Change of conductivity in compounds of Sr 14Cu 24O 41 structure by substitution of calcium for strontium

Abstract

Compounds of the Sr 14− x Ca x Cu 24O 41 structure ( x = 0 to x = 7.0) have been prepared by annealing for 50 h at 900°C in air. The resistivities and thermoelectric power of the compounds at 273 K decrease with the substitution of Ca for Sr. Temperature dependence of the resistivity can be described by an activation law in certain temperature regions. The activation energies decrease with an increasing x value. Making a comparison between the activation energy of the resistivity and thermoelectric power, it has been found that the conductivity of the specimen from x = 0 to x = 2.8 is caused by impurity conduction, and that from x = 4.2 to x = 7.0 by hopping conductivity.