pw0.5. It is, however, necessary to check on the consistency of our results. In the mean-field approximation it follows from the Curie temperature qZK87 K that J Z 3q=2ZSðSC 1ÞZ 2:4 K: On the other hand the soliton energy of the Belavin–Polyakov solution is given

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by the expression ESZ4pJS2. Since we determined from our measurements that ESZ139 K it follows that JZ1.8 K. Giving the crudeness of our approach we get fairly good agreement between the two experimentally determined parameters supporting our assignment. There are at least two possible reasons for the observed weak ferromagnetism in LiMnPO4. While the role of impurities has been refuted by the absence of the EPR signal of Li3PO4 we cannot definitely exclude the presence of intrinsic impurities. Structural investigations suggested that intrinsic impurities might be due to the vacancies at the Li and O3 sites. Such intrinsic impurities could locally change the valence of Mn ions and thus lead to the observed weak spontaneous moment. In order to check on that we have synthesized new impurity free sample (no traces of Li3PO4 were detected in this sample) and magnetization measurements on this sample again demonstrated the magnetic transition with a very small magnetic moment although the magnitude of the magnetic moment and the Neel temperature were slightly smaller. Therefore vacancies at the Li or O3 sites can be responsible for the weak ferromagnetism in LiMnPO4. Still another possibility takes into account the fact that Mn ions are not positioned in a perfect 2D plane as noted above. The ‘buckling’ of the Mn layer should have very important consequences for the magnetic properties: it could lead to the enhancement of the magnetic anisotropy and the staggered magnetic field. For instance a particular orientation of the Jahn–Teller distorted MnO octahedra could result in the weak ferromagnetic moment due to the Ising-type anisotropy. Alternatively, due to the buckling the Dzyaloshinskii–Moriya interaction between Mn spins in the same layer (but not between the layers) is allowed and could account for the occurrence of weak ferromagnetism. If canting of the sublattice magnetizations is responsible for the weak ferromagnetic moment in LiMnPO4 then from the spontaneous moment mSZ 0.01mB and the effective moment meffZ5.8mB one can estimate the canting angle of the sublattice magnetization to be very small, i.e. qZ0.058. We stress out once again that the LiFePO4 sample still undergoes a transition to collinear antiferromagnetic state although it has been prepared in the same way as LiMnPO4 sample. The difference in the magnetic properties of these two samples reflects a sensitivity of the electronic structure of LiMnPO4 to various imperfections that may occur during the Li doping. On the other hand the electronic structure and magnetic properties of LiFePO4 are much more robust. Future theories dealing with the electrochemical differences of these two materials will have to include such instabilities as well. In conclusion, XRD data showed that there are two important observations about the LiMnPO4 structure, which may be very important for the electrochemistry of these materials: (i) we find oxygen vacancies at O3 sites and (ii) large displacements of oxygen atoms at O2 sites which are absent in LiFePO4. The Mn-ions form a 2D square

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˚ . In lattice with the nearest neighbor Mn–Mn distance 3.92 A accordance with the quasi-2D structure we propose that soliton-like excitations with soliton energy ESZ139 K determine the spin dynamics in the wide temperature range 1.1TN%T%2TN. At TNZ43 K we found a transition to a weak ferromagnetic state. We stress out at the end that whatever is the origin of the weak ferromagnetism in our LiMnPO4—either due to the intrinsic impurities or either due to the enhanced magnetic anisotropy—it directly reflects the electronic properties of this material and must be taken into account also to understand the electrochemistry of the LiMPO4 family.

Acknowledgements AD acknowledges a financial support of NATO through a SfP976913 grant. We also thank the Slovenian Ministry of Education, Science and Sports for the provision of financial support through a Russian–Slovenia ‘Joint Research and Technology Program’.

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