Lithium Ion Conducting Boron-Oxynitride Amorphous Thin Films: Synthesis and Molecular Structure by Infrared Spectroscopy and Density Functional Theory Modeling M. Dussauze,* ,†,‡ E. I. Kamitsos,* ,† P. Johansson, § A. Matic, § C. P. E. Varsamis, † D. Cavagnat, ‡ P. Vinatier, ∥ and Y. Hamon ∥ † Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 116 35 Athens, Greece ‡ Institut des Sciences Mole ́ culaires - UMR 5255 CNRS, Universite ́ Bordeaux I, 351 Cours de la Libé ration, 33405 Talence Cedex, France § Department of Applied Physics, Chalmers University of Technology, SE-412 96, Gö teborg, Sweden ∥ CNRS, Universite ́ de Bordeaux, ICMCB site de l’ENSCBP-IPB, 87 Avenue du Dr. A. Schweitzer, Pessac, F-33608, France * S Supporting Information ABSTRACT: Li ion containing oxynitride amorphous thin films are promising materials for electrochemical applications due to their high ionic conductivity, mechanical stability and chemical durability. Here we report on the preparation of Li boron-oxynitride (LiBON) amorphous thin films by rf sputtering of Li-diborate and Li-pyroborate targets in nitrogen atmosphere. The materials produced were subsequently studied by infrared transmittance spectroscopy assisted by density functional theory calculations using representative Li boron-oxide and boron-oxynitride clusters. The combination of experiments and calculations allows us to propose accurate vibrational assignments and to clarify the complex infrared activity of the LiBON films. Both experimental and calculated spectra show that nitrogen incorporation induces significant structural rearrangements, manifested mainly by a change in boron coordination number from four to three, and by the formation of boron−nitrogen-boron bridges. The nature of boron−nitrogen bonding depends on the composition of the sputtering target, with an exponential relationship adequately describing the dependence of B−N stretching frequency on bond length. Besides bonding to two boron atoms by covalent bonds, the nitrogen atoms interact also with Li ions by participating in their coordination sphere together with oxygen atoms. Likely, boron−nitrogen bonding in LiBON films facilitates Li ion transport due to induced charge delocalization within the boron−nitrogen-boron bridges and reduced electrostatic interaction with the Li ions. 1. INTRODUCTION Thin films of ionic conducting oxide glasses constitute potential candidates for solid-state electrochemical applications including batteries, sensors, and electrochromic displays. 1−5 Since ionic conductivity is a key physical property for these applications, the field early focused on improving ionic conductivity by techniques that allow for the development of films with large contents of mobile metal ions. This is, for example, the case of xLi 2 O-(1−x)B 2 O 3 thin films developed by thermal evaporation with the Li 2 O mole fraction spanning the range 0.52 ≤ x ≤ 0.85. 6 The ionic conductivity of ∼0.3 μm thick films was found to increase exponentially with lithium ion content and to reach values as high as 1 × 10 −7 Ω −1 cm −1 at room temperature for x = 0.72. This enhancement of ionic conductivity was attributed to the increasing density and mobility of Li ions as the three- dimensional borate network is gradually disrupted at high Li 2 O contents. A thickness-dependent ionic conductivity was reported 7 for Li-borate films developed by ion-beam sputtering from a 0.2Li 2 O-0.8B 2 O 3 glass target. The direct current (dc) ionic conductivity increased by about 3 orders of magnitude upon reducing the film thickness from 120 to 7 nm. Possible mechanisms proposed to explain this effect included structural modifications at the interfaces, formation of space-charge regions at the interfaces and establishment of randomly distributed ion-conducting channels within the glassy film. Doping Li-borate glasses with lithium salts also improves the ionic conductivity. 1,2 Amorphous films of composition xLi 2 SO 4 - (1−x)LiBO 2 with x = 0.4 to 0.8 were developed by radio frequency (rf) magnetron sputtering, and found to exhibit Received: February 12, 2013 Revised: March 16, 2013 Published: March 21, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 7202 dx.doi.org/10.1021/jp401527x | J. Phys. Chem. C 2013, 117, 7202−7213