Structural and compositional modifications of fayalite Fe 2 SiO 4 under electron irradiation Carine Davoisne a,b , Hugues Leroux a, * a Laboratoire de Structure et Proprie ´te ´s de l’Etat Solide – UMR CNRS 8008, Universite ´ des Sciences et Technologies de Lille, Avenue Paul Langevin, F-59655 Villeneuve d’Ascq-Cedex, France b Laboratoire de Catalyse de Lille – UMR CNRS 8010, Universite ´ des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq-Cedex, France Received 23 May 2005; received in revised form 5 September 2005 Available online 7 November 2005 Abstract Electron irradiation induced phase decomposition in fayalite Fe 2 SiO 4 has been observed in situ in a transmission electron microscope at 300 keV. Structural and chemical changes were monitored by electron diffraction and X-ray energy dispersive spectroscopy (EDS). Fayalite was observed to collapse at a fluence of 4 · 10 23 e  /cm 2 , without significant chemical changes. Further phase decomposition is accompanied by strong mass loss, in particular oxygen, which is found to be fully removed for elevated electron fluences. The residual Si and Fe elements rearranged to form Fe,Si nanoparticles. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.72.F; 64.70.D Keywords: Electron irradiation; TEM; Microstructure; Phase transformation 1. Introduction Electron irradiation is known to damage a number of oxides compounds. Damages include modification of elec- trical properties (e.g. [1]), formation of point defects such as vacancy and interstitials (e.g. [2]), nucleation and growth of clusters of point defects (e.g. [2–4]), phases transition such as amorphisation for some crystalline oxides (e.g. [2,5]) or crystallization for some amorphous materials (e.g. [6,7]), mass loss and composition changes (e.g. [8,9]). Modifications of oxides under electron irradiation are clo- sely related to the nature of radiation-induced point defects, their formation kinetics and further mobility. In multi-component ceramic-like materials the production rate of point defects is complex and strongly depends on the bonding, lattice arrangement or electronic structure of the given material. Modifications can be due to direct atom displacement due to nuclear collision with energetic electrons, as well as ionization. For both processes of atomic displacement, radiation-induced diffusion has been pointed out. Fayalite Fe 2 SiO 4 is an interesting case of study because it contains a high concentration of Fe which is a multivalent transition metal. In fayalite, Fe cations have the ferrous form (Fe 2+ ). Fayalite is the iron end member of the olivine group of minerals. It belongs to the space group Pbnm (orthorhombic), with lattice parameters a = 0.482 nm, b = 1.048 nm and c = 0.609 nm. The structure consists of isolated SiO 4 4 tetrahedra bounds to each other by ionic bonds from Fe cations occupying octahedral sites. The structure of fayalite can be also viewed as a nearly hexagonal close-packed array of oxygen, with the Si cations occupying one eighth of the tetrahedral sites and the Fe cations occu- pying half of the octahedral sites. Fayalite is also an important compound in circumstellar environments. In nature, fayalite is mainly found in solid solution with forsterite Mg 2 SiO 4 , and forms the 0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.09.012 * Corresponding author. Tel.: +33 320 336416; fax: +33 320 436591. E-mail address: Hugues.Leroux@univ-lille1.fr (H. Leroux). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 243 (2006) 371–376 NIM B Beam Interactions with Materials & Atoms