Crystalline solid electrolytes and defect chemistry: Some novel thermodynamic and kinetic results

This work addresses deviations from the most simple defect chemical situation which implies homogeneity, local equilibrium and low defect concentrations in terms of conductivities and diffusivities. (i) For SrTiO3 as a model material it is shown that the low temperature defect chemistry can be understood by taking into account the freezing of the oxygen incorporation reaction within the T-range considered. (ii) Internal defect chemical interaction reactions between defects (such as the valence change of dopants) can have a significant kinetic (buffer) effect on chemical diffusion which has not been paid adequate attention to, so far. Applications to SrTiO3 and ZrO2 are discussed. (iii) Long-range interactions of ionic defects give rise to premelting conductivity enhancements prior to transition points into the molten or partially molten state. It is shown that for AgCl, AgBr, AgI, PbF2 these effects can be understood by a cube-root law. The same relation leads to a surprisingly good prediction of the phase transition temperatures. In this way the inner correlation between lattice energy, dielectric constant, ionic conduction and (pre-)melting temperature is highlighted. (iv) As recent application of defect chemistry in boundary regions the conductivity effects of cation and anion conductors (AgCl, CaF2) due to exposure to Lewis-acidic and Lewis-basic gases (NH3, BF3, SbF5) is discussed. It is shown that in this way a control of the interfacial chemistry is possible. The relevance for chemical sensing of acid-base active gases is addressed. Finally, the problem of phase transition in boundary regions and the perspective of nano-ionics are touched upon.