Structural origin of electrical conductivity of copper lithium metaphosphate glasses C. Mugoni a, ⁎, H. Jain b , M. Montorsi c , M. Montecchi a , A. Kovalskiy d , C. Siligardi a a Department of Engineering Enzo Ferrari (DIEF), University of Modena and Reggio Emilia, Via Pietro Vivarelli 10, 41125 Modena, Italy b Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, United States c Department of Science and Methods for Engineering, University of Modena and Reggio Emilia, Via Amendola 2, 42122 Reggio Emilia, Italy d Austin Peay State University, Department of Physics and Astronomy, Clarksville, United States abstract article info Article history: Received 22 February 2016 Received in revised form 16 April 2016 Accepted 3 May 2016 Available online xxxx We have sought the structural origin of the recently reported electrical conductivity of lithium copper phosphate glass system (50-x) Li 2 O-xCu 2 O-50 P 2 O 5 , as lithium oxide is gradually replaced by Cu 2 O. The structure of these glasses was determined by X-ray photoelectron spectroscopy, Fourier Transform Infrared Spectroscopy and Raman spectroscopy. The results show the presence of both mobile Cu + and relatively immobile Cu 2+ ions. The relative fraction of Cu 2+ [Cu 2+ / Cu tot ] and non-bridging oxygen increases with x; the latter indicating a ten- dency towards depolymerization of the network. On the other hand, there is enhanced crosslinking within the network as P \\ O ⋯Li bonds are replaced by relatively covalent P \\ O⋯Cu bonds. This leads to a more cross- linked structure and a progressive reduction of the optimum sites for the jumping of both Li + and Cu + , in agree- ment with the observed increase of the energy barrier for ion transport as Li 2 O is replaced by Cu 2 O. © 2016 Elsevier B.V. All rights reserved. Keywords: Phosphate glass Structure Raman XPS FTIR 1. Introduction Transition metal oxides (TMO) have been the subject of extensive research in the last decades due to the rich variety of crystal phases and oxidation states that the TM ions can present [1,2]. This leads to a complex scenario of possible properties, which make them appealing systems for magnetic recording, heterogeneous catalysis, insulators or semiconductors, as electrodes or electrolyte in solid state batteries de- pending on their structure and valence, etc. [3–6]. Glasses containing transition metal ions offer parallel prospects for potential applications in memory and photoconducting devices, cathode materials in batteries and magnetic materials [7–9], besides easy formability in various shapes, broader range of compositions, etc. The structure and properties (crystallization behavior, optical absorption and electrical conductivity) of these glasses are influenced by the oxidation-reduction equilibrium, thus on the valence of the metal ions. Hence, both the knowledge and the control of ratio between the lower and the higher valence state of the transition metal ions (TMI) play a fundamental role in the demon- stration and understanding of the ultimate performance. Often this con- trol is difficult to achieve since it depends on several parameters such as the atmosphere, the melting, and the transition-metal ion concentration [10,11]. In this study lithium meta-phosphate glasses containing copper (I) oxide were prepared and analysed in terms of structural characteris- tics using complementary experimental techniques, specifically X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. XPS assessed the chemical structure and estimated the ratio of the different valence states in the TM-oxide glasses, while Raman and FTIR provided structural informa- tion on the short range order [12–17]. The observed structure is then used to explain the electrical properties investigated earlier [18], where the copper lithium phosphate glasses (named copper glasses - CG) exhibited the mixed mobile ion effect (MMIE) when Li 2 O was re- placed by Cu 2 O. 2. Experimental procedure 2.1. Glass preparation The glasses of (50-x)Li 2 O - xCu 2 O - 50P 2 O 5 (x = 5, 10, 15, 20 mol%) compositions were prepared, as listed in Table 1 along with their no- menclature. The raw materials including Cu 2 O (98%, Sigma Aldrich), ammonium dihydrogen orthophosphate (NH 4 H 2 PO 4 , 98%, Sigma Al- drich) and lithium carbonate (Li 2 CO 3 , 98%, Sigma Aldrich) were mixed for 15 min and heated in a muffle furnace from room temperature to 400 °C and maintained at that temperature for 3 h in order to decom- pose NH 4 H 2 PO 4 . The mixture was then heated to and kept at 900 °C for 1 h to ensure good homogeneity of the melt. After that, it was quickly Journal of Non-Crystalline Solids 447 (2016) 91–97 ⁎ Corresponding author. E-mail address: consuelo.mugoni@unimore.it (C. Mugoni). http://dx.doi.org/10.1016/j.jnoncrysol.2016.05.009 0022-3093/© 2016 Elsevier B.V. All rights reserved. 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