LOCAL-STRUCTURE STUDY OF THE UNDERDOPED-OVERDOPED TRANSITION IN YBA2CU3OX (X=6.806-6.984) BY YTTRIUM EXAFS

We have measured the x-ray absorption-fine-structure (EXAFS) at the Y- K edge of YBa2Cu3Ox for x=6.806, 6.826, 6.886, 6.947, 6.968, 6.984 at T=20-300 K. Optimum doping is found to be a notable point in the x=T p hase diagram, also concerning the c-axis related displacements of the planar O2,3 and Cu2 atoms. The O2,3-Cu2 spacing along c is found to be largest near x(opi). In the underdoped regime doping increases the O2 ,3-Cu2 spacing by displacing the Cu2 layer from the O2,3 layer. In the overdoped regime doping reduces the O2,3-Cu2 spacing by displacing th e O2,3 layer towards the Cu2 layer. Possible implications of these c-a xis related displacements for the electronic structure of doped CuO2 l ayers are dicussed.