LOCAL STRUCTURE STUDIES OF THE UNDERDOPED-OVERDOPED TRANSITION IN YBa2Cu3Ox Measurement of the yttrium x-ray absorption-fine-structure

We have measured the extended x-ray absorption-fine-structure (EXAFS) at the Y-K edge of YBa2Cu3Ox for x=6.801, 6.947, 6.968, 6.984 at T=20-300 K. The Y–Cu2 pairs vibrate harmonically but freeze out in the superconducting phase. The Y-O2,3 pairs exhibit strong anharmonicities with a singularity at Tc. With increasing oxygen concentration the Cu2 layer shifts along c towards the Ba layer. Optimum doping is a notable point in the phase diagram, also concerning the structural degrees of freedom. Here the O2,3–Cu2 spacing is largest, the relative displacements of O2,3 and Cu2 layers along c invert upon doping, and the Cu2 position along c is independent on temperature in the superconducting phase.