Electrical properties of MREGeO4 (M=Li, Na, K; RE=rare earth) ceramics

Fourteen kinds of alkali-metal rare earth germanate ceramics, MREGeO4 (M=Li, Na, K; RE=rare earth), were synthesized and their electrical properties were investigated. The crystal structures of the major phases of MREGeO4 could be classified into four groups; hexagonal (apatite type), tetragonal, orthorhombic (olivine type) and orthorhombic (β-K2SO4 type). The tetragonal group exhibited higher ionic conductivity than the other groups in the range 300 to 700◦C. The highest conductivity was achieved for the LiNdGeO4 having a tetragonal structure (2.95 × 10−2 S cm−1 at 700◦C). Analyses on the basis of the Nernst’s equation indicated that the two electron reaction associated with carbon dioxide molecule takes place at the detection electrode above 400◦C regardless of the kind of electrolyte. The electromotive force, EMF, of the potentiometric CO2 gas sensors designed using MLaGeO4 (M=Li, Na, K) increased linearly with an increase in the logarithmic value of CO2 partial pressure, in accordance with the Nernst equation. C 2005 Springer Science + Business Media, Inc.