A Raman investigation of isotope exchange in YBa2Cu3O7−x

Abstract

Raman scattering experiments have been carried out on sintered pellets of YBa2Cu3−16O7−x and YBa2Cu318O7−x. The latter samples were obtained both by gas exchange and by growth with substituted oxides. Three of the observed Raman modes arise from motion of the oxygen atoms only and have nominal frequencies of 500, 435, and 340 cm−1 at room temperature in the 16O compound with x = 0. The frequencies of these modes shift significantly with oxygen isotope exchange, and these frequency shifts have been measured for several sets of samples. The results indicate that in samples prepared primarily by gas exchange the degree of exchange on the O(2) and O(3) planar sites is greater than the degree of exchange on the bridging O(4) site. Linewidth measurements have revealed that the O(4) phonon (500 cm−1) is significantly broadened by 18O exchange, and this is attributed to a random occupation of the O(4) site by 18O and 16O. The results are discussed in the context of recent measurements of oxygen diffusion in YBa2Cu3O7−x and a proposed mode for the oxygen exchange mechanism. Measurements have also been carried out on YBa263Cu316O7−x and YBa265Cu316O7−x. The results confirm that the Raman mode at 150 cm−1 arises from motion of the Cu(2) planar copper atoms.