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⁺, SiO₄^4–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 Q_c value 76±5 kJ/mole for the glass state of all compositions is in excellent agreement with the Na₂SO₄ III Q_c value. The conductivity regime immediately after devitrification with Q_c 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 (Na_3.5Rb_0.5)UO₂(SO₄)₃. A gradual jump in conductivity accompanies the transition in the crystalline sample. The Q_c value is 75±5 kJ/mole for the (HT) phase conductivity in the heating mode but remains constant at 66±5 kJ/mole for the (LT) phase in the heating mode and for both and phases in the cooling mode.The excellent conductivity-volume, i.e. /V correlation is consistent with the free volume contribution to conductivity enhancement and the percolation-type mechanism of transport.This study received partial support from the Natural Sciences and Engineering Research Council of Canada