Relaxation Processes and the Mixed Alkali Effect in Alkali Metasilicate Glasses
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alkali effect.
INTRODUCTION During the past years, many studies, both experimental and theoretical, on slow dynamics near the glass transition temperature have been performed mainly for the so-called "fragile" glasses [1], which have no directional bonds and show an abrupt increase in viscosity upon super-cooling. On the other hand, systems having covalent or hydrogen-bonded frameworks are often classified as "strong" glass systems. In the present work, the slow and fast dynamics of lithium metasilicate (Li.jSiO3) axe examined by means of molecular dynamics (MD) simulation, where the simulation is extended up to a 1 ns region. The
system Li 2 SiO 3 may be regarded as "fragile" for Li+ ions and "strong" for Si and 0 atoms of the chain structure. The study of the "mixed alkali effect" in the mixed alkali system is also helpful for understanding the mechanism of the dynamics in the glass. We have found that not merely the interception of the jump path of each kind of alkali ion but the cooperative jumps play an important role for the mixed alkali effect. The latter was not paid attention previously.
91 Mat. Res. Soc. Symp. Proc. Vol. 455 p1997 Materials Research Society
METHOD The MD calculation was performed in the same way as in our previous studies [2, 3, 4, 5, 6, 7]. The periodic cube contained 144 Li, 72 Si and 216 oxygen atoms. In a mixed alkali system, the cube contained 72 Li and 72 K instead of 144 Li. A Gilbert-Ida [8] type potential function plus an r-6 term was used; i.e.
oij = zizje 2/r + fo(bi + bj)exp[(ai + aj - r)/(b, + bj)] - cici/r 6 ,
(1)
where z is the effective charge number, e the elementary charge; a, b and c are the parameters characteristic of each atom and fo is a normalization constant (4.184 kJ A--mol-1 ). The potential parameters used in this work were previously derived from an ab initio MO calculation [3], and their validity has been checked in the liquid, glassy and crystal states under constant pressure conditions. The system was equilibrated at 4,000 K for more than 10,000 time-steps starting from a random configuration, and then each system was cooled down to lower temperatures. Simulation runs were carried out at 3,000, 2,000, 1,673, 1,173, 973, 800 and 700 K. The step time was 1 fs from 4,000 to 2,000 K and 4 fs from 1,673 K to 700 K. The velocities of all particles were initially set to zero at each temperature, and several thousand steps were carried out at constant temperature and pressure (0.1 MPa). Then, the sizes of the periodic cube were fixed to the equilibrated or quasi-equilibrated volumes. The MD runs (I, II) were carried out at 700 K at a constant energy condition from different initial configurations. Steps of the run I and II were 200,000 and 250,000, respectively. For the mixed alkali system, simulation at 800 K was also performed to enhance the jump frequency. Each system consisted of branched-chain structures composed of Si0 4-tetrahedral units and alkali ions surrounded by oxygen atoms. The glass transition temperature, T, of Li 2 SiO 3 system obtained by
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