Solid State Sciences 12 (2010) 2073e2078
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Aspects of electronic transport in YBaCo4O7þd pellets J.L. Izquierdo a, J.F. Montoya a, A. Gómez b, C. Paucar a, O. Morán a, * a b
Laboratorio de Materiales Cerámicos y Vítreos, Departamento de Física, Universidad Nacional de Colombia, Sede Medellín, A.A. 568, Medellín, Colombia Laboratorio de caracterización de materiales, Universidad Nacional de Colombia, Sede Medellín, A.A. 568, Medellín, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2010 Received in revised form 13 July 2010 Accepted 4 September 2010 Available online 17 September 2010
YBaCo4O7þd powders were obtained by standard solid state reaction und their structural, morphological and electrical properties carefully analyzed. The X-ray powder diffraction patterns showed reflexes corresponding to a pure hexagonal structure (space group P63mc). The lattice parameters resulted to be very close to those reported in the literature for high-quality samples. Raman spectra at room temperature allowed for identifying bands associated with vibrating modes of CoO4 and Y2O6 in tetrahedral and octahedral coordination, respectively. Additional bands, which seemed to stem from CoO in octahedral coordination, were also clearly identified. The dependence of the resistivity on temperature showed a semiconducting-like behavior and no indication of structural phase transition was observed up to temperatures as low as 20 K. The electronic transport mechanism in this material was analyzed within the framework of standard models as thermal activation, polaronic-type conductivity or Mott variablerange hopping. Contrary to some reports in the literature in which thermal activation was reported to be the main transport mechanism, careful analysis of the obtained resistivity data (this work) favored the variable-range hopping conduction model. Certainly, the experimental data recorded in a wide temperature range were well described by the function r(T) ¼ r0exp[(T*/T)1/4]. The fit procedure yielded a temperature scale T* w 106 K, similar to that found in other transition metal oxides. This parameter, in turn, allowed for estimating the density of states at the Fermi level N(EF) for this compound. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Cobaltites Electronic transport Electronic states (localized)
1. Introduction Recently, Co-based compounds have intensively been investigated due to the existence of intriguing magnetic properties [1]. The most representative member of such cobalt oxide systems is the spinel Co3O4 which features antiferromagnetic ordering [2]. Apart from this, Co-rich quaternary systems like Ln2BaCoO5 (Ln ¼ Y, Sm,Er) have also awaked enormous interest because their similarities with the cuprate Y2BaCuO5 discovered in the high temperature superconductor YBa2Cu3O7d [3,4]. In addition, it was also established that Co-rich compounds as (Sr,Ca,Ln)3Co2O6þd (Ln ¼ SmeHo and Y) exhibit long range antiferromagnetism or spineglass properties [5]. Particularly, the double perovskite compounds LnBaCo2O5 resulted to be antiferromagnetic [6]. On the other hand, the new type of magnetic compound YBaCo4O7þd, denoted 114, was reported to exhibit an unusual magnetic behavior, which resembled that of a spin-glass [7]. The crystal structure of this novel cobaltite comprises layers formed by two different types of cobalteoxygen tetrahedra, (Co1)O4 and (Co2)O4 which are
* Corresponding author. E-mail address:
[email protected] (O. Morán). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.09.001
connected by corners and characterized by different bond lengths [8]. Such a feature was interpreted as favoring actual ordering of the cobalt cations in different oxidation states. Certainly, because the crystal field energy differs little from the intratomic exchange energies in crystals containing Co ions, the latter may reside in different spin-states depending on the actual external conditions (temperature and pressure) [9]. For instance, Co3þ ions may exist in the low-spin (LS, S ¼ 0, t62g30g), intermediate-spin (IS, S ¼ 1, t52g31g), and high-spin (HS, S ¼ 2, t42g32g) states. It is precisely the different spin states of Co ions and the layered 2D structure of the cobaltites which account for the rich diversity of properties of this class of compounds [9]. On the other hand, Co-based oxides show large thermoelectric power which is, probably, related to their layered structure and the presence of mixed Co valences [10]. Although the semiconducting character of the 114-phase (from low to high temperatures) has been clearly established from diverse reports, the electronic transport has not been systemically analyzed so far. Several models have been proposed to depict the temperature dependence of electrical resistivity. One approach is the variablerange hopping model (VRH) which describes the disorder-induced localization of charge carriers [11]. The VRH model bases on the fact that the electronic states near the Fermi level are generally localized [12]. As the localized states (LS) do not carry any current in the
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thermodynamic limit, the conduction involving LS may only takes place by means of transitions of electrons from full states to neighboring empty states with the help of phonons [13]. The hopping transition rate in this model may be written as p ¼ n0exp (2aR-bW), where a1 ¼ x represents the inverse localization length (x) of exponentially localized states. Here, one of these states lies below the Fermi energy (EF) and the other one above EF. R, in turn, is the typical hopping distance between localized states, b ¼ 1/ kBT and W the energy separation between the final and initial states. The attempt frequency n0 depends on the strength of the electronephonon coupling and the phonon density of states. Nevertheless, it is almost independent on R and W [14]. It is evident from the expression for p that the electrons will prefer to hop to more distant neighbors where the energy difference between the states is smaller. The transition probability is maximized due to the existence of a trade-off between the hopping distance R and mismatch energy W subjected to the constraints that energy window kBT is quite narrow and both x ¼ a1 and EF remain constant within the narrow window. For a constant density of states in three dimensions, Mott [12] found that the conductivity which is related to the transition probability varies with temperature as s(T) ¼ s0exp[(A/T)1/4]. The physical meaning of 1/4 exponent was interpreted as the reciprocal of the effective dimensionality of 4 (3 spatial and one energy) [15]. Electrons in this language hop in a four-dimensional space (x; y; z; E): a more rigorous treatment of the hopping process was discussed in terms of the percolation theory [16] and by the random resistor network model [17]. The aim of the present work is to explore new aspects of the structural properties and the charge transport process in the novel cobaltite YBaCo4O7þd. Especial attention is devoted to the analysis of the electronic transport mechanism as this issue has not been explored in detail in the existing literature on this complex and intriguing compound. 2. Experiment Powders of YBaCo4O7þd were obtained from stoichiometric mixtures of Y2O3, Ba(CH3COO)2 and Co2O3 reactants. After mixing the constituents thoroughly in an agate mortar, the resulting powder was slowly heated in air (w5 C/min) up to 1200 C and then calcined for 48 h. After this process was accomplished, the sample was cooled slowly inside the furnace to ambient temperature. No reaction occurred between Co and the Pt crucible as it was corroborated by X-ray diffraction (XRD). The black single phase of YBaCo4O7þd powder was grounded and then pressed into pellets (w3 cm in diameter and thickness w3 mm) which were finally sintered at 1300 C for 11 h in air. The structural properties of the samples were studied by XRD spectroscopy with CuKa line radiation at 0.15418 nm and Raman spectroscopy measurements by employing a JY-HR800 instrument, with 633 nm laser excitation (17 mW). The morphological properties were studied by means of scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS). The transport properties of the samples were measured in a standard four-probe configuration using an integrated PPMS system (Quantum Design) equipped with a 14 T superconducting magnet.
those reported in the literature for R-114 compounds with hexagonal symmetry [18]. The corresponding reliability factors are RBragg ¼ 1.35%, c (Rwp/Rexp) w 0.6. The ratio c h Rwp/Rexp, sometimes referred to as the “c factor” (and also as the goodness of fit), is a measure of how well the fitted model accounts for the data. A c factor grater than about 1.5 is a strong indication of an inadequate model or a false minimum. A value of the c less than 1.0, however, is an indicator not of an extremely high quality refinement but of model that contains more parameters than may be justified by the quality of the data. It should be noted that a low value of the c factor may be obtained by a high value of Rexp, due to insufficient counting time, as well as by a low value of Rwp. A low value of Rwp may be obtained if there is a high background than may fit by a relatively crude background function. In the case of the analyzed YBaCo4O7þd powders, the low value obtained for the Rwp factor (0.01721) would be consistent with the high background observed in Fig. 1. Although iodometric titration experiments have not yet been carried out, the value obtained for the c-parameter suggests that d z 0.1 [19]. In this regard, systematic studies have shown that the YBaCo4O7þd lattice monotonically contracts upon the oxygen insertion [19]. Concretely, it was observed that both the c-parameter and cell volume systematically decrease with increasing d as a consequence of shrinkage of the ionic radius of cobalt due to oxidation. The estimated oxygen content was further corroborated by thermogravimetric analysis performed in Ar flow (not shown). At this point, it should be mentioned that experimental results of thermal analysis and coulometric titration as well as p(O2)dependencies of total conductivity and Seebeck coefficient of single-phase powders of YBaCo4O7þd showed that heating above 1070e1110 K in air and/or decreasing oxygen partial pressure resulted in a phase transition accompanied with substantial oxygen losses from the lattice [7]. This, in turn, brought about the formation of essentially stoichiometric YBaCo4O7. Except for these changes, the transport properties and oxygen content were almost p(O2)-independent down to the oxygen pressures of 105 atmosphere. Nevertheless, the great affinity of YBaCo4O7 for oxygen has been firmly established by several research groups [7,20e22] who demonstrated the large oxidation-reduction capabilities of the material at temperatures near 350 C. In this regard, Karppinen et al. [21] showed that YBaCo4O7þd reversibly absorbs and desorbs oxygen up to d z 1.5 within the 470e673 K temperature range. This implies that an amount of oxygen corresponding to w20% of the total oxygen content is readily loaded or removed by just a relatively minor change in temperature or atmosphere. In addition, Tsipis et al. [23] showed that the oxygen hyperstoichiometry is
3. Results and discussion The XRD results (Fig. 1) indicate that YBaCo4O7þd ceramics sintered using the procedure described in the latter section were single phase with hexagonal crystal structure (space group P63mc). The lattice parameters obtained by the Rietveld refinement resulted to be a ¼ 6.3057 A and c ¼ 10.2546 A which are very close to
Fig. 1. Rietveld pattern of YBaCo4O7þd. The difference between experimental and calculated diffraction pattern is shown at the bottom as a solid line. The raw of markers shows the position of allowed reflection for space group P63mc.
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200
850
3
330
3
700
4
630 564 535 530 493 430
380
5
Intensity [x10 a. u.]
closely related to the mixed-valence states of the cobalt atoms. Careful examination of the closely packed structure of YBaCo4O7 (Fig. 2) reveals no obvious sites available for any additional oxygen atoms. Nonetheless, it is possible to identify a significant distance between the adjacent Ba and Co2 kagome layers of w2 A from which extra oxygen atoms may potentially enter the structure and bond with the surrounding cations [24]. The fixed oxidation states of the Y3þ and Ba2þ ions imply that the extra oxygen atoms must primarily bond with some of the Co ions to increase their average oxidation state from 2.25þ to 2.75þ (in RBaCo4O8) [24]. By considering the increased number of oxygen first-neighbors and the well-known Co3þ chemistry [25], it is expected that some of the original tetrahedral would convert to pyramids or octahedral or both. This interesting aspect was further studied by Raman spectroscopy. Fig. 3 shows a Raman spectrum of an YBaCo4O7þd polycrystalline sample recorded at room temperature. A series of strong peaks are clearly seen on this plot. The peaks at w200, and w380 cm1 could be assigned to vibration modes of YeO bonds from tetrahedral coordination of Y2O3. Furthermore, the peaks appearing at w330, and w630 cm1 were attributed to CoO4 tetrahedra. Interestingly, the bands localized at w490 and w530 cm1 could not be identified as vibrations surging from CoO in tetrahedral coordination. They rather seem to stem from CoO in octahedral coordination. Certainly, if there are extra oxygen atoms, it will be easy for the new oxygen atoms to interact inside of the tetrahedral network. This interaction will be enhanced by the Co3þ ions which prefer octahedral coordination [26]. According to these results, the vibration modes found in the Raman spectrum suggest that the cobalt oxide may be found in several oxidation states where the energy required for the allowed vibration modes increases as oxidation state decreases. This seems to be the case of the low energy vibration mode detected at 700 cm1 which would arise from cobalt oxidation state 4þ [CoO4]4 ] [27]. Furthermore, it should be mentioned that the higher energy peaks at 430 cm1 and 530 cm1 may also stem from Co3O4. Nevertheless, more detailed studies indicate that these peaks may be associated with CoO and CoO3 where cobalt has divalent oxidation states like 2þ and 3þ. By using the molecular modeling software HyperChem 7.5 Tm (by Hypercube, Inc.), the optical molecular vibration modes of the different functional groups in YBaCo4O7þd powder were clearly identified (Table 1).
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2 1 1000
800
600
400
200
-1
Raman shift [cm ] Fig. 3. Raman scattering curve of a polycrystalline YBaCo4O7þd sample recorded at room temperature.
Shown in Fig. 4(a) is SEM micrograph of an YBaCo4O7þd polycrystalline sample after reacting the powders at 1200 C in air for 48 h using a Pt crucible followed by slow cooling to room temperature. Hexagon-like prisms of different sizes are readily recognized on this micrograph. Statistical analysis of the grain size based on the large area image yielded an average grain size of 4.5 mm. The corresponding EDS spectrum of this same sample is presented in Fig. 4(b). The measured spectrum (taken with counting time of w1 h) shows no traces of foreign atoms as Pt which might stem from the Pt crucible used for the calcination. The EDS analysis confirms that the cationic composition Y1Ba1Co3.97Ox is in agreement with the nominal one in the limit of experimental errors. Plotted in Fig. 5 is the temperature dependence of the resistivity of an YBaCo4O7þd pellet measured upon cooling and warming processes. No difference between these two processes was observed. The semiconductor-like character of the material is vividly recognized in this plot. At room temperature, the resistivity value amounted to w30 U cm. Interestingly, no anomaly, generally associated with structural phase transitions [28], is observed in the temperature-range measured. The suppression of this transition is, probably, due to the presence of extra oxygen in the structure which tend to increase locally the coordination of the cobalt cations with formal oxidation state Co2.25þ (d ¼ 0). This, in turn, creates a more disordered local crystallographic structure which for some 114 compounds as YbBaCo4O7 seems not to be compatible with a cobalt magnetic ordering [29]. Indeed, by increasing the hole concentration in the O7þd phase, the extra oxygen atoms would hinder the triangular arrangement for the magnetic moments of the tetrahedrally coordinated CoO4 since new polyhedra are created. This suppresses the transition. On the other hand, it is probable that a structural transition takes place at local scale.
Table 1 Different vibration modes of the functional groups in YBaCo4O7þd powder. Functional group
Raman shift [cm1]
Vibration modes
BaO [CoO4]4
656 334 630 700 850 430 530 493 535 564 200 380
Symmetrical stretching bending scissoring Asymmetrical stretching Asymmetrical stretching Asymmetrical stretching Symmetrical stretching Asymmetrical stretching Symmetrical stretching Asymmetrical stretching Asymmetrical stretching Scissoring
[Co3O4] CoO6 Octahedral
Fig. 2. Structure of YBaCo4O7 showing possible oxygen locations (small black circles) according to the model from Ref. [7] and the separation distance of w2 A between the triangular and kagome layers. Solid lines show one hexagonal unit cell.
Y2O3
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Fig. 4. SEM image (a) and EDS spectra (b) taken on an YBaCo4O7þd polycrystalline sample at room temperature.
Nevertheless, if a structural transition came at local scale, it would be very difficult to detect it by normal XRD or transport measurements. The electronic transport mechanism was tested using different models which depict the temperature dependence of the electrical resistivity. The conventional Arrehenius plots, r(T) ¼ Cexp(E/kBT), suggests simple thermal activation transport [30]. While considering the small polaron hopping conduction mechanism, the behavior of electrical resistivity versus temperature should be expressed as r(T)¼CTexp(E/kBT) [31]. Another approach is the Mott’s VRH model in which the dependence of the resistivity on the temperature is given by r(T)¼r0exp(T*/T)1/n [12]. Here, r0, T* and n are constants. The quantity 1/n may take the values1/2, 1/3, or 1/4, depending on the dimensionality D and, in some compounds, on the temperature range. In the present case, the resistivity data measured on the YBaCo4O7þd pellets seemed to obey the VRH model when n ¼ 1/4. By plotting the experimental data r(T) as r(T) ¼ Cexp(E/kBT) and r(T) ¼ CTexp(E/kBT) in the temperature range 350e20 K [Fig. 6(a) and (b), respectively], it is readily recognized that neither simple thermal activation nor small polaron hopping conduction describe well the electronic transport in the reported pellets. Fig. 7, in turn, shows the plot of the resistivity data as a function of T1/4 in the temperature range 350e20 K. The VRH behavior is promptly verified. The fitting procedure yielded a temperature scale T* ¼ 1.3 106 K (R2 ¼ 0.998; slope: stand error ¼ 0.08). The encountered value is the same order of magnitude that that determined for other transition metal oxides [32]. The characteristic temperature T* is related to the parameter x through the expression kBT* ¼ 24/pN(EF)x3 being N(EF) the density A, of states at the Fermi level [12]. By taking x z c ¼ 10.254 a N(EF) z 1.9 1020 eV1 cm3 was estimated for these polycrystalline samples. The fact that the electrical resistivity may be
5
a
5
10
[ cm]
3
b
4
-1
10
/T [ cmK ]
cm
10
described by VRH model points out to the presence of disorder induced localization of charge carriers in the samples. The electron wave functions in the LS decay exponentially over a distance equivalent to the decay length, a1 ¼ x. VRH mechanism normally occurs only in the low-temperature region (below room temperature) wherein the energy is insufficient to excite the charge charier across the Coulomb gap. Hence, conduction is accomplished by hopping within a small region (wkBT) in the vicinity of EF. In this region, the density of states remains almost constant [14]. The results encountered in the present work suggest, nevertheless, that in the YBaCo4O7þd pellets, VRH mechanism may occur also in a temperature region above room temperature. However, the transport mechanism shall rapidly evolve into a simple thermal activation process as now, the energy should be sufficient to excite charge carriers across the Coulomb gap. This fact was verified for YBaCo4O7þd pellets by surveying the Fig. 6(a). The resistivity of RBaCo4O7 (R ¼ Dy, Ho, Y, Er) pellets has been measured at high temperatures (350e1000 K) [10]. The temperature dependence of the electrical resistivity showed a typical semiconducting behavior in the investigated temperature region. The experimental data r(T) were analyzed in the framework of several models as the conventional Arrehenius plot, small polaron hopping and VRH conduction. It was found that the small polaron hopping model yielded a much better fitting that others. Hence, it was suggested that this model may be applicable to RBaCo4O7 in the temperature range 350e650 K. The calculated values of the conductivity activation energy allowed for determining a band gap of about 0.2 eV for the all four compounds. This result was consistent with results obtained in the low-temperature range [33]. The large difference between the conductivity activation energy and the thermopower activation energy suggested that the carriers in the RBaCo4O7 system move by hopping in the localized states at the band edges and may be interpreted in the frame of strong electron-phonon coupling that forms small lattice polarons. It should be recalled that if the transport takes place in the extended states, the two
3
10
10
2
10
0
10
10
1
0
100
200 T [K]
300
Fig. 5. Temperature dependence of the resistivity measured on an YBaCo4O7þd pellet in zero magnetic field.
0
20
40 60 -1 1000/T [K ]
80
0
20
40 60 -1 1000/T [K ]
80
Fig. 6. Dependence of r and r/T on reciprocal temperature [curves (a) and (b), respectively] for YBaCo4O7þd pellets. The temperature range spans 350e20 K.
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J.L. Izquierdo et al. / Solid State Sciences 12 (2010) 2073e2078
10
4
10
2
0.2
0.3
T
0.4
0.5
-1/4
[K
]
Fig. 7. Variable range hopping (VRH) fit to the experimental data r(T) in the temperature range 350e20 K.
activation energies should be equal [10]. By taking these findings into account, it was concluded that the conduction mechanism in the RBaCo4O7 system is due to thermally active hopping in the temperature range 350e650 K. For temperatures above 650 K, the temperature dependence deviated from the straight line which suggested that other mechanism may take place at such high temperatures. It is known that magnetic transitions from ferromagnetic to paramagnetic state, phase transitions or oxygen diffusing into or out of the lattice may produce anomalous behavior in the conductivity. As RBaCo4O7 shows paramagnetic behavior above 100 K [7] and no phase transition above 650 K has been reported, it is possible that the change of transport mechanism at 650 K may be related to oxygen desorption from the lattice or/and the possible change in symmetry as consequence of oxygen adsorption at 650 K [21]. On the other hand, the fact that our samples do not obey an Arrhenius law but rather a VRH at low temperatures might be explained by the presence of oxygen excess in the lattice. The observation of positive thermopower of about 100 mV K1 in more oxidized RBaCo4O7 compounds [29] was considered as indicative of a more like-itinerant behavior (in the sense of band picture) of the charge carriers in that phase. In this regard, the expected more than half-filled eg band of the stoichiometric and hyperstoichiometric samples would lead to a hole-like conduction and positive thermopower in both cases. This conclusion agrees with the smaller resistivity of the oxygen-rich samples and the VRH nature of the conductivity of such samples implying itinerant electron states with the apparent insulating behavior due to magnetic frustration. As it was stated above, the VRH transport comes to pass by hopping over a certain distance in the bulk of the dielectric that is controlled by the thermally available energy window that restricts the selection of possible hopping targets. The resulting average
12
RAv [nm]
10 8 6 4 0
100
200 T [K]
300
Fig. 8. Calculated Rav in the c-direction as a function of T for an YBaCo4O7þd pellet.
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hopping distance (Rav) is then given by Rav ¼ x(T*/T)1/4/2 [10]. The typical Rav decreases with increasing temperature (Fig. 8). For low activation energies the electron has to travel (hop) a long way to find a state with an energy that is close enough to the initial energy. On increasing the activation energy by increasing T, the range of accessible energies broadens. More states near the initial state become available to the electron, and the average Rav decreases. At low T and weak electric fields, the tunneling current is carried by a few hops over a long distance. At higher T or equivalently in higher electric fields, the tunneling current is carried by more hops over a shorter distance. The estimated values for Rav are in accordance with the average distance between two LS, namely [0 ¼ 1/[N A. (EF)]1/3 z 1/[1.9 1020/cm3)]1/3 z 20 4. Summary and conclusions Polycrystalline samples of the new cobaltite YBaCo4O7þd were obtained through standard solid state reaction und their structural, morphological and electrical properties carefully studied. X-ray powder diffraction pattern showed reflexes corresponding to a pure hexagonal structure without traces of possible secondary phases or foreign elements. Raman spectra of the polycrystalline material, in turn, allowed for identifying bands associated with vibrating modes of CoO both in tetrahedral and octahedral configuration. The measuring of the dependence of the resistivity on the temperature showed a semiconductor-like behavior without evidence of any structural transition. The analysis of the electronic transport mechanism in the framework of several conduction models indicated that Mott VRH would be the most appropriated one to describe the experimental data on a wide temperature range. The fitting of this model to the resisitivity data allowed for estimating important physical parameters of the material as the density of states or the average hopping distance.
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