LATTICE DYNAMICAL POTENTIAL OF THE APICAL OXYGEN IN YBA2CU3O7

In order to investigate the behavior of the bridging oxygen in YBa2Cu3 O7 we have calculated the total energy varying the c-coordinate of thi s atom. The calculations were performed by the full potential LAPW met hod which is based on density functional theory. In contrast to the in terpretation of EXAFS measurements we do not find a double well potent ial. From our results we obtain the harmonic vibrational frequency whi ch is in good agreement with Raman measurements of the 500 cm-1 Al(g) phonon mode and calculations done by other groups. The potential is fo und to be anharmonic with a relevant third order term for displacement energies up to 1000 K.