Spectroscopic Investigations of the Network Structure in Borovanadate Glasses
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ABSTRACT Borovanadate glasses with mixed conductivity, i.e., simultaneous ionic and electronic carriers have been obtained in the system (100-x)[B 20 3-Li 2O]-xV 20 5 (0 • x _< 30 mole%). To understand the origin of the conductivity in these glasses, the compositional dependence of the network structure has been studied by Raman and infrared spectroscopy. INTRODUCTION In recent years, fast ionic conducting glasses and cathodic materials based on semiconducting oxide glasses have entered the field of interest as promising candidates in the development of high energy storage systems. According to previous works, lithium borate glasses exhibit a high ionic conductivity which depends on the lithium content and on the nature of the negative sites [1]. Glasses with vanadium are known to have semi-conducting properties due to the presence of vanadium in two valence states, V4+ and V 5+. The hopping of the unpaired 3d1 electrons from V4+ to V5+ site is responsible of the electrical transport [2]. Also, mixed conductivity, i.e. simultaneous ionic and electronic conductivity, was observed in borovanadate glasses [3] and had revealed the possibility of using them as cathode in battery applications [4]. The objective of the present work is to study by Raman scattering and infrared spectroscopy, the compositional dependence of the network structure in borovanadate glasses near the metaborate composition in a wide range of V 20 5 composition (0 - 30 mole%). EXPERIMENTS Glasses of chemical formulae (l00-x)[B2 0 3-Li 2O]-xV 2 0 5 (0 • x < 30 mole%) have been prepared by the process described in ref. 3. For optical experiments, the samples were cut in parallelepipeds with polished faces (4x4x2mm 3) Raman scattering spectra were recorded between 4 and 1800 cm-1 using a Jobin-Yvon U 1000 spectrometer. The excitation source was the 514.5 nm line of an argon-ion laser operating at a power of 100 mW. The scattered light was detected by a cooled photomultiplier ITT FW 130 coupled with a computerised photon-counting system. To get a high signal-noise ratio, each spectrum is the result of the average of 10 successive scans with a spectral resolution of 2 cm' . The Raman spectra in the spectral range 600 to 1200 cm- have been fitted by using the GRAM/386 software from Galactic Industries Co. The curve fitting is based on the original algorithm of non-linear peak fitting described by Marquardt and known as the Levenberg-Marquardt method [5]. Infrared spectra were obtained with the use of a Fourier transform Bruker IFS 113 vacuum interferometer. The reflected interferograms were detected by a nitrogen cooled MCT 18-21 detector. Each spectrum is the average of 100 scans with recorded with a resolution of 2 cm' in the frequency range from 500 to 4000 cm1. 477 Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society
RESULTS AND DISCUSSION Unpolarised Raman spectra of the borovanadate glasses are presented in Fig. 1. Spectra obtained for other intermediate concentrations are shown in Fig. 2 in the range 600 to 1200 cm1. In the pure
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