Electrical conductivity and relaxation frequency of lithium borosilicate glasses Luciana F. Maia, Ana C.M. Rodrigues * Departamento de Engenharia de Materiais, Universidade Federal de Sa ˜o Carlos, Rod. Washington Luiz Km 235, C.P. 676 13565-905 Sa ˜o Carlos, SP, Brazil Received 16 October 2003; received in revised form 16 October 2003; accepted 16 February 2004 Abstract Electrical conductivity of glasses from the system 0.40Li 2 O0.60(xB 2 O 3 (1  x)Si 2 O 4 ) (0 V x V 1) were measured by impedance spectroscopy. The influence of the substitution of the first glass former by the second in electrical conductivity and in the parameters of the Arrhenius expression, i.e., activation energy and pre-exponential factor, is discussed. The electrical conductivity increases from the silica to the boron oxide region, with an anomaly at x = 0.3 but with no evidence of a mixed glass-former effect. The relaxation frequency, which is an intrinsic characteristic of each glass sample and is independent of geometrical parameters, was also deduced from impedance diagrams and are also presented for all compositions. D 2004 Elsevier B.V. All rights reserved. PACS: 66.30.Dn; 61.43.F; 72.80.r Keywords: Electrical conductivity; Relaxation frequency; Impedance spectroscopy; Lithium conductor; Mixed former effect 1. Introduction The ionic conductivity of glasses has been widely investigated in recent decades [1,2] and special attention has been devoted to lithium conductor glasses due to their applicability as solid electrolytes in lithium cells [3].The electrical conductivity of ionic glasses may be increased by dissolving halides or other salts in the glass structure [4,5] or by mixing a second glass former in an originally binary glass constituted by a glass-former and a glass-modifier. In fact, the non-additive increase in electrical conductivity, observed when a second glass-former is added to a binary glass, is known as mixed former effect [1]. The most classical and remarkable example of the mixed former effect was found at the borophosphate system [6] in which the conductivity of the most conductive ternary composi- tions are two orders of magnitude higher than that of the binary glasses. In this system, the binary compositions, lithium borate and lithium phosphate, present very similar electrical conductivities values. The mixed-former effect has also been found in other glass systems such as: silver borophosphate [7], lithium borotelurate [8] lithium silica- telurate [9] and lithium selenoborate [10]. However, if both binary glasses at the given system present very different electrical conductivities, a monotonic change (when the first former is substituted by the second one) of electrical conductivity with no maximum, may not be considered a mixed former effect. Although the lithium borosilicate glasses are mentioned as an example of mixed former effect [1], increased electri- cal conductivity in this system has been observed only in rapidly quenched glasses with concentrations of lithium oxide >60 mol% [11]. Nevertheless, glasses containing smaller concentrations of Li 2 O were investigated by Otto [12]. Although those results show an increase in electrical conductivity when SiO 2 is added to the Li 2 O.B 2 O 3 glasses, Otto did not present data for the entire glass system, precluding conclusions about the existence or absence of a mixed former effect. This work deals with the electrical conductivity and relaxation frequency of glasses in the lithium borosilicate system containing 0.40 mol% of Li 2 O. Silica was written as Si 2 O 4 to ensure that two atoms of the boron former were substituted by two atoms of the silicon former, thus keeping constant the lithium concentration and the number of former atoms in the ternary system 0.40Li 2 O0.60(x- 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.016 * Corresponding author. Tel.: +55-16-260-8524; fax: +55-16-261- 5404. E-mail address: acmr@power.ufscar.br (A.C.M. Rodrigues). www.elsevier.com/locate/ssi Solid State Ionics 168 (2004) 87 – 92