EFFECT OF PRESSURE ON THE ELECTRICAL-CONDUCTIVITY, NA+-ION TRANSPORT, IN NA2SO4

The a.c. electrical conductivity of polycrystalline Na2SO4 has been measured at T between 418 and 914 K and at P between 7 and 50 kbar. The decrease in sigma with P and the diminishing sigma jump at the orthorhombic (III) --> hexagonal (I) transition are consistent with a simple volume-controlled cationic conduction mechanism. A sharp break in the P-dependence of Q(c) at 35 kbar may indicate the presence of a new, distinct, high P, T phase region between phases I and VIII. DELTA-V(ortho)double dagger is strongly T-dependent and decreases through the transition to a value of DELTA-V(hex)double dagger which is independent of T up to 800 K. The calculated compressibility of the orthorhombic phase is approximately 15% higher than the expanded hexagonal phase near the transition. Both the compressibility and strong T-dependence of DELTA-V(ortho)double dagger indicate that thermal expansion plays a much more important role in the enhancement of sigma(ortho) than in sigma(hex).