Structural study of Ni-substituted YBaCo4−xNixO7 frustrated cobaltites
Graphical abstract
Thermal evolution of the diffractogram for a sample YBaCo3.9Ni0.1O7.00 collected on cooling at the LNLS synchrotron source. The Bragg reflections were indexed in the trigonal P31c and orthorhombic Pbn21 space groups above and below the transition temperature TS, indicated with an arrow. The region of coexistence of both phases is indicated as . A projection along the c-axes for both crystal structures is shown on the right.
Introduction
Cobalt oxides have always constituted a focus of interest, due to the wide spectrum of physical properties they exhibit. These properties originate from the very strong interplay among crystal structure, magnetism and electronic transport, giving rise to rich phase diagrams tuned by temperature, pressure, magnetic fields, oxygen content and cationic size. A quite novel member of the cobalt-rich oxides family is the so-called “114-cobaltite” RBaCo4O7, discovered in 2002 for R=Y [1] and later synthesized with R=lanthanides and calcium. This compound soon gained attention because it displays two very attractive features. First, it offers the possibility to study frustrated magnetism in the unique topology of a three-dimensional framework of Co tetrahedra in interconnected triangular and Kagomé lattices. Geometric frustration is related to intensively studied phenomena in spin ices and multiferroic materials [2], [3], and this compound opens a new possible scenario for its study. A second remarkable property is its capacity of absorption and desorption of gases, which turns these cobaltites outstanding candidates for oxygen-storage and gas-sensors applications [4], [5]. They show an extreme affinity for oxygen, being able to uptake up to 1.5 extra atoms per formula unit at 350 °C, and release them again, very abruptly, at 400–450 °C [6], [7], [8].
The stoichiometric compound YBaCo4O7 has a nominal valence for Co of 2.25, so it is assumed that Co is present as Co2+ and Co3+ in a 3:1 proportion. The structure of this compound is shown in Fig. 1. In its high–temperature form, in the trigonal space group P31c, the symmetry imposes a high magnetic frustration. However, it has been shown that for the strictly stoichiometric compound with δ=0, a first–order phase transition to an orthorhombic Pbn21 space group at a temperature (close to room temperature for YBaCo4O7 [9], [10]) lifts this frustration and allows for the development of a long-range antiferromagnetic (AFM) structure below [9]. The origin and driving force for the structural transition at remain under debate. It was originally suggested that a charge ordering mechanism would occur at [11]; it was also proposed that the transition is driven by the condensation of a phonon mode associated to the rigid-body rotation of the tetrahedra in the Kagomé layers [12]. Finally, Huq et al. [13] proposed that it is driven by a severe under-bonding of Ba2+ ions. In Fig. 2 we show the schematic geometrical relation between the unit cells of the P31c and Pbn21 phases in the a–b plane. The lattice parameters are related as , and .
It is interesting to note that the oxygen stoichiometry plays an important role in the trigonal to orthorhombic transition, which is suppressed already for very small values of δ (as low as 0.02 [14]). A second variable which modifies the transition temperature is the R-cation size, as has been shown and discussed by various authors [11], [15]. Less studied has been the effect on the structural transition of replacing Co with other magnetic ions [16], [17]. In this paper, we report a structural study of Ni-doped YBaCo4O7 cobaltites based on synchrotron X-ray powder diffraction (SXPD).
Section snippets
Synthesis
Polycrystalline samples of nominal YBaCo4O7 (hereafter called Ni-0) and YBaCo4−xNixO7 with x=0.10 and 0.20 (alias Ni-10 and Ni-20) were prepared by conventional solid–state reaction. High-purity powders of Y2O3, BaCO3, Co3O4 and metallic Ni were thoroughly mixed in an agate mortar at stoichiometric weights. After a de-carbonation process at 1173 K for 12 h, the mixtures were pressed into pellets, annealed during 24 h at 1473 K and slowly cooled at 1 K/min in the furnace. The compression and
Oxygen stoichiometry
The TGA measurements show that when heating the samples for the first time after their synthesis, those that had been slowly cooled from 1473 K during the synthesis present a loss of mass occurring between 473 and 523 K, as shown for sample Ni-10 in Fig. 3(b) and for Ni-20 in Fig. 3(c). This behavior is in very good agreement with that reported for the parent compound (x=0) in Ar or N2 flow [6], [7], [18], [14]. This mass loss is related to the release of oxygen in excess from the stoichiometric δ
Conclusions
In this paper we have studied the structure of samples YBaCo4O7.00 partially substituted with Ni at the Co site. When starting from a metallic Ni precursor, the solubility limit has been found to lie below 5 at.% Ni/Co. This was detected in magnetization measurements which are extremely sensitive to small amounts of ferromagnetic impurities. Although we do not observe any distortion of the crystal lattices upon Ni substitution, the phase stability is strongly modified, as had been shown
Acknowledgment
This work is part of a research project supported by Agencia Nacional de Promoción Científica y Tecnológica (Argentina), under grant PICT-2011-0752, by Conicet under grant PIP 490 2012-2014 and by Universidad Nacional de Cuyo. Authors are very grateful to F. Castro at CAB for performing the TGA measurements, and to F. Pomiro for his participation in the preparation of samples. EG acknowledges CNPq and FAPESP for financial support. We acknowledge LNLS synchrotron for the beamtime allocation and
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