International Journal of Material and Mechanical Engineering, 2012, 1: 44-49 -44- Published Online April 2012 www.ijm-me.org 1. INTRODUCTION Cobalt oxides have particular interest, not only because of the unique features of Co ions, but also due to their technological applications, such as solid oxide fuel cells and membranes for gas separation.[1–5]. Moreover, perovskite cobaltates exhibit rich magnetic properties including different types of ordering phenomena such as paramagnetic (PM), antiferromagnetic (AFM), or ferromagnetic (FM). Double exchange interaction between Co +3 − O − Co +4 is known to be FM while super-exchange interaction between Co ions with the same oxidation of states is AFM [6–9] The crystal-field splitting energy of Co d-states (E cf ) and Hund energy (E ex ) are comparable for perovskite cobaltates. This means that the energy gap between the t 2g and e g states is small and the electrons in t 2g can be thermally exited into the e g state. As a result high spin states can be archived [10]. In addition, the differences in electric or magnetic properties of cobaltates can be understood by considering the relationship between the average ionic radius of R 1−x A x (R is the rare earth ions and A is the alkalic ions) and the energy difference between the low spin and intermediate spin states. Similar to isotropic perovskite compounds R 1−x Sr x CoO 3 , the physical properties of layered perovskite system R 2−x Sr x CoO 4 , R = La, Nd or Pr are fascinating. For instance Nd 1−x Sr 1+x CoO 4 (0.25 ≤ x ≤ 0.6) show spin-glass (SG) at 18K and Griffiths singularity around 210 K [11]. While the magnetic state of La 2−x Sr x CoO 4 alters from AFM state to FM state when the doping level x ≥ 0.6 [12]. Doping- dependent charge and superstructures in Pr 2−x Ca x CoO 4 (0.39≤ x≤0.70) and La 2−x Sr x CoO 4 (x = 0.4, 0.61) were suggested [13]. It was found that the magnetic ordering of these systems is established at low temperature and has t wice larger periodicity , indicating a dominant AFM correlation between the nearest Co 2+ spins. The electrical behavior of Nd 1−x Sr 1+x CoO 4 system obeys Arrhenius law for x = 0.25, 0.33 and 0.60 due to hopping of small lattice polarons as in the case of manganites. A systematic increase of the resistivity with decreasing A- site rare earth ionic radius r +3 in cobaltates was reported earlier [14] implying that upon narrowing the band width, the mobility of e g electrons is decreased. For Pr 1- x Sr 1+x CoO 4 samples, the resistivity and thermal activation energy decreases irregularly with increasing x [15]. Because the layered perovskite compounds exhibit interesting properties so that a good motivation is to through light on Pr 1-x Sr 1+x CoO 4 system , especially at 0.60, i.e. in the Sr rich side. Through the publications [15,16] there is no details about the physical properties of this system Structural, electric and magnetic properties of PrSrCoO 4 and Pr 0.4 Sr 1.6 CoO 4 layered perovskites (Abstract) The structural, electric and magnetic properties of PrSrCoO 4 and Pr 0.4 Sr 1.6 CoO 4 layered perovskites have been investigated. The Rietveld refinements of x-ray powder diffraction (XRD) patterns at room temperature indicate that the samples crystallize in K 2 NiF 4 -type structure with group symmetry I4/mmm. The specimens exhibit semi-conducting behavior. The electric mechanism of PrSrCoO 4 obeys Arrhenius law and might be understood by small polaron models. On the other hand, the conduction of Pr 0.4 Sr 1.6 CoO 4 follows two-dimensional variable range hopping model (2D-VRH). The magnetic properties of PrSrCoO 4 show Curie-Weiss paramagnetic behavior within the studied range of temperature. While Pr 0.4 Sr 1.6 CoO 4 exhibits two magnetic transitions; one is due to Griffiths phase and second because of a mixture between ferromagnetic (FM) and antiferromagnetic (AFM) transitions. These transitions become broad and shifted towards low temperatures with increasing applied field. No complete FM state was observed for this higher content of Sr. Comparison with similar compounds of the same Sr content is also discussed models. Keywords: layered perovskites , electric resistivity, semiconductors, magnetic measurements A. Hassen Department of Physics, Faculty of Science, Fayoum University, 63514 El Fayoum, Egypt Department of Physics, Faculty of Science and Education, Taif University, KSA Email: arafahassen@yahoo.com and ash02@fayoum.edu.eg