Pressure dependence of the ionic conductivity of Na- and Na–Rb borate glasses Á.W. Imre a,c, ⁎ , S. Voss a,1 , F. Berkemeier a,c , H. Mehrer a,c , I. Konidakis b , M.D. Ingram b,c a Institut für Materialphysik, Universität Münster, Wilhelm-Klemm-Straβe 10, D 48149 Münster, Germany b Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, UK c Sonderforschungsbereich 458, Germany Received 2 November 2005; received in revised form 13 February 2006; accepted 4 March 2006 Abstract The ionic conductivity of Na- and Na–Rb borate glasses is investigated as a function of hydrostatic pressure. Activation volumes of the dc conductivity are obtained from pressure-dependent ac conductivity measurements at a constant temperature of 180 °C. The activation volume of Na borate glasses decreases with increasing alkali content. It depends linearly on a separation parameter which is defined as the ratio of the average ionic distance and the average network atom distance. The activation volume of Na–Rb borate glasses increases from both end-members towards the mixed-alkali regime and passes through a maximum near the relative content Na/(Na + Rb) = 0.4. In addition to mobility anomalies, observed earlier in these borate glasses, the maximum in the activation volume is a further important fingerprint of the mixed-alkali effect. © 2006 Elsevier B.V. All rights reserved. Keywords: Alkali borates; Mixed-alkali effect; Impedance spectroscopy; Ionic conductivity; Pressure dependence 1. Introduction Ionic conductivity and diffusivity are thermally activated, and temperature-dependent measurements reveal important features of ionic transport in glasses [1]. Pressure-dependent measurements provide complementary information to the temperature-dependent measurements. An important parameter deduced therefrom is the activation volume. Ionic conductivity and diffusivity below the glass transition temperature depend exponentially on temperature and pressure. The pressure dependence of ionic conductivity and diffusivity is much smaller than the temperature dependence, at least for pressures available in standard high-pressure devices. Never- theless, the effect of pressure on conductivity and diffusivity contains information on the ion-jump dynamics and the mechanism of diffusion not accessible from temperature- dependent measurements. High-pressure experiments have already played a key role in the identification of atomic diffusion mechanisms in several crystalline and glassy metals [2–6], in ionic crystals [7,8], and recently also in ionic glasses [9,10]. For interstitial diffusion in a crystal such as carbon diffusion in iron small activation volumes of migration are associated with the jump process, because interstitial diffusion proceeds without formation of thermal defects like vacancies. If the diffusion process is mediated by thermal vacancies as in the case of self-diffusion in metals, the activation volume is the sum ΔV = V F + V M of the formation volume V F and the migration volume V M of the vacancy. Typical values of the activation volume for vacancy-mediated diffusion in metals lie between 0.5 and 1 atomic volumes [2,3]. Large activation volumes of the order of one molar volume are also characteristic of ionic conduction in the intrinsic region of ionic crystals with Schottky disorder [7]. In glasses the translational symmetry is missing and defects like vacancies are not well defined. However, a short-range order exists. Recent molecular dynamics simulations have shown that in ionic glass the number of sites for ions is just 8– Solid State Ionics 177 (2006) 963 – 969 www.elsevier.com/locate/ssi ⁎ Corresponding author. Tel.: +49 251 8339001; fax: +49 251 8338346. E-mail address: imre@nwz.uni-muenster.de (Á.W. Imre). 1 At present: Infineon Technologies AG, P.O. Box 80 09 49, 81609 Munich, Germany. 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.009