I. INTRODUCTION
La1 −x Sr x CoO3 ceramics attract much interest owing to the unusual structural, magnetic and transport properties, which are correlated with the spin-state configuration of the cobalt ion (Senaris-Rodriguez and Goodenough, Reference Senaris-Rodriguez and Goodenough1995; Radaelli and Cheong, Reference Radaelli and Cheong2002; Zobel et al., Reference Zobel, Kriener, Bruns, Baier, Grüninger, Lorenz, Reutler and Revcolevschi2002; Maris et al., Reference Maris, Ren, Volotchaev, Zobel, Lorenz and Palstra2003; Knížek et al., Reference Knížek, Jirak, Hejtmanek, Veverka, Marysko and Maris2005; Sazonov et al., Reference Sazonov, Troyanchuk, Gamari-Seale, Sikolenko, Stefanopoulos, Nicolaides and Atanassova2009). In the ground state, the parent compound LaCoO3 contains the Co3+ ions in the low-spin electronic configuration t 2g 6 e g 0 (LS, S = 0). The electronic configuration of Co3+ gradually changes to the intermediate (IS, t 2g 5 e g 1, S = 1) or high (HS, t 2g 4 e g 2, S = 2) spin state with temperature growth. The evolution of the electronic configuration of the cobalt ions can be probed experimentally owing to the significant difference in the ionic radii of the different spin states. In the HS, Co3+ has a considerably larger radius (0.61 Å) than in the LS (0.54 Å) and in the IS (0.55 Å) states (Shannon, Reference Shannon1976).
The redistribution of the electrons between the t 2g and e g levels is a result of the competition between the crystal field splitting energy Δ cf and the intra-atomic Hund exchange energy J ex, which are comparable in cobaltites. The energy Δ cf strongly depends on the Co–O bond length and therefore a balance between Δ cf and J ex can be easily adjusted by changing the temperature, external pressure or chemical substitution (Zobel et al., Reference Zobel, Kriener, Bruns, Baier, Grüninger, Lorenz, Reutler and Revcolevschi2002; Maris et al., Reference Maris, Ren, Volotchaev, Zobel, Lorenz and Palstra2003; Knížek et al., Reference Knížek, Jirak, Hejtmanek, Veverka, Marysko and Maris2005).
The magnetic and transport properties of La1−x Sr x CoO3 cobaltites with the perovskite-like structure are similar to those of the La1−x Sr x MnO3 manganites (Maris et al., Reference Maris, Ren, Volotchaev, Zobel, Lorenz and Palstra2003; Troyanchuk et al., Reference Troyanchuk, Balagurov, Sikolenko, Efimov and Sheptyakov2013a). In both systems, the substitution of La with divalent Sr ion induces a paramagnetic (x < 0.15) to ferromagnetic (x > 0.3) transition, as the dopant concentration increases. The ionic radius of Sr2+ is significantly larger than that of the La3+ ion, so it is possible to expect a stabilization of the IS state of the cobalt ions by substituting Sr2+ with La3+ ions. However, such heterovalent substitution results in the formation of Co4+ ions and leads consequently to the ferromagnetic metallic ground state (Knížek et al., Reference Knížek, Jirak, Hejtmanek, Veverka, Marysko and Maris2005). In order to prevent formation of the Co4+ ions, diamagnetic Nb ions can be introduced, which in the presence of Co3+ ions have the oxidation state of 5+. The simultaneous doping with both Sr2+ and Nb5+ ions preserves the oxidation state of the cobalt ions and therefore, the conductivity of La1–x Sr x Co1–y Nb y O3 solid solutions would decrease with dopant concentration (Sikolenko et al., Reference Sikolenko, Efimov, Efimova, Sazonov, Ritter, Kuzmin and Troyanchuk2009). Thus, the transition of the cobalt ions spin state from LS to the mixture of IS and HS, leads to modification of the exchange interaction Co3+–O–Co3+ (Potze et al., Reference Potze, Sawatzky and Abbate1995; Korotin et al., Reference Korotin, Anisimov, Khomskii, Ezhov, Solovyev, Khomskii and Sawatzky1996; Sundaram et al., Reference Sundaram, Jiang, Anderson, Belanger, Booth, Bridges, Mitchell, Proffen and Zheng2009).
In this work, we discuss the effects of temperature and doping on the structural and electronic properties of the cobalt ions in the LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 compounds, based on neutron powder diffraction (NPD), X-ray powder diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) measurements.
II. EXPERIMENTAL
The synthesis of the powder samples of LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 is described elsewhere (Sikolenko et al., Reference Sikolenko, Efimov, Efimova, Sazonov, Ritter, Kuzmin and Troyanchuk2009). XRD experiments were carried out at the synchrotron facility HASYLAB/DESY (Hamburg, Germany) using the powder diffractometer at the B2 beamline in the temperature range of 17–300 K. The neutron diffraction experiments for the La0.5Sr0.5Co0.75Nb0.25O3 sample were performed using the high-resolution powder diffractometer D2B at the Institute Laue-Langevin with the neutron wavelength of λ = 1.594 Å. The XRD and NPD data were analyzed by a Rietveld method using the FullProf program (Rodriguez-Carvajal, Reference Rodriguez-Carvajal1993).
EXAFS experiments have been performed at the beamline BM29 of the European Synchrontron Radiation Facility (ESRF) (Grenoble, France). The EXAFS spectra were measured at the Co K-edge in the energy range 7400–9500 eV in the standard transmission mode, simultaneously with a reference sample (9 µm cobalt) in the temperature range 20–300 K. Each temperature point was measured three times with a count rate of 2.5 s per point. To reduce the harmonic content in the X-ray beam, we detuned the monochromator crystals 40% at 7900 eV. The powder samples were deposited on the millipore cellulose membranes with thicknesses specially selected to obtain an X-ray absorption edge jump Δμ·x ~ 1 at the Co K-edge.
A curve-fitting procedure by the EDA software package (Kuzmin, Reference Kuzmin1995) was used to determine the average R(Co–O) distance and the parallel mean-square relative displacement (MSRD||) ∆σ 2 Co–O (or EXAFS Debye–Waller parameter). The energy position E 0, used in the definition of the photoelectron wave number k = [(2m e/ћ 2)(E − E 0)]1/2, was set at the threshold energy E 0 = 7714 eV. The Fourier transforms (FTs) of the EXAFS χ(k)k 2 spectra were calculated in the wave number intervals up to k = 1.0–20 Å−1 with a 10% Gaussian-type window function. At low temperatures up to 20 Å−1 the signal-to-noise ratio is very good, however it deteriorates at high k as the temperature increases and it is rather poor beyond ~18.0 Å−1 at 300 K. Consequently, in all the fits the upper end of the FT k range is restricted to 17.5 Å−1.
Experimental scattering amplitude and phase shift functions for the Co–O atom pair were used in the EXAFS analysis. They were obtained from the EXAFS spectra of a reference Co-foil sample at T = 20 K. We assumed that under these conditions, anharmonicity effects in the dynamics of the CoO6 octahedron can be neglected, and the sample was assumed to be composed of regular CoO6 octahedra. The cobalt coordination number and Co–O distance were fixed to N ref = 6 and R ref = 1.925 Å, respectively, based on the Rietveld refinement of NPD data on the same LaCoO3 sample.
III. RESULTS AND DISCUSSIONS
Figure 1 shows an example of the diffraction pattern for La0.5Sr0.5Co0.75Nb0.25O3 at 5 K. All observed Bragg peaks for La0.5Sr0.5Co0.75Nb0.25O3 and LaCoO3 were indexed within the rhombohedral R
$\bar 3$
c space group and the structure does not change in the temperature range between 10 and 300 K.
(Colour online) Rietveld refinement of NPD pattern collected for La0.5Sr0.5Co0.75Nb0.25O3 at 5 K, with experimental data as open circles, the calculated pattern – black solid line, and the difference curve marked – blue line. The vertical ticks mark the corresponding positions of the Bragg reflections.
Figure 2 shows the temperature dependence of the Co–O bond lengths for LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 calculated using the refinement of the diffraction and the EXAFS data. We note that the local interatomic distance 〈r Co−O〉 = 〈|r O − r Co|〉 (where r O and r Co are equilibrium interatomic distance between absorbing cobalt and backscattering oxygen atoms) probed by EXAFS is normally larger than the equilibrium crystallographic distance R Co–O = |〈r O〉 − 〈r Co〉| (where 〈r O〉 and 〈r Co〉 are average positions of oxygen and cobalt in unit cell) measured by diffraction techniques. For initial compound LaCoO3 the Co–O bond lengths determined from the EXAFS analysis in our experiment are always shorter with respect to those obtained from the diffraction, and this deviation increases with temperature. Usually owing to perpendicular vibrations, the EXAFS bond lengths increase with temperature faster than that calculated from diffraction in framework of the same symmetry. Therefore the anomalous decrease of local atomic EXAFS Co–O bond lengths compared to long-range Co–O bond lengths calculated from the diffraction data above ~80 K could be explained by the gradually spin-state transition from LS (basic volume of 500 nm powder grains) +HS (distorted surface layer of 500 nm powders) to LS +highly hybridized IS (from HS-distorted surface layer) that occurs at ~80 K and the IS/HS ratio is further increasing in LS with temperature. It is in agreement with similar radii of the LS (0.54 Å) and IS (0.56 Å) ions, compared with the HS ion radius (0.61 Å) (Shannon, Reference Shannon1976). In other words, the Co–O distance in the HS sate is essentially longer than that of in the LS and IS states. As a consequence, an appearance of highly hybridized IS leads to a decrease of the perpendicular vibrations amplitude (compared with the HS ion and its longer radius), which is responsible for the increase of Co–O bond length.
(Colour online) The temperature dependence of the Co–O bond lengths for LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 obtained by EXAFS, NPD, and XRD.
It is interesting to note that the Co–O bond length obtained by EXAFS in the La0.5Sr0.5Co0.75Nb0.25O3 sample is a little smaller up to room temperature than the ones obtained by diffraction. Presumably, it is associated with increased fraction of cobalt ions being in IS and especially HS states (which are mainly located in the surface layer of the grains), while a dominant amount of the cobalt ions retain the LS configuration in contrast to the situation occurred in LaCoO3 at low temperatures (Figure 2).
The temperature dependencies of correlated MSRD|| and of the uncorrelated mean-squared displacement (MSD or Debye–Waller parameter, calculated from diffraction data) for the Co–O bond in LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 are shown in Figure 3. The MSD of Co–O bond exhibits an anomalous increase below ~50 K for both samples. This transition is more pronounced in the LaCoO3 sample owing to considerably higher slope of the MSD at room temperature as compared to that attributed to the La0.5Sr0.5Co0.75Nb0.25O3 sample.
(Colour online) Temperature dependence of uncorrelated MSD (empty circles) and the correlated MSRD (full circles) for Co–O bond in LaCoO3 and MSD (empty squares) and MSRD (full red squares) for Co–O bond in La0.5Sr0.5Co0.75Nb0.25O3. DCF is a displacement correlation function (difference between MSD and MSRD).
The displacement correlation function (DCF) (i.e. difference between MSD and MSRD||), reflecting the correlation in atomic motion of distant cobalt and oxygen atoms increases gradually as a function of temperature (Figure 3) for both samples. Such increase of the interaction strength between atoms in the Co–O pairs can be associated with a gradual transition from HS Co3+ ions to a highly hybridized IS state (Korotin et al., Reference Korotin, Anisimov, Khomskii, Ezhov, Solovyev, Khomskii and Sawatzky1996; Pirogov et al., Reference Pirogov, Teplykh, Voronin, Balagurov, Pomjakushin, Sikolenko and Filonova1999a; Sazonov et al., Reference Sazonov, Troyanchuk, Gamari-Seale, Sikolenko, Stefanopoulos, Nicolaides and Atanassova2009). In the La0.5Sr0.5Co0.75Nb0.25O3 sample the DСF temperature dependence is less pronounced. The essentially large Co–O distance (Figure 2) and MSRD (Figure 3) measured in this sample (Figure 2) presumably leading to a thicker surface HS and IS layers compared with the ones in LaCoO3 sample. The fact that no increase of MSRDII was found in the temperature range from 5 to 20 K for La0.5Sr0.5Co0.75Nb0.25O3 can be associated with a high correlation and low-amplitude motion between Co3+ ions in either LS or HS with the oxygen.
IV. CONCLUSIONS
We have observed an anomalous increase of the MSD of the Co–O bond in LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 samples in temperature range from 50 to 17 K. Such increase of the MSD can be explained by the contribution of HS Co3+ located in the surface layer of powder grains (~500 nm) stabilized by the influence of structural defects and disruptions of chemical bonds (Fornasini et al., Reference Fornasini, Beccara, Dalba, Grisenti, Sanson and Vaccari2004) assuming basic configuration of the cobalt ions as LS one, i.e. mix of different spin states from maximal ion radius in HS up to minimal one in LS.
A combined analysis of EXAFS and diffraction results above ~100 K have revealed an anomalous temperature dependence of the Co–O bond lengths in LaCoO3, while such dependence is absent in La0.5Sr0.5Co0.75Nb0.25O3. Moreover, the temperature dependence of DCF is more pronounced in LaCoO3 as compared to La0.5Sr0.5Co0.75Nb0.25O3, particularly in vicinity of room temperature. The effects observed in La0.5Sr0.5Co0.75Nb0.25O3, can be associated with essentially large amount of HS cobalt ions being in surface layers of crystallites compared to initial LaCoO3 at low temperatures. Major part of cobalt ions retain LS-state configuration. It is owing to a significantly larger Co–O bond length for La0.5Sr0.5Co0.75Nb0.25O3 sample.
Our experimental results are well described by a thermally induced spin-state transition of cobalt ions located in the surface layer of LaCoO3 and La0.5Sr0.5Co0.75Nb0.25O3 powders from HS to a highly hybridized IS spin state (Potze et al., Reference Potze, Sawatzky and Abbate1995).
The main conclusions of our work are in good agreement with existing data about magnetic susceptibility, magnetic circular dichroism and inelastic neutron scattering measurements, as well as with structural properties measured under high pressure (Potze et al., Reference Potze, Sawatzky and Abbate1995; Korotin et al., Reference Korotin, Anisimov, Khomskii, Ezhov, Solovyev, Khomskii and Sawatzky1996; Fornasini et al., Reference Fornasini, Beccara, Dalba, Grisenti, Sanson and Vaccari2004; Knížek et al., Reference Knížek, Jirak, Hejtmanek, Veverka, Marysko and Maris2005; Haverkort et al., Reference Haverkort, Hu, Cezar, Burnus, Hartmann, Reuther, Zobel, Lorenz, Tanaka, Brookes, Hsieh, Lin, Chen and Tjeng2006; Podlesnyak et al., Reference Podlesnyak, Streule, Mesot, Medarde, Pomjakushina, Conder, Tanaka, Haverkort and Khomskii2006; Pandey et al., Reference Pandey, Kumar and Prabhakaran2008; Herklotz et al., Reference Herklotz, Rata, Schultz and Doerr2009; Sundaram et al., Reference Sundaram, Jiang, Anderson, Belanger, Booth, Bridges, Mitchell, Proffen and Zheng2009; Troyanchuk et al., Reference Troyanchuk, Balagurov, Sikolenko, Efimov and Sheptyakov2013a, Reference Troyanchuk, Bushinsky, Sikolenko, Efmov, Ritter, Hansen and Többens2013b).
ACKNOWLEGEMENTS
This work was supported by Russian Science Foundation project no. 15-19-20038.
