E.P.R. study of electron irradiation defects in the red molybdenum bronze of potassium K0.33MoO3

Defects induced by electron irradiation in the red molybdenum bronze of potassium K0.33MoO3 have been studied by E.P.R. spectroscopy. Two main types of centers (A and B) have been observed. They both have been identified as unpaired 4d electrons localized on one molybdenum atom with a weak interaction with two other ones. A very simple electronic model is proposed to explain qualitatively the main features of these two paramagnetic centers. Measurements of the spin-lattice relaxation time of defect A show a temperature variation much slower than expected from the standard theory. This unusual dependence (laws in T3 or T5) can be explained by an extended theory which takes into account the contributions of lattice vibrations at the defect sites.Nous avons étudié par R.P.E. les défauts créés dans K0,33MoO 3 par irradiation aux électrons. Nous avons ainsi mis en évidence l'existence de deux types de défauts (A et B) présentant de nombreux points communs : tous deux sont constitués d'un électron 4d non apparié localisé préférentiellement sur un atome de molybdène et couplé beaucoup plus faiblement avec deux autres atomes de molybdène. Nous proposons un modèle très simple pour la structure électronique de ces centres paramagnétiques. Les mesures du temps de relaxation spin-réseau ont montré une dépendance en température très inhabituelle (lois en T3 et T5) qui peut être expliquée en considérant un modèle de phonons localisés au voisinage du défaut

Low-flux neutron irradiation of ceramic YBa2Cu3O7−δ

Pellets of sintered YBa 2Cu 3O 7-δ with three different oxygen contents have been irradiated with fast neutron beams of energies 6.5, 3.3 and 4.4 MeV at fluences of 7.7·10 4, 1.3·10 5 and 1.4·10 9 n/cm 2, respectively. The radiation damage has been investigated by comparing the critical temperature (T c mid ), the zero resistivity and the onset temperature before and after neutron irradiation. The critical current has been measured for a few samples in the same experimental conditions. In all transport measurements two different responses to the neutron radiation are observed and discussed