Relaxation and Prigogine-Defay ratio of compressed glasses with negative viscosity-pressure dependence.

Differential scanning calorimetry and density measurements were employed to study relaxation processes in compressed NaAlSi(3)O(8) and Na(0.34)Ca(0.31)Al(0.96)Si(3.04)O(8) glasses with negative dependence of viscosity (eta) on pressure (p). Isobaric configurational heat capacities, coefficients of thermal expansion and compressibility of the melts, and real and apparent fictive temperatures are reported for the pressure regime from 0.1 to 700 MPa and cooling rates from 0.1 to 400 K/min. The onset of glass transition, measured on the compressed glass under normal pressure, is found to shift with increasing pressure of freezing in accordance with in situ viscosity data. Based on the pressure-derivative of the compression-induced excess enthalpy (or apparent fictive temperature), a condition for which a negative dependence of fictive temperature on pressure occurs is developed. It is further shown that in this case, also d eta/d p is negative. We suggest that in the region of glass transition, glasses with large Prigogine-Defay (PD) ratio have a high probability to also exhibit a negative dependence of viscosity on pressure. However, the criterion cannot be applied inversely: particularly glasses with a high sensitivity of short-range structural parameters to pressure may exhibit a relatively low PD ratio, but negative d eta/d p.