Hyperfine Interactions 66 (1991) 429-434 429 JAHN-TELLER CALCULATIONS FOR Cut 4 AND FeO 4 CLUSTERS IN THE NdLssCeo.lsCuo.99Feo.010 4_ 8 A. CALLES 1, E. YEPEZ 2, j.j. CASTRO 3, A. SALCIDO a, A. CABRERA l, R. GOMEZ 1, S. ABURTO 1, V. MARQUINA 1, M.L. MARQUINA 1, M. JIMENEZ 1 and R. ESCUDERO 4 I Departarnento de Fisica, Facultad de Ciencias, UNAM, Apdo. Postal 70-646, 04510 M~xico D.F. 2 Escuela Superior de Fisica y Matemrticas, IPN, 07738 M~xico D.F. Departamento de Fisica, CINVESTA V, IPN, Apdo. Postal 14-740, 07340 Mdxico D.F. 4 Instituto de Investigacibn en Materiales, UNAM, 04510 Mdxico D.F. The Jahn-Teller distortion mechanism in the Nd2_xCexCuO4 is analyzed. This analysis is based on a quasi-molecular model for the Cut 4 cluster having D4h symmetry. In order to compare with experimental MiSssbauer measurements in the doped Ndl.85CeoAsCuo.99Feo.01 04_ 8 superconductor, the FeO4 cluster is also analyzed for the case of Fe 2+ in the high spin state. The results show not evidence of a Jahn-Teller effect in these superconductors. 1. Introduction Since the discovery of high T~ superconductivity, the understanding of the mechanism responsible for the pairing condensation still remains as an open problem. In their original paper, Bednorz and Mtiller [1] suggested that the Jahn-Teller [2] (JT) effect might be a possible mechanism to explain the electron- phonon coupling in these cuprates. In order to have an understanding of whether this effect plays a role in the Nd-Ce-Cu-O superconductor, in this paper we present a JT analysis in the CuO4 cluster based on a symmetry classification of the actual electronic ground state wave function as determined by a quasi-molecular model [3] in an extended H0ckel calculation, and the normal modes of vibration obtained by solving the corresponding dynamic matrix. In section 2 we present the normal modes for the CuO4 cluster. Section 3 presents the electronic states for the MO 4 cluster (M = Cu, Fe) with the relevant oxidation states. The FeO4 analysis is presented in order to compare with preliminary experimental data obtained by M/Sssbauer spectroscopy which are included in section 4. 2. Normal modes of vibration The normal modes of vibration for the Cut 4 were obtained by solving the dynamic matrix. The equilibrium position for the ions were taken from the work O J.C. Baltzer A.G. Scientific Publishing Company