Ionic Motion in Materials with Disordered Structures
Mixed cation effects in alkali alkaline-earth silicate glasses
Compared to the investigations conducted on mixed-alkali glasses which until today stimulate the discussion
about the underlying microscopic jump mechanisms, little is known about the mobility of alkaline-earth ions in
silicate glasses. This is surprising since many silicate glasses used for technical applications, such as the float
glass produced by Pilkington plc. with 72 mol% SiO2, 13 mol%
Na2O, 9 mol% CaO, and 4 mol% MgO, contain alkali and
alkaline-earth oxides. Diffusion
of the radioactive isotopes 45Ca, 133Ba, and 85Sr in alkali alkaline-earth
silicate glasses of compositions A2O·2MO·4SiO2
with A=Li, Na, K, Cs and M=Mg, Ca, Sr, Ba was investigated at temperatures below the respective
glass-transition temperatures. The diffusion profiles of the radiotracers were recorded by means of an ion-beam
sputtering technique in conjunction with activity measurements. Analysis of the radiotracer profiles is based on
the thin-film solution of the diffusion equation and a contribution which reflects the instrumental broadening of
the profiles caused by sputtering effects. The diffusion coefficients of the alkaline-earth ions were determined
down to values of as low as 10-21m2s-1. Our diffusion data enables us
to determine accurate values for the activation enthalpy of diffusion and the pre-exponential factor. The
activation enthalpies reveal a distinct correlation with the radius ratio of the alkaline-earth ions to the alkali ions
and show that the alkaline-earth ions are most mobile when the radii of the alkaline-earth and alkali ions are
similar. Our results provide strong evidence that the faster alkali ions play an important role for the dynamics
and the diffusion of the slower alkaline-earth ions.
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