Local structure studies of the underdoped-overdoped transition in YBa_2Cu_3O_x. Measurement of the yttrium y-ray absorption fine structure
We have measured the extended x-ray absorption-fine-structure (EXAFS) at the Y-K edge of YBa_2Cu_3O_x for x=6.801, 6.947, 6.968, 6.984 at T=20-300 K. Across the underdoped-overdoped transition the Y-Cu2 pairs exhibit harmonic vibrations which freeze out in the superconducting phase, whereas the Y-O2,3 pairs exhibit strong anharmonicities with a singularity at T_c. In the underdoped regime the average spacing O2,3-Cu2 increases with x due to displacements of Cu2 towards Ba, but the average position of O2,3 along c is almost unaffected. Optimum doping is a notable point in the x-T phase diagram, also concerning the lattice degrees of freedom. Here the O2,3-Cu2 spacing (buckling) is largest, and in the superconducting phase the Cu2 position along the c-axis is independent on temperature. In the overdoped regime increasing x displaces the Cu2 along c further towards Ba, but the O2,3 layer starts to shift in the same direction, thus decreasing the O2,3-Cu2 spacing.