FAST NA+ ION CONDUCTIVITY IN GLASS, INTERMEDIATE AND CRYSTALLINE PHASES OF NA4UO2(SO4)3

The ac electrical conductivity, DSC calorimetry and density data for pure Na4UO2(SO4)3 and for compound incorporating guest ions Rb+, Cd2+, Gd3+, SiO44-in the glass, quasi-crystalline intermediate and crystalline phases are reported. The glass phase conductivity data show an increase in Na+ conductivity by a factor ≧103 relative to pure crystalline Na2SO4 in the low temperature (LT) region, i.e. 180°C. There is no onset of phase transition up to ∼260°C. The distinct conductivity regimes prior to devitrification in the glass suggest that higher energy or excited structural states/configurations can exist in the glass phase. The apparent activation energy Qc value 76±5 kJ/mole for the glass state of all compositions is in excellent agreement with the Na2SO4 III Qc value. The conductivity regime immediately after devitrification with Qc of 40±3 kJ/mole represents the stable intermediate phase. The conductivity of the final product of devitrification on cooling resembles crystalline behavior except for (Na3.5Rb0.5)UO2(SO4)3. A gradual jump in conductivity accompanies the β⇆α transition in the crystalline sample. The Qc value is 75±5 kJ/mole for the α(HT) phase conductivity in the heating mode but remains constant at 66±5 kJ/mole for the β (LT) phase in the heating mode and for both α and β phases in the cooling mode. The excellent conductivity-volume, i.e. Δσ/ΔV correlation is consistent with the "free" volume contribution to conductivity enhancement and the percolation-type mechanism of transport. © 1990 Springer-Verlag.