J. Phys.: Condens. Matter 2 (1990) 3135-3139. Printed in the UK LETTER TO THE EDITOR Normal state charge redistribution as determined by the Raman scattering from orthorhombic and tetragonal single crystals of Y Ba,Cu,O, - Milko N Iliev and Victor G Hadjiev Faculty of Physics, Sofia University, 1126 Sofia, Bulgaria Received 22 January 1990 Abstract. A polarised Raman study of YBa,Cu307_b single crystals revealed significant changes of light scattering anisotropy with reducing oxygen content. Strongly (y, y) and ( x , x) polarised scattering from free carriers and its coupling with the phonon system (Fano interference) are characteristic for the orthorhombic crystal (6 = 0). In contrast, in the tetragonal crystal (6 = 0.8) enhanced continuum scattering and asymmetry lines were observed only in the tetragonal (z, z) polarisation. These findings are attributed to the reduction of free carriers in the xy Cu02 planes due to chains-to-planes charge redistribution initiated by the decreased oxygen concentration. The electronic light scattering of superconducting YBa2Cu307 - single crystals mani- fests itself through an anisotropic background continuum which exhibits interference with the 118 and 335 cm-' vibrational modes [ 1,2]. The existence of electronic scattering in (x, x) and Cy, y) polarisation geometries correlates with the fact that the charge carriers (holes) are distributed within the 02-Cu2-03 planes [3]. The stronger scattering in Cy,y) polarisation provides evidence that the Cul-04 chains along the b axis also influence the scattering process as well [l]. It has been recently suggested [4] on the basis of the step-like stoichiometric depen- dence of some bonding lengths and interatomic distances that the electronic charge transfer from chains to the planes plays a significant role in the suppression of super- conductivity in oxygen deficient R B a 2 C ~ 3 0 7 - 6 compounds (0 < 6 < 1). According to the picture described in [4], the removal of 0 4 atoms gives excess electrons which still stay on the Cul-04 chains at low 6. With increasing 6 the capacity of Cul-04 chains to hold charge is exhausted and electron transfer from the chains to the planes begins. The net effect of this mechanism is a reduction of the hole concentration in the pyramidal planes. Another experimental proof for charge redistribution in the Cu02 planes of RBa2C~307-6 (R = Er, Ho; 0 < 6 < 1) has been obtained by means of neutron crystal- field spectroscopy [5]. The charge redistribution should affect both the phonon and electronic scattering. In particular, the asymmetry (the Fano line shape) of the Raman line at 339 cm-l (Big) is lost in the tetragonal phase [6,7] and it could be attributed to the strong reduction of the free carrier concentration in the Cu2-02,3 planes. It should be noted that no conclusions about the existence of a (z, z) polarised electronic continuum (arising from interband excitations [2]) could be drawn in this case as the B1, phonon is not seen in 0953-8984/90/133135 + 05 $03.50 @ 1990 IOP Publishing Ltd 3135