Influences of interlayer distance and cobalt oxidation state on superconductivity of Na xCoO 2

Abstract

Sodium cobalt oxide with bilayer-hydrate structure, Na x CoO 2 · yH 2O ( x≈0.35, y≈1.3), is the first superconductor in cobalt oxide system. Several sodium cobalt oxide hydrates related to the superconducting one were synthesized in order to explore the influence of the structure and composition upon the superconductivity. Partial dehydration brought about shrinkage of the interlayer distance to yield a monolayer hydrate, in which the superconductivity vanished. Modification of the carrier density, or lowering the Co oxidation state, lowered the superconducting transition temperature from 5 to 2 K.