J. Morillo

Phonon Soft Modes and Damage Production by High Electronic Excitations in Pure Metals

It is now well known that during high-energy heavy-ion irradiation, the very high-energy deposition in electronic excitation induces a spectacular damage creation in some types of metallic targets. A selected number of pure metals has been irradiated by GeV ions in order to test some possible criteria which might be pertinent to explain such effects: electron-phonon interaction, electrical conductivity, existence of various allotropic phases,.... The present results show that the latter criterion or more precisely that the existence of a displacive transformation associated with a soft mode in the phonon spectrum seems to favour efficient energy transfers between highly excited electrons and target atoms. For titanium targets, electron microscopy observations show striations which are parallel to the incident ion beam direction.

Neutron and electron radiation defects in titanium and tantalum monocarbides: An electrical resistivity study

Abstract The electrical resistivities of nearly stoichiometric titanium and tantalum monocarbides (TiC0.97 and TaC0.99) have been measured during fast neutron and 2.5 MeV electron irradiations at 21 K. The recovery of defects was followed up to 400 K. The radiation-induced resistivity increases were 10 times larger in TiC than in TaC. Only slight differences were observed between post-electron and neutron irradiations recovery spectra with a single wide stage centered around 270 K for TiC0.97 and three well defined stages around 80, 175 and 280 K for TaC0.99. Recovery was only partial at 400 K (between 60 and 80%). Results on yttrium monosulphide are given for comparison. The contributions of the different defects to the damage rates and the recovery spectra are tentatively separated.

Electron irradiation damage in stoichiometric and substoichiometric tantalum carbides TaC x part 1: Thershold displacement energies

Abstract The damage rate (electrical resistivity increase per fluence unit δρ/δΦ) has been measured in cubic tantalum carbides TaC0.99 and TaC0.80, during irradiation at 21 K by electrons of energies E ranging from 0.4 to 2.7 MeV. Both experimental δρ/δΦ versus E curves clearly display two thresholds at E < 0.35 MeV and E < 1.5 MeV. Comparison with theoretical displacement cross-sections calculated for TaC allowed us to attribute unambiguously the first threshold to carbon displacements and the second one to tantalum. Surprisingly, in the largely substoichiometric TaC0.80 sample, irradiation still creates carbon Frenkel pair type defects. Taking into account the real irradiation conditions, the deduced parameters are TC d = 28 ± 6eV and Td Ta = 42 ± 2eV for the threshold displacement energies in TaC0.99 (resp. 28 ± 6 and 32 ± 2eV in TaC0.80), and ρC F = 26 ± 5 μΩ-cm/at.%. for the specific mean resistivity of carbon Frenkel pair in TaC0·99.

Semimetallic behaviour of SmS at high pressure

Abstract The transition from semiconducting to metallic intermediate-valence behaviour occurring at 20 kbar in SmS in studied by resistivity and Hall coefficient measurements (1.2 K ⪕ T ⪕300 K, 4 kbar ⪕ P ⪕21 kbar). The data are analyzed in a two-band-conduction model. It is concluded that an indirect band gap closes at 20 kbar and that a direct one subsist above this transition.

Radiation damage in SmS, SmS1−xPx and SmB6

Abstract Large conductivity increases under electron or neutron irradiations at 21 K are observed in SmS and SmS 1− x P x . It is shown that they are related to Sm defects. A possible mechanism is 4f electron delocalization around radiation defects. In SmB 6 , the low temperature resistivity increase disappears under irradiation at 21 K. The thermal stability of the defects is also investigated up to room temperature.

Semiconductor to metal transition in the black phase of SmS under pressure

Abstract Hall coefficient and resistivity of black phase of SmS were measured as functions of temperature and pressure up to 10 kbar. An interpretation of these electronic transport properties involving acceptor-like bound magnetic polaron levels is presented. It implies that the semiconductor to metal transition observed at 4 kbar originates from an overlap of 4f and 5d bands.

Electron Radiation Defects in TaC1−x and TiC0.97

The electrical resistivity changes of TaC0.99 and TaC0.80 have been measured at 21 K during irradiation with electrons of incident energies ranging from 2.5 to 0.25 MeV: a non-zero production rate is observed, even at the lowest energies. The recovery of defects was followed up to 400 K for TaC0.99 and TiC0.97 irradiated with 2.25 MeV electrons and up to 160 K for TaC0.80 irradiated with 0.75 MeV electrons. The results are compared to fast neutron radiation damage data. For TiC0.97 and TaC0.99, the contributions of the different defects to the production rates and recovery spectra are tentatively separated, and a rough estimate of Frenkel pair resistivities is given.