NEUTRON-SCATTERING STUDY OF CHAIN-OXYGEN VIBRATIONS IN YBA2CU3O7

The ab-plane-polarized transverse chain-oxygen vibrations in YBa2Cu3O7 were investigated by inelastic neutron scattering on an untwinned sin gle crystal at different temperatures. From the rather high frequencie s of these modes (nu almost-equal-to 5 THz), the moderate phonon linew idth, and the absence of an appreciable frequency shift with temperatu re, we conclude that the chain-oxygen vibrations are not strongly anha rmonic. Concurrently, diffraction data are incompatible with a wide do uble-well potential for the chain-oxygen atoms.