ANOMALOUS BEHAVIOR OF THE 147 CM(-1) CU(2) RAMAN MODE IN YBA2CU4O8 UNDER HIGH-PRESSURE - SIGNATURE OF CHANGE IN THE ELECTRONIC-STATE OF THE CUO2 PLANE

The effect of pressure on Raman modes of ceramic superconducting samples of YBa2Cu4O8 has been investigated up to 16 GPa at room temperature. While frequencies of the modes at 101, 250, 310, 339, 500 and 605 cm(-1) increase almost linearly with increasing pressure, the mode at 147 cm(-1) corresponding to the z-motion of Cu(2) atoms in the CuO2 plane exhibited a nonlinearity at pressure exceeding 6 GPa. A correlation is observed between the pressure-induced changes in T-c and the change in the frequency of the Cu(2) mode. We propose that the anomalous nonlinear behaviour of the Cu(2) mode in the high pressure regime is a signature of changes in the electronic state of the CuO2 plane resulting from a charge carrier redistribution between Cu(2) and O(2/3) in the conducting CuO2 plane.