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X-ray absorption spectroscopy and neutron diffraction study of the perovskite-type rare-earth cobaltites
Physica B xxx (xxxx) xxx–xxx
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Physica B journal homepage: www.elsevier.com/locate/physb
X-ray absorption spectroscopy and neutron diffraction study of the perovskite-type rare-earth cobaltites ⁎
V. Sikolenkoa, E. Efimovaa, , A. Franzb, C. Ritterc, I.O. Troyanchukd, D. Karpinskyd, Y. Zubavichuse, A. Veligzhanine, S.I. Tiutiunnikova, A. Sazonovf, V. Efimova a
Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia Helmholtz Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany c Institute Laue Langevin, 71 Avenue des Martyrs, 38042 Grenoble, France d Scientific-Practical Materials Research Center of NAS of Belarus, 220072 Minsk, Belarus e NRC "Kurchatov Institute", Acad. Kurchatov sq. 1, 123182 Moscow, Russia f Institute of Crystallography, RWTH Aachen University and JCNS at MLZ, 85747 Garching, Germany b
A R T I C L E I N F O
A BS T RAC T
Keywords: EXAFS Neutron diffraction Powders Spin state
Correlations between local and long-range structure distortions in the perovskite-type RE1−xSrxCoO3-δ (RE = La, Pr, Nd; x = 0.0 and 0.5) compounds have been studied at room temperature by extended X-ray absorption fine structure (EXAFS) at the Co K–edge and high-resolution neutron powder diffraction (NPD). The use of two complementary experimental techniques allowed us to explore the influence of the type of rare-earth element and strontium substitution on unusual behavior of static and dynamic features of both the Co–O bond lengths.
1. Introduction The discovery of giant magnetoresistance effect in manganites with perovskite-type structure has stimulated studies of similar phenomena in cobaltites [1–12]. In RECoO3 oxides (RE = rare-earth element), the Co3+ ions in the ground state are characterized by the low spin (LS) electronic configuration t2g6eg0 (S = 0). This LS state gradually transforms to the intermediate (IS: t2g5eg1, S = 1) or high (HS: t2g4eg2, S = 2) spin state upon temperature increase. For example, in the case of LaCoO3, the spin state of the cobalt ions gradually changes from LS to IS or HS upon a temperature raise from about 20 K up to 120 K [1,2]. For the PrCoO3 and NdCoO3 compounds containing rare-earth ions with smaller ionic radii, the characteristic temperatures of the spinstate transition are ~ 220 K and ~ 275 K, respectively [2,3]. The decrease of rare-earth ionic radius in RECoO3 (RE = La, Pr, Nd) perovskite family results in the crystal structure transformation from rhombohedral (R-3c space group) symmetry for LaCoO3 [4,5] to the orthorhombic one (space group Pbnm) for PrCoO3 and NdCoO3 [6–9]. The heterovalent substitution of RE3+ with the divalent Sr2+ ion in RE1−xSrxCoO3-δ induces a transition from the paramagnetic to the ferromagnetic state at x – 0.18. Since the ionic radius of Sr2+ is significantly larger than that of the RE3+ ions, one can expect stabilization of the IS/HS state of the cobalt ions upon the Sr2+ substitution for the RE3+ ions [7–11]. Such a heterovalent substitution
⁎
should result also in the appearance of Co4+ ions, which are supposed to be responsible for the ferromagnetic metallic ground state at x – 0.5 [12,13]. The origin of ferromagnetism in the metallic cobaltites remains a subject of discussion for a long time. Most studies on the spin state transitions of cobalt ions suggest that the trivalent and tetravalent cobalt ions coexist in the mixed LS and IS/HS states [1]. The majority of researchers suppose that ferromagnetism in cobaltites is caused by the “superexchange” interaction between Co3+ and Co4+, which results in the hole-type carriers in the hybridized 3d–2p bands strongly coupled with the 3d spins [14]. The aim of this work is to study correlations between the local and long-range crystal structure parameters and to analyze the “chemical pressure” effects in the perovskite-type RE1−xSrxCoO3-δ polycrystalline samples at room temperature. 2. Experimental procedure Polycrystalline samples of RE1−xSrxCoO3-δ (RE = La, Pr, Nd) have been synthesized by conventional ceramic technique. No impurity phases were detected by laboratory X-ray diffraction analysis in all samples. The neutron powder diffraction (NPD) measurements of RE1−xSrxCoO3-δ (x = 0.0 and 0.5) samples were carried out at room temperature on the high-resolution neutron diffractometers E9 at the
Corresponding author. E-mail address: [email protected] (V. Efimov).
http://dx.doi.org/10.1016/j.physb.2017.09.104 Received 29 June 2017; Received in revised form 22 September 2017; Accepted 25 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Sikolenko, V., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.104
Physica B xxx (xxxx) xxx–xxx
V. Sikolenko et al.
1.160 Å [16], with smaller ions Pr3+ (1.126 Å) and Nd3+ (1.109 Å) results in a compression of the lattice, which is manifested as a decrease of the unit cell volume and mean Co–O–Co angle. Moreover, the crystal structure transforms upon substitution from the rhombohedral phase in LaCoO3 to the orthorhombic phase in NdCoO3 and PrCoO3. Such structural transition is accompanied by a decrease of mean Co–O bond lengths by about 0.005 Å (Fig. 1a). The substitution of the rare-earth RE3+ ions in RECoO3 by larger 2+ Sr ions (1.44 Å [16]) leads to a decrease of the average Co–O bond length by about 0.011 Å (Fig. 1a) due to the effect of “chemical pressure” on the cobalt-oxygen sublattice. Note also that the structural transition from orthorhombic to rhombohedral phase takes place for RE = Pr, whereas no phase transition occurs for RE = La and Nd; The La-compound remains in the rhombohedral and the Nd-compound in the orthorhombic phase. EXAFS spectra were treated using the EDA software package following the standard procedure [17]. The energy position E0, used in the definition of the photoelectron wave number k = [(2me/ћ2)(E E0)]1/2, was set at the threshold energy 7714 eV. The Fourier transforms (FTs) of the EXAFS χ(k)k2 spectra were calculated in the wave number interval k = 1.5 ÷ 14 Å−1 with a Gaussian-type window function. A curve fitting procedure [17] was used to determine the R(Co–O) distance in the first coordination shell and the mean square relative displacement (MSRD) Δσ2(Co–O) [18,19]. The fits of the EXAFS χ(k)k2 spectra from the first coordination shell are shown only for LaCoO3 and NdCoO3 at the 290 K are shown in Fig. 2. The experimental scattering amplitude and phase shift functions for the Co-O atom pair were used in the analysis from the EXAFS spectra of the reference LaCoO3 sample, measured at T = 16 K under the assumption that there is no significant anharmonicity in the dynamics of the CoO6 octahedron and that it is completely regular: the cobalt coordination number Nref = 6 and the Co–O distance Rref = 1.9254 Å were set according to the results of the Rietveld refinement of X-ray diffraction data [20]. The values of the interatomic distance R(Co–O) and the relative MSRD Δσ2(Co–O), obtained by the fitting procedure, are given in Fig. 3(a, b), respectively. Both R(Co–O) and the relative MSRD Δσ2(Co–O) show similar dependences on the rare-earth ion size: their values are the largest in the La-containing cobaltites and the smallest in the case of Nd ions. Note that this dependence is less evident for the R(Co–O) values in RECoO3 as observed by the EXAFS, due to unexpectedly shorter interatomic distance value in LaCoO3. One should take into account that the local interatomic distance probed by EXAFS is usually larger than the equilibrium crystallographic distance measured by diffraction. The difference between Co–O bond lengths obtained by EXAFS and diffraction is associated with the influence of the perpendicular MSRD
Fig. 1. NPD pattern of Nd0.5Sr0.5CoO3 at 290 K: experimental curve (open circles), refined curve (solid line) and residual curve (solid line at the bottom). The tick marks indicate the calculated positions of the Bragg peaks.
BER-II reactor in Helmholtz Center Berlin for Materials and Energy and the D2B at the Institute Laue Langevin with the neutron wavelengths of λ = 1.797 Å and λ = 1.594 Å, respectively. The NPD data were analyzed using the Rietveld method by the FullProf suite [15]. EXAFS spectra of RE1−xSrxCoO3 (RE = La, Pr, Nd; x = 0.0 and 0.5) at the Co K-edge were measured preliminary at the Kurchatov Center for Synchrotron Radiation and Nanotechnology (Moscow, Russia) and mainly at the beam line BM29 of ESRF (Grenoble, France) in the energy range 7400–9500 eV in standard transmission mode simultaneously with reference sample (cobalt thickness 9 µm metallic foil – in order to fix E0 not bigger 0.005 Å) at the temperature 16 K and 300 K. For EXAFS measurements, the powder samples were deposited on the millipore cellulose membranes with thicknesses specially selected to obtain an X-ray absorption edge jump of about 1.0 ÷ 1.5. 3. Results and discussion An example of the refined typical NPD pattern for Nd0.5Sr0.5CoO3 at 290 K is displayed in Fig. 1. All observed Bragg peaks for RE1−xSrxCoO3-δ (RE = La at x = 0.0, 0.5 and RE = Pr at x = 0.5) were successfully indexed in the rhombohedral R 3 c space group in hexagonal axes setting, whereas the rest of the members of the family RE1−xSrxCoO3-δ (RE = Nd at x = 0.0, 0.5 and RE = Pr at x = 0) exhibited an orthorhombic crystallographic structure with the Pbnm space group. Note that the refined bond lengths for all the measured samples agree well (within ± 10−3 Å) with the published results [6–13]. In RECoO3, the substitution of La3+, having the ionic radius of
Fig. 2. (Solid symbols) Experimental and (open circles) model Co K-edge EXAFS signals χ(k)k2 of first coordination shell for (a) LaCoO3 and (b) NdCoO3 at room temperature.
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Fig. 3. a. The average Co–O distance in RECoO3 (solid symbols) and RE0.5Sr0.5CoO3 (open symbols) (RE = La, Pr, Nd) obtained by NPD (squares) and EXAFS (circles) at room temperature. b. The relative MSRD Δσ2(Co-O) in RECoO3 (solid circles) and RE0.5Sr0.5CoO3 (open circles) (RE = La, Pr, Nd) at room temperature.
Δσ2(Co–O), i.e. the thermal atomic displacement of oxygen ions in the direction perpendicular to the Co–O–Co bond. However, only in the LaCoO3 case the Co–O bond lengths determined at room temperature from the EXAFS analysis are slightly shorter compared to those obtained from the diffraction (Fig. 3). This deviation can be ascribed to the Co3+ spin-state transition from HS (with ion radius 0.61 Å [16] measured at 16 K as a reference LaCoO3) to IS (with 0.56 Å at 300 K) in LS (0.54 Å) basic phase. An alternative potential explanation of the anomaly related to strongly anisotropic vibrations of oxygen atoms normal to the Со–О–Со bonds. This assumption however contradicts to conclusions by [4], which rather implies spin transitions. Furthermore, Raman, IR, and neutron inelastic scattering data show vibrational anomalies for the Со–О bonds neither at 300 K nor at 20 K. In the case of PrCoO3 and NdCoO3 and especially Pr0.5Sr0.5CoO3 and Nd0.5Sr0.5CoO3 we do not observe such effect due to “chemical pressure” influence, i.e. essentially smaller Co–O bond length (compared to LaCoO3) that leads to an absence of HS domains (clusters) at 16 K. Note that our experimentally obtained values MSRD Δσ2(Co–O) and structural parameters for RECoO3 are in agreement with the previously obtained EXAFS and diffraction data [4–13]. A substitution of the rare-earth ions with strontium or a decrease of the rare-earth ions size leads also to an increase of the relative MSRD Δσ2(Co–O) by about 0.001 Å2 (Fig. 3b) at room temperature, which is mainly associated with a mixture of Co3+ and Co4+ in LS basic phase and very small part IS spin states.
Acknowledgments Authors thank Dr. Roman Chernikov (Canadian Light Source) for helpful discussion. Authors are indebted to Dr. Sakura Pascarelli and Dr. Olivier Mathon (ESRF, France) for their help with EXAFS experiments at the BM29 beamline. This work was supported by the JINR-BFBR Research Grant no. T16D-009. References [1] P.M. Raccah, J.B. Goodenough, Phys. Rev. 155 (1967) 932. [2] M.A. Korotin, V.I. Anisimov, D.I. Khomskii, S.Y. Ezhov, I.V. Solovyev, D.I. Khomskii, G.A. Sawatzky, Phys. Rev. B 54 (1996) 5309. [3] K. Knížek, Z. Jirák, J. Hejtmánek, M. Maryško, G. Maris, Eur. Phys. J. B 47 (2005) 213. [4] P.G. Radaelli, S.W. Cheong, Phys. Rev. B 66 (2002) 094408. [5] J.-S. Zhou, J.-Q. Yan, J.B. Goodenough, Phys. Rev. B 71 (2005) 220103. [6] K. Knížek, J. Hejtmánek, Z. Jirák, P. Tomeš, P. Henry, Phys. Rev. B 79 (2009) 134103. [7] M. Kriener, C. Zobel, A. Reichl, J. Baier, M. Cwik, K. Berggold, H. Kierspel, O. Zabara, A. Freimuth, T. Lorenz, Phys. Rev. B 69 (2004) 094417. [8] J.C. Burley, J.F. Mitchell, S. Short, Phys. Rev. B 69 (2004) 054401. [9] A.P. Sazonov, I.O. Troyanchuk, V. Sikolenko, G.M. Chobot, H. Szymczak, J. Phys.: Condens. Matter 17 (2005) 4181. [10] T. Takami, J.-S. Zhou, J.B. Goodenough, H. Ikuta, Phys. Rev. B 76 (2007) 144116. [11] E. Efimova, V. Efimov, D. Karpinsky, A. Kuzmin, J. Purans, V. Sikolenko, S. Tiutiunnikov, I. Troyanchuk, E. Welter, D. Zajac, V. Simkin, A. Sazonov, J. Phys. Chem. Solids 69 (2008) 2187–2190. [12] T. Saitoh, T. Mizokawa, A. Fujimori, M. Abbate, Y. Takeda, Phys. Rev. B 56 (1997) 1290. [13] N. Sundaram, Y. Jiang, I.E. Anderson, D.P. Belanger, C.H. Booth, F. Bridges, J.F. Mitchell, Th Proffen, H. Zheng, Phys. Rev. Lett. 102 (2009) 026401. [14] Y. Jiang, F. Bridges, N. Sundaram, D.P. Belanger, Phys. Rev. B 80 (2009) 144423. [15] J. Rodriguez-Carvajal, Physica B 55 (1993) 192. [16] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [17] A. Kuzmin, Physica B 208/209 (1995) 175. [18] M. Vaccari, P. Fornasini, J. Synchrotron Radiat. 13 (2006) 321–325. [19] G. Dabla, P. Fornasini, et al., J. Phys. Condens. Matter 7 (1995) 1199–1213. [20] V. Sikolenko, E. Efimov, D. Efimova, I. Karpinsky, S. Troyanchuk, O. Pascarelli, D. Zaharko, D. Aquilanti, Prabhakaran, J. Phys.: Conf. Ser. 712 (2016) 038519. [21] A. Herklotz, A.D. Rata, L. Schultz, K. Doerr, Phys. Rev. B 79 (2009) 092409. [22] M.V. Haverkort, Z. Hu, J.C. Cezar, T. Burnus, H. Hartmann, M. Reuther, C. Zobel, T. Lorenz, A. Tanaka, N.B. Brookes, H.H. Hsieh, L.H. Tjeng, Phys. Rev. Lett. 97 (2006) 176405. [23] A. Podlesnyak, S. Streule, J. Mesot, M. Medarde, E. Pomjakushina, K. Conder, A. Tanaka, M.V. Haverkort, D.I. Khomskii, Phys. Rev. Lett. 97 (2006) 247208. [24] I. Troyanchuk, M. Bushinsky, V. Sikolenko, V. Efmov, C. Ritter, T. Hansen, D.M. Többens, Eur. Phys. J. B 86 (2013) 435. [25] I. Troyanchuk, A. Balagurov, V. Sikolenko, V. Efimov, D. Sheptyakov, J. Appl. Phys. 113 (2013) 053909. [26] I.O. Troyanchuk, M.V. Bushinsky, L.S. Lobanovsky, J. Appl. Phys. 114 (2013) 213910.
4. Conclusions We have observed an unusual behavior of the Co–O bond length in LaCoO3 at 300 K by a combined analysis of EXAFS and diffraction results. This behavior is well described by a thermally induced spinstate transition from HS at 16 K (as a reference sample) to the IS at 300 K in LS matrix [1–13]. An existence of HS at 16 K can be explained by influence of structural defects due to oxygen vacancies and scraps of chemical bonds at the boundary between surface and core in powder grains [20]. Ionic radii of LS and IS are essential smaller with respect to the HS ion radius, i.e. the HS Co-O distance essentially bigger than LS and IS Co-O ones. The existing of essential fraction of HS states in LS basic matrix at low temperature can be also confirmed by an increase of Debye-Waller factor at low temperatures [20]. The main results of our study are supported by the existing magnetic susceptibility data [3,21], the MCD measurements [22], and inelastic neutron data [23] as well as by structural properties under high pressure [24–26].
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
X-ray absorption spectroscopy and neutron diffraction study of the perovskite-type rare-earth cobaltites ⁎
V. Sikolenkoa, E. Efimovaa, , A. Franzb, C. Ritterc, I.O. Troyanchukd, D. Karpinskyd, Y. Zubavichuse, A. Veligzhanine, S.I. Tiutiunnikova, A. Sazonovf, V. Efimova a
Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia Helmholtz Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany c Institute Laue Langevin, 71 Avenue des Martyrs, 38042 Grenoble, France d Scientific-Practical Materials Research Center of NAS of Belarus, 220072 Minsk, Belarus e NRC "Kurchatov Institute", Acad. Kurchatov sq. 1, 123182 Moscow, Russia f Institute of Crystallography, RWTH Aachen University and JCNS at MLZ, 85747 Garching, Germany b
A R T I C L E I N F O
A BS T RAC T
Keywords: EXAFS Neutron diffraction Powders Spin state
Correlations between local and long-range structure distortions in the perovskite-type RE1−xSrxCoO3-δ (RE = La, Pr, Nd; x = 0.0 and 0.5) compounds have been studied at room temperature by extended X-ray absorption fine structure (EXAFS) at the Co K–edge and high-resolution neutron powder diffraction (NPD). The use of two complementary experimental techniques allowed us to explore the influence of the type of rare-earth element and strontium substitution on unusual behavior of static and dynamic features of both the Co–O bond lengths.
1. Introduction The discovery of giant magnetoresistance effect in manganites with perovskite-type structure has stimulated studies of similar phenomena in cobaltites [1–12]. In RECoO3 oxides (RE = rare-earth element), the Co3+ ions in the ground state are characterized by the low spin (LS) electronic configuration t2g6eg0 (S = 0). This LS state gradually transforms to the intermediate (IS: t2g5eg1, S = 1) or high (HS: t2g4eg2, S = 2) spin state upon temperature increase. For example, in the case of LaCoO3, the spin state of the cobalt ions gradually changes from LS to IS or HS upon a temperature raise from about 20 K up to 120 K [1,2]. For the PrCoO3 and NdCoO3 compounds containing rare-earth ions with smaller ionic radii, the characteristic temperatures of the spinstate transition are ~ 220 K and ~ 275 K, respectively [2,3]. The decrease of rare-earth ionic radius in RECoO3 (RE = La, Pr, Nd) perovskite family results in the crystal structure transformation from rhombohedral (R-3c space group) symmetry for LaCoO3 [4,5] to the orthorhombic one (space group Pbnm) for PrCoO3 and NdCoO3 [6–9]. The heterovalent substitution of RE3+ with the divalent Sr2+ ion in RE1−xSrxCoO3-δ induces a transition from the paramagnetic to the ferromagnetic state at x – 0.18. Since the ionic radius of Sr2+ is significantly larger than that of the RE3+ ions, one can expect stabilization of the IS/HS state of the cobalt ions upon the Sr2+ substitution for the RE3+ ions [7–11]. Such a heterovalent substitution
⁎
should result also in the appearance of Co4+ ions, which are supposed to be responsible for the ferromagnetic metallic ground state at x – 0.5 [12,13]. The origin of ferromagnetism in the metallic cobaltites remains a subject of discussion for a long time. Most studies on the spin state transitions of cobalt ions suggest that the trivalent and tetravalent cobalt ions coexist in the mixed LS and IS/HS states [1]. The majority of researchers suppose that ferromagnetism in cobaltites is caused by the “superexchange” interaction between Co3+ and Co4+, which results in the hole-type carriers in the hybridized 3d–2p bands strongly coupled with the 3d spins [14]. The aim of this work is to study correlations between the local and long-range crystal structure parameters and to analyze the “chemical pressure” effects in the perovskite-type RE1−xSrxCoO3-δ polycrystalline samples at room temperature. 2. Experimental procedure Polycrystalline samples of RE1−xSrxCoO3-δ (RE = La, Pr, Nd) have been synthesized by conventional ceramic technique. No impurity phases were detected by laboratory X-ray diffraction analysis in all samples. The neutron powder diffraction (NPD) measurements of RE1−xSrxCoO3-δ (x = 0.0 and 0.5) samples were carried out at room temperature on the high-resolution neutron diffractometers E9 at the
Corresponding author. E-mail address: [email protected] (V. Efimov).
http://dx.doi.org/10.1016/j.physb.2017.09.104 Received 29 June 2017; Received in revised form 22 September 2017; Accepted 25 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Sikolenko, V., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.104
Physica B xxx (xxxx) xxx–xxx
V. Sikolenko et al.
1.160 Å [16], with smaller ions Pr3+ (1.126 Å) and Nd3+ (1.109 Å) results in a compression of the lattice, which is manifested as a decrease of the unit cell volume and mean Co–O–Co angle. Moreover, the crystal structure transforms upon substitution from the rhombohedral phase in LaCoO3 to the orthorhombic phase in NdCoO3 and PrCoO3. Such structural transition is accompanied by a decrease of mean Co–O bond lengths by about 0.005 Å (Fig. 1a). The substitution of the rare-earth RE3+ ions in RECoO3 by larger 2+ Sr ions (1.44 Å [16]) leads to a decrease of the average Co–O bond length by about 0.011 Å (Fig. 1a) due to the effect of “chemical pressure” on the cobalt-oxygen sublattice. Note also that the structural transition from orthorhombic to rhombohedral phase takes place for RE = Pr, whereas no phase transition occurs for RE = La and Nd; The La-compound remains in the rhombohedral and the Nd-compound in the orthorhombic phase. EXAFS spectra were treated using the EDA software package following the standard procedure [17]. The energy position E0, used in the definition of the photoelectron wave number k = [(2me/ћ2)(E E0)]1/2, was set at the threshold energy 7714 eV. The Fourier transforms (FTs) of the EXAFS χ(k)k2 spectra were calculated in the wave number interval k = 1.5 ÷ 14 Å−1 with a Gaussian-type window function. A curve fitting procedure [17] was used to determine the R(Co–O) distance in the first coordination shell and the mean square relative displacement (MSRD) Δσ2(Co–O) [18,19]. The fits of the EXAFS χ(k)k2 spectra from the first coordination shell are shown only for LaCoO3 and NdCoO3 at the 290 K are shown in Fig. 2. The experimental scattering amplitude and phase shift functions for the Co-O atom pair were used in the analysis from the EXAFS spectra of the reference LaCoO3 sample, measured at T = 16 K under the assumption that there is no significant anharmonicity in the dynamics of the CoO6 octahedron and that it is completely regular: the cobalt coordination number Nref = 6 and the Co–O distance Rref = 1.9254 Å were set according to the results of the Rietveld refinement of X-ray diffraction data [20]. The values of the interatomic distance R(Co–O) and the relative MSRD Δσ2(Co–O), obtained by the fitting procedure, are given in Fig. 3(a, b), respectively. Both R(Co–O) and the relative MSRD Δσ2(Co–O) show similar dependences on the rare-earth ion size: their values are the largest in the La-containing cobaltites and the smallest in the case of Nd ions. Note that this dependence is less evident for the R(Co–O) values in RECoO3 as observed by the EXAFS, due to unexpectedly shorter interatomic distance value in LaCoO3. One should take into account that the local interatomic distance probed by EXAFS is usually larger than the equilibrium crystallographic distance measured by diffraction. The difference between Co–O bond lengths obtained by EXAFS and diffraction is associated with the influence of the perpendicular MSRD
Fig. 1. NPD pattern of Nd0.5Sr0.5CoO3 at 290 K: experimental curve (open circles), refined curve (solid line) and residual curve (solid line at the bottom). The tick marks indicate the calculated positions of the Bragg peaks.
BER-II reactor in Helmholtz Center Berlin for Materials and Energy and the D2B at the Institute Laue Langevin with the neutron wavelengths of λ = 1.797 Å and λ = 1.594 Å, respectively. The NPD data were analyzed using the Rietveld method by the FullProf suite [15]. EXAFS spectra of RE1−xSrxCoO3 (RE = La, Pr, Nd; x = 0.0 and 0.5) at the Co K-edge were measured preliminary at the Kurchatov Center for Synchrotron Radiation and Nanotechnology (Moscow, Russia) and mainly at the beam line BM29 of ESRF (Grenoble, France) in the energy range 7400–9500 eV in standard transmission mode simultaneously with reference sample (cobalt thickness 9 µm metallic foil – in order to fix E0 not bigger 0.005 Å) at the temperature 16 K and 300 K. For EXAFS measurements, the powder samples were deposited on the millipore cellulose membranes with thicknesses specially selected to obtain an X-ray absorption edge jump of about 1.0 ÷ 1.5. 3. Results and discussion An example of the refined typical NPD pattern for Nd0.5Sr0.5CoO3 at 290 K is displayed in Fig. 1. All observed Bragg peaks for RE1−xSrxCoO3-δ (RE = La at x = 0.0, 0.5 and RE = Pr at x = 0.5) were successfully indexed in the rhombohedral R 3 c space group in hexagonal axes setting, whereas the rest of the members of the family RE1−xSrxCoO3-δ (RE = Nd at x = 0.0, 0.5 and RE = Pr at x = 0) exhibited an orthorhombic crystallographic structure with the Pbnm space group. Note that the refined bond lengths for all the measured samples agree well (within ± 10−3 Å) with the published results [6–13]. In RECoO3, the substitution of La3+, having the ionic radius of
Fig. 2. (Solid symbols) Experimental and (open circles) model Co K-edge EXAFS signals χ(k)k2 of first coordination shell for (a) LaCoO3 and (b) NdCoO3 at room temperature.
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Fig. 3. a. The average Co–O distance in RECoO3 (solid symbols) and RE0.5Sr0.5CoO3 (open symbols) (RE = La, Pr, Nd) obtained by NPD (squares) and EXAFS (circles) at room temperature. b. The relative MSRD Δσ2(Co-O) in RECoO3 (solid circles) and RE0.5Sr0.5CoO3 (open circles) (RE = La, Pr, Nd) at room temperature.
Δσ2(Co–O), i.e. the thermal atomic displacement of oxygen ions in the direction perpendicular to the Co–O–Co bond. However, only in the LaCoO3 case the Co–O bond lengths determined at room temperature from the EXAFS analysis are slightly shorter compared to those obtained from the diffraction (Fig. 3). This deviation can be ascribed to the Co3+ spin-state transition from HS (with ion radius 0.61 Å [16] measured at 16 K as a reference LaCoO3) to IS (with 0.56 Å at 300 K) in LS (0.54 Å) basic phase. An alternative potential explanation of the anomaly related to strongly anisotropic vibrations of oxygen atoms normal to the Со–О–Со bonds. This assumption however contradicts to conclusions by [4], which rather implies spin transitions. Furthermore, Raman, IR, and neutron inelastic scattering data show vibrational anomalies for the Со–О bonds neither at 300 K nor at 20 K. In the case of PrCoO3 and NdCoO3 and especially Pr0.5Sr0.5CoO3 and Nd0.5Sr0.5CoO3 we do not observe such effect due to “chemical pressure” influence, i.e. essentially smaller Co–O bond length (compared to LaCoO3) that leads to an absence of HS domains (clusters) at 16 K. Note that our experimentally obtained values MSRD Δσ2(Co–O) and structural parameters for RECoO3 are in agreement with the previously obtained EXAFS and diffraction data [4–13]. A substitution of the rare-earth ions with strontium or a decrease of the rare-earth ions size leads also to an increase of the relative MSRD Δσ2(Co–O) by about 0.001 Å2 (Fig. 3b) at room temperature, which is mainly associated with a mixture of Co3+ and Co4+ in LS basic phase and very small part IS spin states.
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4. Conclusions We have observed an unusual behavior of the Co–O bond length in LaCoO3 at 300 K by a combined analysis of EXAFS and diffraction results. This behavior is well described by a thermally induced spinstate transition from HS at 16 K (as a reference sample) to the IS at 300 K in LS matrix [1–13]. An existence of HS at 16 K can be explained by influence of structural defects due to oxygen vacancies and scraps of chemical bonds at the boundary between surface and core in powder grains [20]. Ionic radii of LS and IS are essential smaller with respect to the HS ion radius, i.e. the HS Co-O distance essentially bigger than LS and IS Co-O ones. The existing of essential fraction of HS states in LS basic matrix at low temperature can be also confirmed by an increase of Debye-Waller factor at low temperatures [20]. The main results of our study are supported by the existing magnetic susceptibility data [3,21], the MCD measurements [22], and inelastic neutron data [23] as well as by structural properties under high pressure [24–26].
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