Defect Chemistry in Solid State Electrochemistry
Solid electrolytes and mixed ionic–electronic conductors (MIECs) have been known for quite some time, going back at least to Faraday’s observations in the 1830s that lead fluoride when heated to red hot conducts electricity similar to platinum. The defect chemistry of ternary compounds has also attracted widespread attention, and the more fundamental concepts were developed mainly in the 1960s and 1970s. The quantitative relations between the point defect concentrations and the compound activities are very useful in interpreting electrical properties of solid electrolytes and MIECs. Defect chemistry is a chemistry within the solid state that is analogous to the long-familiar chemistry in the liquid phase, and arises from departures from the ideal crystal structure which are thermodynamically unavoidable, the point defects. Point defects fall into two main categories: intrinsic defects, which are internal to the crystal in question, and extrinsic defects, which are created when an impurity atom or ion is inserted into the lattice.