Effects of pH and sintering temperature on the synthesis and electrical properties of the bilayered LaSr 2 Mn 2 O 7 manganite prepared by the sol–gel process M. H. Ehsani • M. E. Ghazi • P. Kameli Received: 19 December 2011 / Accepted: 5 April 2012 / Published online: 25 April 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The bilayered LaSr 2 Mn 2 O 7 manganite was synthesized by the sol–gel process at different pH values (3.5, 7, and 9.5) and different sintering temperatures (1553, 1623, and 1723 K) to investigate the effects of growth conditions upon the structural and electrical properties of the samples under investigation. X-Ray diffraction and FT-IR spectroscopy techniques conﬁrm the phase forma- tion for all samples. However, samples sintered at 1553, 1623, and 1723 K, and formed at pH = 7 are single phased having a tetragonal structure. Study of these single-phased samples indicates that with increase in the sintering tem- perature from 1553 to 1723 K, the average grain size increases from *200 to *1000 nm. To investigate the inﬂuence of grain size on the conduction mechanism, resistivity of the samples was measured as a function of temperature. The data obtained was analyzed by the adia- batic small polaron hopping model. These analyses show the inﬂuence of grain size on the parameters obtained by ﬁtting the data by the above models. Introduction Mn-based Ruddlesdon-Popper [1] (RP) compounds with the general formula (La,Sr) n?1 Mn n O 3n?1 are well known, having layered-type structures. The n = 1 member (La 1-x Sr 1?x MnO 4 ) having the two-dimensional (2D) K 2 NiF 4 structure is an insulator without showing ferromagnetic (FM) phase [2]. The n = 2 member, La 2-2x Sr 1?2x Mn 2 O 7 , with a tetragonal Sr 3 Ti 2 O 7 -type structure, and the 3D manganites La 1-x Sr x MnO 3 ) the n = ? member (with a perovskite structure have a rich variety of physical prop- erties depending on the amount of hole doping (x)[3–5]. The bilayered (n = 2) manganites La 2-2x Sr 1?2x Mn 2 O 7 have a 2D character, and show an anisotropic reduction in the one-electron (e g ) band width. The physical properties observed for these materials along the ab plane and c axis are different, which may give important clues on the colossal magnetoresistance (CMR) effect, tunneling magnetoresis- tance, and fascinating electronic and magnetic properties [6, 7]. Also the n = 2 series show a rich magnetic phase diagram, which depends strongly on the doping level x [8]. The half-doped bilayered manganite LaSr 2 Mn 2 O 7 (x = 0.5) exhibits a charge ordered (CO) phase. The so-called charge ordering state is one of the most inter- esting phenomena frequently found at some manganite systems, provided that the one-electron band width (W) is not large. In the LaSr 2 Mn 2 O 7 compound, the A-type AFM order and CE-type charge ordering have been observed experimentally around T N,CO CE & 210 K, and then another transition to CE-type charge and spin-ordered states below T N,CO A & 145 K [9]. Various methods have been used to synthesize bilayered manganites including ﬂoating zone methods for single crystal growth [10, 11], conventional solid-state reaction for polycrystalline compounds [12, 13], pulse laser M. H. Ehsani  M. E. Ghazi (&) Department of Physics, Shahrood University of Technology, 3619995161 Shahrood, Iran e-mail: ebrahim_ghazi@yahoo.com M. H. Ehsani Department of Physics, Semnan University, 35195-363 Semnan, Iran P. Kameli Department of Physics, Isfahan University of Technology, 84156-8311 Isfahan, Iran 123 J Mater Sci (2012) 47:5815–5822 DOI 10.1007/s10853-012-6481-4