The electrical conductivity of calcium and strontium oxides

Abstract

Measurements have been carried out of the electrical conductivity of single crystals of calcium oxide from 300 to 1400°K, and of strontium oxide from 600 to 1050°K. It was measured at 1·59 kc/s for both oxides, and also with d.c. for calcium oxide. For calcium oxide the conductivity was 1 × 10-8 ω-1 cm-1 at 1050°K and for strontium oxide it was 3 × 10-8 ω-1 cm-1 at 900°K. This was predominantly bulk conductivity, but with some surface conduction through a hydroxide layer on the surface of at least some of the crystals. The high temperature activation energies were 3·5 eV and 2·0 eV respectively. The evidence is discussed for considering that these thermal activation energies may refer to charge transport by electrons `hopping' from donor to donor through a compensated semiconductor containing approximately equal concentrations of interstitial negative oxygen ions and oxygen ion vacancies. Alternatively, they may refer to conduction by negative oxygen ions migrating through the crystal. The dielectric constants of these crystals at 1·59 kc/s were estimated to be at least 7·5 for calcium oxide and 6·5 for strontium oxide. Results are also given for the conductivity of sintered powders of strontium oxide: these acted as pore conductors with low temperature activation energies from 0·25 to 1·15 eV (below about 550°K) and with high temperature activation energies from 0·8 to 1·45 eV, both being dependent on the previous heat treatment of the oxide.