Evidence for phase transition in Cu-O ligands of the ladder material Sr14-xCaxCu24O41

Measurements of the temperature dependence of the resistance in Sr14-xCaxCu24O41, a novel cuprate having quasi-one-dimensional, two-leg Cu2O3 ladder planes and edge-shared CuO2 chains, show semiconducting behaviour with a transition to a smaller band gap occurring at temperatures where large surface resistance drops have been observed. At the same temperature changes in the Curie-Weiss dependence of the susceptibility are observed as well as changes of the g values of the EPR spectrum of the Cu2+ ions in the chains. The effects are suggested to be due to a structural phase transition in the CuO2 chains possibly resulting from a charge ordering transition in the chains.