The mixed alkali effect in IOX SLS glass processing

When comparing the change in DOL between compositions A and B in Table 2, it is evident composition B, which had no K₂O in the base starting glass, showed much smaller effects. We attribute the larger effects in composition A to the mixed alkali effect.^a

In terms of ionic conductivity, a simple definition of the mixed alkali effect is that the conductivity decreases upon substitution of a second alkali. While this definition is technically correct, it does not explain why this decrease occurs.

The reason for this decrease is that ionic conductivity is controlled by the fastest-diffusing ion, i.e., sodium, while interdiffusion of the ions is controlled by the slowest diffusing ion, i.e., potassium. As more potassium ions are substituted for sodium ions, the potassium diffusion coefficient increases while the sodium diffusion coefficient decreases due to blocking by the potassium ions.

As we know from Equation 1, the diffusion coefficient D is based on the effective diffusion coefficients of the two mobile ions. Because this value is larger in composition A, which contained K₂O in the base starting glass, the ions can achieve a deeper DOL than in composition B.

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