Abstract

The YBa2Cu3O7−y superconducting system at variable O deficiency (0.05 < y < 0.85), prepared by means of the gettered annealing technique, has been investigated by IR absorption in the 3000–400-cm−1 spectral range. The phase transition between the tetragonal and the orthorhombic structures that occurs in the narrow range of O deficiency between y = 0.69 and y = 0.80 is accompanied by a dramatic change of the IR phonon spectra. Because the formation of the 60-K superconducting plateau occurs at y = 0.5, the phase transition does not have any correlation with the onset of superconductivity. A strong band at 591 cm−1 in the tetragonal phase, which vanishes after the occurrence of the phase transition, is assigned to a defect-induced transition at k ≠ 0, marking the phonon density-of-states profile of the Cu(1)–O(1) and Cu(1)–O(5) stretching modes. The nature of the defects is related to the disordered arrangement of the O(1) and O(5) atoms situated between the Ba planes. The 640-cm−1 band of the orthorhombic phase is assigned to the k = 0 Cu(1)–O(1) IR stretching mode. A new IR phonon mode at 573 cm−1 (for y ≤ 0.38) is tentatively assigned to the Cu–O axial antisymmetric stretching mode.

© 1989 Optical Society of America

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