LOW-TEMPERATURE RAMAN-STUDY OF THE YBA2CU3OX ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-6.98) SUPERCONDUCTOR

Microcrystallites of the series of YBa2Cu3Ox superconductors with oxyg en doping in the range 6.44 less than or equal to x less than or equal to 6.98 have been studied by Raman spectroscopy at low temperatures. The abrupt softening of the O(2,3) in-phase phonon above optimum dopin g, previously reported at room temperature persists to the lowest temp eratures studied (similar to 5K). The dependence of the linewidth of t his mode on the oxygen concentration at low temperatures agrees, in ge neral, with the already known room temperature dependence, except at t he optimum doping where the phonon remains very wide, an indication of a phase transition. The temperature dependence of the phonon characte ristics of the in-phase plane oxygen mode indicate an anharmonic behav iour which increases with decreasing x, in agreement with previously r eported data at room temperature obtained with oxygen isotopic substit ution. A similar anharmonicity but to a lesser degree is also observed for the apical phonon.