Swift heavy ion amorphization of quartz — a comparative study of the particle amorphization mechanism of quartz

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Previously we studied the modifications of the atomic and electronic structure induced in monocrystalline quartz by a swift heavy ion bombardment. Quartz samples were irradiated with 58Ni, 86Kr, and 208Pb ions. Kinetic energies correspond to the electronic regime, i.e. several MeV/amu and typical fluences were of the order of 1011–1012 ions/cm2. Structural modifications were investigated by X-ray absorption spectroscopies (XANES, EXAFS). Modifications of the valence band were studied by X-ray photoelectron spectroscopy (XPS). Electronic spin resonance (ESR) was used to measure the concentration of bulk paramagnetic E′ point defects.With increasing absorbed dose, quartz samples undergo a crystalline to amorphous transition. Irradiation damages consist mainly of extended defects. These extended defects are composed of an amorphous SiO2 state. These amorphized regions concentrate the most of the E′ point defects and appear slightly densified when compared with common relaxed silica (a-SiO2). The measurement of the densification indicates a value of about 4%.This work deals with the amorphization mechanism of quartz. This poorly densified silica is similar to that obtained by neutron irradiation or ion irradiation (within the nuclear regime) or electron irradiation or photon irradiation of quartz and silica. The fact that various bombardment processes give identical final amorphization products, suggests that particle amorphization of quartz proceeds through a specific mechanism. We suggest that this specific mechanism consists of a structural relaxation process of a highly damaged quartz matrix, triggered by a critical point defect concentration. Comparison of significant particle irradiation experiments allows the paramagnetic part of this critical point defect concentration to be estimated to be at least equal to 1 × 1018E′ point defects/g. The various irradiations (ions in electronic and nuclear regime, neutrons, electrons and photons) provide different damaging cross sections; they scale with point defect cross sections. The proposed two-step mechanism ((1) point defect creation, (2) intrinsic relaxation process) is a generalization of that one proposed by Hobbs and Pascucci for electron irradiated quartz.Previous experimental results, such as structural differences between particle amorphized quartz and plastically densified silica, are discussed within the proposed mechanism.