Refinement of Incommensurate Misfit Compounds: Sr_14-xCa_xCu₂₄O₄₁

Rietveld refinements of incommensurate misfit compounds Sr0.4Ca13.6Cu24O41 and Sr8Ca6Cu24O41 at 10 and 300K allow to calculate the effective valences of the copper ions. For the copper located in CuO4 squares the valence remains constant over the modulation period in Sr0.4Ca13.6Cu24O41 which becomes superconducting at high pressure, whereas it its strongly modulated in Sr8Ca6Cu24O41.