Temperature and pressure behavior of the structural relaxation time in glass formers R. Casalini a, * , S. Capaccioli a , M. Lucchesi a , M. Paluch b , S. Corezzi c , P.A. Rolla a a INFM e Dipartimento di Fisica, Universit  a di Pisa, Via Buonarroti 2, I-56127 Pisa, Italy b Institute of Physics, Silesian University, ul.Uniwersytecka 4, 40-007 Katowice, Poland c INFM e Dipartimento di Fisica, Universit  a di Perugia, Via Pascoli, I-06100 Perugia, Italy Abstract Starting from the original model of Adam and Gibbs, which gives a relationship between the structural relaxation time s and the configurational entropy, an equation to describe the temperature and pressure behavior of s has been recently derived. Herein a test of the validity of this new equation is presented by means of dielectric, calorimetric and volumetric data. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 64.70.Pf; 64.90.1b; 77.22.Gm 1. Introduction The model of Adam and Gibbs (AG) gives an expression of the relaxation time s in terms of the configurational entropy S c [1] s ¼ s 0 exp C AG Dl TS c   ; ð1Þ where Dl is the free energy barrier (per molecule per cooperative rearranging region) to rearrange- ments, T is the absolute temperature, s 0 is the re- laxationtimeinthelimitofhightemperaturesand C AG isaconstant.Inthemodel S c isdefinedasthe excess entropy, S c ¼ S melt  S crystal ,anditmeasures all apart from the vibrational contribution to the entropyofthemelt.Byusingthisdefinition S c can be estimated as [2] S c ðT ; P Þ¼ Z T T k DC P ðT 0 Þ T 0 dT 0  Z P 0 D oV oT   P 0 dP 0 ; ð2Þ where DC p and D oV oT   P aretheexcessheatcapacity and excess thermal expansivity of the melt respect to the crystal, respectively. In particular the temperature behavior of DC p has been found to be well described as DC p ðT Þ¼ k=T (where k is a constant) and consequently at P ¼ 0 the temperature behavior of S c ðT Þ¼ S 1  k=T where S 1 is the limit of S c at very high tem- peratures [3,4]. By substituting this expression of Journal of Non-Crystalline Solids 307–310 (2002) 264–269 www.elsevier.com/locate/jnoncrysol * Corresponding author. E-mail addresses: riccardo.casalini@df.unipi.it, casalini@ ccs.nrl.navy.mil (R. Casalini). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01474-6