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Neutron, synchrotron X-ray diffraction and X-ray absorption studies of CaLaFeMnO6 double perovskite
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Solid State Sciences 10 (2008) 1634e1639 www.elsevier.com/locate/ssscie
Neutron, synchrotron X-ray diffraction and X-ray absorption studies of CaLaFeMnO6 double perovskite M. Nasir Khan*, R. Shaheen, J. Bashir Physics Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan Received 13 November 2007; received in revised form 31 January 2008; accepted 8 February 2008 Available online 15 February 2008
Abstract High-resolution neutron and synchrotron X-ray powder diffraction data of CaLaFeMnO6 double perovskite have been used in a joint Rietveld refinement method to obtain accurate room temperature structure which includes a precise set of atomic positions and thermal motion parameters. The valence states of Mn and Fe in the octahedra were also determined by X-ray absorption spectroscopy by measuring Mn and Fe K edges at room temperature. The simultaneous refinement of both synchrotron X-ray and neutron diffraction data reaffirmed that the system indeed possesses orthorhombic structure with space group Pbnm. X-ray absorption spectroscopic measurements reveal that both Mn and Fe occupy the octahedral sites with valence state of þ4 and þ3, respectively. No cationic ordering arrangement was found and Fe/Mn was found to be randomly distributed over the octahedral sites. The results agree well with the earlier published neutron diffraction data. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Oxides; Neutron scattering; X-ray diffraction; XANES; Crystal structure
1. Introduction The ideal perovskite structure has an ABO3 stoichiometry and belongs to cubic symmetry with Pm3m space group. The perovskite structure has the potential to accommodate huge varieties of ions by the multiple ion substitution for single or more than one original cations. The double perovskites 0 0 of the type AA BB O6 are derived from ABO3 perovskites when half of the 12 coordinated A-site cations and six coordi0 0 nated B-site cations are replaced by suitable A and B cations, respectively. These are usually manganites and exhibit diverse properties such as ferroelectricity, piezoelectricity, non-linear optical properties and superconductivity [1e3]. The renewed interests in these compounds arise because of the room temperature colossal magneto-resistance (CMR) discovery in Sr2FeMoO6 [4,5]. Owing to their potential applications arising from their interesting electrical and magnetic properties, extensive studies relating to different aspects of double * Corresponding author. Tel.: þ92 51 2207278; fax: þ92 51 9290275. E-mail address: [email protected] (M. Nasir Khan). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.02.007
perovskites have been made [6,7]. In addition to their technological applications, these perovskites are also of crystallographic interest [8e11]. The structural and magnetic properties of double perovskite 0 0 0 materials with either A2BB O6 or AA BB O6 stoichiometry are 0 shown to depend on how the BB cations are distributed over the octahedral sites, degree of cation inversion as well as on the size and electronic structure of transition metal cations B 0 and B [12e15]. Depending on the tolerance factor, t, which is the function of the ionic radii of the cations, the structure of the perovskites distorts from the cubic symmetry. For t z 1, the perovskite adopts a cubic symmetry (space group Pm3m). For t > 1 or t < 1, the structure is distorted resulting in symmetry lower than cubic. Three different types of distortions have been identified [16]: (i) distortions of the BO6 octahedral units, (ii) B-cation displacements within the octahedra, and (iii) the tilting of the BO6 octahedra relative to one another as practically rigid corner-linked units. The third type of distortion, octahedral tilting, is the most common type of distortion. The rotation and tilting of octahedra in the perovskite compounds have been studied in detail earlier by
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many researchers [8,9,17]. The octahedral tilting and the presence of cationic ordering produce superlattice reflections. Depending on the degree of ordering and tilting of the octahedra, superlattice reflections can be very weak and hence difficult to ascertain. Anderson and Poeppelmeier [18] based on their findings reported that ordered structure in such complicated system is most likely to be formed if charge difference be0 tween B and B cations is 2 or higher. For rational understanding of the solid-state properties of these new materials and for establishing proper structuree property relationships, crystal structure determination is a pre-requisite. However, the limited resolution of the diffrac0 tion instruments, presence of relative BB cation disorders, and pseudocubic structure exhibited by these materials make it difficult to determine their real crystal structure. Fortunately, the exceptional resolution and luminosity offered by synchrotron radiation source make it possible to determine the real crystal structure of these materials with high degree of accuracy. For example, Gateshki et al. [19] have employed synchrotron X-ray diffraction technique for the determination of crystal structure of La2NiRuO6 where earlier neutron diffraction measurements provided contradictory information about the crystal structure of the material. Synthesis and unit-cell parameters of CaLaFeMnO6 (CLFMO) were previously reported by Ramesha et al. [5] and Shaheen et al. [20] using X-ray and neutron diffraction investigations, respectively. From the neutron diffraction investigations, CLFMO was reported to be orthorhombic whereas laboratory X-ray measurements [5] revealed it to be cubic/ pseudocubic. In order to resolve such controversy, joint refinement of both the room temperature powder synchrotron X-ray and high-resolution neutron diffraction data was carried out. Since X-ray diffraction is generally more sensitive to cationic ordering whereas neutron diffraction is more sensitive to octahedral tilting, combined refinement of both data provides best means of establishing the crystal structure and tilts in this material. The valence state of transition metal cations was also determined using X-ray absorption near edge structure (XANES ) data. This provides an additional tool to establish the presence or absence of ordering in the materials. 2. Experimental CaLaFeMnO6 samples were synthesized by solid-state reaction. Stoichiometric amounts of La2O3, Fe2O3, MnO2 and CaCO3 were mixed thoroughly and ground. The calcination and sintering conditions have been reported elsewhere [20]. Neutron powder diffraction data were measured at 50 MW research reactor at NFL, Studsvik, Sweden in the 2q range 4e140 with a step size of 0.08 using wavelength of ˚ . The synchrotron data were collected at ELETTRA 1.47 A synchrotron source, Trieste, Italy at XRD1 beam line with ˚ from Si (111) crystal monochromator in nonl ¼ 0.67 A dispersive configuration followed by toroidal focusing mirror. Quartz capillary with 0.3 mm diameter was used as the sample holder. MARS 345 imaging plate was used to record the diffraction pattern with sample to detector distance of 85 mm.
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During each exposure the sample was rotated through a 90 angle. Any preferred orientation of the powder should have been eliminated through a combination of the specimen rotation, use of a capillary sample holder and full intensity integration of the diffraction rings, as obtained using the programme Fit2D [21]. The calibration of wavelength and distance was carried out using standard LaB6 sample. The Rietveld structural refinement of both the data was carried out simultaneously using a Multi Rietveld analysis programme Rietica [22]. The background was defined by a fifth order polynomial in 2q and was refined simultaneously with other structural and profile parameters. Refined parameters included unit-cell parameters, positions and isotropic thermal parameters (Biso) of all the atoms as well as the usual profile parameters describing the pseudo-Voigt peaks’ shape function. In total 43 parameters were simultaneously refined. Room temperature Mn K-edge and Fe K-edge X-ray absorption spectroscopic (XAS) experiments were performed at the XAFS beam line at ELETTRA Synchrotron Radiation Source, Trieste, Italy. Well-dispersed sample powder in cyclohexane was deposited on porous membranes. The amount of the sample was pre-calculated to obtain optimum absorption jump (Dm z 1). X-ray beam energies were tuned using a double crystal, focusing Si (111) monochromator crystals. The incident (Io) and transmitted (It) beam intensities were recorded with ionization chambers filled with appropriate gases. Data were collected in two regions: (1) the pre-edge region with a step size of 5 eV to allow pre-edge background calculation and subtraction; (2) the XANES region from 30 eV below the edge to 50 eV above the absorption edge with a step size of 0.3 eV. The reproducibility in the determination of the edge positions was found to be better than 0.2 eV. Scanning electron microscope (SEM) with an energy dispersive X-ray (EDX) analyzer and inductive coupled plasma (ICP) were the tools used to check the chemical composition of the synthesized samples. The average results of several spot measurements of EDX analysis and ICP measurements confirmed nominal composition of the sample to be Ca0.99(1)La0.98(2)Mn0.98(2)Fe0.99(1)O5.97(3). These results were further confirmed by Rietveld analysis where the oxygen occupancies varied along with other cation elements but we had found no deficiency of either oxygen or other cation elements. Subsequently, all occupancies for cations and anions were fixed at their nominal composition values during refinement. 3. Results and discussion 3.1. Joint refinement for neutron/SXRD The crystal structure has been refined using space group Pbnm with an orthorhombic structure, which is a non-standard setting of Pnma (# 62). The refined parameters obtained from the joint refinement of the two data sets are listed in Table 1. The model of the Rietveld refinement was built on the basis of atomic arrangements reported in Ref. [20]. In the refined structure the assigned occupation sites are 4c for Ca/La and O1, 4b
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Table 1 Room temperature refined structural parameters, reliability factors, and interatomic distances for CaLaMnFeO6 system Parameters
NDb results [20]
Present results
˚) a (A ˚) b (A ˚) c (A
5.4468(5) 5.4386(5) 7.6862(5)
5.4658(3) 5.4501(3) 7.7321(5)
Ca/La at (xy1/4) x y ˚ 2) BLa/Ca (A
0.006(2) 0.021(1) 0.89(5)
0.0025(1) 0.0176(1) 0.99(3)
1.1(3)
0.81(3)
O(1) at (xy1/4) x y ˚ 2) B (A
0.063(2) 0.487(1) 0.5(1)
0.066 (2) 0.502(2) 1.32(1)
O(2) at (x,y,z) x y z ˚ 2) B (A
0.221(1)a 0.279(1) 0.029(1) 1.5(1)
0.216(1) 0.268(2) 0.029(1) 1.32(1)
1.949(2) 1.952(2) 1.952(2) 2.426(8) 2.447(8) 2.658(8) 2.562(8) 2.727(7) 2.992(7) 3.031(8) 3.112(5) 89.3(1) 90.7(1) e e 1.1 106 0.0082
1.8914(2) 1.9664(2) 2.0146(2) 2.3602(1) 2.4893(3) 2.6897(1) 2.6627(1) 2.8314(1) 2.7168(1) 3.0731(1) 3.1085(1) 89.46(2) 90.54(1) 162.26(1) 158.85(1) 0.00402 0.00794
Mn/Fe at (1/2,0,0) ˚ 2) BMn/Fe (A
˚) Interatomic distances (A Fe/MneO2(B) 2 Fe/MneO1 2 Fe/MneO2(A) 2 A/LaeO1 1 A/LaeO2 2 A/LaeO2 2 A/LaeO1 1 A/LaeO1 1 A/LaeO2 2 A/LaeO2 2 A/LaeO1 1 :O1eMn/FeeO2 :O1eMn/FeeO2 :Mn/FeeO2eFe/Mn :Mn/FeeO1eMn/Fe DBeO6 DAeO12
The numbers in the parenthesis represent errors in the last significant digit. Joint R factors for the two histograms are: Rp ¼ 7.72%, Rwp ¼ 9.87% with RBragg ¼ 1.89%. a Denotes that the value of 0.721(1) quoted in Ref. [20] is a typing mistake and the actual value is 0.221(1). b ND ¼ Neutron diffraction.
for Mn/Fe and 8d for O2. No evidence was found for ordering of cations in either of the structural sites in the neutron or the synchrotron X-ray diffraction while refining simultaneously both the data. The observed, calculated and difference for the two patterns are shown in Fig. 1. The data fitted well using the arrangements mentioned above and easily converged resulting in the lowest R values. The lattice parameters are ˚ , b ¼ 5.4501(3) A ˚ and c ¼ 7.7321(5) A ˚ with a ¼ 5.4658(3) A total joint R factors of Rp ¼ 7.72%, Rwp ¼ 9.87 with RBragg ¼ 1.89% for the two histograms. The structure obtained is similar to that of CaTiO3 and a large number of ABO3 compounds that adopt the GdFeO3 structure. In the GdFeO3 structures octahedra are rotated about more than one pseudocubic Cartesian axes compared to that of an ideal cubic
Fig. 1. Observed (þ), calculated (), and below the difference of observed and calculated patterns. (a) Synchrotron and (b) neutron jointly refined by Rietveld refinement method.
perovskite [23e25]. The polyhedral view along (001) axis is shown in Fig. 2(a), using refined parameters listed in Table 1. This clearly indicated the tilting of BO6 octahedra in the aacþ tilt system in the Glazer tilt notation scheme [9]. The calculated bond distances obtained from the combined refinement are listed in Table 1, which indicates that CLFMO consists of a frame work of tilted and distorted (Mn/Fe)O6 polyhedra with La and Ca cations occupying the distorted 12-fold coordinated sites within this framework. If all the oxygen atoms that surround the Mn/Fe atoms possess the same bond distances then there is no distortion of the octahedra and if the angle between OeMn/FeeO is 180 in the ab plane and along c-axis then the octahedra are not tilted [26]. The rotation angle j around the c-axis (:Fe/MneO2eFe/Mn) is 162.26 , while the octahedral tilt angle relative to (001) plane is q w 158.85 . The (Fe/Mn)O6 polyhedra exhibit a similar style of distortion to that found in CaTiO3 and other ABO3 perovskite compounds. This can be revealed by the relatively short MneO(2) bonds showing the compression along the semi-minor axis designated as O2(B) of the equatorial plane of the polyhedron shown in Fig. 2(b). The (Ca/La)O12 cubo-octahedron is a highly distorted site as is evident from the spread of 12 bond distances of oxygen atoms. This is also in agreement with the observed tolerance factor criterion mentioned above, which becomes smaller due to A-site compression. The observed tolerance factor in the present CaLaMnFeO6 system is 0.959. In short, we have observed significant changes in the crystallographic and atomic parameters with joint refinements when compared to earlier neutron diffraction studies [20]. The lattice constant shows more unit-cell volume compared
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Fig. 2. (a) Polyhedral view along (001) of the LaCaMnFeO6 system. (b) Octahedral tilt and rotation along with bond distances; view along (001) axis.
to individual neutron diffraction refinement. The atomic positions also show slight variations in the joint refinement. Also the temperature factors of the two oxygen atoms are higher as compared to the values determined from neutron diffraction data. This is in agreement with tilting of octahedra which induces a large shortening of two AeO1 bond distances and its rotation shortens two large AeO2 distances as can be seen in Table 1. Therefore, the Biso of oxygen increases [26]. The polyhedral distortion is more pronounced in the joint refinement. The Mn/FeO6 octahedra are more distorted and tilted when compared with neutron diffraction data, where these seem to be rather regular. The Ca/LaO12 cubo-octahedra also show more distortion when compared to individual neutron data. The polyhedral distortion was also calculated using the formula as reported in Ref. [27] showing the relation between bond lengths and bond strength. The polyhedral distortion using refined parameters of the joint refinement shows DFe/MnO6 w 0.00402, and for cubo-octahedron DCa/ LaO12 w 0.00794, while these values are 1.1 106 and 0.0082 when calculated from the individual neutron diffraction refined parameters [20]. The degree of distortion in this system is greater than that found in CaTiO3 and other GdFeO3 type perovskite compounds [23]. The combined use of X-ray and neutron scattering was found to improve significantly the determination of both light and heavy atom positions and thermal parameters and are thus more useful in the structural characterization of complex compounds containing both heavy
and light elements. The site ordering of Mn/Fe thus can only be predicted through neutron scattering whereas X-ray diffraction alone is insufficient to determine it uniquely. We have shown that technique of joint neutron and synchrotron X-ray diffraction Rietveld refinement is a powerful tool and allows more in depth, complete and precise structural determination, and virtually eliminates problems encountered with false minima.
3.2. XANES The oxidation states of manganese and iron in CLFMO were investigated by XANES spectroscopy. The comparison of K-edge XANES of Fe and Mn in CLFMO with the model compounds of known oxidation state and local coordination environment provides a way to assess their chemical environment. LaMnO3 (for Mnþ3) and CaMnO3 (for Mnþ4) are suitable reference materials for manganese based perovskites because both of these materials contain manganese in the octahedral environment. In Fig. 3, the Mn K-edge absorption spectra for the series of Mnþn (n ¼ 3, 4) along with that of CLFMO are shown. All these materials contain Mn in octahedral environment. The XANES spectra of LaMnO3 and CaMnO3 are in agreement with those already reported in the literature [28e33]. The comparison of XANES spectra of CLFMO with LaMnO3
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Table 3 Threshold energies (E0) for model compounds and CaLaMnFeO6 at Fe K-edge
Fig. 3. Comparison of Mn K-edge XANES spectra of CaLaFeMnO6 with standard samples LaMnþ3O3 and CaMnþ4O3. The vertical and horizontal lines show the valence state of Mn determined from value of energy comparison with standard samples when normalised absorption equals 0.5.
Compound
E0 (eV) Present work
From literature
Feþ2O Fe3O4 Feþ3 2 O3 CaLaMnFeO6
7119.8 7122.3 7123.9 7124.2
7120.0 [36] 7124.3 [36]
Mn K-edge spectra of CLFMO and standard samples show similar features. The main difference among the spectra is the position of the Mn and Fe absorption edges (E0) which was determined from the value of energy when normalised absorption equals 0.5. The threshold energies summarised in Tables 2 and 3 are in good agreement with those reported in the literature [28,32]. It is evident from Fig. 3 that there is a substantial chemical shift of the edges with increasing valence. The chemical shift of CLFMO is close to that of CaMnO3 thereby indicating that valence of Mn in CLFMO is þ4. The Fe pre-edge peak around 7112 eV and edge jump peak around 7127 eV represent 1s to 3d (dipole forbidden process) and 1s to 4p transitions, respectively [35]. The threshold energies for FeO and Fe2O3 are 7119.8 eV and 7123.9 eV, respectively, and are in good agreement with values reported in literature [36]. Based on standard FeO (Fe2þ) and Fe2O3 (Fe3þ) spectra (Fig. 4), valence state of Fe in CLFMO is found to be þ3.
and CaMnO3 suggests that Mn occupies only the octahedral sites in the perovskite structure. Like other CMR compounds, three energy regions are visible in the Mn K-edge spectra of CLFMO. In the preedge region (6535e6545 eV), the peaks in Fig. 3 are attributed to mixture of 1s / 3d quadrupole and 1s / 3d dipole transitions (weakly allowed by a hybridization between 3d states and 4p states) [30]. The most intense peak at 6550 eV in these materials is associated with 4p bands. The other resonance extending from 6560 eV to 6570 eV arises due to the multiple scattering events of the photoexcited electron. Joly et al. [34] have suggested that these pre-peaks arise from the mixing of d states between different atoms, through hybridization of p band.
Table 2 Threshold energies (E0) for model compounds and CaLaMnFeO6 at Mn K-edge Compound
E0 (eV) Present work
From literature
þ3
LaMn O3
6551.2
6550.6 [28] 6550.7 [32]
CaLaMnFeO6 CaMnþ4O3
6554.7 6555.3
6554.8 [28] 6554.6 [32]
Fig. 4. Comparison of Fe K-edge XANES spectra of CaLaFeMnO6 with standard samples FeO (Fe2þ) and Fe2O3 (Fe3þ). The vertical and horizontal lines show the valence state of Fe determined from value of energy comparison with standard samples when normalised absorption equals 0.5.
M. Nasir Khan et al. / Solid State Sciences 10 (2008) 1634e1639
4. Conclusions We have simultaneously refined high-resolution synchrotron X-ray and neutron diffraction data for CaLaFeMnO6 system. The Rietveld refinement reaffirmed that the system could be best described in orthorhombic structure with Pbnm space group. The results are in agreement with previous neutron diffraction data [20], but with a slight change in crystallographic and atomic parameters. Mn and Fe exist with the þ4 and þ3 valencies on the octahedral sites, respectively. Mn and Fe are randomly distributed over the B site. This random distribution of Mn and Fe is supported by the magnetic susceptibility measurements of Ramesha et al. [5] showing that these compounds are paramagnetic insulators. The cation ordering otherwise would have revealed this perovskite to be a ferromagnetic insulator. The random distribution of transition metal cations over the six coordinated octahedral sites as revealed by diffraction techniques and the valence state difference between two cations of less than 2 as determined by XANES measurements validate the criterion set by Anderson and Poeppelmeier [18] stated earlier. Acknowledgements The authors are thankful to ICTP for the generous support to visit ELETTRA synchrotron radiation source facility under ICTPeELETTRA user programme where the experimental part of this work was carried out. The authors also thank Dr. Mariziou Paletinou (XRD1 beamline) and Dr. Luca Olivi (XAFS beamline) at ELETTRA for their help in the experiments. References [1] R.E. Newnham, G.R. Ruschau, J. Am. Ceram. Soc. 74 (1991) 463. [2] R.J. Cava, R.B. van Dover, B. Batlogg, E.A. Rietman, Phys. Rev. Lett. 58 (1987) 408. [3] J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyon, B. Raveau, J.L. Soubeyroux, J.L. Tholence, R. Tournier, Europhys. Lett. 3 (1987) 1301. [4] G.Q. Gong, C. Canedy, G. Xiao, J.Z. Sun, A. Gupta, W.J. Gallagher, Appl. Phys. Lett. 67 (1995) 1783. [5] K. Ramesha, V. Thangadurai, D. Sutar, S.V. Subramanyam, G.N. Subbanna, Gopalakrishnan, Mater. Res. Bull. 35 (2000) 559.
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[6] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [7] M. DeMarco, H.A. Blackstead, J.D. Dow, M.K. Wu, D.Y. Chen, F.Z. Chien, M. Haka, S. Toorongian, J. Fridmann, Phys. Rev. B 62 (2000) 14301. [8] C.J. Howard, H.T. Stokes, Acta Crystallogr. B 54 (1998) 782. [9] A.M. Glazer, Acta Crystallogr. B 28 (1972) 3384. [10] O. Bock, U. Muller, Acta Crystallogr. B 58 (2002) 594. [11] C.J. Howard, B.J. Kennedy, P.M. Woodward, Acta Crystallogr. B 59 (2003) 463. [12] P.D. Battle, C.W. Jones, F. Studer, J. Solid State Chem. 90 (1991) 301. [13] P.D. Battle, W.J. Macklin, J. Solid State Chem. 52 (1984) 138. [14] W.R. Gemmill, M.D. Smith, H.-C. Zur Loye, J. Solid State Chem. 177 (2004) 3560. [15] E. Quarez, F. Abraham, O. Mentre´, J. Solid State Chem. 176 (2003) 137 and references cited therein. [16] H.D. Megaw, Crystal Structures e A Working Approach, W.B. Saunders, Philadelphia, 1973. [17] P.M. Woodward, Acta Crystallogr. B 53 (1997) 32. [18] M.T. Anderson, K.R. Poeppelmeier, Chem. Mater. 3 (1991) 476. [19] M. Gateshki, J.M. Igartua, E. Hernandez-Bocanegra, J. Phys.: Condens. Matter 15 (2003) 6199. [20] R. Shaheen, J. Bashir, H. Rundlof, A.R. Rennie, Mater. Lett. 59 (2005) 2296. [21] A.P. Hammersley, S.O. Svensson, M. Hanfland, A.N. Fitch, D. Hausermann, High Pressure Res. 14 (1996) 235. [22] C.J. Howard, B.A. Hunter, A Computer Programme for Rietveld Analysis of X-ray and Neutron Powder Diffraction Patterns, Lucas Heights Research Laboratories, NSW, Australia, 1998, pp. 1e27. [23] S. Sasaki, C.T. Prewitt, J.D. Bass, W.A. Schulze, Acta Crystallogr. C 43 (1987) 1668. [24] R.H. Buttner, E.N. Melsen, Acta Crystallogr. B 48 (1992) 644. [25] B.G. Hyde, S. Anderson, Inorganic Crystal Structures, John Wiley & Sons, NY, 1989. [26] T. Yamanaka, N. Hirai, Y. Komatsu, Am. Mineral. 87 (2001) 183. [27] I.D. Brown, R.D. Shannon, Acta Crystallogr. B 29 (1973) 266. [28] G. Subı´as, J. Garcı´a, M.G. Proietti, J. Blasco, Phys. Rev. B 56 (1997) 8183. [29] C. Monesi, C. Meneghini, F. Bardelli, M. Benfatto, S. Mobilio, U. Manju, D.D. Sarma, Nucl. Instrum. Methods Phys. Res., Sect. B 246 (2006) 158. [30] F. Bridges, C.H. Booth, G.H. Kwei, J.J. Neumeier, G.A. Sawatzky, Phys. Rev. B 61 (2000) R9237. [31] M. Croft, D. Sills, M. Greenblatt, C. Lee, S.-W. Cheong, K.V. Ramanujachary, D. Tran, Phys. Rev. B 55 (1997) 8726. [32] G. Dezanneau, M. Audier, H. Vincent, C. Meneghini, E. Djurado, Phys. Rev. B 69 (2004) 014412. [33] T. Shibata, B.A. Bunker, J.F. Mitchell, Phys. Rev. B 68 (2003) 024103. [34] Y. Joly, D. Cabaret, H. Renevier, C.R. Natoli, Phys. Rev. Lett. 82 (1999) 2398. [35] A. Deb, U. Bergmann, E.J. Cairns, S.P. Cramer, J. Phys. Chem. B 108 (2004) 7046. [36] A. Ide-Ektessabi, T. Kawakami, F. Watt, Nucl. Instrum. Methods Phys. Res., Sect. B 213 (2004) 590.
Solid State Sciences 10 (2008) 1634e1639 www.elsevier.com/locate/ssscie
Neutron, synchrotron X-ray diffraction and X-ray absorption studies of CaLaFeMnO6 double perovskite M. Nasir Khan*, R. Shaheen, J. Bashir Physics Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan Received 13 November 2007; received in revised form 31 January 2008; accepted 8 February 2008 Available online 15 February 2008
Abstract High-resolution neutron and synchrotron X-ray powder diffraction data of CaLaFeMnO6 double perovskite have been used in a joint Rietveld refinement method to obtain accurate room temperature structure which includes a precise set of atomic positions and thermal motion parameters. The valence states of Mn and Fe in the octahedra were also determined by X-ray absorption spectroscopy by measuring Mn and Fe K edges at room temperature. The simultaneous refinement of both synchrotron X-ray and neutron diffraction data reaffirmed that the system indeed possesses orthorhombic structure with space group Pbnm. X-ray absorption spectroscopic measurements reveal that both Mn and Fe occupy the octahedral sites with valence state of þ4 and þ3, respectively. No cationic ordering arrangement was found and Fe/Mn was found to be randomly distributed over the octahedral sites. The results agree well with the earlier published neutron diffraction data. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Oxides; Neutron scattering; X-ray diffraction; XANES; Crystal structure
1. Introduction The ideal perovskite structure has an ABO3 stoichiometry and belongs to cubic symmetry with Pm3m space group. The perovskite structure has the potential to accommodate huge varieties of ions by the multiple ion substitution for single or more than one original cations. The double perovskites 0 0 of the type AA BB O6 are derived from ABO3 perovskites when half of the 12 coordinated A-site cations and six coordi0 0 nated B-site cations are replaced by suitable A and B cations, respectively. These are usually manganites and exhibit diverse properties such as ferroelectricity, piezoelectricity, non-linear optical properties and superconductivity [1e3]. The renewed interests in these compounds arise because of the room temperature colossal magneto-resistance (CMR) discovery in Sr2FeMoO6 [4,5]. Owing to their potential applications arising from their interesting electrical and magnetic properties, extensive studies relating to different aspects of double * Corresponding author. Tel.: þ92 51 2207278; fax: þ92 51 9290275. E-mail address: [email protected] (M. Nasir Khan). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.02.007
perovskites have been made [6,7]. In addition to their technological applications, these perovskites are also of crystallographic interest [8e11]. The structural and magnetic properties of double perovskite 0 0 0 materials with either A2BB O6 or AA BB O6 stoichiometry are 0 shown to depend on how the BB cations are distributed over the octahedral sites, degree of cation inversion as well as on the size and electronic structure of transition metal cations B 0 and B [12e15]. Depending on the tolerance factor, t, which is the function of the ionic radii of the cations, the structure of the perovskites distorts from the cubic symmetry. For t z 1, the perovskite adopts a cubic symmetry (space group Pm3m). For t > 1 or t < 1, the structure is distorted resulting in symmetry lower than cubic. Three different types of distortions have been identified [16]: (i) distortions of the BO6 octahedral units, (ii) B-cation displacements within the octahedra, and (iii) the tilting of the BO6 octahedra relative to one another as practically rigid corner-linked units. The third type of distortion, octahedral tilting, is the most common type of distortion. The rotation and tilting of octahedra in the perovskite compounds have been studied in detail earlier by
M. Nasir Khan et al. / Solid State Sciences 10 (2008) 1634e1639
many researchers [8,9,17]. The octahedral tilting and the presence of cationic ordering produce superlattice reflections. Depending on the degree of ordering and tilting of the octahedra, superlattice reflections can be very weak and hence difficult to ascertain. Anderson and Poeppelmeier [18] based on their findings reported that ordered structure in such complicated system is most likely to be formed if charge difference be0 tween B and B cations is 2 or higher. For rational understanding of the solid-state properties of these new materials and for establishing proper structuree property relationships, crystal structure determination is a pre-requisite. However, the limited resolution of the diffrac0 tion instruments, presence of relative BB cation disorders, and pseudocubic structure exhibited by these materials make it difficult to determine their real crystal structure. Fortunately, the exceptional resolution and luminosity offered by synchrotron radiation source make it possible to determine the real crystal structure of these materials with high degree of accuracy. For example, Gateshki et al. [19] have employed synchrotron X-ray diffraction technique for the determination of crystal structure of La2NiRuO6 where earlier neutron diffraction measurements provided contradictory information about the crystal structure of the material. Synthesis and unit-cell parameters of CaLaFeMnO6 (CLFMO) were previously reported by Ramesha et al. [5] and Shaheen et al. [20] using X-ray and neutron diffraction investigations, respectively. From the neutron diffraction investigations, CLFMO was reported to be orthorhombic whereas laboratory X-ray measurements [5] revealed it to be cubic/ pseudocubic. In order to resolve such controversy, joint refinement of both the room temperature powder synchrotron X-ray and high-resolution neutron diffraction data was carried out. Since X-ray diffraction is generally more sensitive to cationic ordering whereas neutron diffraction is more sensitive to octahedral tilting, combined refinement of both data provides best means of establishing the crystal structure and tilts in this material. The valence state of transition metal cations was also determined using X-ray absorption near edge structure (XANES ) data. This provides an additional tool to establish the presence or absence of ordering in the materials. 2. Experimental CaLaFeMnO6 samples were synthesized by solid-state reaction. Stoichiometric amounts of La2O3, Fe2O3, MnO2 and CaCO3 were mixed thoroughly and ground. The calcination and sintering conditions have been reported elsewhere [20]. Neutron powder diffraction data were measured at 50 MW research reactor at NFL, Studsvik, Sweden in the 2q range 4e140 with a step size of 0.08 using wavelength of ˚ . The synchrotron data were collected at ELETTRA 1.47 A synchrotron source, Trieste, Italy at XRD1 beam line with ˚ from Si (111) crystal monochromator in nonl ¼ 0.67 A dispersive configuration followed by toroidal focusing mirror. Quartz capillary with 0.3 mm diameter was used as the sample holder. MARS 345 imaging plate was used to record the diffraction pattern with sample to detector distance of 85 mm.
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During each exposure the sample was rotated through a 90 angle. Any preferred orientation of the powder should have been eliminated through a combination of the specimen rotation, use of a capillary sample holder and full intensity integration of the diffraction rings, as obtained using the programme Fit2D [21]. The calibration of wavelength and distance was carried out using standard LaB6 sample. The Rietveld structural refinement of both the data was carried out simultaneously using a Multi Rietveld analysis programme Rietica [22]. The background was defined by a fifth order polynomial in 2q and was refined simultaneously with other structural and profile parameters. Refined parameters included unit-cell parameters, positions and isotropic thermal parameters (Biso) of all the atoms as well as the usual profile parameters describing the pseudo-Voigt peaks’ shape function. In total 43 parameters were simultaneously refined. Room temperature Mn K-edge and Fe K-edge X-ray absorption spectroscopic (XAS) experiments were performed at the XAFS beam line at ELETTRA Synchrotron Radiation Source, Trieste, Italy. Well-dispersed sample powder in cyclohexane was deposited on porous membranes. The amount of the sample was pre-calculated to obtain optimum absorption jump (Dm z 1). X-ray beam energies were tuned using a double crystal, focusing Si (111) monochromator crystals. The incident (Io) and transmitted (It) beam intensities were recorded with ionization chambers filled with appropriate gases. Data were collected in two regions: (1) the pre-edge region with a step size of 5 eV to allow pre-edge background calculation and subtraction; (2) the XANES region from 30 eV below the edge to 50 eV above the absorption edge with a step size of 0.3 eV. The reproducibility in the determination of the edge positions was found to be better than 0.2 eV. Scanning electron microscope (SEM) with an energy dispersive X-ray (EDX) analyzer and inductive coupled plasma (ICP) were the tools used to check the chemical composition of the synthesized samples. The average results of several spot measurements of EDX analysis and ICP measurements confirmed nominal composition of the sample to be Ca0.99(1)La0.98(2)Mn0.98(2)Fe0.99(1)O5.97(3). These results were further confirmed by Rietveld analysis where the oxygen occupancies varied along with other cation elements but we had found no deficiency of either oxygen or other cation elements. Subsequently, all occupancies for cations and anions were fixed at their nominal composition values during refinement. 3. Results and discussion 3.1. Joint refinement for neutron/SXRD The crystal structure has been refined using space group Pbnm with an orthorhombic structure, which is a non-standard setting of Pnma (# 62). The refined parameters obtained from the joint refinement of the two data sets are listed in Table 1. The model of the Rietveld refinement was built on the basis of atomic arrangements reported in Ref. [20]. In the refined structure the assigned occupation sites are 4c for Ca/La and O1, 4b
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Table 1 Room temperature refined structural parameters, reliability factors, and interatomic distances for CaLaMnFeO6 system Parameters
NDb results [20]
Present results
˚) a (A ˚) b (A ˚) c (A
5.4468(5) 5.4386(5) 7.6862(5)
5.4658(3) 5.4501(3) 7.7321(5)
Ca/La at (xy1/4) x y ˚ 2) BLa/Ca (A
0.006(2) 0.021(1) 0.89(5)
0.0025(1) 0.0176(1) 0.99(3)
1.1(3)
0.81(3)
O(1) at (xy1/4) x y ˚ 2) B (A
0.063(2) 0.487(1) 0.5(1)
0.066 (2) 0.502(2) 1.32(1)
O(2) at (x,y,z) x y z ˚ 2) B (A
0.221(1)a 0.279(1) 0.029(1) 1.5(1)
0.216(1) 0.268(2) 0.029(1) 1.32(1)
1.949(2) 1.952(2) 1.952(2) 2.426(8) 2.447(8) 2.658(8) 2.562(8) 2.727(7) 2.992(7) 3.031(8) 3.112(5) 89.3(1) 90.7(1) e e 1.1 106 0.0082
1.8914(2) 1.9664(2) 2.0146(2) 2.3602(1) 2.4893(3) 2.6897(1) 2.6627(1) 2.8314(1) 2.7168(1) 3.0731(1) 3.1085(1) 89.46(2) 90.54(1) 162.26(1) 158.85(1) 0.00402 0.00794
Mn/Fe at (1/2,0,0) ˚ 2) BMn/Fe (A
˚) Interatomic distances (A Fe/MneO2(B) 2 Fe/MneO1 2 Fe/MneO2(A) 2 A/LaeO1 1 A/LaeO2 2 A/LaeO2 2 A/LaeO1 1 A/LaeO1 1 A/LaeO2 2 A/LaeO2 2 A/LaeO1 1 :O1eMn/FeeO2 :O1eMn/FeeO2 :Mn/FeeO2eFe/Mn :Mn/FeeO1eMn/Fe DBeO6 DAeO12
The numbers in the parenthesis represent errors in the last significant digit. Joint R factors for the two histograms are: Rp ¼ 7.72%, Rwp ¼ 9.87% with RBragg ¼ 1.89%. a Denotes that the value of 0.721(1) quoted in Ref. [20] is a typing mistake and the actual value is 0.221(1). b ND ¼ Neutron diffraction.
for Mn/Fe and 8d for O2. No evidence was found for ordering of cations in either of the structural sites in the neutron or the synchrotron X-ray diffraction while refining simultaneously both the data. The observed, calculated and difference for the two patterns are shown in Fig. 1. The data fitted well using the arrangements mentioned above and easily converged resulting in the lowest R values. The lattice parameters are ˚ , b ¼ 5.4501(3) A ˚ and c ¼ 7.7321(5) A ˚ with a ¼ 5.4658(3) A total joint R factors of Rp ¼ 7.72%, Rwp ¼ 9.87 with RBragg ¼ 1.89% for the two histograms. The structure obtained is similar to that of CaTiO3 and a large number of ABO3 compounds that adopt the GdFeO3 structure. In the GdFeO3 structures octahedra are rotated about more than one pseudocubic Cartesian axes compared to that of an ideal cubic
Fig. 1. Observed (þ), calculated (), and below the difference of observed and calculated patterns. (a) Synchrotron and (b) neutron jointly refined by Rietveld refinement method.
perovskite [23e25]. The polyhedral view along (001) axis is shown in Fig. 2(a), using refined parameters listed in Table 1. This clearly indicated the tilting of BO6 octahedra in the aacþ tilt system in the Glazer tilt notation scheme [9]. The calculated bond distances obtained from the combined refinement are listed in Table 1, which indicates that CLFMO consists of a frame work of tilted and distorted (Mn/Fe)O6 polyhedra with La and Ca cations occupying the distorted 12-fold coordinated sites within this framework. If all the oxygen atoms that surround the Mn/Fe atoms possess the same bond distances then there is no distortion of the octahedra and if the angle between OeMn/FeeO is 180 in the ab plane and along c-axis then the octahedra are not tilted [26]. The rotation angle j around the c-axis (:Fe/MneO2eFe/Mn) is 162.26 , while the octahedral tilt angle relative to (001) plane is q w 158.85 . The (Fe/Mn)O6 polyhedra exhibit a similar style of distortion to that found in CaTiO3 and other ABO3 perovskite compounds. This can be revealed by the relatively short MneO(2) bonds showing the compression along the semi-minor axis designated as O2(B) of the equatorial plane of the polyhedron shown in Fig. 2(b). The (Ca/La)O12 cubo-octahedron is a highly distorted site as is evident from the spread of 12 bond distances of oxygen atoms. This is also in agreement with the observed tolerance factor criterion mentioned above, which becomes smaller due to A-site compression. The observed tolerance factor in the present CaLaMnFeO6 system is 0.959. In short, we have observed significant changes in the crystallographic and atomic parameters with joint refinements when compared to earlier neutron diffraction studies [20]. The lattice constant shows more unit-cell volume compared
M. Nasir Khan et al. / Solid State Sciences 10 (2008) 1634e1639
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Fig. 2. (a) Polyhedral view along (001) of the LaCaMnFeO6 system. (b) Octahedral tilt and rotation along with bond distances; view along (001) axis.
to individual neutron diffraction refinement. The atomic positions also show slight variations in the joint refinement. Also the temperature factors of the two oxygen atoms are higher as compared to the values determined from neutron diffraction data. This is in agreement with tilting of octahedra which induces a large shortening of two AeO1 bond distances and its rotation shortens two large AeO2 distances as can be seen in Table 1. Therefore, the Biso of oxygen increases [26]. The polyhedral distortion is more pronounced in the joint refinement. The Mn/FeO6 octahedra are more distorted and tilted when compared with neutron diffraction data, where these seem to be rather regular. The Ca/LaO12 cubo-octahedra also show more distortion when compared to individual neutron data. The polyhedral distortion was also calculated using the formula as reported in Ref. [27] showing the relation between bond lengths and bond strength. The polyhedral distortion using refined parameters of the joint refinement shows DFe/MnO6 w 0.00402, and for cubo-octahedron DCa/ LaO12 w 0.00794, while these values are 1.1 106 and 0.0082 when calculated from the individual neutron diffraction refined parameters [20]. The degree of distortion in this system is greater than that found in CaTiO3 and other GdFeO3 type perovskite compounds [23]. The combined use of X-ray and neutron scattering was found to improve significantly the determination of both light and heavy atom positions and thermal parameters and are thus more useful in the structural characterization of complex compounds containing both heavy
and light elements. The site ordering of Mn/Fe thus can only be predicted through neutron scattering whereas X-ray diffraction alone is insufficient to determine it uniquely. We have shown that technique of joint neutron and synchrotron X-ray diffraction Rietveld refinement is a powerful tool and allows more in depth, complete and precise structural determination, and virtually eliminates problems encountered with false minima.
3.2. XANES The oxidation states of manganese and iron in CLFMO were investigated by XANES spectroscopy. The comparison of K-edge XANES of Fe and Mn in CLFMO with the model compounds of known oxidation state and local coordination environment provides a way to assess their chemical environment. LaMnO3 (for Mnþ3) and CaMnO3 (for Mnþ4) are suitable reference materials for manganese based perovskites because both of these materials contain manganese in the octahedral environment. In Fig. 3, the Mn K-edge absorption spectra for the series of Mnþn (n ¼ 3, 4) along with that of CLFMO are shown. All these materials contain Mn in octahedral environment. The XANES spectra of LaMnO3 and CaMnO3 are in agreement with those already reported in the literature [28e33]. The comparison of XANES spectra of CLFMO with LaMnO3
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Table 3 Threshold energies (E0) for model compounds and CaLaMnFeO6 at Fe K-edge
Fig. 3. Comparison of Mn K-edge XANES spectra of CaLaFeMnO6 with standard samples LaMnþ3O3 and CaMnþ4O3. The vertical and horizontal lines show the valence state of Mn determined from value of energy comparison with standard samples when normalised absorption equals 0.5.
Compound
E0 (eV) Present work
From literature
Feþ2O Fe3O4 Feþ3 2 O3 CaLaMnFeO6
7119.8 7122.3 7123.9 7124.2
7120.0 [36] 7124.3 [36]
Mn K-edge spectra of CLFMO and standard samples show similar features. The main difference among the spectra is the position of the Mn and Fe absorption edges (E0) which was determined from the value of energy when normalised absorption equals 0.5. The threshold energies summarised in Tables 2 and 3 are in good agreement with those reported in the literature [28,32]. It is evident from Fig. 3 that there is a substantial chemical shift of the edges with increasing valence. The chemical shift of CLFMO is close to that of CaMnO3 thereby indicating that valence of Mn in CLFMO is þ4. The Fe pre-edge peak around 7112 eV and edge jump peak around 7127 eV represent 1s to 3d (dipole forbidden process) and 1s to 4p transitions, respectively [35]. The threshold energies for FeO and Fe2O3 are 7119.8 eV and 7123.9 eV, respectively, and are in good agreement with values reported in literature [36]. Based on standard FeO (Fe2þ) and Fe2O3 (Fe3þ) spectra (Fig. 4), valence state of Fe in CLFMO is found to be þ3.
and CaMnO3 suggests that Mn occupies only the octahedral sites in the perovskite structure. Like other CMR compounds, three energy regions are visible in the Mn K-edge spectra of CLFMO. In the preedge region (6535e6545 eV), the peaks in Fig. 3 are attributed to mixture of 1s / 3d quadrupole and 1s / 3d dipole transitions (weakly allowed by a hybridization between 3d states and 4p states) [30]. The most intense peak at 6550 eV in these materials is associated with 4p bands. The other resonance extending from 6560 eV to 6570 eV arises due to the multiple scattering events of the photoexcited electron. Joly et al. [34] have suggested that these pre-peaks arise from the mixing of d states between different atoms, through hybridization of p band.
Table 2 Threshold energies (E0) for model compounds and CaLaMnFeO6 at Mn K-edge Compound
E0 (eV) Present work
From literature
þ3
LaMn O3
6551.2
6550.6 [28] 6550.7 [32]
CaLaMnFeO6 CaMnþ4O3
6554.7 6555.3
6554.8 [28] 6554.6 [32]
Fig. 4. Comparison of Fe K-edge XANES spectra of CaLaFeMnO6 with standard samples FeO (Fe2þ) and Fe2O3 (Fe3þ). The vertical and horizontal lines show the valence state of Fe determined from value of energy comparison with standard samples when normalised absorption equals 0.5.
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4. Conclusions We have simultaneously refined high-resolution synchrotron X-ray and neutron diffraction data for CaLaFeMnO6 system. The Rietveld refinement reaffirmed that the system could be best described in orthorhombic structure with Pbnm space group. The results are in agreement with previous neutron diffraction data [20], but with a slight change in crystallographic and atomic parameters. Mn and Fe exist with the þ4 and þ3 valencies on the octahedral sites, respectively. Mn and Fe are randomly distributed over the B site. This random distribution of Mn and Fe is supported by the magnetic susceptibility measurements of Ramesha et al. [5] showing that these compounds are paramagnetic insulators. The cation ordering otherwise would have revealed this perovskite to be a ferromagnetic insulator. The random distribution of transition metal cations over the six coordinated octahedral sites as revealed by diffraction techniques and the valence state difference between two cations of less than 2 as determined by XANES measurements validate the criterion set by Anderson and Poeppelmeier [18] stated earlier. Acknowledgements The authors are thankful to ICTP for the generous support to visit ELETTRA synchrotron radiation source facility under ICTPeELETTRA user programme where the experimental part of this work was carried out. The authors also thank Dr. Mariziou Paletinou (XRD1 beamline) and Dr. Luca Olivi (XAFS beamline) at ELETTRA for their help in the experiments. References [1] R.E. Newnham, G.R. Ruschau, J. Am. Ceram. Soc. 74 (1991) 463. [2] R.J. Cava, R.B. van Dover, B. Batlogg, E.A. Rietman, Phys. Rev. Lett. 58 (1987) 408. [3] J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyon, B. Raveau, J.L. Soubeyroux, J.L. Tholence, R. Tournier, Europhys. Lett. 3 (1987) 1301. [4] G.Q. Gong, C. Canedy, G. Xiao, J.Z. Sun, A. Gupta, W.J. Gallagher, Appl. Phys. Lett. 67 (1995) 1783. [5] K. Ramesha, V. Thangadurai, D. Sutar, S.V. Subramanyam, G.N. Subbanna, Gopalakrishnan, Mater. Res. Bull. 35 (2000) 559.
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