Phase equilibria, electrical conductance, and density in the glass-forming system zinc chloride + pyridinium chloride. A detailed low-temperature analog of the silicon dioxide + sodium oxide system

In a study of ZnCl₂-based binary chloride melts which may serve as analogs of the well known and technologically important glass-forming binary systems based on SiO₂ and BeF₂ as first component, a detailed phase-equilibrium, electrical conductance, and density study of the system zinc chloride + pyridinium chloride has been carried out. In contrast to the better known zinc chloride + alkali halide systems, this present system reproduces in great detail the phase relations and physicochemical behavior of the classic Na₂O + SiO₂ system, though at temperatures reduced by a factor of about 1/3. Electrical conductance data have been analyzed in terms of the three parameter equation κ = AT^-1/2 exp B/(T -T₀), and the "ideal" glass transition temperature T₀ found to closely parallel the experimentally measured glass transition temperature T_g. T₀ and T_g show complex composition dependences. A minimum at 33.3 mol % ZnCl₂ is interpreted in terms of formation of the orthochlorozincate ion ZnCl₄^2-, an approximately linear increase from 33.3 to 66 mol % is probably due to the formation of polymeric chains based on linked ZnCLi tetrahedra, and a plateau region at ZnCl²-rich compositions (65-90 mol %) is associated with the tendency to, or occurrence of, subliquidus liquid-liquid phase separation. The classical concept of "network-breaking" satisfactorily explains dramatic changes in conductivity in the region 90-100 mol % ZnCl₂. There is some suggestion that the rapid decrease in "activation energy" for transport in this region may be associated primarily with changes in equilibrium thermodynamic properties (configurational heat capacity) rather than with changes in a purely kinetic energy barrier as is generally assumed.