Phaseseparation in overdoped Y{sub 1-0.8}Ca{sub 0-0.2}Ba{sub 2}Cu{sub 3}O{sub 6.96-6.98}

The dimpling in the CuO{sub 2} planes of overdoped Y{sub 1-y}Ca{sub y}Ba{sub 2}Cu{sub 3}O{sub 6.96-6.98}, (y=0.02-0.2) has been measured by x-ray absorption-fine-structure spectroscopy (Y-K EXAFS). A step-like decrease around 12% Ca indicates a percolation threshold for distorted sites of 5 cells, and thus phase segregation. We conclude the charge carriers added by substitution of Y{sup 3+} by Ca{sup 2+} to be trapped at the Ca sites and their nn environment.