Local structure study of the underdoped-overdoped transition in YBa 2Cu 3O x (x=6.806-6.984) by yttrium EXAFS
We have measured the x-ray absorption-fine-structure (EXAFS) at the Y- K edge of YBa 2Cu 3O x 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 phase 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 xopt. 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 the O2,3 layer towards the Cu2 layer. Possible implications of these c-axis related displacements for the electronic structure of doped CuO 2 layers are discussed.