Structural changes induced by swift heavy ions in non-metallic compounds Paolo M. Ossi * Istituto Nazionale per la Fisica della Materia, INFM – Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy Abstract Irradiation of crystalline non-metallic binary A x B y compounds with swift heavy-ions results in electronic excitation and damage track formation, beyond a threshold value of stopping power. As the cores of the cylindrical damage tracks are experimentally shown to be homogeneous, each ion is supposed to deposit its energy over all atoms of the crystal involved in the interaction process. We assume that within an ionisation cylinder coaxial with the damage cylinder each atom, taken as isolated, undergoes n multiple ionisation events. Both A and B target atoms are progressively stripped of the same number n of electrons, beginning with the outer shell. Thus they change atomic configuration from e.g. A to ðA  nÞ¼ A  . All ionised atoms (A  and B  ) are ejected out of the ionisation cylinder, being spread in the damage cylinder where they form the ½A  B and the ½AB   starting compounds. By the segregation-charge transfer model, the interface between the crystalline A x B y matrix and the damage cylinder, containing the starting compounds, is enriched in one starting compound constituent, giving rise to non-equilibrium compositional and electronic density profiles. Charge transfer reactions simulate the local trend towards equilibrium restoration. Each reaction product is a dimer, considered as a cluster of an effective compound. The energy cost to introduce in the matrix an effective compound dimer and the difference between the sum of the formation enthalpies of both effective compounds and that of the irradiated compound are calculated. Qualitative differences are found between compounds undergoing amorphisation, or crystal structure formation under swift heavy-ion irradiation. Ó 2003 Elsevier B.V. All rights reserved. PACS: 61.80.Az; 61.82.Ms; 64.70.Kb Keywords: Non-metallic compounds; Irradiation; GeV heavy ions; Amorphisation 1. Introduction When inelastic collisions play the major role to the slowing down of a highly energetic ion in an irradiated solid the electronic excitation channel prevails. As a rule in this kind of experiments ion fluxes and fluences (of the order of 10 8 ions cm 2 s 1 and 10 12 ions cm 2 , respectively) are so low that no superposition of the effects of different projectiles occurs. Indeed TEM micrographs of damaged regions show that ion tracks are homo- geneous cylinders with average radius r d of a few nm, each one corresponding to an incoming pro- jectile. In the case of binary non-metallic com- pounds the observation is ubiquitous, irrespective of the structural changes suffered by the target * Tel.: +39-02-23996319; fax: +39-02-23996309. E-mail address: paolo.ossi@polimi.it (P.M. Ossi). 0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(02)02060-8 Nuclear Instruments and Methods in Physics Research B 209 (2003) 55–61 www.elsevier.com/locate/nimb