High Pressure Research, Vol. 23, No. 1–2, March=June 2003, pp. 111–115 HIGH PRESSURE RAMAN STUDY OF Bi-2212 E. LIAROKAPIS a, *, D. LAMPAKIS a , C. PANAGOPOULOS b and T. NISHIZAKI c a Department of Physics, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece; b Cavendish Laboratory and IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK; c Institute for Materials Science, Tohoku University, Ka\tahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan (Received 4 September 2002; In final form 2 October 2002) The hydrostatic pressure dependence of the Raman spectra of Bi 2 Sr 2 CaCu 2 O 8 single crystals has been investigated. The energy of the A g and B 1g modes was found to increase with pressure in agreement with previously reported measurements, except the strong mode at 465 cm 1 , which softens with pressure, while another peak at 458 cm 1 appears more pronounced at low temperatures and high pressures. The energy of both modes does not seem to change with increasing pressure, up to 5 GPa, although the average energy of the wide band has been found to soften, which is in disagreement with previously published results. Based on the modifications observed in some phonons at 1.8 GPa, which correlate with the reduction of T c and the deformations of the CuO 5 pyramids, we attribute the mode at 465 cm 1 to the vibrations of the apex oxygen atoms. All modes due to the oxygen atoms were found to be strongly anharmonic. Keywords: Raman; Phonons; High pressure; Superconductors; Bi 2 Sr 2 CaCu 2 O 8 INTRODUCTION The compound Bi 2 Sr 2 CaCu 2 O 8 is one of the phases of the Bi compounds, which has two CuO 2 planes as YBa 2 Cu 3 O x , with the Ca atom occupying the central position of the cell, as Y. In the latter the assignment of modes is well known, while for the Bi compound there is a controversy even for the strong A g symmetry modes. Some authors based on the expected similarities between the Bi and Y compounds have assigned the high-energy modes at 630 cm 1 and 658 cm 1 to the apical and the in-phase vibrations of the plane oxygen atoms along the c-axis [1, 2]. Another strong mode at 465 cm 1 was subse- quently assigned by the same authors to the c-axis vibrations of the oxygen atoms in the BiO planes. The inverse assignment, the 630 cm 1 to the Bi oxygen atoms and the 465 cm 1 to the apex oxygen, has been also suggested [3], while others have assigned the mode at 465 cm 1 to the in-phase phonon [4]. In this work we have carried out a hydrostatic pressure and low temperature study of the A g and the B 1g modes. We have found that some modes show modifications in their dependence on pressure around 1.8 GPa. Besides, we find that the mode at 465 cm 1 is accompanied by * Corresponding author. Fax: (þ30210) 7722928; E-mail: eliaro@central.ntua.gr ISSN 0895-7959 print; ISSN 1477-2299 online # 2003 Taylor & Francis Ltd DOI: 10.1080=0895795031000156768