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High-temperature synchrotron X-ray diffraction study of LaMn7O12
Solid State Sciences 11 (2009) 1211–1215
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High-temperature synchrotron X-ray diffraction study of LaMn7O12 H. Okamoto a, M. Karppinen b, c, H. Yamauchi b, c, H. Fjellvåg a, * a
Centre for Materials Science and Nanotechnology, Department of Chemistry, P.O. Box 1033, University of Oslo, Blindern, N-0315 Oslo, Norway Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan c Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, FI-02015 TKK, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2008 Received in revised form 16 March 2009 Accepted 19 March 2009 Available online 28 March 2009
The crystal structure of the 1:3 A-site ordered perovskite LaMn7O12 with manganese atoms on both the A- and B-sites, was investigated with high-temperature in-situ synchrotron X-ray diffraction (SXRD) from room temperature up to 803 K as well as with high-resolution SXRD at room temperature. A first order structural phase transition from a body centered monoclinic I2/m to a cubic Im3 structure (from aþbþcþ to aþaþaþ tilt system) occurs around Tc ¼ 653 K as evidenced by discontinuous jumps in unit cell dimensions and differential scanning calorimetric data. The temperature dependence of the Mn–O bond distances suggests that the driving force for the transition is relaxation of the Jahn–Teller distortion and subsequent change in the tilting pattern of the MnO6 octahedra. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskite Manganese Synchrotron X-ray diffraction Phase transition
Manganese(III) based perovskite oxides typically exhibit Jahn– 3 e1 Þ the Teller (JT) distortions at low temperatures. For Mn3þ ðt2g g octahedral Oh symmetry is then lifted and the eg orbitals become non-degenerate. This is exemplified by the REMnO3 perovskites which show cooperative JT distortion, orbital ordering, antiferromagnetic order and insulator properties [1,2]. For instance, LaMnO3 takes an orthorhombic structure with cooperative JT distortion associated with orbital ordering and with A-type antiferromagnetic order below TN ¼ 140 K [3]. Upon heating, a first order structural phase transition to a rhombohedral structure occurs at 750 K [4,5] with loss of the cooperative distortion. Among manganese containing perovskite oxides the AA0 3B4O12type phases with Mn at both the A- and the B-sites are extraordinary with regard to chemistry and properties. The phases exhibit interesting superstructures with respect to the simple ABO3 perovskite. For the AA0 3B4O12 phases (A ¼ monovalent alkali metal, divalent alkaline earth, or lanthanoide; A0 ¼ divalent Cu or trivalent Mn; B ¼ transition metal) complete A-site order is anticipated. The normal cubooctahedral 12-coordination is retained for the A atoms, while the A0 atoms are displaced and achieve highly distorted icosahedral 12-coordination with respect to oxygen. This is illustrated in Fig. 1 for LaMn7O12. The AA0 3B4O12 phases are known with Cu2þ and/or Mn3þ as A0 -site cations and with various A-site cations
* Corresponding author. Tel.: þ47 22 85 55 64; fax: þ47 22 85 54 41. E-mail address: [email protected] (H. Fjellvåg). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.03.012
(mono-, di-, tri-, and tetravalent). They take either cubic-, trigonalor monoclinic symmetry at room temperature [6–9]. Interestingly, the location of Cu2þ and/or Mn3þ cations with preference for the distorted local symmetry at the A0 -site seems to be a prerequisite for stabilization of this structure. This A/A0 -site ordered structure shows in-phase tilting of the BO6 octahedra. When the A0 -site is occupied by manganese, i.e. AMn7O12, the crystal symmetry correlates with the oxidation state of the A cation and becomes lower for higher valent A cations; i.e. cubic for monovalent Naþ, trigonal for divalent Ca2þ, Sr2þ and Cd2þ, and monoclinic for trivalent La3þ and Nd3þ. The corresponding average oxidation state of the B-site Mn is Mn3.5þ, Mn3.25þ, and Mn3þ, respectively, while the A0 -site Mn is always Mn3þ [7]. NaMn7O12 shows charge separation of Mn3þ and Mn4þ at the B-sites in a ratio of 1:1. This charge ordering and corresponding Jahn–Teller distortion of Mn3þ are lost during the symmetry change from I2/m to Im3 between 168 K and 176 K [10]. In the case of CaMn7O12, the charge separation of Mn3þ and Mn4þ in a 3:1 ratio at the B-site is lost between 410 K and 450 K upon changing the symmetry from R3 to Im3 [11]. In the present work, the structural properties of LaMn7O12 have been investigated by means of high-temperature powder X-ray synchrotron diffraction (SXRD) as well as by high-resolution SXRD at room temperature. During the in-situ SXRD measurements, diffraction patterns were collected with a 2D-detector at time intervals of 2 min (corresponding to DT ¼ 3.4 K) using a continuous heating rump for the heater gun. A structural first order phase transition is observed at around 659 K without any sign of
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H. Okamoto et al. / Solid State Sciences 11 (2009) 1211–1215 Table 1 Space group, unit cell parameters, and figure of merits of the Rietveld refinement of LaMn7O12 as based on high-resolution SXRD data at room temperature (upper table). Refined atomic coordinates and displacement parameters are given in lower table. Calculated standard deviations in parentheses. Phase
LaMn7O12 at R.T.
Space group a (Å) b (Å) c (Å) b ( ) Volume (Å3) Z Rwp (%) Rp (%) RF2 (%)
I2/m 7.5146(10) 7.3772(10) 7.5224(10) 91.283(10) 416.91(17) 2 12.16 7.39 5.850 8.923
c2 La Mn1 Mn2 Mn3 Mn4 Mn5 O1 O2 O3 O4 Fig. 1. Schematics of the crystal structure of LaMn7O12. (a) octahedral MnO6, (b) 12coordinated cubooctahedral LaO12, and (c) 12-coordinated icosahedral MnO12.
a two-phase region. This first order nature of the transition is supported by differential scanning calorimetry data. The LaMn7O12 phase was synthesized by a solid-state highpressure synthesis method. In order to remove any carbonate or hydroxide, the starting materials La2O3 (>99.9%, Kojundo Chemical Laboratory) and Mn2O3 (>99.9%, Kojundo Chemical Laboratory) were annealed for around 12 hours at 1273 K in air. Stoichiometric amounts of La2O3 and Mn2O3 were thereafter carefully ground and mixed, then ca. 100 mg of precursor powder was packed into Au capsule. The high-pressure synthesis was carried out at 5 GPa and 1473 K for 30 min. The phase purity of the as-synthesized samples was investigated by conventional X-ray diffraction using CuKa radiation (Rigaku: RINT2000þ ultraX18). In-situ high-temperature
Fig. 2. Observed, calculated and difference profiles from Rietveld refinement of highresolution SXRD data for LaMn7O12 at room temperature. The inset shows reflections of importance for space group and tilt system determination.
Wyckoff position
x
y
z
100 Uiso (Å2)a
2a 2b 2c 2d 4e 4f 4i 4i 8j 8j
0 0 0 0 1/4 1/4 0.3070(6) 0.3186(6) 0.0135(4) 0.1758(4)
0 1/2 0 1/2 1/4 1/4 0 0 0.1761(4) 0.3088(4)
0 0 1/2 1/2 3/4 1/4 0.1698(6) 0.1746(6) 0.3151(4) 0.0183(4)
0.39(18) 0.28(2) 0.28(2) 0.28(2) 0.03(17) 0.03(17) 0.11(4) 0.11(4) 0.11(4) 0.11(4)
a The displacement parameters of La and Mn at A-site, Mn at B-site, and O were constrained at same values, respectively.
SXRD data were collected at beam line BM01A at the European Synchrotron Radiation Facility (ESRF). A hot-air blower was programmed to heat the sample and data were collected every 5 min between room temperature and 803 K using a heating rate of 10 K/ min and an MAR342 2D-detector. The samples were kept in 0.50 mm rotating borosilicate capillaries, open to the air. The adopted wavelength was 0.71096(2) Å. The diffraction data were analyzed in the range 7.0 2q 40.9 and rebinned into 1905 data points in steps of D2q ¼ 0.0179 . High-resolution SXRD data were
Fig. 3. Exo- and endothermic signals observed by DSC for LaMn7O12. Open and filled symbols refer to measurements on decreasing and increasing temperatures, respectively. Inset shows endothermic and exothermic transition temperatures for the different heating rates of 40, 20, and 10 K/min.
Fig. 4. In-situ SXRD patterns for LaMn7O12, (a) diffraction profiles derived from 2D patterns as function of temperature, (b) SXRD diagram at 636 K (upper) and 655 K (lower). Observed (þ), calculated (solid line) and difference pattern are shown. Enlarged 2q region in inset.
collected at BM01B high-resolution diffractometer, ESRF, l ¼ 0.49990(2) Å. The high-resolution pattern was measured in the range 3.0 2q 30.0 with 0.0020 /min sweeping scan mode. Unit cell dimensions and atomic coordinates were derived from refinements according to the Rietveld method [12] by means of the GSAS code [13]. One scale factor, zero point, five (high-resolution SXRD data) or four (in-situ SXRD data) pseudo-Voigt profile parameters, four (monoclinic phase) or one (cubic phase) unit cell dimensions, positional parameters, together with isotropic displacement factors, entered into the final least-squares refinements. Three isotropic displacement parameters were refined for the cations; respectively, for La, the A0 - and the B-site Mn atoms. For oxygen, they were all constrained to one parameter. Thermal property data were measured with a Perkin–Elmer Pyris 1 differential scanning calorimetry (DSC) up to 723 K at heating rates of 10, 20, and 40 K/min in He atmosphere. The temperature of the sample chamber was calibrated by measuring melting points of commercial In and Zn standard materials. The high-resolution room-temperature SXRD pattern of LaMn7O12 shows systematic reflection conditions hkl: h þ k þ l ¼ 2n corresponding to a body centered monoclinic cell, see Fig. 2, in full accordance with space group I2/m reported by Bochu et al. [7]. The refined structural parameters and atomic coordinates are listed in Table 1. Weak impurity peaks in the diffraction pattern were identified as mainly Mn3O4 with phase fraction of ca. 0.04 wt% and Mn2O3 (in probably even less amounts, yet not well quantified from the refinements). The inset of Fig. 2 shows Miller indices for characteristic reflections that help in determination of the octahedral tilting
Fig. 5. Temperature dependence of unit cell dimensions for LaMn7O12.
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H. Okamoto et al. / Solid State Sciences 11 (2009) 1211–1215
scheme. Glazer [14] has shown for a 2 2 2 supercell of the simplest Pm3m perovskite structure that in-phase tilting gives rise to reflections with indices of type odd–odd–even, whereas odd– odd–odd reflections indicate anti-phase tilting. The observed reflections in the high-resolution SXRD at room temperature, e.g. 013 (even–odd–odd, k s l), 103 (odd–even–odd, h s l), and 031 (odd–odd–even, h s k) indicate that the octahedral tilting of LaMn7O12 below Tc is aþbþcþ by the Glazer notification [14]. The MnO6 octahedra for Mn4 and Mn5 at the 4e and 4f sites, respectively, are strongly distorted in terms of Mn–O bond lengths. The long Mn–O bond for Mn4 [Mn5] is 2.1497(3) Å [2.1210(3)], the intermediate bonds are 1.9937(2) Å [1.9904(17)] while the short bonds are 1.912(4) Å [1.870(4)]. The distortion parameters D [15] are 23.8(6) 104 and 26.4(8) 104 for Mn4 and Mn5,
respectively. These values can be compared with similar distorted isomorphic oxides, such as LaMnO3 (33.1 104 with the C-type orbital ordering at 300 K) [5], KCrF3 (46.2 104 at 295 K) [16], NaMn7O12 (20.9 104 and 20.5 104 for Mn4 and Mn5 with dx2 y2 orbital ordering at 150 K) [10]. These findings of strongly distorted MnO6 octahedra suggest the orbital ordering of the trivalent Mn at the B-site. In Fig. 3, we represent DSC data measured at rumping rates of 40, 20, and 10 K/min upon heating and cooling. The difference in the estimated endothermic and exothermic reaction temperatures as function of heating rate, see inset to Fig. 3, shows that the extrapolated endothermic transition temperature is higher than the exothermic one. The DSC data hence indicate that LaMn7O12 undergoes a first order phase transition.
Fig. 6. Temperature dependence of Mn–O bond lengths and angles for (a) Mn4O6, (b) Mn5O6 for LaMn7O12 (values obtained from high-resolution SXRD data are shown as filled symbols), and (c) the largest and smallest bond angles among all Mn–O–Mn bonds. Schematic illustration of polyhedra along (d) b-axis and (e) a-axis for Mn4O6 and Mn5O6.
H. Okamoto et al. / Solid State Sciences 11 (2009) 1211–1215
The high-temperature in-situ SXRD patterns of LaMn7O12 show immediately a structural phase transition around 653 K, see Fig. 4(a), which corresponds well to the observations by DSC. The in-situ SXRD pattern of LaMn7O12 at 636 K is consistent with the systematic extinctions observed in the high-resolution SXRD pattern at room temperature, see Figs. 2 and 4(b). The distinct peak splitting for the low-temperature I2/m phase remains up to the transition temperature, Tc, Fig. 4(a). All Bragg reflections at 655 K were well indexed on a cubic unit cell. It is worth to mention that no reflections from the low-temperature I2/m phase remain in the pattern at 655 K. The pattern is consistent with a cubic body centered space group with extinctions rules: h þ k þ l ¼ 2n, corresponding to a body centered cubic cell, cf. Fig. 4 (b). The possible space groups are I23, I213, Im3, I432, I4132, I43m and Im3m. According to symmetry and octahedra tilting considerations, the 2 2 2 supercell in space group Im3 with aþaþaþ tilts represents a subgroup to the ideal Pm3m structure with the a0a0a0 tilt in the Glazer-scheme [14]. The analysis below was conducted according to space group Im3. The Wyckoff positions for the B-site of LaMn7O12 in Im3 (No. 204) are the same as in the simple perovskite structure, i.e. no off-center displacements. The thermal evolution of unit cell dimensions of LaMn7O12 was evaluated from the in-situ SXRD patterns, see Fig. 5. The thermal expansion shows linear behavior for temperatures well below Tc, however, becoming more enhanced above some 570 K. The volumetric thermal expansion coefficients aV ¼ DV/(VDT) for I2/m and Im3 phases are 2.38(2) 105 and 2.79(5) 105 (K1), for normalized temperatures at 297 K and 650 K, respectively. The most pronounced expansion occurs for the b-axis, Fig. 5. The unit cell dimensions of a, b and c approach each other just below Tc. This smooth evolution of the unit cell dimensions clearly indicates that the octahedral tilting as well as main features of the Mn–O polyhedra are retained up to Tc. No coexistence of both phases was observed around Tc. The observed discontinuity in the unit cell dimensions proves a first order character of the I2/m to Im3 transition. At the transition, the tilting changes from aþbþcþ to aþaþaþ and the JT distortion of Mn at the B-site vanishes. More information is provided by showing the variation in Mn–O bond lengths and selected bond angles in Fig. 6. Thermal evolutions of Mn–O bond lengths and angles were deduced from refinement of the in-situ SXRD patterns by means of the Rietveld method. It is first noticed that there is no major change in relative intensities of Bragg reflections of the I2/m phase below Tc and that all these patterns hence could be well fitted by just small perturbations of the coordinate values derived from the highresolution data at room temperature. The distortions of the Mn4O6 (Mn5O6) octahedra are mainly due to relative elongation of the Mn4–O4 bond (Mn5–O3 bond) and shortening of the Mn4–O3 bond (Mn5–O4), see Fig. 6(a) and (b). The tilting of Mn4O6 (Mn5O6) octahedra is reflected in the bond angles Mn4–O2–Mn4 and Mn5– O1–Mn5, see Fig. 6(c). Note that only the extremes’ (largest and
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smallest) bond angles are shown in Fig. 6(c). The rest of the bond angles show intermediate behavior with almost no temperature dependence. Preliminary data for the magnetization below room temperature show that LaMn7O12 exhibits two magnetic transitions; the higher temperature transition is ferromagnetic-like, while the lower temperature transition shows antiferromagneticlike behavior. These observations are incompatible with the mSR study by Prodi et al. [17] who claimed onset of antiferromagnetic ordering of Mn at respectively the B- and A-sites at the two different ordering temperatures. However, through slightly different synthesis procedures we obtained samples where the features of the lower temperature antiferromagnetic-like transition have disappeared. These significant differences in magnetic behavior indicate the possibility of a complex defect situation (including intersite cation mixing) for the LaMn7O12 phase. A detailed investigation is therefore now in progress to clarify the magnetic characteristics at low temperatures and their correlation to chemistry and crystal structure.
Acknowledgements This work was supported by the Research Council of Norway, Grant. No. 158518/431 (NANOMAT), Grants-in-aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), and also by Tekes (No. 1726/31/07) and Academy of Finland (No. 116254). We acknowledge the assistance from the research group member at the Swiss-Norwegian Beam lines, ESRF.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
J.B. Goodenough, Phys. Rev. 100 (1955) 564. J.B. Goodenough, Magnetism and the Chemical Bond, Wiley, New York, 1963. E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. P. Norby, I.G.K. Andersen, E.K. Andersen, N.H. Andersen, J. Solid State Chem. 119 (1995) 191. J. Rodrı´guez-Carvajal, M. Hennion, F. Moussa, A.H. Moudden, Phys. Rev. B 57 (1998) R3189. M. Marezio, P.D. Dernier, J. Chenavas, J.C. Joubert, J. Solid State Chem. 6 (1973) 16. B. Bochu, J. Chenavas, J.C. Joubert, M. Marezio, J. Solid State Chem. 11 (1974) 88. J. Chenavas, J.C. Joubert, M. Marezio, B. Bochu, J. Solid State Chem. 14 (1975) 25. A. Collomb, D. Samaras, B. Bochu, J. Chenavas, M.N. Deschizeaux, G. Fillion, J.C. Joubert, M. Marezio, Physica B 86–88 (1977) 927. A. Prodi, E. Gilioli, A. Gauzzi, F. Licci, M. Marezio, F. Bolzoni, Q. Huang, A. Santoro, J.W. Lynn, Nat. Mater. 3 (2004) 48. R. Przenioslo, I. Sosnowska, E. Suard, A. Hewat, A.N. Fitch, J. Phys.: Condens. Matter 14 (2002) 5747. H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748. A.M. Glazer, Acta Crystallogr., Sect. A 31 (1975) 756. R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. S. Margadonna, G. Karotsis, J. Am. Chem. Soc. 128 (2006) 16436. A. Prodi, G. Allodi, E. Gilioli, F. Licci, M. Marezio, F. Bolzoni, A. Gauzzi, R. De Renzi, Physica B 374 (2006) 55.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
High-temperature synchrotron X-ray diffraction study of LaMn7O12 H. Okamoto a, M. Karppinen b, c, H. Yamauchi b, c, H. Fjellvåg a, * a
Centre for Materials Science and Nanotechnology, Department of Chemistry, P.O. Box 1033, University of Oslo, Blindern, N-0315 Oslo, Norway Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan c Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, FI-02015 TKK, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2008 Received in revised form 16 March 2009 Accepted 19 March 2009 Available online 28 March 2009
The crystal structure of the 1:3 A-site ordered perovskite LaMn7O12 with manganese atoms on both the A- and B-sites, was investigated with high-temperature in-situ synchrotron X-ray diffraction (SXRD) from room temperature up to 803 K as well as with high-resolution SXRD at room temperature. A first order structural phase transition from a body centered monoclinic I2/m to a cubic Im3 structure (from aþbþcþ to aþaþaþ tilt system) occurs around Tc ¼ 653 K as evidenced by discontinuous jumps in unit cell dimensions and differential scanning calorimetric data. The temperature dependence of the Mn–O bond distances suggests that the driving force for the transition is relaxation of the Jahn–Teller distortion and subsequent change in the tilting pattern of the MnO6 octahedra. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskite Manganese Synchrotron X-ray diffraction Phase transition
Manganese(III) based perovskite oxides typically exhibit Jahn– 3 e1 Þ the Teller (JT) distortions at low temperatures. For Mn3þ ðt2g g octahedral Oh symmetry is then lifted and the eg orbitals become non-degenerate. This is exemplified by the REMnO3 perovskites which show cooperative JT distortion, orbital ordering, antiferromagnetic order and insulator properties [1,2]. For instance, LaMnO3 takes an orthorhombic structure with cooperative JT distortion associated with orbital ordering and with A-type antiferromagnetic order below TN ¼ 140 K [3]. Upon heating, a first order structural phase transition to a rhombohedral structure occurs at 750 K [4,5] with loss of the cooperative distortion. Among manganese containing perovskite oxides the AA0 3B4O12type phases with Mn at both the A- and the B-sites are extraordinary with regard to chemistry and properties. The phases exhibit interesting superstructures with respect to the simple ABO3 perovskite. For the AA0 3B4O12 phases (A ¼ monovalent alkali metal, divalent alkaline earth, or lanthanoide; A0 ¼ divalent Cu or trivalent Mn; B ¼ transition metal) complete A-site order is anticipated. The normal cubooctahedral 12-coordination is retained for the A atoms, while the A0 atoms are displaced and achieve highly distorted icosahedral 12-coordination with respect to oxygen. This is illustrated in Fig. 1 for LaMn7O12. The AA0 3B4O12 phases are known with Cu2þ and/or Mn3þ as A0 -site cations and with various A-site cations
* Corresponding author. Tel.: þ47 22 85 55 64; fax: þ47 22 85 54 41. E-mail address: [email protected] (H. Fjellvåg). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.03.012
(mono-, di-, tri-, and tetravalent). They take either cubic-, trigonalor monoclinic symmetry at room temperature [6–9]. Interestingly, the location of Cu2þ and/or Mn3þ cations with preference for the distorted local symmetry at the A0 -site seems to be a prerequisite for stabilization of this structure. This A/A0 -site ordered structure shows in-phase tilting of the BO6 octahedra. When the A0 -site is occupied by manganese, i.e. AMn7O12, the crystal symmetry correlates with the oxidation state of the A cation and becomes lower for higher valent A cations; i.e. cubic for monovalent Naþ, trigonal for divalent Ca2þ, Sr2þ and Cd2þ, and monoclinic for trivalent La3þ and Nd3þ. The corresponding average oxidation state of the B-site Mn is Mn3.5þ, Mn3.25þ, and Mn3þ, respectively, while the A0 -site Mn is always Mn3þ [7]. NaMn7O12 shows charge separation of Mn3þ and Mn4þ at the B-sites in a ratio of 1:1. This charge ordering and corresponding Jahn–Teller distortion of Mn3þ are lost during the symmetry change from I2/m to Im3 between 168 K and 176 K [10]. In the case of CaMn7O12, the charge separation of Mn3þ and Mn4þ in a 3:1 ratio at the B-site is lost between 410 K and 450 K upon changing the symmetry from R3 to Im3 [11]. In the present work, the structural properties of LaMn7O12 have been investigated by means of high-temperature powder X-ray synchrotron diffraction (SXRD) as well as by high-resolution SXRD at room temperature. During the in-situ SXRD measurements, diffraction patterns were collected with a 2D-detector at time intervals of 2 min (corresponding to DT ¼ 3.4 K) using a continuous heating rump for the heater gun. A structural first order phase transition is observed at around 659 K without any sign of
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H. Okamoto et al. / Solid State Sciences 11 (2009) 1211–1215 Table 1 Space group, unit cell parameters, and figure of merits of the Rietveld refinement of LaMn7O12 as based on high-resolution SXRD data at room temperature (upper table). Refined atomic coordinates and displacement parameters are given in lower table. Calculated standard deviations in parentheses. Phase
LaMn7O12 at R.T.
Space group a (Å) b (Å) c (Å) b ( ) Volume (Å3) Z Rwp (%) Rp (%) RF2 (%)
I2/m 7.5146(10) 7.3772(10) 7.5224(10) 91.283(10) 416.91(17) 2 12.16 7.39 5.850 8.923
c2 La Mn1 Mn2 Mn3 Mn4 Mn5 O1 O2 O3 O4 Fig. 1. Schematics of the crystal structure of LaMn7O12. (a) octahedral MnO6, (b) 12coordinated cubooctahedral LaO12, and (c) 12-coordinated icosahedral MnO12.
a two-phase region. This first order nature of the transition is supported by differential scanning calorimetry data. The LaMn7O12 phase was synthesized by a solid-state highpressure synthesis method. In order to remove any carbonate or hydroxide, the starting materials La2O3 (>99.9%, Kojundo Chemical Laboratory) and Mn2O3 (>99.9%, Kojundo Chemical Laboratory) were annealed for around 12 hours at 1273 K in air. Stoichiometric amounts of La2O3 and Mn2O3 were thereafter carefully ground and mixed, then ca. 100 mg of precursor powder was packed into Au capsule. The high-pressure synthesis was carried out at 5 GPa and 1473 K for 30 min. The phase purity of the as-synthesized samples was investigated by conventional X-ray diffraction using CuKa radiation (Rigaku: RINT2000þ ultraX18). In-situ high-temperature
Fig. 2. Observed, calculated and difference profiles from Rietveld refinement of highresolution SXRD data for LaMn7O12 at room temperature. The inset shows reflections of importance for space group and tilt system determination.
Wyckoff position
x
y
z
100 Uiso (Å2)a
2a 2b 2c 2d 4e 4f 4i 4i 8j 8j
0 0 0 0 1/4 1/4 0.3070(6) 0.3186(6) 0.0135(4) 0.1758(4)
0 1/2 0 1/2 1/4 1/4 0 0 0.1761(4) 0.3088(4)
0 0 1/2 1/2 3/4 1/4 0.1698(6) 0.1746(6) 0.3151(4) 0.0183(4)
0.39(18) 0.28(2) 0.28(2) 0.28(2) 0.03(17) 0.03(17) 0.11(4) 0.11(4) 0.11(4) 0.11(4)
a The displacement parameters of La and Mn at A-site, Mn at B-site, and O were constrained at same values, respectively.
SXRD data were collected at beam line BM01A at the European Synchrotron Radiation Facility (ESRF). A hot-air blower was programmed to heat the sample and data were collected every 5 min between room temperature and 803 K using a heating rate of 10 K/ min and an MAR342 2D-detector. The samples were kept in 0.50 mm rotating borosilicate capillaries, open to the air. The adopted wavelength was 0.71096(2) Å. The diffraction data were analyzed in the range 7.0 2q 40.9 and rebinned into 1905 data points in steps of D2q ¼ 0.0179 . High-resolution SXRD data were
Fig. 3. Exo- and endothermic signals observed by DSC for LaMn7O12. Open and filled symbols refer to measurements on decreasing and increasing temperatures, respectively. Inset shows endothermic and exothermic transition temperatures for the different heating rates of 40, 20, and 10 K/min.
Fig. 4. In-situ SXRD patterns for LaMn7O12, (a) diffraction profiles derived from 2D patterns as function of temperature, (b) SXRD diagram at 636 K (upper) and 655 K (lower). Observed (þ), calculated (solid line) and difference pattern are shown. Enlarged 2q region in inset.
collected at BM01B high-resolution diffractometer, ESRF, l ¼ 0.49990(2) Å. The high-resolution pattern was measured in the range 3.0 2q 30.0 with 0.0020 /min sweeping scan mode. Unit cell dimensions and atomic coordinates were derived from refinements according to the Rietveld method [12] by means of the GSAS code [13]. One scale factor, zero point, five (high-resolution SXRD data) or four (in-situ SXRD data) pseudo-Voigt profile parameters, four (monoclinic phase) or one (cubic phase) unit cell dimensions, positional parameters, together with isotropic displacement factors, entered into the final least-squares refinements. Three isotropic displacement parameters were refined for the cations; respectively, for La, the A0 - and the B-site Mn atoms. For oxygen, they were all constrained to one parameter. Thermal property data were measured with a Perkin–Elmer Pyris 1 differential scanning calorimetry (DSC) up to 723 K at heating rates of 10, 20, and 40 K/min in He atmosphere. The temperature of the sample chamber was calibrated by measuring melting points of commercial In and Zn standard materials. The high-resolution room-temperature SXRD pattern of LaMn7O12 shows systematic reflection conditions hkl: h þ k þ l ¼ 2n corresponding to a body centered monoclinic cell, see Fig. 2, in full accordance with space group I2/m reported by Bochu et al. [7]. The refined structural parameters and atomic coordinates are listed in Table 1. Weak impurity peaks in the diffraction pattern were identified as mainly Mn3O4 with phase fraction of ca. 0.04 wt% and Mn2O3 (in probably even less amounts, yet not well quantified from the refinements). The inset of Fig. 2 shows Miller indices for characteristic reflections that help in determination of the octahedral tilting
Fig. 5. Temperature dependence of unit cell dimensions for LaMn7O12.
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scheme. Glazer [14] has shown for a 2 2 2 supercell of the simplest Pm3m perovskite structure that in-phase tilting gives rise to reflections with indices of type odd–odd–even, whereas odd– odd–odd reflections indicate anti-phase tilting. The observed reflections in the high-resolution SXRD at room temperature, e.g. 013 (even–odd–odd, k s l), 103 (odd–even–odd, h s l), and 031 (odd–odd–even, h s k) indicate that the octahedral tilting of LaMn7O12 below Tc is aþbþcþ by the Glazer notification [14]. The MnO6 octahedra for Mn4 and Mn5 at the 4e and 4f sites, respectively, are strongly distorted in terms of Mn–O bond lengths. The long Mn–O bond for Mn4 [Mn5] is 2.1497(3) Å [2.1210(3)], the intermediate bonds are 1.9937(2) Å [1.9904(17)] while the short bonds are 1.912(4) Å [1.870(4)]. The distortion parameters D [15] are 23.8(6) 104 and 26.4(8) 104 for Mn4 and Mn5,
respectively. These values can be compared with similar distorted isomorphic oxides, such as LaMnO3 (33.1 104 with the C-type orbital ordering at 300 K) [5], KCrF3 (46.2 104 at 295 K) [16], NaMn7O12 (20.9 104 and 20.5 104 for Mn4 and Mn5 with dx2 y2 orbital ordering at 150 K) [10]. These findings of strongly distorted MnO6 octahedra suggest the orbital ordering of the trivalent Mn at the B-site. In Fig. 3, we represent DSC data measured at rumping rates of 40, 20, and 10 K/min upon heating and cooling. The difference in the estimated endothermic and exothermic reaction temperatures as function of heating rate, see inset to Fig. 3, shows that the extrapolated endothermic transition temperature is higher than the exothermic one. The DSC data hence indicate that LaMn7O12 undergoes a first order phase transition.
Fig. 6. Temperature dependence of Mn–O bond lengths and angles for (a) Mn4O6, (b) Mn5O6 for LaMn7O12 (values obtained from high-resolution SXRD data are shown as filled symbols), and (c) the largest and smallest bond angles among all Mn–O–Mn bonds. Schematic illustration of polyhedra along (d) b-axis and (e) a-axis for Mn4O6 and Mn5O6.
H. Okamoto et al. / Solid State Sciences 11 (2009) 1211–1215
The high-temperature in-situ SXRD patterns of LaMn7O12 show immediately a structural phase transition around 653 K, see Fig. 4(a), which corresponds well to the observations by DSC. The in-situ SXRD pattern of LaMn7O12 at 636 K is consistent with the systematic extinctions observed in the high-resolution SXRD pattern at room temperature, see Figs. 2 and 4(b). The distinct peak splitting for the low-temperature I2/m phase remains up to the transition temperature, Tc, Fig. 4(a). All Bragg reflections at 655 K were well indexed on a cubic unit cell. It is worth to mention that no reflections from the low-temperature I2/m phase remain in the pattern at 655 K. The pattern is consistent with a cubic body centered space group with extinctions rules: h þ k þ l ¼ 2n, corresponding to a body centered cubic cell, cf. Fig. 4 (b). The possible space groups are I23, I213, Im3, I432, I4132, I43m and Im3m. According to symmetry and octahedra tilting considerations, the 2 2 2 supercell in space group Im3 with aþaþaþ tilts represents a subgroup to the ideal Pm3m structure with the a0a0a0 tilt in the Glazer-scheme [14]. The analysis below was conducted according to space group Im3. The Wyckoff positions for the B-site of LaMn7O12 in Im3 (No. 204) are the same as in the simple perovskite structure, i.e. no off-center displacements. The thermal evolution of unit cell dimensions of LaMn7O12 was evaluated from the in-situ SXRD patterns, see Fig. 5. The thermal expansion shows linear behavior for temperatures well below Tc, however, becoming more enhanced above some 570 K. The volumetric thermal expansion coefficients aV ¼ DV/(VDT) for I2/m and Im3 phases are 2.38(2) 105 and 2.79(5) 105 (K1), for normalized temperatures at 297 K and 650 K, respectively. The most pronounced expansion occurs for the b-axis, Fig. 5. The unit cell dimensions of a, b and c approach each other just below Tc. This smooth evolution of the unit cell dimensions clearly indicates that the octahedral tilting as well as main features of the Mn–O polyhedra are retained up to Tc. No coexistence of both phases was observed around Tc. The observed discontinuity in the unit cell dimensions proves a first order character of the I2/m to Im3 transition. At the transition, the tilting changes from aþbþcþ to aþaþaþ and the JT distortion of Mn at the B-site vanishes. More information is provided by showing the variation in Mn–O bond lengths and selected bond angles in Fig. 6. Thermal evolutions of Mn–O bond lengths and angles were deduced from refinement of the in-situ SXRD patterns by means of the Rietveld method. It is first noticed that there is no major change in relative intensities of Bragg reflections of the I2/m phase below Tc and that all these patterns hence could be well fitted by just small perturbations of the coordinate values derived from the highresolution data at room temperature. The distortions of the Mn4O6 (Mn5O6) octahedra are mainly due to relative elongation of the Mn4–O4 bond (Mn5–O3 bond) and shortening of the Mn4–O3 bond (Mn5–O4), see Fig. 6(a) and (b). The tilting of Mn4O6 (Mn5O6) octahedra is reflected in the bond angles Mn4–O2–Mn4 and Mn5– O1–Mn5, see Fig. 6(c). Note that only the extremes’ (largest and
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smallest) bond angles are shown in Fig. 6(c). The rest of the bond angles show intermediate behavior with almost no temperature dependence. Preliminary data for the magnetization below room temperature show that LaMn7O12 exhibits two magnetic transitions; the higher temperature transition is ferromagnetic-like, while the lower temperature transition shows antiferromagneticlike behavior. These observations are incompatible with the mSR study by Prodi et al. [17] who claimed onset of antiferromagnetic ordering of Mn at respectively the B- and A-sites at the two different ordering temperatures. However, through slightly different synthesis procedures we obtained samples where the features of the lower temperature antiferromagnetic-like transition have disappeared. These significant differences in magnetic behavior indicate the possibility of a complex defect situation (including intersite cation mixing) for the LaMn7O12 phase. A detailed investigation is therefore now in progress to clarify the magnetic characteristics at low temperatures and their correlation to chemistry and crystal structure.
Acknowledgements This work was supported by the Research Council of Norway, Grant. No. 158518/431 (NANOMAT), Grants-in-aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), and also by Tekes (No. 1726/31/07) and Academy of Finland (No. 116254). We acknowledge the assistance from the research group member at the Swiss-Norwegian Beam lines, ESRF.
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