ANOMALOUS BEHAVIOR OF THE 147 CM(-1) CU(2) RAMAN MODE IN YBA2CU4O8 UNDER HIGH-PRESSURE - SIGNATURE OF CHANGE IN THE ELECTRONIC-STATE OF THECUO2 PLANE

The effect of pressure on Raman modes of ceramic superconducting sampl es 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 14 7 cm(-1) corresponding to the z-motion of Cu(2) atoms in the CuO2 plan e 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 non linear behaviour of the Cu(2) mode in the high pressure regime is a si gnature 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.