Aspects of electronic transport in YBaCo 4 O 7þd pellets J.L. Izquierdo a , J.F. Montoya a , A. Gómez b , C. Paucar a , O. Morán a, * a Laboratorio de Materiales Cerámicos y Vítreos, Departamento de Física, Universidad Nacional de Colombia, Sede Medellín, A.A. 568, Medellín, Colombia b Laboratorio de caracterización de materiales, Universidad Nacional de Colombia, Sede Medellín, A.A. 568, Medellín, Colombia article info Article history: Received 13 February 2010 Received in revised form 13 July 2010 Accepted 4 September 2010 Available online 17 September 2010 Keywords: Cobaltites Electronic transport Electronic states (localized) abstract YBaCo 4 O 7þd 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 P6 3 mc). The lattice parameters resulted to be very close to those reported in the literature for high-quality samples. Raman spectra at room temper- ature allowed for identifying bands associated with vibrating modes of CoO 4 and Y 2 O 6 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 variable- range 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 r 0 exp[(T * /T) 1/4 ]. The fit procedure yielded a temperature scale T * w 10 6 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(E F ) for this compound. Ó 2010 Elsevier Masson SAS. All rights reserved. 1. Introduction Recently, Co-based compounds have intensively been investi- gated due to the existence of intriguing magnetic properties [1]. The most representative member of such cobalt oxide systems is the spinel Co 3 O 4 which features antiferromagnetic ordering [2]. Apart from this, Co-rich quaternary systems like Ln 2 BaCoO 5 (Ln ¼ Y, Sm,Er) have also awaked enormous interest because their similar- ities with the cuprate Y 2 BaCuO 5 discovered in the high temperature superconductor YBa 2 Cu 3 O 7d [3,4]. In addition, it was also estab- lished that Co-rich compounds as (Sr,Ca,Ln) 3 Co 2 O 6þd (Ln ¼ SmeHo and Y) exhibit long range antiferromagnetism or spineglass prop- erties [5]. Particularly, the double perovskite compounds LnBa- Co 2 O 5 resulted to be antiferromagnetic [6]. On the other hand, the new type of magnetic compound YBaCo 4 O 7þd , denoted 114, was reported to exhibit an unusual magnetic behavior, which resem- bled that of a spin-glass [7]. The crystal structure of this novel cobaltite comprises layers formed by two different types of cobalteoxygen tetrahedra, (Co1)O 4 and (Co2)O 4 which are 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, Co 3þ ions may exist in the low-spin (LS, S ¼ 0, t 2g 6 3 g 0 ), intermediate-spin (IS, S ¼ 1, t 2g 5 3 g 1 ), and high-spin (HS, S ¼ 2, t 2g 4 3 g 2 ) 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 variable- range 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 local- ized [12]. As the localized states (LS) do not carry any current in the * Corresponding author. E-mail address: omoranc@unal.edu.co (O. Morán). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.09.001 Solid State Sciences 12 (2010) 2073e2078