COEXISTENCE OF VALENCE FLUCTUATING AND STABLE PR IONS IN PR6O11

Inelastic scattering experiments were performed with thermal neutrons on Pr6O11 in order to investigate the magnetic excitation spectrum. Two different kinds of behaviour were found in the response: first a rather broad feature, which is purely quasielastic for high temperatures and becomes inelastic at low temperatures, and second very narrow inelastic excitations which are interpreted as crystal-field (CF) splitting. This different behaviour can be explained as a consequence of vacancies on the oxygen sites in the cubic CaF2 structure. This yields Pr sites with at least two different coordination numbers. On sites with the full cubic coordination number of eight the Pr ion is intermediate valent (broad feature) and on sites with a lower coordination number the Pr ion is trivalent (CF excitations).