Mass transport in low melting point ionic solids.

The mass transport properties or the low melting point salts, sodium bromate and cadmium iodide, are examined. A detailed atudy of the purification and growth of large single crystals of both these materials are reported. The ionic conductivity of both these solids are investigated. Sodium bromate, because of its high resistance, is examined by D.C, techniques. The conductivity of cadmium iodide is quite high and is measured by an A,C, method. The errors introduced by the D.C, method are quite large; however, from the limited number of conductivity runs performed, it is deduced that, with sodium bromate, sodium vacancies are the mobile species (A h.;= 0,47 eV), The enthalpy of formationof the predominant defects is shown to be 1,41 eV. Thermal decomposition of the bromate is shown to be the major obstacle to further studies and to the elucidation of whether the defects are Schottl.;r or Cationic Prenlcel in origin.The ionic conductivity of cadmium iodide is shown to be anisotropic. The conductivity in pure crystals, in the direction of the c axis, is found to be more typical of a heavily doped ionic conductor than a pure one. The equations describing the three regions are Region Comparison of the conductivity and diffusion results (via the Nernst-Einstein equation), in the direction of the c axis, shows that cadmium iodide is a Schottky disordered solid 'iwlth both cadmium and iodine vacancies appreciably mobile, and with the former dominant at high temperatures.The expected aliovalent impurity dependence is not found and a number of postulates are proposed to explain this effect. The ionic conductivity in a direction perpendicular to the c axis is uncharacteristic, only showing evidence of an intrinsic range v/ith doped crystals, A tentative explanation, invoking the effects of dislocations, is proposed to explain this phenomenon. Polarisation current measurements on cells of the Type in directions both parallel and perpendicular to the c axis, show that the electronic contribution to the total A,C. conductivity is, at high temperatures at least, minimal.