Electrical properties of fluorine-intercalated layered manganite: La 1.4 Sr 1.6 Mn 2 O 7 F 2 Ankam Bhaskar, C.-S. Sheu, Chia-Jyi Liu ⇑ Department of Physics, National Changhua University of Education, Changhua 500, Taiwan article info Article history: Received 14 August 2014 Received in revised form 8 October 2014 Accepted 20 October 2014 Available online 28 October 2014 Keywords: Ceramics Sintering Exchange and superexchange X-ray diffraction abstract Fluorinating agent NH 4 F is employed to intercalate the bi-layered manganite La 1.4 Sr 1.6 Mn 2 O 7 to form La 1.4 Sr 1.6 Mn 2 O 7 F 2 using solid state reaction. The insertion of fluorine results in a contraction of 0.116 Å in the a axis and an expansion of 3.152 Å in the c axis with respect to the parent compound La 1.4 Sr 1.6 Mn 2 O 7 . The Mn–O bond lengths are altered due to the electrostatic repulsion between O 2 and F  . Rietveld analysis indicates that a longer bond length of Mn–O(1) and shorter bond lengths of Mn–O(2) and Mn–O(3) has been found in La 1.4 Sr 1.6 Mn 2 O 7 F 2 when compared to La 1.4 Sr 1.6 Mn 2 O 7 . The electrical resistivity of La 1.4 Sr 1.6 Mn 2 O 7 F 2 shows insulating behavior and exhibits remarkably different behavior from La 1.4 Sr 1.6 Mn 2 O 7 . For the first time, we report the temperature dependence of electrical resistivity of La 1.4 Sr 1.6 Mn 2 O 7 F 2 . Magnetic measurements indicate that the magnetic order of local moments is suppressed by fluorine insertion into interstitial sites. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years, great attention has been paid to the series of Ruddlesden–Popper compounds (RE, AE) n+1 Mn n O 3n+1 with perov- skite-like structure, where RE is a trivalent rare-earth cation and AE a divalent alkaline-earth cation, due to the existence of an extraordinary colossal magnetoresistance (CMR) effect [1,2].A large magnetoresistance is subsequently discovered in La 22x Sr 1+2x Mn 2 O 7 with x = 0.3 and 0.4 [3–8]. Fluorine insertion reaction has been shown by modifying the structure of materials [9]. Recently, the insertion of fluorine into mixed copper oxides is adopted to control the carrier density and, hence, induces superconductivity [10,11]. This idea has been implemented to other oxide systems such as Sr 2 TiO 3 F 2 [12], Ba 2 ZrO 3 F 2 . 0.5 H 2 O [13], Sr 3 Mn 2 O 6 F y with y = 1, 2 and 3 [14], Sr 2 TiO 3 F 2 and Ca 2 CuO 2 F 2 [15], Ln 1.2 Sr 1.8 Mn 2 O 7 F 2 (Ln = Pr, Nd, Sm, Eu, and Gd) [16]. The fluorine intercalation layered manganites LaSrMnO 4 F, La 1.2 Sr 1.8 Mn 2 O 7 F and La 1.2 Sr 1.8 Mn 2 O 7 F 2 can be prepared from LaSrMnO 4 (n = 1) and La 1.2 Sr 1.8 Mn 2 O 7 [9,16–19]. The fluorination proceeds in an oxidative manner with fluorine being inserted into interstitial sites between the (La/Sr)O rocksalt layers. This leads to a significant expansion in the c-axis as the separation between perovskite blocks is increased. The overall structure of the perovskite blocks is preserved, but is noticeably distorted with the apical Mn–O bond directed towards the rocksalt layers being significantly shorter than that in the center of the perovskite blocks [17]. A variety of fluorinating agents such as F 2 gas, NH 4 F, CuF 2 or poly(vinylidene fluoride) (PVDF) can be used to prepare LaSrMnO 4 F and Ln 1.2 Sr 1.8 Mn 2 O 7 F 2 [16–18]. To the best of our knowledge, there is neither previous report on the electrical properties of the LaSrMnO 4 F, La 1.2 Sr 1.8 Mn 2 O 7 F, Ln 1.2 Sr 1.8 Mn 2 O 7 F 2 [16–19] nor the synthesis of La 1.4 Sr 1.6 Mn 2 O 7 F 2 . Therefore, it would be interesting to investigate the electrical properties of La 1.4 Sr 1.6 Mn 2 O 7 F 2 . 2. Experimental details La 1.4 Sr 1.6 Mn 2 O 7 powder was synthesized by quantitatively mixing high-purity powders of La 2 O 3 , SrCO 3 and Mn 2 O 3 . The mixed and ground powders were then calcined at 1490 °C in oxygen for 36 h with intermediate grindings. The resulting powders were sintered at 1500 °C in air for 24 h, followed by annealing at 800 °C under nitrogen atmosphere. La 1.4 Sr 1.6 Mn 2 O 7 was ground with fluorinating agent (NH 4 F) (1:3 molar ratio). The resulting mixing powders were made into a parallel- epiped and then sintered at 350 °C for 24 h in the flowing N 2 gas [16–18]. The resulting parallelepiped sample was then ground into powders and then mixed with additional NH 4 F (1:1 molar ratio). The resulting mixture was ground and pressed into a parallelepiped, which is then sintered at 350 °C for another 24 h in the flowing N 2 gas [16–18]. In order to deprive some oxygen extent, the parallele- piped was loaded into a Pyrex ampoule along with titanium metal, which is used as a getter for trace amounts of oxygen in the ampoule. The ampoule was evacuated using a diffusion pump to reach a vacuum up to 10 5 to 10 6 torr and then sealed. After that the ampoule was placed into a box furnace for sintering at various tem- peratures. The phase purity of resulting powders was examined by a Shimadzu XRD-6000 powder X-ray diffractometer equipped with Cu Ka radiation. The electri- cal resistance measurements were carried out using the constant voltage method with a Keithley 6512 electrometer. A commercial superconducting quantum interference device magnetometer (Quantum Design) was used to characterize the magnetic properties of the samples. http://dx.doi.org/10.1016/j.jallcom.2014.10.099 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +886 4 723 2105x3337; fax: +886 4 728 0698. E-mail address: liucj@cc.ncue.edu.tw (C.-J. Liu). Journal of Alloys and Compounds 623 (2015) 324–327 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom