Influence of the crystallite size on the phase transformation of yttria irradiated with swift heavy ions

The irradiation effects induced by swift heavy ions are now widely describe d in `bulk` materials. It is shown here that tile behaviour of matter under irradiation depends on its crystallille state in the sense that a given ma terial is all the more sensitive to swift heavy ion irradiations as tile me an crystallite size L is small. The present paper relates the experimental results obtained in yttrium oxide from 'in situ' X-ray diffraction measurem ents. Three kinds of sample have been irradiated: sintered samples (L = 1 m um), non-ground powders (L = 45 nm) and ground powders (L = 28 nm). A cubic to monoclinic phase transformation appears if the electronic energy loss o f the incident particle is higher than a threshold. The comparison between the different kinds of samples reveals that this phase transformation is al l the easier as the mean crystallite size of the target is weak.