THERMAL-PROPERTIES OF LITHIUM-SULFATE

The thermal conductivity and diffusivity of lithium sulphate have been measured simultaneously, using the transient plane source technique o ver the temperature range 300-900 K. The thermal conductivity decrease s slowly up to about 640 K, whereupon a distinct rise occurs, indicati ng the onset of a pre-transitional behaviour, which causes a continuou s growth of the conductivity up to the structural phase transition at 851 K, whereupon a very sharp increase occurs. A similar behaviour has been observed for the thermal diffusivity, for which a very sharp dip occurs at the transition point due to the exceptionally large transit ion enthalpy. The pre-transitional behaviour of heat transport is asso ciated with the librational disorder of the sulphate anions known from Raman scattering studies of both phases (and neutron scattering from the cubic phase), whereas the translational disorder of lithium cation s is of hardly any importance. It is thus possible to link the 'paddle -wheel' concept of ion migration in the cubic phase to the enhancement of heat transport observed in the 'pre-transition' region, as well as to the large difference in heat-transport rates between the monoclini c and cubic phases.