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Synchrotron X-ray powder diffraction studies on the phase transitions in LiMn2O4
Journal of Alloys and Compounds 362 (2004) 231–235
Synchrotron X-ray powder diffraction studies on the phase transitions in LiMn2 O4 P. Piszora∗ , J. Darul, W. Nowicki, E. Wolska Laboratory of Magnetochemistry, Adam Mickiewicz University, Pozna´n Grunwaldzka 6, PL-60780 Pozna´n, Poland Received 20 June 2002; received in revised form 4 November 2002; accepted 5 December 2002
Abstract Structural phase transitions of the spinel lithium-manganese oxide, LiMn2 O4 (Fd3m at room temperature) in the temperature range of 300–20 K, were investigated using synchrotron and laboratory X-ray powder diffraction. Splitting of the spinel X-ray lines 311, 400, 440, 531, 533 and 622, recorded during the test, evidenced the reversible formation of cubic, orthorhombic and tetragonal structures. The first transformation starts at ∼290 K as a cubic (Fd3m) → orthorhombic (Fddd) symmetry reduction, and both phases coexist down to 260 K. Below 100 K a second structural transition, orthorhombic → tetragonal (F41 /ddm) was observed, with the coexistence of these two phases down to ∼40 K. The supercell corresponding to 3a × 3a × a (a, spinel unit-cell constant) has been considered for both orthorhombic and tetragonal polymorphs of LiMn2 O4 , based on the appearance of (2.10.0) superstructure X-ray reflection. © 2003 Elsevier B.V. All rights reserved. Keywords: Electrode materials; Crystal structure and symmetry; X-ray diffraction; Synchrotron radiation
1. Introduction A large number of studies on the synthesis and structure of the spinel-type lithium-manganese oxides have been carried out, since they are important as the most promising low-cost cathode materials for rechargeable lithium batteries [1,2]. High-resolution powder diffraction methods make possible the investigations of structural phase transitions and enable the structure refinement of polycrystalline Lix Mn3−x O4 . It has become generally acknowledged that the lithium extraction/intercalation reaction, both by electrochemical and chemical mathods, are largely reversible, resulting in structural changes of cubic LiMn2 O4 to the tetragonally distorted spinel for x < 1, and to a rock-salt LiMnO2 phase for x > 1 [2–4]. On the other hand, the occurrence of different structural phase transformations below the room temperature in a stoichiometric LiMn2 O4 phase has been reported. The cubic → tetragonal phase transition was observed at 280 K, based on the experimental results of thermal analysis and X-ray powder diffraction [5]. Subsequent studies have shown, however, that the low-temperature phase has rather
∗
Corresponding author. E-mail address: [email protected] (P. Piszora).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00590-5
an orthorhombic structure [6,7]. The existence of a structural phase-transition in LiMn2 O4 near the room temperature has been confirmed by the differential scanning calorimetry and by the electrical conductivity experiments [5,8]. Stoichiometric spinel oxide, LiMn2 O4 , has the Li+ ions occupying the tetrahedral 8a sites, and the 1:1 mixture of Mn3+ and Mn4+ ions randomly distributed over the octahedral 16d Wyckoff’s positions. It displays a cubic, normal-spinel structure (space group Fd3m) at room temperature. Results presented in this paper deal with the low-temperature phase transformations in this lithium-manganese spinel oxide, investigated with conventional (laboratory) and synchrotron X-ray powder diffraction techniques.
2. Experimental details The LiMn2 O4 sample has been obtained by solid state reaction of Li2 CO3 with the manganese oxide precursor, ␣-Mn2 O3 (Ia3, bixbyite structure). After successive thermal treatment for 4 h in air, at 973 and 1023 K, the preparation was quenched rapidly in the solid CO2 . Laboratory X-ray powder diffraction experiments were performed with a computerized TUR-61 (HZG-3)
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P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
diffractometer, employing the Mn-filtered Fe K␣ radiation (λ = 1.93604 Å). For precise determination of the lattice parameters, and for measurements of the integrated intensities, the powder diffraction patterns were recorded in the range 10◦ ≤ 2θ ≤ 155◦ by step scanning, using 2θ increments of 0.04◦ and fixed counting time of 7.5 s/step. The phase identification and structure refinement were performed using the programs PowderCell [9] and GSAS [10]. Data from 18◦ to 145◦ (2θ) were included into calculations. The investigations with synchrotron radiation were executed on the HASYLAB (beamline B2) high-resolution X-ray diffractometer equipped with He-cryostat. Sample in form of a powder disc underwent the cooling and heating procedures, in the temperature range 20–300 K. X-ray powder diffraction data were recorded in the region corresponding to 311, 222, 400, 440, 531, 533, 622 and 444 cubic spinel reflections (in the range of 20◦ ≤ 2θ ≤ 60◦ ). The wavelength, determined by calibration using a NIST silicon standard (640b, a = 5.43094 Å), was 1.12422 Å. The geometry included a long parallel-foil collimator at the detector arm, ascertaining resolution suitable for analysis of overlapping peaks of LiMn2 O4 phases.
Fig. 1. X-ray powder diffraction patterns of the cubic spinel form of LiMn2 O4 (space group Fd3m), recorded at the temperature of 300 K, with: (a) laboratory X-ray (TUR-61, HZG-3), Fe K␣ radiation, λ = 1.93604 Å; (b) synchrotron radiation (B2, Hasylab), λ = 1.12422 Å (indexed as cubic Fd3m structure).
3. Results and discussion The structural phase transformation arising for the LiMn2 O4 below the room temperature may be observed using X-ray powder diffraction techniques, both with the conventional diffractometer and synchrotron radiation facility. Fig. 1 shows the sections of X-ray powder diffraction patterns of LiMn2 O4 , for the reflections with the d-space 1.0–2.5 Å, obtained at 300 K. The X-ray lines are characteristic for the cubic spinel (Fd3m) structure. The data for the same sample recorded in the laboratory and on synchrotron at the temperature of 200 K, presented in Fig. 2, reveal a reduction of symmetry to the orthorhombic (Fddd) structure. The splitting of spinel X-ray lines 311, 400, 440, 533 and 622 evidences the reversible cubic–orthorhombic transition. Investigations with the synchrotron X-ray show that both phases coexist in the temperature range of 290–260 K. The orthorhombic form of LiMn2 O4 is stable down to ∼100 K, but below that temperature a second phase transition begins (orthorhombic → tetragonal), with the coexistence of these two crystalline forms down to about 40 K. The low-temperature tetragonal phase displays the F41 /ddm space group. The body-centred been replaced by the face-centred unit cell I41 /amd has √ F41 /ddm with aF = aI 2 which is more convenient for
Fig. 2. X-ray powder diffraction patterns of the orthorhombic polymorph of LiMn2 O4 (space group Fddd), recorded at the temperature of 200 K, with: (a) laboratory X-ray (TUR-61, HZG-3), Fe K␣ radiation, λ = 1.93604 Å; (b) synchrotron radiation (B2, Hasylab), λ = 1.12422 Å (indexed as orthorhombic Fddd structure).
P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
233
Fig. 3. Sections of X-ray powder diffraction patterns of LiMn2 O4 , in the region of 533 and 622 spinel reflections, taken with synchrotron radiation (B2, Hasylab) at the temperatures increasing from 20 to 300 K.
the spinel type structure. Sections of X-ray patterns in Fig. 3 illustrate the F41 /ddm → Fddd → Fd3m phase transformations, showing changes in the 533 and 622 spinel X-ray lines with the increase of temperature from 20 to 300 K. The lowering of symmetry, caused by the partial ordering of manganese Mn3+ and Mn4+ ions, results in the superstructure corresponding to the enlarged unit cell 3a × 3a × a (where a is spinel lattice parameter) [6]. The superstructure X-ray reflections in the region of 2 = 26.5–27.0◦ , indexed as 2.10.0 and 10.2.0 appear for both orthorhombic and tetragonal forms (see Fig. 4.). The lattice constants were obtained on the basis of synchrotron X-ray data, using GSAS program package [10] by the following procedure. The profile function parameters for pseudo-Voigt function have been fitted for single phase cubic spinel (300 K), then the fixed parameters were applied to determine the lattice constants from X-ray patterns recorded at other temperatures. The two phase fitting was performed only in case where it improved the R-parameters. Results of accurate measurements of the spinel unit cell parameters, based on the synchrotron data, for all three polymorphs of LiMn2 O4 in the temperature region of their existence, are shown in Fig. 5 and in Table 1. The X-ray diffraction data recorded during the laboratory measurements at the temperature of 200 K give the unit cell parameters of the orthorhombic (Fddd) phase: a = 24.7432(9) Å,
Fig. 4. The superstructure reflections arising in the region 26.5◦ < 2θ < 27.0◦ on the X-ray patterns of orthorhombic and tetragonal form of LiMn2 O4 .
b = 24.8566(10) Å and c = 8.19849(32) Å. A more detailed report on the structure refinement will be published before long. The measurements of integrated intensities of X-ray powder diffraction lines provide information concerning the distribution of cations in the spinel lattice [11]. Changes in the intensity ratios of 400 reflection, dependent on the distribution of cations in both tetrahedral (8a) and octahedral (16d) positions of spinel structure, compared to the 222 line intensity, only octahedral-sites dependent, confirm that the Li+ ions distribution does not change during the structural phase transitions, and that the lithium ions occupy the 8a positions exclusively (see Fig. 6).
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Table 1 Lattice constants of the spinel-like phases T (K)
300 290 280 270 260 250 200 120 100 60 40 20
Cubic Fd3m a (Å)
8.24483(4) 8.24385(21) 8.24321(21) 8.2423(8) – – – – – – – –
Orthorhombic Fddd
Tetragonal F41 /ddm
a (Å)
b (Å)
c (Å)
a (Å)
c (Å)
– 24.7471(12) 24.7456(12) 24.7443(6) 24.7427(6) 24.7425(4) 24.7380(7) 24.7359(9) 24.7364(9) 24.7414(7) 24.7444(20) –
– 24.8656(9) 24.8636(7) 24.8595(6) 24.8563(7) 24.8529(5) 24.8348(8) 24.8138(9) 24.8080(9) 24.7963(7) 24.7822(22) –
– 8.20057(30) 8.19912(23) 8.19845(22) 8.19787(22) 8.19744(15) 8.19471(26) 8.19258(30) 8.19212(29) 8.19198(27) 8.1913(6) –
– – – – – – – – – 24.768(15) 24.7647(42) 24.76737(11)
– – – – – – – – – 8.193(12) 8.1909(35) 8.19314(18)
Fig. 6. Variation of the integrated intensity ratios of spinel X-ray lines 400 and 222, during the phase transformations in the sequence tetragonal → orthorhombic → cubic, with increasing temperature. For the tetragonal and orthorhombic phases the superlattice should be considered. The regions of the phase coexistence: (I) Fd3m; (II) Fd3m/Fddd; (III) Fddd; (IV) Fddd/F41 /ddm; (V) F41 /ddm.
Fig. 5. Lattice constants of the orthorhombic, tetragonal and cubic LiMn2 O4 , obtained with the GSAS program, plotted as a function of temperature. For standard deviations compare Table 1.
4. Conclusions • The stoichiometric lithium manganese oxide, LiMn2 O4 , undergoes two subsequent temperature phase transformations; • In the room-temperature region the structural transition from cubic (Fd3m) to orthorhombic (Fddd) lattice
take place, with the coexistence of both phases down to 260 K; • Below the temperature of 100 K a second phase transition, from orthorhombic Fddd to tetragonal F41 /ddm, appears, and the two phases coexist presumably down to about 40 K. • Both orthorhombic and tetragonal structures display a superlattice caused by partial ordering of Mn3+ and Mn4+ ions.
Acknowledgements This work was supported by the IHP-Contract HPRI-CT1999-00040 of the European Commission. The authors
P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
would like to thank Dr. C. Baehtz from HASYLAB for his assistance. One of the authors is grateful to the Committee of Scientific Research of Poland for financial support under research project No. 4 T09A 164 23 (2002-2004). References [1] P. Piszora, C.R.A. Catlow, S.M. Woodley, E. Wolska, Comput. Chem. 24 (2000) 609. [2] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. [3] K. Kanamura, H. Naito, T. Yao, Z. Takehara, J. Mater. Chem. 6 (1996) 33.
235
[4] T.J. Richardson, S.J. Wen, K.A. Striebel, P.N. Ross, E.J. Cairns, Mater. Res. Bull. 32 (1997) 609. [5] A. Yamada, M. Tanaka, Mater. Res. Bull. 30 (1995) 715. [6] J. Rodriguez-Carvajal, G. Rousse, C. Masquelier, M. Hervieu, Phys. Rev. Lett. 81 (1998) 4660. [7] H. Haykawa, T. Takada, H. Enoki, E. Akiba, Powder Diffract. 15 (2000) 19. [8] J. Sugiyama, T. Atsumi, A. Koiwai, T. Sasaki, T. Hioki, S. Noda, N. Kamegashira, J. Phys.: Condens. Matt. 9 (1997) 1729. [9] W. Kraus, G. Nolze, PowderCell 2.3 (1998). [10] A.C. Larson, R.B. Von Dreele, Generalized Structure Analysis System (GSAS), Los Alamos National Lab. Rep. No. LAUR-86-748 (2000). [11] E. Wolska, Defect Diffus. Forum 134/135 (1996) 89.
Synchrotron X-ray powder diffraction studies on the phase transitions in LiMn2 O4 P. Piszora∗ , J. Darul, W. Nowicki, E. Wolska Laboratory of Magnetochemistry, Adam Mickiewicz University, Pozna´n Grunwaldzka 6, PL-60780 Pozna´n, Poland Received 20 June 2002; received in revised form 4 November 2002; accepted 5 December 2002
Abstract Structural phase transitions of the spinel lithium-manganese oxide, LiMn2 O4 (Fd3m at room temperature) in the temperature range of 300–20 K, were investigated using synchrotron and laboratory X-ray powder diffraction. Splitting of the spinel X-ray lines 311, 400, 440, 531, 533 and 622, recorded during the test, evidenced the reversible formation of cubic, orthorhombic and tetragonal structures. The first transformation starts at ∼290 K as a cubic (Fd3m) → orthorhombic (Fddd) symmetry reduction, and both phases coexist down to 260 K. Below 100 K a second structural transition, orthorhombic → tetragonal (F41 /ddm) was observed, with the coexistence of these two phases down to ∼40 K. The supercell corresponding to 3a × 3a × a (a, spinel unit-cell constant) has been considered for both orthorhombic and tetragonal polymorphs of LiMn2 O4 , based on the appearance of (2.10.0) superstructure X-ray reflection. © 2003 Elsevier B.V. All rights reserved. Keywords: Electrode materials; Crystal structure and symmetry; X-ray diffraction; Synchrotron radiation
1. Introduction A large number of studies on the synthesis and structure of the spinel-type lithium-manganese oxides have been carried out, since they are important as the most promising low-cost cathode materials for rechargeable lithium batteries [1,2]. High-resolution powder diffraction methods make possible the investigations of structural phase transitions and enable the structure refinement of polycrystalline Lix Mn3−x O4 . It has become generally acknowledged that the lithium extraction/intercalation reaction, both by electrochemical and chemical mathods, are largely reversible, resulting in structural changes of cubic LiMn2 O4 to the tetragonally distorted spinel for x < 1, and to a rock-salt LiMnO2 phase for x > 1 [2–4]. On the other hand, the occurrence of different structural phase transformations below the room temperature in a stoichiometric LiMn2 O4 phase has been reported. The cubic → tetragonal phase transition was observed at 280 K, based on the experimental results of thermal analysis and X-ray powder diffraction [5]. Subsequent studies have shown, however, that the low-temperature phase has rather
∗
Corresponding author. E-mail address: [email protected] (P. Piszora).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00590-5
an orthorhombic structure [6,7]. The existence of a structural phase-transition in LiMn2 O4 near the room temperature has been confirmed by the differential scanning calorimetry and by the electrical conductivity experiments [5,8]. Stoichiometric spinel oxide, LiMn2 O4 , has the Li+ ions occupying the tetrahedral 8a sites, and the 1:1 mixture of Mn3+ and Mn4+ ions randomly distributed over the octahedral 16d Wyckoff’s positions. It displays a cubic, normal-spinel structure (space group Fd3m) at room temperature. Results presented in this paper deal with the low-temperature phase transformations in this lithium-manganese spinel oxide, investigated with conventional (laboratory) and synchrotron X-ray powder diffraction techniques.
2. Experimental details The LiMn2 O4 sample has been obtained by solid state reaction of Li2 CO3 with the manganese oxide precursor, ␣-Mn2 O3 (Ia3, bixbyite structure). After successive thermal treatment for 4 h in air, at 973 and 1023 K, the preparation was quenched rapidly in the solid CO2 . Laboratory X-ray powder diffraction experiments were performed with a computerized TUR-61 (HZG-3)
232
P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
diffractometer, employing the Mn-filtered Fe K␣ radiation (λ = 1.93604 Å). For precise determination of the lattice parameters, and for measurements of the integrated intensities, the powder diffraction patterns were recorded in the range 10◦ ≤ 2θ ≤ 155◦ by step scanning, using 2θ increments of 0.04◦ and fixed counting time of 7.5 s/step. The phase identification and structure refinement were performed using the programs PowderCell [9] and GSAS [10]. Data from 18◦ to 145◦ (2θ) were included into calculations. The investigations with synchrotron radiation were executed on the HASYLAB (beamline B2) high-resolution X-ray diffractometer equipped with He-cryostat. Sample in form of a powder disc underwent the cooling and heating procedures, in the temperature range 20–300 K. X-ray powder diffraction data were recorded in the region corresponding to 311, 222, 400, 440, 531, 533, 622 and 444 cubic spinel reflections (in the range of 20◦ ≤ 2θ ≤ 60◦ ). The wavelength, determined by calibration using a NIST silicon standard (640b, a = 5.43094 Å), was 1.12422 Å. The geometry included a long parallel-foil collimator at the detector arm, ascertaining resolution suitable for analysis of overlapping peaks of LiMn2 O4 phases.
Fig. 1. X-ray powder diffraction patterns of the cubic spinel form of LiMn2 O4 (space group Fd3m), recorded at the temperature of 300 K, with: (a) laboratory X-ray (TUR-61, HZG-3), Fe K␣ radiation, λ = 1.93604 Å; (b) synchrotron radiation (B2, Hasylab), λ = 1.12422 Å (indexed as cubic Fd3m structure).
3. Results and discussion The structural phase transformation arising for the LiMn2 O4 below the room temperature may be observed using X-ray powder diffraction techniques, both with the conventional diffractometer and synchrotron radiation facility. Fig. 1 shows the sections of X-ray powder diffraction patterns of LiMn2 O4 , for the reflections with the d-space 1.0–2.5 Å, obtained at 300 K. The X-ray lines are characteristic for the cubic spinel (Fd3m) structure. The data for the same sample recorded in the laboratory and on synchrotron at the temperature of 200 K, presented in Fig. 2, reveal a reduction of symmetry to the orthorhombic (Fddd) structure. The splitting of spinel X-ray lines 311, 400, 440, 533 and 622 evidences the reversible cubic–orthorhombic transition. Investigations with the synchrotron X-ray show that both phases coexist in the temperature range of 290–260 K. The orthorhombic form of LiMn2 O4 is stable down to ∼100 K, but below that temperature a second phase transition begins (orthorhombic → tetragonal), with the coexistence of these two crystalline forms down to about 40 K. The low-temperature tetragonal phase displays the F41 /ddm space group. The body-centred been replaced by the face-centred unit cell I41 /amd has √ F41 /ddm with aF = aI 2 which is more convenient for
Fig. 2. X-ray powder diffraction patterns of the orthorhombic polymorph of LiMn2 O4 (space group Fddd), recorded at the temperature of 200 K, with: (a) laboratory X-ray (TUR-61, HZG-3), Fe K␣ radiation, λ = 1.93604 Å; (b) synchrotron radiation (B2, Hasylab), λ = 1.12422 Å (indexed as orthorhombic Fddd structure).
P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
233
Fig. 3. Sections of X-ray powder diffraction patterns of LiMn2 O4 , in the region of 533 and 622 spinel reflections, taken with synchrotron radiation (B2, Hasylab) at the temperatures increasing from 20 to 300 K.
the spinel type structure. Sections of X-ray patterns in Fig. 3 illustrate the F41 /ddm → Fddd → Fd3m phase transformations, showing changes in the 533 and 622 spinel X-ray lines with the increase of temperature from 20 to 300 K. The lowering of symmetry, caused by the partial ordering of manganese Mn3+ and Mn4+ ions, results in the superstructure corresponding to the enlarged unit cell 3a × 3a × a (where a is spinel lattice parameter) [6]. The superstructure X-ray reflections in the region of 2 = 26.5–27.0◦ , indexed as 2.10.0 and 10.2.0 appear for both orthorhombic and tetragonal forms (see Fig. 4.). The lattice constants were obtained on the basis of synchrotron X-ray data, using GSAS program package [10] by the following procedure. The profile function parameters for pseudo-Voigt function have been fitted for single phase cubic spinel (300 K), then the fixed parameters were applied to determine the lattice constants from X-ray patterns recorded at other temperatures. The two phase fitting was performed only in case where it improved the R-parameters. Results of accurate measurements of the spinel unit cell parameters, based on the synchrotron data, for all three polymorphs of LiMn2 O4 in the temperature region of their existence, are shown in Fig. 5 and in Table 1. The X-ray diffraction data recorded during the laboratory measurements at the temperature of 200 K give the unit cell parameters of the orthorhombic (Fddd) phase: a = 24.7432(9) Å,
Fig. 4. The superstructure reflections arising in the region 26.5◦ < 2θ < 27.0◦ on the X-ray patterns of orthorhombic and tetragonal form of LiMn2 O4 .
b = 24.8566(10) Å and c = 8.19849(32) Å. A more detailed report on the structure refinement will be published before long. The measurements of integrated intensities of X-ray powder diffraction lines provide information concerning the distribution of cations in the spinel lattice [11]. Changes in the intensity ratios of 400 reflection, dependent on the distribution of cations in both tetrahedral (8a) and octahedral (16d) positions of spinel structure, compared to the 222 line intensity, only octahedral-sites dependent, confirm that the Li+ ions distribution does not change during the structural phase transitions, and that the lithium ions occupy the 8a positions exclusively (see Fig. 6).
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Table 1 Lattice constants of the spinel-like phases T (K)
300 290 280 270 260 250 200 120 100 60 40 20
Cubic Fd3m a (Å)
8.24483(4) 8.24385(21) 8.24321(21) 8.2423(8) – – – – – – – –
Orthorhombic Fddd
Tetragonal F41 /ddm
a (Å)
b (Å)
c (Å)
a (Å)
c (Å)
– 24.7471(12) 24.7456(12) 24.7443(6) 24.7427(6) 24.7425(4) 24.7380(7) 24.7359(9) 24.7364(9) 24.7414(7) 24.7444(20) –
– 24.8656(9) 24.8636(7) 24.8595(6) 24.8563(7) 24.8529(5) 24.8348(8) 24.8138(9) 24.8080(9) 24.7963(7) 24.7822(22) –
– 8.20057(30) 8.19912(23) 8.19845(22) 8.19787(22) 8.19744(15) 8.19471(26) 8.19258(30) 8.19212(29) 8.19198(27) 8.1913(6) –
– – – – – – – – – 24.768(15) 24.7647(42) 24.76737(11)
– – – – – – – – – 8.193(12) 8.1909(35) 8.19314(18)
Fig. 6. Variation of the integrated intensity ratios of spinel X-ray lines 400 and 222, during the phase transformations in the sequence tetragonal → orthorhombic → cubic, with increasing temperature. For the tetragonal and orthorhombic phases the superlattice should be considered. The regions of the phase coexistence: (I) Fd3m; (II) Fd3m/Fddd; (III) Fddd; (IV) Fddd/F41 /ddm; (V) F41 /ddm.
Fig. 5. Lattice constants of the orthorhombic, tetragonal and cubic LiMn2 O4 , obtained with the GSAS program, plotted as a function of temperature. For standard deviations compare Table 1.
4. Conclusions • The stoichiometric lithium manganese oxide, LiMn2 O4 , undergoes two subsequent temperature phase transformations; • In the room-temperature region the structural transition from cubic (Fd3m) to orthorhombic (Fddd) lattice
take place, with the coexistence of both phases down to 260 K; • Below the temperature of 100 K a second phase transition, from orthorhombic Fddd to tetragonal F41 /ddm, appears, and the two phases coexist presumably down to about 40 K. • Both orthorhombic and tetragonal structures display a superlattice caused by partial ordering of Mn3+ and Mn4+ ions.
Acknowledgements This work was supported by the IHP-Contract HPRI-CT1999-00040 of the European Commission. The authors
P. Piszora et al. / Journal of Alloys and Compounds 362 (2004) 231–235
would like to thank Dr. C. Baehtz from HASYLAB for his assistance. One of the authors is grateful to the Committee of Scientific Research of Poland for financial support under research project No. 4 T09A 164 23 (2002-2004). References [1] P. Piszora, C.R.A. Catlow, S.M. Woodley, E. Wolska, Comput. Chem. 24 (2000) 609. [2] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. [3] K. Kanamura, H. Naito, T. Yao, Z. Takehara, J. Mater. Chem. 6 (1996) 33.
235
[4] T.J. Richardson, S.J. Wen, K.A. Striebel, P.N. Ross, E.J. Cairns, Mater. Res. Bull. 32 (1997) 609. [5] A. Yamada, M. Tanaka, Mater. Res. Bull. 30 (1995) 715. [6] J. Rodriguez-Carvajal, G. Rousse, C. Masquelier, M. Hervieu, Phys. Rev. Lett. 81 (1998) 4660. [7] H. Haykawa, T. Takada, H. Enoki, E. Akiba, Powder Diffract. 15 (2000) 19. [8] J. Sugiyama, T. Atsumi, A. Koiwai, T. Sasaki, T. Hioki, S. Noda, N. Kamegashira, J. Phys.: Condens. Matt. 9 (1997) 1729. [9] W. Kraus, G. Nolze, PowderCell 2.3 (1998). [10] A.C. Larson, R.B. Von Dreele, Generalized Structure Analysis System (GSAS), Los Alamos National Lab. Rep. No. LAUR-86-748 (2000). [11] E. Wolska, Defect Diffus. Forum 134/135 (1996) 89.